Binaphthyl-based macromolecules: a review

Cite this: RSC Advances, 2013, 3, 6717

Binaphthyl-based macromolecules: a review

Received 5th October 2012,Accepted 14th January 2013

DOI: 10.1039/c3ra22418j

www.rsc.org/advances

Abbas Shockravi,* Ali Javadi and Ebrahim Abouzari-Lotf

Binaphthyl-based systems have been under extensive investigation during the past years as sensors, host

molecules, asymmetric catalysts, chiral-conjugated polymers, and high performance materials. This review

mainly focuses on the synthesis, structural investigation, and applications of binaphthyl-based

macromolecules. It begins with a brief overview on design and synthesis of direct or indirect-linked

binaphthyl-based monomers. Fundamentals of the synthesis and macromolecular structure of binaphthyl

hosts as chiral catalysts for various asymmetric organic reactions are then provided. In addition, recent

complexation studies of some podands and macrocycles derived from binaphthyls are highlighted.

Furthermore, the synthesis and properties of main chain chiral conjugated polymers having optically active

1,19-binaphthyls are discussed. Finally, we focus on recent work involving the incorporation of binaphthyl

units into the macromolecules backbone to prepare processable high performance polymers.

1 Introduction

Binaphthyl-based systems represent an important class ofmaterials as a result of their bulkiness and possible restrictedrotation in some isomers including 1,19-binaphthyls. In thesesystems, non-coplanar conformation of two bulky naphthylgroups and restricted rotation about the transannular bondlead to special properties such as optical activity. Since 1970s,Cram started using these optically active moieties to preparevarious molecular hosts for chiral recognition. For example, heand co-workers synthesized the first chiral crown ethers by

introducing chiral binaphthyl units into the cyclic polyetherbackbone. Their work made it possible to direct crown etherchemistry into the scientific fields such as asymmetricsynthesis and enzyme-mimicking. In recent years, chirality ofcoordination compounds has received considerable attention.This can be attributed to several factors including the role ofchiral metal complexes in asymmetric synthesis, advances inmetallosupramolecular chemistry, and development of bioi-norganic chemistry. In the 1980s, research on the transitionmetal complexes of 1,19-binaphthyl-based ligands for theasymmetric hydrogenation of alkenes and carbonyl com-pounds was initiated by Noyori and Takaya. In addition,several reports by other scientists on using the 1,19-binaphthyl-based systems for molecular recognition and asymmetric

Faculty of Chemistry, Kharazmi University (former Tarbiat Moallem University),

Tehran, Iran. E-mail: [email protected]; [email protected];

Fax: +98-21-88820993; Tel: +98-912-2596174

Prof. Abbas Shockravi receivedhis Master’s degree in OrganicChemistry from Eastern MichiganUniversity (USA) in 1977 and hisPhD degree in Organic Chemistryfrom the University of Manchester(UK) in 1992. He has been aFaculty member at the KharazmiUniversity (Tehran, Iran) since1977, where he is currently aFull Professor of Organic-PolymerChemistry. His research teamfocuses on supramolecular chem-istry, sulfur-containing materials,

podands and macrocyclic compounds, high performance polymers,membranes, and nanocomposites.

Dr Ali Javadi received hisPhD degree in Organic-PolymerChemistry from KharazmiUniversity (Tehran, Iran) underthe supervision of Prof. Shockraviand Prof. Kuckling in 2012 on thesynthesis and characterization offluorinated aramids and thinhydrogel layers. He was a visitingresearch scholar in 2011 whilstworking in Prof. Kuckling’s groupat the University of Paderborn inGermany. His current researchinterests include high perfor-

mance polymeric materials, fluorinated polymers, membranes,smart polymers, thin polymer films, hydrogels, and targeted drugdelivery.

Abbas Shockravi Ali Javadi

RSC Advances

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catalysis have appeared and a large number of enantioselectivecatalysts and molecular receptors have been synthesized.

The use of optically active 1,19-binaphthyls has not beenrestricted to the field of host–guest complexation chemistry.Since the 1990s, Pu’s laboratory has started a plan for usingthese materials to generate new chiral materials including theenantioselective fluorescent sensors, light harvesting chiraldendrimers, monomeric and polymeric chiral catalysts, andmain chain chiral-conjugated polymers. Chiral-conjugatedpolymers are a class of conjugated materials with greatpotentials. These materials have several applications inelectrodes, ferroelectric liquid crystals, polarized light emis-sions, non-linear optical (NLO) materials, and chiral sensorsfor the detection of chiral molecules. Incorporation of1,19-binaphthyl units into the main chain of conjugatedpolymers results in new main chain chiral-conjugated poly-mers with stable chiral configuration.

Aromatic polyamides and polyimides are well known fortheir high thermal stability, good mechanical strength andoutstanding chemical resistance that qualify them as highperformance materials. However, most of them are difficult toprocess due to their limited solubility in common organicsolvents and high melting or glass transition temperaturescaused by the highly rigid and regular polymer backbones andthe strong intermolecular interactions. Therefore, variousattempts have been made to improve their solubilities andprocessabilities either by introducing flexible linkages, bulkypendant groups, or fluorinated substituents into the polymerbackbones. Recent studies demonstrated that the introductionof naphthalene groups as rigid, bulky, and heat resistantstructures, into the polymer chains can increase the solubilityand processability of aromatic polyamides, polyimides, andtheir copolymers. Another attractive method employed toimprove the solubility of polyamides and polyimides withoutdeteriorating their inherent excellent properties is the incor-poration of less symmetric units such as ortho-linked aromatic

rings in the main chains, which leads to a reduction incrystallinity. 1,19-Binaphthyls functionalised at the 2- and29-positions are considered as good candidates for synthesiz-ing high performance polyamides and polyimides. Indeed, theformation of non-coplanar conformation because of thepresence of direct or indirect ortho-linkage between twoaromatic naphthyl rings can disturb interchain packing,decrease glass transition temperature and increase solubility.

The unique structure of the chiral direct-linked binaphthylshas led to extensive studies on the use of these compounds ina variety of fields, especially in chiral catalysts for asymmetricorganic reactions and main chain chiral-conjugated polymers.Over a decade ago, our laboratory launched a program to usethe indirect-linked binaphthyls to study the complexations ofthese compounds with a variety of transition or heavy metals.We have also been particularly interested in using thesecompounds as bulky units to construct structurally diversehigh performance polymers. The aim of this review is not tomake a full compilation of studies on all binaphthyl-basedsystems but instead to cover major structural aspects, proper-ties, and applicative potential of these materials. We begin thisreview with an overview of direct- and indirect-linkedbinaphthyl-based monomers. The design and synthesis ofsome binaphthyl-containing chiral catalysts for various asym-metric organic reactions are then highlighted. In addition, thecomplexation studies of some binaphthyl-based hosts with avariety of guests are provided. Moreover, the synthesis andstructure of the main chain chiral-conjugated polymers isdescribed here. Finally, synthesis and properties of highperformance binaphthyl-based polyamides, polyimides, andtheir copolymers are discussed.

2 1,19-Binaphthyl-based monomers

2.1 Monomers derived from direct-linked 1,19-binaphthyls

1,19-Binaphthyl compounds are a unique class of biarylmolecules. For an unsubstituted biphenyl molecule, itsrotation barrier around the phenyl–phenyl bond in gas phasewas determined to be y1.4 kcal mol21.1 1,19-Binaphthylincorporated with fused benzene rings to biphenyl has asignificantly increased rotation barrier of 23.5 kcal mol21.2

Although the biphenyl molecule has the plane of symmetry atthe orthogonal conformation, but 1,19-binaphthyl molecule nolonger has such a plane of symmetry. Because of this fact, theisolation of the optically active 1,19-binaphthyl enantiomers ispossible. A chiral 1,19-binaphthyl system has a chiral axisinstead of a chiral center. The optically active 1,19-binaphthylhas a racemization half-life of 14.5 min at 50 uC. In fact,1,19-binaphthyl systems have two possible racemizationroutes. A syn-interaction state where there are close contactsfor the 2,29-hydrogens and 8,89-hydrogens is one of theseroutes. Moreover, another route goes through an anti-interac-tion state where there are close contacts for the 2,89-hydrogensand 29,8-hydrogens. The anti-route gives lower steric hin-drance than the syn-route when theoretical models areapplied.

Dr Ebrahim Abouzari-Lotf earnedhis PhD in Organic-PolymerChemistry from KharazmiUniversity (Tehran, Iran) underthe supervision of Prof. Shockraviand Dr Ghassemi in 2010. As avisiting research scholar, heworked in Prof. Schiraldi’s andProf. Zawodzinski’s groups atCase Western Reserve University,USA in 2009. He is currently anAssistant Professor in theMembrane Research Group atACECR (Kharazmi University).His research interests includeproton-exchange membranes, fuelcells, aerogels, high performancepolymers, and nanocomposites.

Ebrahim Abouzari-Lotf

6718 | RSC Adv., 2013, 3, 6717–6746 This journal is � The Royal Society of Chemistry 2013

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In 1971, it was found that when the racemic 1,19-binaphthylcrystallized from the melt it underwent spontaneous resolu-tion to produce the optically active R or S enantiomer.3 Theprobability to produce the R or S enriched enantiomer wasalmost same when out of the 200 crystallization experimentsof rac-1,19-binaphthyl examined. Besides, when substituentswere introduced to the 2,29-positions of 1,19-binaphthylmolecule, the chiral configuration of the 1,19-binaphthylcompounds became very stable. For instance, (S)-1,19-binaphthyl-2,29-dicarboxylic acid did not racemize at 175uC in N,N-dimethyl formamide (DMF), and 2,29-dimethyl-1,19-binaphthyl did not racemize after 40 h at 240 uC.4,5

As shown in Fig. 1, an optically pure 1,19-binaphthyl moleculecan exist in two conformations, i.e. cisoid and transoid. Thedihedral angle between the two naphthalene rings is less than90u in the (S)-cisoid conformation, whereas in the (S)-transoidconformation, the dihedral angle is greater than 90u.According to the crystal structure of rac-1,19-binaphthyl, itexists in the cisoid conformation with a dihedral angle of68u.6,7 However, its optically active crystals exist in the transoidconformation with a dihedral angle of 103u. Research on

various 2,29-substituted 1,19-binaphthyl molecules has demon-strated that when the 2,29-substituents L are either small orcapable of intramolecular hydrogen bonding, such as –OH,–CH2OH, –OCH2COOH, –NH2, –OCH3, –CH3 or –OCH2Ph, thecisoid conformation is preferred. On the other hand, whenthe 2,29-substituents are large, such as –CH2Br or –CHBr2, thetransoid conformation is preferred.8–11

The 2,29-substituted 1,19-binaphthyls have been extensivelyused to control many asymmetric processes and have demon-strated excellent chiral discrimination properties because oftheir highly stable chiral configuration. Many of1,19-binaphthyl molecules are C2 symmetric having twoidentical naphthyl groups. The C2 symmetry as well as therigid structure of the chiral binaphthyl molecules are found tobe important for their contributions in chiral induction. Also,many binaphthyl-based C1 symmetric catalysts or reagentshave been synthesized and studied.12–14 Since the early 1970s,studies on the use of chiral binaphthyl-based macrocyclesincluding crown ethers, cyclophanes, and cyclic amides ashost compounds for molecular recognition have been con-ducted in Cram’s research group.15–37 These compoundseither undergo complexation-induced organization in thepresence of guests or have pre-organized cavities based onthe rigid structures. They can interact with the functionalgroups of guest molecules through weak forces like hydrogenbonding, p–p stacking, and van der Waals forces. In thesehosts, the chirality of the binaphthyl units results in theirenantioselective complexation with chiral guest molecules.Such a chiral recognition has been used to the resolution ofracemic molecules such as amino esters, amino acids, amines,sugars, and other chiral aryl or alkyl compounds.

For instance, it was found that the bisbinaphthyl macro-cyclic ether (R,R)-2.1 with specific optical rotation of [a]578 =+152 is an outstanding chiral host in the differentiation of theenantiomeric salts of chiral amines, chiral amino acids, andamino esters.17,18,21,22 The chiral recognition factors, DA/DB,(DA is the distribution coefficient of the enantiomer morecomplexed and DB is that of the less complexed) as high as 52for PhCH(NH3

+)CO2HClO42, 48 for

p-HOC6H4CH(NH3+)CO2HClO4

2, and 31 forPhCH(CO2CH3)NH3

+PF62 were observed when (R,R)-2.1 or

(S,S)-2.1 was used for extraction of these ammonium saltsfrom their water solution into the organic phase. TheD-enantiomers of the amino acid or ester salts bound withthe host (R,R)-2.1 more favorably than the L-enantiomers did.This high chiral recognition was achieved because of thecomplementary complexation between the host and the guest.According to the Corey–Pauling–Koltun (CPK) model, thepreferred complex between a R chiral amino ester and (R,R)-2.1 had a structure as shown by the structure 2.2. In this host–guest complex, the binding between the amino ester salt andthe crown ether involved a p–p attractive interaction of theester group with a naphthalene ring and three O–H–Nhydrogen bonds. Such a four-point binding was necessary forthe observed high chiral discrimination.

Fig. 1 Conformations of 2,29-substituted 1,19-binaphthyls.


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Also, the macrocyclic compound (R,R)-2.1 was covalentlybound to polymer resins to prepare the polymeric material(R,R)-2.3 as a chiral stationary phase for enantioselectivechromatography.26 These chromatography columns gave base-line separation for the enantiomers of several amino acid andamino ester salts. The chiral recognition behavior of theimmobilized host corresponded that of the macrocycle itself.

In addition to the research on the binaphthyl-based crownethers for chiral recognition, the discovery of 2,29-bis(diphe-nylphosphino)-1,19-binaphthyl (BINAP)-based chiral catalystsin asymmetric hydrogenation is another important issue indevelopment of the binaphthyl chemistry. As an example, therhodium complex of BINAP, (S)-2.4, catalyzed the hydrogena-tion of enamides to produce the a-amino acid derivatives withoutstanding enantiomeric excess (Scheme 1).38

The dihydride complex 2.5 was also proposed as anintermediate for the asymmetric hydrogenation process. Thehydride and alkene ligands could undergo migratory insertionand then reductive elimination to produce the N-acyl aminoacid product. The chelated chiral BINAP ligand governed thestereochemistry for the hydride-alkene reaction.

In the study of 1,19-binaphthy-based systems, 1,19-bi-2-naphthol (BINOL) usually serves as the starting material tochiral binaphthyl compounds.12–14 The 2,29-hydroxyl groups ofBINOL can be simply converted to other functional groups.Also, the 3,39-, 4,49- and 6,69-positions of this molecule can beselectively functionalized to generate a variety of binaphthylderivatives. Due to the importance of this molecule, significantattention has been devoted to synthesize it in the opticallypure forms and several procedures have been reported.36,39–44

Among these procedures, the use of (8S,9R)-(2)-N-benzylcinchonidinium chloride to resolve racemic BINOLinto its optically pure (R)- and (S)-enantiomers received muchattention due to its simplicity and efficiency when used inorganic laboratory (Scheme 2).45–48 This procedure was

initially developed by Toda et al.46,47 and was later improvedby Pu’s laboratory.48 Both (R)-BINOL and (S)-BINOL could beprepared in large scale with high optical purity by using thisprocedure. After the resolution, the chiral resolving agent waseasily recovered. This resolving agent is commercially availableand can be prepared from the reaction of (2)-cinchonidinewith benzyl chloride.49 Racemic BINOL can be synthesized inlarge scale from the oxidative coupling of 2-naphthol in air inthe presence of a copper catalyst (Scheme 2).47,50

The chiral configuration of BINOL is thermally stable andafter heating till 100 uC for 24 h in a dioxane–water solution,(S)-BINOL exhibited no sign of racemization.51 However, inacid or base media the thermal racemization of BINOL can beoccurred. For example, in 1.2 N HCl solution, there was 72%racemization for (S)-BINOL when it was heated at 100 uC for 24h. Also, in butanol containing 0.67 M KOH, (S)-BINOLunderwent 69% racemization after 23 h at 118 uC. In diphenylether, the racemization half-life of BINOL was 60 min (DG? =158 kJ mol21) at 220 uC.16 The racemization of BINOL might gothrough an anti-route as shown by a density functionalcalculation.16,52

Scheme 1 Representation of asymmetric hydrogenation catalyzed by a BINAPcomplex.38

Scheme 2 Synthesis of racemic BINOL and its optical resolution.47,50


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The structures of (+)-(R)- and racemic BINOL were determinedby X-ray analysis of the single crystals of these molecules.53

The crystal of (R)-BINOL has a cisoid conformation with adihedral angle of 80.8u between the two naphthol groups. Onthe other hand, the crystal of racemic BINOL has a moreorthogonal conformation with a dihedral angle of 91.4u.Besides, the C1–C1’ bond distances for these two structuresare 1.494 and 1.500, respectively. The C1–C2–O bond angles inboth structures are larger than those of C3–C2–O due to thesteric interaction between the OH functional group and theother naphthol moiety at position 1.

2.2 Monomers derived from indirect-linked 1,19-binaphthyls

In the last few decades, research on sulfur-containingmaterials increased considerably and a number of sulfur-containing substances have been investigated for the firsttime. Recently, Mague and co-workers54 have been able toobtain the single crystal and X-ray structure of 1,19-sulfide-bis(2-naphthol) (S-BINOL) in which the cisoid conformationwas preferred (Fig. 2). This observation might be related to thehydrogen bonding formation between two hydroxyl groups.Due to the free rotations between sulfur and two naphtholrings, chirality is not induced in this molecule compared tochiral BINOL at room and higher temperatures. We synthe-sized this molecule according to the reported procedure55 andused it in our research projects for the preparation of differentligands, podands, macrocycles, and polymeric materials whichwill be discussed in the coming sections.

It is interesting that we found unexpected structuralobservations when we synthesized corresponding biphenylmolecules. In fact, the X-ray structure of 2,29-sulfinyl-bis(4-methyl phenol) indicated transoid conformation while thecisoid conformation was preferred in its corresponding sulfidederivative (Fig. 3).56

Sulfur can also exist in different oxidation states and in eachoxidation state it is able to form different functions, thus thereare many different sulfur-containing chemical functions withan extensive array of chemical and physical properties.57

Sulfoxides and sulfones have fascinated organic chemists for along time because of their various reactivities as functionalgroups for transformation into a variety of organo sulfurcompounds.58–60 Several methods are available for conversionof sulfides to sulfoxides.61–64 A direct method for the synthesisof diaryl sulfoxides is the Friedel–Crafts sulfinylation of arenesusing a catalysts such as AlCl3 or trifluromethane sulfonicacid.65,66 However, direct synthesis of sulfoxides often suffersfrom the formation of mixtures of products containingsulfonium salts and chlorinated by-products along with thedesired sulfoxides.67 We investigated the direct solvent-freesynthesis of 1,19-sulfinyl-bis(2-naphthol) (SO-BINOL) by thereaction of thionyl chloride with 2-naphthol in the solid-stateon the SiO2 in the presence of AlCl3 in moderate yield(Scheme 3).68

It is well-known that aryl sulfides can be readily oxidized toproduce aryl sulfones. Various aromatic and aliphatic sulfideswere selectively oxidized to sulfoxides and sulfones in highyields using 30% H2O2 in the presence of a recyclable silica-based tungstate interphase catalyst at room temperature.69

Oxidation of sulfides with 30% H2O2 and tantalum carbide ascatalyst provided the corresponding sulfoxides in good toexcellent yields, whereas niobium carbide as catalyst efficientlyafforded the corresponding sulfones.70 In our previousstudies, we also synthesized 1,19-sulfone-bis(2-naphthol)

Fig. 2 Schematic and X-ray structure of 1,19-sulfide-bis(2-naphthol) (S-BINOL).(Reprinted from ref. 54, Copyright 2007, with permission from InternationalUnion of Crystallography).

Fig. 3 X-Ray structure of 2,29-sulfinyl-bis(4-methyl phenol) and its sulfidecounterpart 2,29-sulfide-bis(4-methyl phenol) (shown only for comparison withS-BINOL). (Reprinted from ref. 56, Copyright 2003, with permission from Taylor& Francis).

Scheme 3 A solvent free route to the synthesis of 1,19-sulfinyl-bis(2-naphthol)(SO-BINOL).68


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(SO2-BINOL) by the oxidation of S-BINOL with hydrogenperoxide in acid medium (Scheme 4).71

Besides our work in this field, many other researchers haveconducted extensive studies on the design and synthesis ofindirect ortho-linked binaphthyl-based monomers. While theresearch on the monomeric chiral binaphthyl compoundscontinues to be an active issue in many laboratories, multiplebinaphthyl units have also been joined together to develop thechiral differentiation role of the binaphthyl structure and togenerate molecular objects and polymers having uniquestructures and properties. Over a decade ago, several groupshave explored the use of binaphthyl-based systems toconstruct structurally diverse hosts and chiral catalysts. Theapplication of these materials in main chain chiral conjugatedpolymers and high performance materials have also beeninvestigated.

3 Binaphthyl-based systems as hostcompounds

3.1 Chiral catalysts for asymmetric organic reactions

Hosts are defined as organic compounds containing con-vergent binding sites, and guests as ions or moleculescontaining divergent binding sites. Complexes are hosts andguests held together in solution in structural relationships byelectrostatic forces other than those of ionic crystals or of fullcovalent bonds.72,73 These electrostatic forces include pole–pole, pole–dipole, and/or dipole–dipole that attract host toguest. In solution, the enthalpic attractions are supplementedby the desolvation of host and guest binding surfaces to turnorganized solvent into drowned solvent. The so-called solvo-phobic effect is a factor when complexes form in solution, butis often outweighed, especially for complexes with smallshared surfaces dissolved in non-protic media.74

In 1967, Pedersen published his work on crown ethers.75 Hedemonstrated that a rational approach to host–guest com-plexation was possible. He prepared a series of macrocyclicpolyethers based on CH2CH2O repeating units, of which 18-crown-6 was the prototype. This molecule could strongly bindK+ due to the stereoelectronic complementarity between hostand guest. As it is suggested by structure 3.1, the K+ guest nestsin the center of a wreath-like complex ligated by six oxygen

atoms. Also, structure 3.2 illustrates the ability of 18-crown-6to bind RNH3

+ salts.This early work of Pedersen encouraged Cram and co-

workers to enter into the field of host–guest complexationchemistry. Between 1970 and 1974 they had designed systemswhich incorporated one or more 1,19-binaphthyl units intocrown ethers (3.3 and 3.4) for complexing chiral amine saltsenantioselectively.15 The binaphthyl system was also useful forattaching the first side chains to crown ethers, which mightaid in complexing several ions by intramolecular ion-pairing.The structures 3.5 and 3.6 are examples of such complexes ofbinaphthyl hosts with metal ions as guests. The charges onhost and guest of these complexes are matched and both ofthem show parent ions in their mass spectra.37

In complexes 3.7 and 3.8, the binaphthyl host with twoterminal carboxylate groups is able to bind both the aminoand carboxylate groups of the valine as guest. The racemic hostwas totally resolved by a continuous liquid–liquid extractioninvolving (S)-valine dissolved in AcOH–H2O adsorbed on silicagel as the stationary phase, with racemic host in the C6H6–AcOH mobile phase. (S)-(CH3)2CHCH(NH2)CO2H and the twoenantiomers as a racemate of this host were distributedbetween the organic and aqueous layers in a one-plateexperiment. According to the comparison of the steric strainassociated with CPK molecular models of complexes 3.7 and3.8, (S)-valine complexed the (S)-binaphthyl host more stronglythan the (R)-host in the aqueous layer by a factor of 1.7. Inthese models, the R group (CH3)2CH– of valine of the complex3.7 in the (S)(S)-diastereomer had more room than in thecomplex 3.8 in the (S)(R)-diastereomer.76

Scheme 4 Synthesis of 1,19-sulfone-bis(2-naphthol) (SO2-BINOL) fromS-BINOL.71


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Compound 3.9 was the first ditopic (containing two separatearrays of binding sites) host to be synthesized in which the twosets of binding sites acted cooperatively. This host contains aC2 axis and is chiral. In CPK models, the two jaws werecomplementary to polyfunctional salts such as 3.10–3.13. Asolution of 3.9 in CDCl3 extracted one mole of each of theseguests to prepare complexes in which the two remotefunctional groups of the guest were each bound to one ofthe jaws of the hosts. The crystal structure of complex 3.9–3.10was also obtained and investigated.19,77

Catalytic enantioselective carbon–carbon bond forming reac-tions are studied reactions in asymmetric synthesis. Asmentioned before, the application of BINOL and its functio-nalized derivatives in asymmetric catalysis has been studiedextensively over the past two decades. These studies on BINOLsystems have focused primarily on the 3-monosubstituted- andthe 3,39-disubstituted derivatives. In fact, the introduction ofsubstituents to these positions can modify both the electronicand steric environments around the chiral BINOL, which hasled to various catalytic properties in many asymmetricreactions. In addition, this design is favorable to keep the 2-and 29-OH groups free for effective complexation withorganometallic reagents.

Lin Pu and his research group has been a pioneer instudying BINOL and its functional derivatives in combinationwith Lewis acidic metal complexes to catalyze the asymmetricreactions of prochiral aldehydes with alkylzincs, alkynylzincs,arylzincs, trimethylsilyl cyanide (TMSCN) and conjugateddienes. Highly enantioselective catalytic processes have beendeveloped. It was reported that ZnEt2 is able to react withterminal alkynes at room temperature to produce alkynylzincsin highly polar aprotic solvents.78 Hence, Pu and co-workerstested the use of these compounds as additives in theasymmetric alkyne addition.79–81 The Zn–Ti complex basedon BINOL (3.14) was proposed as one of the possibleintermediates for the alkyne addition to aldehydes catalyzed

by the (S)-BINOL system. The PC Spartan Semiemperical PM3program was used to obtain the molecular modeling structure.The aldehyde carbonyl oxygen is coordinated to both the Zn(II)and Ti(IV) center in this intermediate. The aldehyde n-Bugroup is up in order to avoid the steric interaction with theethyl group, and the ethyl group on Zn is down in order toavoid the steric interaction with the 3-hydrogen of (S)-BINOL.Therefore, the subsequent alkyne migration to the si face ofthe aldehyde gave the observed R product.

In addition to using BINOL systems for the asymmetricalkyne addition to aldehydes, Pu and co-workers have alsoinvestigated the application of functionalized BINOLligands.82,83 They studied the preparation of ligand (R)-3.15and its use in the ZnEt2 addition to aldehydes. This ligand andits different derivatives were used to catalyze the asymmetricalkyne addition to aldehydes. However, all of these 3,39-anisylsubstituted BINOL systems gave only 0–67% ee for thephenylacetylene addition to benzaldehyde in the presence ofZnEt2 with or without Ti(O-iPr)4. Ligand (S)-3.16 was subse-quently synthesized and it was found that this compoundcatalyzed the reaction of phenylacetylene with benzaldehyde inthe presence of ZnEt2 and Ti(O-iPr)4 at room temperature toproduce the desired propargylic alcohol product with 80% ee.Since the study of the ligand (R)-3.15 and its various derivativesindicated that neither the electron-withdrawing groups nor theelectron-donating groups on the 3,39-anisyl substituents couldgive high enantioselectivity, Pu and co-workers suspected thatthe increased enantioselectivity of (S)-3.16 might be because ofthe increased size of the phenyl substituents.82

In order to improve the enantioselectivity, ligand (S)-3.17containing t-butyl groups at the para-position of the 3,3’-anisylsubstituents of BINOL was prepared in Pu’s Laboratory. When10 mol% of (S)-3.17, 20 mol% of Ti(O-iPr)4 and two equivalentsof ZnEt2 were used for the reaction of phenylacetylene (2.1equivalents) with benzaldehyde in tetrahydrofuran (THF), thepropargylic alcohol product was achieved with 85% ee. It was


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also found that the zinc complex produced from the reactionof (R)-3.17, the enantiomer of (S)-3.17, with ZnEt2 couldcatalyze the formation of the alkynylzinc and decreased thereaction temperature. After this step, the mixture was stirredwith Ti(O-iPr)4 (0.5 equivalents) for 1 h and then withbenzaldehyde for 3 h. This generated the propargylic alcoholproduct with 89% ee. Another ligand (S)-3.18 that contains asmaller methyl group was also tested. The catalytic propertiesof (S)-3.18 were compared with those of (R)-3.17 and it wasfound that the enantioselectivity of (R)-3.17 was generallymuch better than that of (S)-3.18 in the reaction ofphenylacetylene with aromatic aldehydes. Besides, the cataly-tic activity of (R)-3.17 was considerably higher than that of (S)-3.18.82

To further improve the catalytic properties of BINOL-basedligands for the asymmetric alkyne addition to aldehydes, Puand Yu prepared compound (R)-3.19 containing two bulkysubstituents closer to the BINOL center.84 They found that (R)-3.19 in combination with ZnEt2 could catalyze the reaction ofphenylacetylene with benzaldehyde without Ti(O-iPr)4. In thepresence of (R)-3.19 (30 mol%) and ZnEt2 (two equivalents),phenylacetylene (1.5 equivalents) reacted with benzaldehyde toproduce the R propargylic alcohol product with 91% ee. Thecompound (R)-3.19 was also used to catalyze the reaction ofphenylacetylene with various aromatic aldehydes under thesame conditions and high enantioselectivities (86–92% ee)were observed (Scheme 5).

According to a proposed mechanism, when (R)-3.19 istreated with excess ZnEt2, complex 3.21 could be producedvia complex 3.20. It seems that the central ZnEt2 unit in 3.21 isactivated by the two Lewis basic oxygen atoms of the ligandand it is able to deprotonate phenylacetylene to generate thealkynylzinc unit in 3.22. Then, an aldehyde molecule can beactivated by coordination with two of the zinc centers of 3.22to produce complex 3.23. The migration of the alkynyl group tothe carbonyl followed by displacement with ZnEt2 can formthe zinc propargyloxide aggregates, which aqueous work-upgives the propargylic alcohol product.

Chiral a-substituted benzyl alcohols that exist in manyorganic structures can be prepared by arylzinc addition toaldehydes. The use of the 3,39-bisanisyl-substituted BINOLsand the 3,39-bismorpholinomethyl-substituted BINOLs as twotypes of BINOL derivatives for catalysis of the reaction ofarylzincs with aldehydes has been studied previously.85–88

Scheme 5 The proposed mechanism for the asymmetric phenylacetylene addition to aldehydes in the presence of (R)-3.19 and ZnEt2.84


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During this period, other catalysts have also been synthesizedfor the enantioselective arylzinc addition to aldehydes andketones.89 As shown in Scheme 6, the first highly enantiose-lective diphenylzinc addition to aldehydes has been reportedby using the ligand (R)-3.24. In the presence of 10 mol% of (R)-3.24, diphenylzinc can react with propionaldehyde in tolueneto produce (S)-1-phenylpropanol with 87% ee. The use of (R)-3.24 to catalyze the reaction of diphenylzinc with aromaticaldehydes has also been further studied. In order to obtainhigh enantioselectivity, the reaction conditions such astemperature, solvent, concentration, and additive needed tobe varied for different aldehydes. For instance, in the reactionof cinnamaldehyde, in addition to ZnEt2, methanol alsoneeded to be added and the reaction mixture needed to beheated at reflux, which produced the diphenylzinc additionproduct with 83% ee.

One of the main challenges in the asymmetric diphenylzincaddition to aldehydes is the uncatalyzed reaction which isfrequently in competition with the chiral catalyst-catalyzedreaction and subsequently leads to decreased enantioselec-tivity. Thus, electron-withdrawing groups were introduced tothe 3,39-anisyl substituents for increasing the catalytic activityof (R)-3.24. This approach could increase the Lewis acidity ofthe corresponding zinc complex and increased the catalyticactivity. Therefore, a series of 3,39-bisanisyl BINOL ligands (S)-3.25-(S)-3.30 were synthesized and tested for the reaction ofdiphenylzinc with cinnamaldehyde without using methanol asadditive.

A proposed mechanism for the diphenylzinc additioncatalyzed by (S)-3.27 is shown in Scheme 7. Treatment of thisligand with two equivalents of ZnEt2 followed by coordinationof diphenylzinc can produce the zinc complex 3.31. Analdehyde molecule coordinates to the zinc centers to produce3.32. In fact, the fluorine atoms on the anisyl substituent canmake the Zn center more Lewis acidic and hence provide morecatalyst control for the diphenylzinc addition to aldehydes.This leads to the much higher enantioselectivity of (S)-3.27than (R)-3.24. However, it was found that the additionalelectron-withdrawing fluorine atoms in (S)-3.28 could notfurther increase the enantioselectivity. On the other hand, thesignificantly lower enantioselectivity of (S)-3.29 than (S)-3.26showed that the electron-withdrawing bromine atoms on theBINOL system were not promising for the diphenylzincaddition.

The enantioselective alkylation of aldehydes with organozincreagents has attracted considerable attention due to its utilityand simplicity for the preparation of chiral secondaryalcohols.90 Several chiral catalysts have been used successfullyfor the asymmetric addition of dialkylzinc to aldehydes.91–97

Although research on BINOL-based catalysts has beenrestricted primarily in 3-monosubstituted- and the 3,39-dis-ubstituted derivatives, recently Judeh and co-workers reportedthe synthesis of novel 2,29-disubstituted BINOL ligands andexamined their application in the asymmetric additions ofdiethylzinc to various aldehydes.98 Chiral ligands (S)-3.33 and(S)-3.34 were designed intentionally to investigate the effect of

Scheme 6 Asymmetric diphenylzinc addition to propionaldehyde catalyzed by(R)-3.24.86

Scheme 7 The proposed mechanism for the catalytic asymmetric diphenylzincaddition by (S)-3.27.88


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expansion of the chirality and to determine the importance ofthe hydrogens of the 2- and 29-OH groups. The catalytic activityof these ligands was examined under typical conditions for theaddition of diethylzinc to aldehydes in the presence of Ti(O-iPr)4. Preliminary studies on the alkylation of benzaldehydeusing 10 mol% of (S)-3.33 and 120 mol% of Ti(O-iPr)4 gave onlytraces of the corresponding 1-phenyl propyl alcohol. However,when the same reaction was repeated using (S)-3.34, 1-phenylpropyl alcohol was obtained in 90% yield and in 67% ee(Scheme 8). Also, it was found that the complex (S)-3.33–Ti(O-iPr)4, in which the ligand does not bear OH groups, did notcatalyze the reaction, whereas an outstanding yield andreasonable enantioselectivity were achieved when the (S)-3.34–Ti(O-iPr)4 complex was employed. Hence, these resultsshowed that the presence of a free OH, as in (S)-3.34, wasindispensable for catalytic activity.

Cyanohydrins are very useful synthetic precursors to organiccompounds including a-hydroxyketones, a-hydroxyacids,a-amino acids, b-amino alcohols, and 1,2-diols. Variouscatalysts have been developed for the asymmetric reaction ofaldehydes with TMSCN to produce chiral cyanohydrins.99 Asshown in Scheme 9, the morpholinomethyl-substituted BINOLligand (S)-3.35 is one of the catalysts used in combination withMe2AlCl to catalyze the TMSCN addition to benzaldehyde.100

When Ti(O-iPr)4 was used in place of Me2AlCl only moderate eewas observed. Switching Ph3PLO with HMPA decreased thereaction time and slightly increased the yield. Under theoptimized conditions, the reaction of TMSCN with benzalde-hyde gave the cyanohydrin product with 92% yield and 92% ee.The reactions of TMSCN with other aromatic aldehydes werealso conducted and good enantioselectivity was observed forvarious aromatic aldehydes. The structure 3.36 is a workinghypothesis for the transition state of the TMSCN addition toan aldehyde catalyzed by (S)-3.35. In this structure, thealdehyde is activated by coordination to the Al(III) center andthe TMSCN is activated by the morpholinyl nitrogen and theLewis basic HMPA.

The Lewis-acid catalyzed hetero-Diels–Alder (HDA) reactionof Danishefsky’s diene with the enamide aldehyde 3.37 cansynthesize compound 3.38 that is potentially worthwhile in thenatural product synthesis (Scheme 10). Although some BINOL-based chiral Lewis acids such as BINOL–Ti(O-iPr)4 and BINAP–Cu(OTf)2 have been successfully used in different asymmetricHDA reactions, but these compounds could not catalyze thisreaction.101,102

In 2000, Pu and co-workers reported the use of some chiralLewis acid complexes made from the combination of BINOL-based ligands (R)-3.39–(R)-3.43 with AlMe3 to catalyze theasymmetric HDA of Danishefsky’s diene with the enamidealdehyde 3.37.103 These reactions were strongly influenced bysolvents. In solvents such as THF and diethylether which havecoordinating oxygen atoms, no reaction was observed. A polarsolvent such as dichloromethane was found to be moreappropriate than a less polar toluene. Incorporation of the6,69-bromine atoms in (R)-3.43 should increase the acidity ofthe hydroxyl groups, but did not show an improvement on theenantioselectivity.

Scheme 8 Enantioselective addition of diethylzinc to benzaldehyde catalyzedby Ti(IV) complexe of chiral (S)-3.34.98

Scheme 9 Catalytic TMSCN addition to benzaldehyde in the presence of (S)-3.35 and its transition state 3.36 as a working hypothesis.100

Scheme 10 The HDA reaction of Danishefsky’s diene with enamide aldehyde3.37.


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3.2 Complexation studies with different guests

Coordination of the metal cations by the ionophores facilitatestransport across membrane barriers which results in a rangeof significant biological activity. The similarity between alkalimetal cations as featureless charged spheres, which are theguests for ionophore antibiotics, and lanthanide3+ cations,which lack any directional preference for ligand binding, hasled several research groups to consider simple chiral polyetherpodands as potential ligands for lanthanide catalysis. BINOLhas been the most effective and versatile ligands forenantioselective lanthanide catalysis and so Aspinall andGreeves investigated its use as a chiral unit in polyetherpodands 3.44–3.46.104

These podands were capable of forming complexes withlanthanide triflates of a range of sizes from La to Yb. Theyfound Ln(OTf)3 complexes with all of these ligands showedLewis acidity, but enantioselectivities in the Diels–Alder andcarbonyl allylation reactions were very poor. Although a degreeof pre-organisation had been achieved in these podands viathe chiral substituents, the flexibility required to allowcoordination to the full range of Ln3+ ions indicated thatchiral binding sites were not sufficiently well-defined to obtaingood enantioselectivities in catalytic reactions.

As a recognition motif, podand based receptors have beenreported to be used successfully as recognition components inoptical or electrochemical sensors.105–109 In fact, variousmacrocyclic compounds, such as crown ethers, cryptands,and calixarenes, have been synthesized and structurallycharacterized to generate ionophore model systems. Inaddition to these macrocyclic compounds, podands have alsobeen employed as non-cyclic ionophore models. In 2007, wereported the synthesis of sulfur-containing S-BINOL-basedpodands 3.47–3.49 as modified BINOL podands, in which thepodand 3.47 was used in determination of Ag+, Hg2+, Cr3+,Pb2+, and Ni2+ cations.110 The electronic absorption spectra ofthe podand 3.47 with various concentrations indicated that theelectronic spectra of this ligand were largely dependent on theconcentration. This behavior could be ascribed to theassociation of molecule through intermolecular hydrogen

bonding. A non-linear, least-squares, curve fitting programKINFIT was used for evaluation of the formation constantsfrom the absorbance vs. CM/CL mole ratio data.111 One of thetypical fitting curves of podand 3.47 with cations using thissoftware clearly showed that a 1 : 1 Pb2+ to 3.47 complex wasformed in solution. The results of formation constantsindicated that the complexes stabilities of these cations with3.47 decreased in the order of Pb2+ > Ni2+ > Hg2+ > Cr3+ > Ag+.

Potentiometric procedures based on ion selective electrodesare suited for accurate, reproducible, and selective determina-tion of various metal ions. These potentiometric proceduresoffer advantages such as sensitivity, selectivity, simplicity, andso on. These methods allow on-line monitoring of concentra-tion of selected species without any pretreatment.112,113 Silveris an important element that is generally used in imaging andphotographic industry, medical products, electronic equip-ment, and other products like coins, jewellers, and mirrors.114

Consequently, the determination of silver in various media isof importance. Over the years, various Ag+-selective electrodeshave been reported.115,116 Recently, the podand 3.50 based onS-BINOL was synthesized and used as a suitable carrier forAg(I) ion selective electrode in our research group.117 Inpreliminary experiments, the complexation of 3.50 with a widevariety of cations including hard and soft metal ions wasinvestigated conductometrically. The results indicated thatthis podand could act as a highly selective ionophore for Ag+

ion in the membrane phase of a PVC-based ion-selectiveelectrode for the cation. As shown in Fig. 4, among cationsexamined, Ag+ with the most sensitive response seems to besuitably determined with the membrane electrode based onthe podand 3.50. This finding was due to the high selectivity ofthe ionophore for silver ion over other metal ions as well as therapid exchange kinetics of the resulting complex. In addition,the effects of several parameters such as the membranematrix, carrier concentration, and pH were investigated andthe optimized membrane was used to obtain the responsecharacteristics and selectivity.

Lead (Pb) is one of the most toxic elements and continuousexposure to this element may cause adverse and poisonous


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effects to the blood, brain, kidneys, and nervous andreproductive system along with others diseases.118 Hence,there has been an increasing need for scientists to evaluate theenvironmental and healthy quality based on accurate deter-mination of Pb in biological and environmental samples.Potentiometric procedures using an ion sensor as an indicatorelectrode are suitable for determination of Pb. In recent years,much attention has been devoted to the advantages of carbonpaste electrodes (CPEs). Potentiometric CPEs offer veryinteresting properties for the electrochemical investigation oforganic and inorganic species over polymeric membraneelectrodes.119,120 Due to the ability of forming complexes withmetal ions possessing the compatible dimensions, manymacrocyclic compounds are being employed as the carrier inphase transfer catalysis, metal selective extraction, membranetransport, and in polymeric sensors as ionophore for thedetermination of different ions.121–123 In 2010,Mashhadizadeh and co-workers reported some modifiedCPEs in which gold nanoparticle were self-assembled ontosome sulfur-containing ionophores. These electrodes weresuccessfully applied for potentiometric determination ofultratrace levels of Al(III) and Cu(II) cations.124,125 Quiterecently, in collaboration with Mashhadizadeh, we reportedthe synthesis of a novel S-BINOL-based podand 3.51 and usedit as an excellent carrier for construction of Pb2+ modified CPE(Scheme 11).126 The detection limit of the proposed electrodewas superior to those reported for other Pb(II) ion-selectiveelectrodes, and its selectivity was among the most selectivePb(II) ion sensor reported. Also, it could be accuratelyemployed for the estimation of lead content in real samples.

Organic dithiocarbamates (DTCs) have received muchattention because of their interesting biological activities such

as fungicidal and antibacterial effects.127,128 DTCs also havestrong ability to absorb metal ions and complexation withmetals as monodentate and bidentate ligands.129 The classicalsynthesis of DTCs involves the use of thiophosgene130 and itssubstituted derivatives,131 which are toxic and expensivereagents. General methods for the synthesis of vinyl and arylesters of dithiocarbamic acids are based on the reactions ofhypervalent iodine compounds with a sodium salt of dithio-carbamic acid.132–134 Because of the importance of DTCs, wewere encouraged to synthesize the DTC podands 3.52–3.60based on S-BINOL or its –CH2–NLCH2– or –CH2–NH–CH2–linked counterparts.135 Currently, the complexations of thesecompounds with a variety of transition and/or heavy metals areunder study. Due to the observation of two doublets for eachmethylene hydrogen (first methylene group linked withthioether and carbonyl groups; second one linked withnitrogen iminic group and aromatic ring) in 1H NMR spectraof the podands 3.55 and 3.57, it was demonstrated that both ofthese methylene groups behaved as diastereotopic nuclei.Such diastereotopic evidences were not found in otherprepared podands. In addition, we acquired spectra at severaltemperatures up to the coalescence points. The coalescencetemperatures of these podands were obtained from theircorresponding DNMR spectra at about 308–353 K.

A logical design of macromolecular receptors is governed byseveral factors such as the number, the nature, the relativestructural and spatial placement of various ligating units, andso on. Also, the combination of these factors induces afundamental balance of noncovalent binding forces optimumfor specificity in host–guest recognition.136,137 The amidegroup has acquired a unique status in the design of receptorsdue to its geometrical rigidity, dual (O or N and NH) ligatingcharacter, and higher negative charge on oxygen than for etherand ester groups.138 Macrocycles containing amide groups forselective recognition of metal cations139 and organic mole-cules140 adopt preorganization of their binding sites throughconfigurational rigidity around the amide carbon-nitrogenbond or hydrogen bonding. Aza macrocyclic compounds havereceived much attention because of their widespread applica-tions in chemistry, microanalysis, sensoring, metal separation,and biology.141 In addition, there is an increasing interest inthe preparation of new crown ethers due to their highcapability in selective and effective complexation with varietyof transition and heavy metals. It is also recognized thatincorporation of polar amide donors into a complexone plays asignificant role in the improvement of the selectivity in cationbinding.

The amide group in many hosts directs the specificity of thehost toward softer alkaline earth cations through N–CLO–metal interactions and plays a key role in the complexation ofvarious guests in aza macrocycles. In our previous studies wereported the synthesis of dibenzosulfide and dibenzosulf-oxides aza macrocycles as metal cations receptors.109,142,143

According to the inhibitor effects of some of these macrocyclesin biological studies144 and previously mentioned applicationof S-BINOL-based podand 3.47 containing amide groups in thedetermination of Ag+, Hg2+, Cr3+, Pb2+, and Ni2+ cations,110 wewere encouraged to synthesize various S-BINOL aza macro-cycles 3.61–3.67 and carry out the complexation studies.145 In

Scheme 11 Synthesis of the sulfur-containing podand 3.51 based onS-BINOL.126

Fig. 4 Potential response of various ion-selective electrodes based on podand3.50. (Reprinted from ref. 117, Copyright 2009, with permission from Wiley-VCH).


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order to have a clue about the selectivity of the synthesizedmacrocycles, we also reported the conductance study ofcomplexation of several different metal ions with 3.66 inmethanol solution. The molar conductance of the perchloratesalts of Tl+, Mg2+, Ca2+, Sr2+, Ba2+, Ag+, Pb2+, Hg2+ and thenitrate salts of Ni2+, Cu2+, Zn2+ and Cd2+, at a constant saltconcentration was monitored while increasing 3.66 crownether concentration at room temperature. The resulting molarconductance vs. ligand/cation mole ratio plots are shown inFig. 5. In all cases, there was a gradual decrease in the molarconductance with an increase in the macrocycle concentra-tion. This behavior exhibited the lower mobility of thecomplexed cations compared to the solvated ones. It was alsofound that in the case of Pb2+-3.66 and especially Hg2+-3.66systems, the addition of ligand to the metal ion solutionscaused a continuous decrease in the molar conductance,which began to level off at a mole ratio greater than one. Thisconductance behavior was indicative of the formation of fairlystable 1 : 1 complexes in solution. However, in other cases, themolar conductance did not show any tendency for levelling offeven at a molar ratio of about 4 and the corresponding molarratio data did not show substantial changes in their slopes at amolar ratio of about one, which emphasized the formation ofsome weaker 1 : 1 complexes.

In 2011, in collaboration with Taghdiri, we developedinvestigations on two of the previously reported S-BINOL-substituted macrocyclic diamides 3.61 and 3.66 which wereconsidered as multidentate macrocycles including amidicnitrogens. The conductometric behavior of complexation andquantum chemical calculations of 15-membered macrocyclicdiamide 3.61 containing –CH2CH2– group attached to theamide groups were reported and compared with previousstudied 18-membered macrocyclic diamide 3.66 containing –(CH2)2NH(CH2)2– group attached to the amide groups.146

The amide groups in the structures of these ligands makethem able to form intra- and intermolecular hydrogen bonding.Therefore, we optimized their conformations and indicated thatthe intramolecular hydrogen bonds were formed between NHamide-carbonyl and NH amide-ethereal oxygen. In addition, theability of inter- and intramolecular hydrogen bonding affectedconductometric behavior of these macrocycles in methanol. Also,the conductometric behavior of complexation of ligands 3.61 and

3.66 in methanol as a protic solvent was interpreted on the basisof their ability to form inter- and intramolecular hydrogen bonds.This interpretation was confirmed by quantum chemicalcalculations of intramolecular hydrogen bonds in free ligandsand intermolecular hydrogen bonds between free ligands andmethanol as a solvent, and between free ligands and perchlorateion as a counter ion. The experimental results showed that forsome cations, there was not any interaction between the ligandsand cations, while for some others there was a strong interaction.Two different behaviors of the complexation were observed. Inthe first one, after 1 : 1 mole ratio of ligand/cation, a horizontalline was obtained which indicated that with addition of more freeligands, a new process was not occurred. For the second one,after 1 : 1 mole ratio of ligand/cation the slope increasedabruptly. To prove the conjectured mechanism, we used atomsin molecules (AIM) theory. AIM data demonstrated that there wasmore tendency for ligand 3.61 to interact with methanol, bywhich the counter ion was released. With increasing the mobilityof this ion the conductance increased.

Among chromogenic sensors, sensors with fluorescent signal-ing units have a noticeable edge over others because of theirability to detect the desired analytes that are present in ultratracequantities.147 Many fluorescence signaling systems for metalions have been developed. In recent years, much attention hasbeen devoted to fluorescent chemosensors for detection of Zn+2

because of their ease of use in solution as well as their highsensitivity and selectivity to trace analytes. Quite recently,Azadbakht and co-workers reported the synthesis of a –O–CH2CH2–O– linked binaphthyl-based macrocyclic derivative 3.68(Scheme 12).148 The recognition properties of 3.68 toward metalions of Na+, Ag+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ andPb2+ were investigated through fluorescence titration experi-ments. The results indicated that this receptor had sensitivefluorescent response to Zn2+ ions, while this change was notfound when the receptor interacted with other metal ions.

Anion receptor chemistry has attracted considerable atten-tion because of its positive roles in biological systems such asmembrane transport agents, ion-pair receptors, sensors for the

Fig. 5 The S-BINOL aza macrocycles 3.61–3.67 and the molar conductance (Scm2 mol21) vs. [3.66]/[Mn+] for various cations in methanol at 25 uC. (Reprintedfrom ref. 145, Copyright 2008, with permission from Springer).


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detection of biologically important anionic species and in avariety of other applications.149,150 Applications of suchreceptors as lipid-bilayer membrane transport agents forchloride or HCl are also attracting growing interest. In 2008,an excellent review was published by Gale and co-workers thatfocused on the design of anion receptors based on organicframeworks.151

Secondary amides have been extensively studied as hydro-gen bond donor groups in anion receptors.152,153 In 2006,Hiratani and co-workers investigated the molecular recogni-tion properties of a series of binaphthyl-based crownophanescontaining two hydroxy groups and two secondary amidegroups 3.69–3.71 and model compound 3.72 with anions suchas halides, dihydrogen phosphate and acetate.154 1H NMRtechniques were used to measure the anion coordinationability of these species, and it was demonstrated thatamidocrownophanes 3.69 and 3.70 bound anions withmoderate affinity with the following order of selectivity:H2PO4

2 > F2 > CH3COO2 > Cl2 & Br2 and I2, whilstcompounds 3.71 and 3.72 had no affinity for anions underthese conditions. The intramolecular hydrogen bondinginteractions in the phthalamide derivatives 3.71 and 3.72 thatwere not present in the isophthalamide derivatives 3.69 and3.70 were probably responsible for this lack of affinity.

4 Binaphthyl-based systems as polymericmaterials

4.1 Main chain chiral-conjugated polymers

The study of conjugated polymers has become an attractingfield of research since the discovery of the high conductivity ofdoped polyacetylenes. These polymers can be used to preparesensors, organic conductors, non-linear optical devices, solarcells, etc.155–157 Chiral-conjugated polymers are a special classof conjugated materials with great potentials and over theyears, various chiral-conjugated polymers have been pre-pared.158–161 However, these chiral-conjugated polymers haveoften shared one usual characteristic that their chirality hasgenerally been based on their side chain substituents and theirmain chains have not been chiral inherently. Consequently,the chiral chain conformations of these polymers have notbeen stable and they have frequently undergone majorchanges with respect to external factors such as temperature,solvent, and pH. In order to solve this problem,1,19-binaphthyl units have been incorporated into the mainchain of a variety of conjugated polymers including arylene-vinylenes, arylenes, aryleneethynylenes, and thiophenes toconstruct various main chain chiral-conjugated polymers.These polymers had stable main chain chiral configurationdue to the presence of the chiral binaphthyl units. Thesynthesis and structure of these materials are described here.

4.1.1 Polymers with conjugated linkers.

4.1.1.1Binaphthyl-based polyarylenevinylenes. In 1996, Pu andco-workers162 prepared a binaphthyl-based 6,69-diboronic acidmonomer (R)-4.1. The specific optical rotation [a]D of thiscompound was 35.6 (c = 0.22, DMSO). The Suzuki coupling ofthis monomer with a phenylenevinylene linker molecule 4.2containing both trans and cis double bonds with a trans : cisratio of 1 : 1.2 in the presence of Pd(PPh3)4 lead to the mainchain chiral conjugated polyphenylenevinylene (R)-4.3 in 95%yield (Scheme 13). The specific optical rotation [a]D of theobtained polymer was 2351 (c = 0.38, THF). The NMR analysisof this polymer indicated that it still contained both cis andtrans double bonds with a trans : cis ratio of 1 : 0.45. That is,some of the cis double bonds were converted to trans duringthe polymerization. In addition, the p-phenylene units in (R)-4.3 gave a characteristic strong C–H out-of-plane bendingabsorption at 814 cm21 in the IR spectrum. On the other hand,the polymerization of the racemic monomer rac-4.1 with 4.2gave polymer rac-4.3. This polymer most likely contained arandom distribution of the (R)- and (S)-binaphthyl units in themain chain.

The thin films of the resulting polymers could be obtainedby spin-coating their solutions onto glass slides. When theprepared thin films were exposed to the acetonitrile solution ofNOBF4, the main chain chiral-conjugated polymers wereoxidatively doped. The conductivity of these doped thin filmsof (R)-4.3 and rac-4.3 was in the range of 4 6 1025 to 7 6 1025

scm21. Atomic force microscopy studies showed that thesethin films were amorphous. Polymer (R)-4.3 was stable up to340 uC under nitrogen as shown by thermogravimetric analysis(TGA). Its glass transition temperature (Tg) was 210 uC asdetermined by differential scanning calorimetry (DSC). The

Scheme 12 The synthetic route of binaphthyl-based macrocyclic derivative3.68.148


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thermal stability of polymer rac-4.3 was found to be higherthan that of polymer (R)-4.3. TGA showed that rac-4.3 wasstable up to 390 uC under nitrogen and its Tg was at 203 uC.

4.1.1.2 Binaphthyl-based polyarylenes. From the Suzuki cou-pling of (R)-4.1 with p-dibromobenzene, the main chain chiral-conjugated polyarylene (R)-4.4 was obtained (Scheme 14).163

This polymer was soluble in common organic solvents andGPC measured its molecular weight as Mw = 41 000 and Mn =20 000. The specific optical rotation [a]D of this polymer was2289.4 (c = 0.5, CH2Cl2). When a toluene solution of (R)-4.4was heated at reflux under nitrogen for 40 h, there was only 5%decrease in the optical rotation. This demonstrated that thechiral configuration of the obtained polymer was very stable.The methylene chloride solution of (R)-4.4 gave UV absorptionat lmax = 328 nm. The thin film of the spin-coated polymer onglass slide exhibited only a slightly blue shifted UV signal atlmax = 324 nm. This indicated that there was no significant p–pinteraction in the solid state. This observation could beattributed to the presence of the chiral non-planar binaphthylunits which reduced the intermolecular interactions.

On the other hand, polymerization of (R)-4.1 with 4,49-dibro-mobiphenyl gave another main chain chiral-conjugated poly-arylene (R)-4.5 in a way similar to the preparation of (R)-4.4(Scheme 14). This polymer was also soluble in commonorganic solvents and GPC showed its molecular weight as Mw =47 000 and Mn = 14 000. The specific optical rotation [a]D of(R)-4.5 was 2275 (c = 0.22, CH2Cl2). The UV absorption of (R)-

4.5 in methylene chloride solution was observed at lmax = 334nm. The thin film of this polymer gave the same UVabsorption as the solution, indicating a minimum p–pinteraction in the solid state. While excited at 334 nm, thepolymer solution emitted at 396 and 415 nm. Polymer rac-4.5was obtained from rac-4.1 with Mw = 92 000 and Mn = 23 000(PDI = 4.0). This polymer showed properties similar to those of(R)-4.5 except the optical rotation. Both the optically activepolymers (R)-4.4 and (R)-4.5 exhibited strong Cotton effects intheir circular difference (CD) spectra.

4.1.1.3 Binaphthyl-based polyaryleneethynylenes. Polyary-leneethynylenes are conjugated materials composed of aro-matic rings and alkyne functions. These rigid rod polymershave exhibited photoluminescence as well as strong thirdharmonic generation.164 They can be synthesized by thepalladium-catalyzed coupling of aryl halides and terminalalkynes.165,166

In 1996, Pu and co-workers167 used the Sonogashiracoupling of the alkylated 6,69-dibromo-BINOL (R)-4.6 withp-diethynylbenzene to produce the main chain chiral-conju-gated polyaryleneethynylene (R)-4.7 (Scheme 15). It wasdemonstrated that the long chain octadecyloxy groups in themonomer were necessary to make the obtained polymersoluble in organic solvents. In fact, the binaphthyl monomerscontaining shorter chain alkoxy groups could not produce asoluble polymer. Polymer (R)-4.7 was soluble in commonorganic solvents. In addition, polymerization of the racemicmonomer rac-4.6 with p-diethynylbenzene gave rac-4.7. On theother hand, from the coupling of (R)-4.6 with 4,49-diethynyl-biphenyl, polymer (R)-4.8 was obtained. Also, polymer rac-4.8was prepared by using the racemic monomer rac-4.6. The useof Pd(PPh3)4 in place of PdCl2 was also tested for thepolymerization, which led to the formation of polymers havinghigher molecular weights.

4.1.1.4 Binaphthyl-thiophene copolymers. In order to system-atically tune the absorption and emission properties ofpolybinaphthyls, coupling of the binaphthyl-based 6,69-diboro-nic acid monomer rac-4.1 with some a,a9-dibromooligothio-phene monomers 4.9–4.13 was used to generate a series ofbinaphthyl-thiophene copolymers 4.14–4.18 (Scheme 16).168

The binaphthyl-monothiophene copolymer 4.14 was obtainedas a greenish-yellow solid in 93% yield. GPC measured its

Scheme 14 Synthesis of the chiral-conjugated binaphthyl-based polyarylenes(R)-4.4 and (R)-4.5.163

Scheme 15 Synthesis of the chiral-conjugated binaphthyl-based polyaryle-neethynylenes (R)-4.7 and (R)-4.8.167

Scheme 13 Synthesis of the chiral-conjugated binaphthyl-based polyarylenevi-nylene (R)-4.3.162


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molecular weight as Mw = 35 500 and Mn = 13 900. Thebinaphthyl-bithiophene copolymer 4.15 was also synthesizedas a yellow solid in 97% yield. GPC exhibited its molecularweight as Mw = 30 300 and Mn = 18 100. The authors claimedthat since the tetrathiophene monomer 4.11 had lowsolubility, its coupling with rac-4.1 resulted in a lowermolecular weight polymer 4.16 as an orange solid in 88%yield. GPC showed its molecular weight as Mw = 5500 and Mn =5100. Due to the good solubility of the alkylated pentathio-phene monomer 4.12, its coupling with rac-4.1 gave polymer4.17 as a red solid in 94% yield. GPC showed its molecularweight as Mw = 28 000 and Mn = 17 400. In addition, the lowsolubility of the alkylated heptathiophene monomer 4.13resulted in the low yield polymer 4.18 as a dark red solid.This low yield was attributed to the formation of insoluble andprobably higher molecular weight polymers.

All the prepared polymers 4.14–4.18 showed good solubilityin common organic solvents and gave well-resolved 1H and 13CNMR spectra consistent with the expected structures. Ingeneral, increase in the repeating unit length of the polymersresulted in an increase in both absorption and emissionwavelengths. Moreover, 9-anthracene carboxylic acid was usedas a reference for estimation of the fluorescence quantumyields of the polymers in methylene chloride solution. Thepolymer 4.18 containing heptathiophene units showed a muchlower quantum yield than other polymers. The polymers 4.14,4.15, and 4.17 showed similar thermal decomposition patternsaccording to their thermogravimetric analysis (TGA) thermo-grams. In fact, these polymers lost their hexyl groups in thetemperature range of 380–480 uC under nitrogen. Furtherheating resulted in a small additional mass loss (y10%).However, the two low-molecular-weight polymers 4.16 and 4.18

were less stable and began to decompose at much lowertemperatures.

As shown in Scheme 17, the polymerization of a1,19-binaphthyl-2,29-diamine (BINAM)-derived monomer (R)-4.19 with a thiophene-containing conjugated linker molecule4.20 was conducted in the presence of Pd2(dba)3, tri-tert-butylphosphine, and cesium carbonate to produce polymer(R)-4.21 as a dark red solid in 85% yield.169 The two alkylgroups on each of the nitrogens made the binaphthyl-basedmonomer more air stable. Also, these groups contributed tothe solubility of the corresponding chiral-conjugated polymer.This polymer was soluble in common organic solvents andGPC showed its molecular weight as Mw = 7500 and Mn = 3000(DPI = 2.5). The Heck coupling of this binaphthyl-basedmonomer with the divinyl monomer 4.22 was also conductedto generate polymer (R)-4.23. The polymerizations involved theuse of Pd(OAc)2, CuI and tri-o-tolylphosphine in DMF andtriethylamine. 2,6-Di-tert-butyl-4-methylphenol (BHT) was alsoadded to inhibit a possible radical polymerization of 4.22. GPCshowed the molecular weight of (R)-4.23 as Mw = 12 000.Therefore, it was demonstrated that both the halogen atomsand the size of the alkyl groups on this monomer did notsignificantly influence the polymerization. The UV and CDspectral data of the resulting polymers (R)-4.21 and (R)-4.23showed that the more extended conjugation in the repeatingunits of polymer (R)-4.23 resulted in its longer wavelengthabsorption than that of polymer (R)-4.21.

4.1.2 Polymers without conjugated linkers. Polybinaphthylsthat do not contain any conjugated linkers as described in theprevious sections are also synthesized. Polymerization of (R)-4.24 in the presence of a stoichiometric amount of bis(1,5-cyclooctadiene)nickel(0), 2,29-bipyridine, 1,5-cyclooctadiene,

Scheme 16 The binaphthyl-thiophene copolymers 4.14–4.18.168

Scheme 17 Synthesis of the BINAM-based chiral conjugated polymers (R)-4.21and (R)-4.23.169


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and DMF gave the polybinaphthyl (R)-4.25 (Scheme 18).170 GPCexhibited the molecular weight of (R)-4.25 as Mw = 15 000 andMn = 6800 (PDI =2.2). TGA showed that the resulting polymerwas stable up to 392 uC before it started to lose mass. Inaddition, polymerization of the racemic monomer gavepolymer rac-4.25 with a molecular weight of Mw = 11 000and Mn = 4700 (PDI = 2.3). The NMR spectra of rac-4.25 werealmost identical to those of (R)-4.24. The methylene chloridesolution of polymer (R)-4.25 displayed UV absorptions at lmax =240 (sh), 262, 284 and 316 nm. Comparison of the UVabsorptions of polymer (R)-4.25 with those of 2,29-bishexyloxy-1,19-binaphthyl and 2-hexyloxy naphthalene showed thatpolymer (R)-4.25 did not have more extended conjugationthan the small molecules. This may be due to the fact that thenaphthalene groups in the polymer had mutually orthogonalconformation with respect to the 1,19- and 6,69-bonds. StrongCotton effects were observed in the CD spectrum of (R)-4.25and when this polymer was heated at reflux in toluene for 24 h,there was only 6% loss of its optical rotation. Hence, its chiralconfiguration was very stable.

The Suzuki coupling of (R)-4.24 with (R)-4.1 was conductedto produce the polybinaphthyl (R)-4.26, which had the samestructure as (R)-4.25 (Scheme 19).171 GPC showed themolecular weight of (R)-4.26 as Mw = 17 600 and Mn = 7600(PDI = 2.3). In addition, its specific optical rotation [a]D was2281 (c = 0.50, THF). The NMR and UV spectra of this polymerwere almost the same as those of (R)-4.25. The coupling of theracemic monomer rac-4.24 with rac-4.1 gave rac-4.26 with Mw =21 500 and Mn = 8100 (PDI = 2.6). It was demonstrated thatusing the Suzuki coupling generated higher molecular weightpolymers than using the Ni(0) complex.

The coupling of two equivalents of rac-4.24 with oneequivalent of (R)-4.1 was also conducted, which produced anoligomer with Mw = 3600 and Mn = 2300 (PDI = 1.5). Thespecific optical rotation of this oligomer [a]D was 297.7 (c =1.0, THF). About 20% of the unreacted rac-4.24 was recoveredat the end of the reaction whose specific optical rotation [a]D

was 20.7. This demonstrated that (R)-4.1 coupled with bothenantiomers of rac-4.24 equally to produce the oligomer withno enrichment of either enantiomer in the unreacted rac-4.24.Consequently, the polymerization of rac-4.24 with rac-4.1produced polymer rac-4.26 which should contain randomlydistributed R and S binaphthyl groups in the polymer chain.The oligomer from the coupling of rac-4.24 and (R)-4.1 was stilloptically active due to the fact that using (R)-4.1 allowed theoligomer to contain excess (R)-binaphthyl groups.

Additionally, it is important to note that a discussion onbinaphthyl-based chiral-conjugated polymers regarding pro-peller-like polybinaphthyls, binaphthyl-based polysalophens,and helical ladder polybinaphthyls has been recently pub-lished in a book entitled ‘‘1,19-binaphthyl-based ChiralMaterials: Our Journey’’172 that are not covered in this review.

4.2 High performance polyamides, polyimides, and theircopolymers

High performance materials are characterized by particularcriteria, such as possessing excellent thermal stability and/ormechanical strength, good conductivity, outstanding thermal,electrical, or sound insulation properties, low specific density,and superior flame resistance. Consequently, aromatic poly-amides, polyimides, and their copolymers are considered to behigh-performance materials because of their superior mechan-ical and thermal properties, which make them practical foradvanced technologies. These materials are finding increasingdemand for use as useful replacements for ceramics or metalsin current goods, or even as new materials in uniquetechnological applications.173–175

However, the extremely high melting or glass transitiontemperatures (Tgs) of the commercial polyamides and poly-imides, which lie above their decomposition temperatures,and their limited solubility in common organic solvents resultin processing difficulties and limit their widespread applica-tions. Therefore, basic and applied research has recentlyfocused on improving their solubilities and processabilities inorder to expand the technological applications of thesematerials. There are currently various attempts directedtoward exploiting the specific high performance characteris-tics of the polyamides and polyimides to obtain reverseosmosis, gas or ion-exchange membranes, electro- or photo-luminescent materials, nanocomposites, optically active mate-rials, etc. having superior thermo-mechanical performances.The enhanced solubility and processability of these materialsis achieved by introducing flexible linkages, bulky groups,

Scheme 18 Synthesis of optically active poly(binaphthyl) (R)-4.25.170

Scheme 19 The Suzuki polymerization to synthesize polybinaphthyl (R)-4.26.171


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fluorinated substituents, or amide–imide moieties into thepolymer backbones.176–181

Recent studies demonstrated that bulky units in thepolymer backbone or as pedant groups can enhance thesolubility and processability of aromatic polyamides andpolyimides, without any significant reduction in thermalstability.182–184 This approach results in a weakening ofhydrogen bonding, a separation of chains, and a lowering ofchain packing with a gain of free volume, which diminishesthe Tgs. On the other hand, bulky units restrict molecularmobility and the overall observable effect is an increase in Tgs.This counteracts the chain separation effect and simulta-neously improves the solubility and processability.Naphthalene is a bulky and rigid structure which also hashigh heat resistance.5,185 Incorporation of naphthalene groupsmay disrupt the crystal packing, reducing intermolecularinteractions and enhancing solubility of the resulting poly-mers.186 Binaphthyl-based systems have been used as eitherbulky main chain units or bulky pendant groups to prepare avariety of high performance polyamides, polyimides, and theircopolymers.

4.2.1 Binaphthyls as bulky main chain units. In 2004,Mehdipour-Ataei187 reported the preparation of a pyridine-based ether diamine called 2,6-bis(5-amino-1-naphthoxy)pyridine (4.27) via reaction of 5-amino-1-naphthol with 2,6-dichloro pyridine. The diimide-diacid 4.28 was prepared byreaction of the diamine with trimellitic anhydride.Polycondensation reaction of 4.28 with different types of

diamines using triphenyl phosphite (TPP) afforded differentpoly(amide imide)s with high thermal stability and modifiedsolubility (Scheme 20). To study structure-property relations inthe resulting polymers, some structural modifications wereconsidered. Incorporation of ether linkages, pendant naphthylgroups, and pyridine heterocyclic rings were applied toprepare diimide-diacid 4.28. For estimation of molecularweights of polymers, inherent viscosity of the polymers inNMP at a concentration of 0.5 dL g21 at 30u C was measured. Itwas in the range of 0.44–0.50 dL g21 and from these results itcould be concluded that the polymers showed reasonablemolecular weights. The Tgs of the polymers were about 138–225 uC according to differential scanning calorimetry (DSC)technique. The initial decomposition temperature and thetemperature for 10% weight loss were about 155–240 uC, and326–432 uC, respectively. It was quite clear that polymersderived from fully aromatic diamines (polymers 4.29–4.31)were more thermally stable than polymers derived fromaromatic diamines with flexible moieties (polymers 4.32–4.34). Also, these polymers were more heat resistant than 4.35derived from aliphatic diamine with flexible units. To studythe crystalline characteristics of the polymers, Wide-angleX-ray scattering (WAXS) measurements at room temperaturewere performed. According to the results, polymers 4.29–4.33showed semi-crystalline patterns and polymers 4.34 and 4.35were almost amorphous.

In another study, a binaphthyl-based diamine monomer4.36 was designed and prepared in Mehdipour-Ataei’s

Scheme 20 Preparation of poly(amide imide)s 4.29–4.35.187


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Laboratory.188 As shown in Scheme 21, the polycondensationreactions of 4.36 with aromatic and aliphatic diacid chloridesled to preparation of new poly(ether ester amide)s 4.37–4.40.The presence of flexible ether linkages, bulky naphthyl groups,and pyridine polar rings on the polymer chains were effectivefactors in improving the solubility of the polyamides. The Tgsof the resulting polyamides were found in the range of 170–202 uC. The initial decomposition temperatures were about200–260 uC and the temperatures for 10% gravimetric losswere in the range of 309–355 uC. Also, the char yields of thepolyamides at 600 uC were about 19–40%.

In 2004, two main monomers including CF3-bis(etheramine)s 4.41 and 4.42 were prepared in two steps by Liouand co-workers.189 Two series of new aromatic fluorinatedpolyamides 4.43(a–g) and 4.44(a–g) were synthesized from the

CF3-containing diamines 4.41 and 4.42 with various aromaticdicarboxylic acids a–g (Scheme 22). The polymerization wascarried out via solution polycondensation using TPP andpyridine as condensing agents in the NMP solution containingdissolved calcium chloride. The resulting 4.43 and 4.44 seriespolyamides had inherent viscosities in the range of 0.43–0.62dL g21 and 0.36–0.74 dL g21, respectively. These polyamidescould be fabricated into flexible and transparent films,indicating high molecular weight polymer formation. All thepolymers were characterized by wide-angle X-ray diffraction(WAXD) studies in the film form and showed a nearlycompletely amorphous pattern. Because of the amorphousnature and the decreased intermolecular interactions, all the4.43 and 4.44 series polyamides were highly soluble in polarsolvents such as DMF, DMAc, NMP, DMSO, m-cresol, and even

Scheme 21 Preparation of poly(ether ester amide)s 4.37–4.40.188

Scheme 22 Synthesis of polyamides 4.43(a–g) and 4.44(a–g).189


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in less polar solvents such as pyridine and THF. The tensilestrengths, elongations at break, and initial moduli ofpolyamide films of the 4.47 series were in the range of 49–71MPa, 5–9%, and 1.7–1.9 GPa, and polyamide films of the 4.48series were 48–72 MPa, 5–12%, and 1.4–2.1 GPa, respectively.DSC, thermomechanical analysis (TMA), and TGA were used toevaluate the thermal properties of these polyamides (Table 1).The Tgs of the 4.44 series polyamides derived from 4.42 werehigher than those of the corresponding 4.43 series polyamidesderived from 4.41 because of the higher bulky effect of the1,19-binaphthyl units. Moreover, almost all polyamides exhib-ited well-defined softening temperatures (Ts) on the TMAtraces. In most cases, the Ts values obtained by TMA werecomparable to the Tg values observed by DSC, and the 4.43series polyamides also exhibited lower Ts values comparedwith the corresponding 4.44 series ones.

In addition to using binaphthyl-based monomer 4.42 for thesynthesis of high performance polyamides, Liou and co-workers have also investigated the synthesis and characteriza-tion of organosoluble and light-colored fluorinated polyimides4.45 from this monomer with various aromatic dianhydrides(a–f) by a conventional, two-step thermal- or chemical-imidization method (Scheme 23).190 All the polyimides werecharacterized by WAXD studies. They failed to show anycrystallinity apparently due to the noncoplanar structure andthe presence of bulky CF3 and naphthyl pendent groups. Theamorphous nature of these polyimides was also reflected intheir outstanding solubility. The Tg values of the resultingpolyimides ranged from 257 to 351 uC and the increasing orderof Tg correlated with that of chain rigidity. Monomer 4.42contains the twisted 2,29-disubstituted binaphthyl units andtwo bulky CF3 groups and hence provides a highly packing-disruptive structure to the polyimides. This interfered with thedense packing of molecular chains and led to light-coloredpolyimide films. All the fluorinated polyimides revealed low

cutoff wavelengths and high optical transparency, with apercentage of transmittance higher than 80% at 500 nm.

A binaphthyl-based diamine monomer containing sulfone,ether, and ester linkages (4.46) was prepared by a three-stepmethod in Mehdipour-Ataei’s Laboratory.191 Nucleophilicreaction of 1,5-dihydroxy naphthalene with 4-nitrobenzoylchloride in the presence of NaOH afforded 5-hydroxy-1-naphthyl-4-nitrobenzoate. Reduction of nitro group using ironpowder and HCl resulted in preparation of 5-hydroxyl-1-naphthyl-4-aminobenzoate. Reaction of two moles of thiscompound with bis(4-chlorophenyl) sulfone led to preparationof the diamine monomer 4.46. Polycondensation reaction of4.46 with aromatic dianhydrides (a–c) resulted in preparationof related poly(amic acid)s and subsequent chemical dehydra-tion led to preparation of fully aromatic poly(sulfone etherester imide)s 4.47(a–c) (Scheme 24). The viscosities of thesepolyimides were in the range of 0.40–0.43 dL g21 that revealedreasonable molecular weights. Furthermore, the presence offlexible linkages and bulky naphthyl groups on the polymerchains were effective factors in improving the solubility of thepolyimides. The Tgs of polyimides were found in the range of239–278 uC. The initial decomposition temperatures wereabout 332–352 uC and the temperatures for 5% gravimetricloss were in the range of 369–392 uC. Moreover, char yields ofthe polyimides at 600 uC were about 49–52%.

In 2007, we reported the synthesis and characterization ofpolyamides obtained from a binaphthyl-based monomercontaining sulfide and ether linkages.192 As shown inScheme 25, treatment of S-BINOL with the methylcholoroace-tate gave corresponding diester 4.48. Basic hydrolysis of 4.48in alcoholic solution followed by acidification afforded thediacid monomer 4.49. The binaphthyl containing aramids4.50(a–e) were prepared according to phosphorylation techni-que described by Yamazaki and co-workers.193 Except for4.50(a), the aramids were dissolved not only in highly polarsolvents such as NMP, DMAc, DMF, and DMSO, but also in

Table 1 Thermal characterizations of polyamides 4.43(a–g) and 4.44(a–g)189

Polymer code DSC TMATGA

Decomposition temperaturea (uC) Char yieldb (%)

Tgc (uC) Ts

d (uC) In air In nitrogen

4.43a —e 199 450 460 564.43b 190 196 420 426 554.43c 233 210 462 488 594.43d — 221 442 490 594.43e 215 209 433 473 564.43f 237 227 440 474 564.43g 240 230 463 505 554.44a — 291 476 500 514.44b 255 260 462 500 564.44c 255 258 478 488 614.44d 251 — 476 549 664.44e 253 258 460 501 614.44f 248 241 458 513 594.44g 254 257 485 520 58

a Decomposition temperature at which a 10% weight loss was recorded by TGA at a heating rate of 20 uC min21. b Residual weight% at 800 uCin nitrogen. c Midpoint temperature of baseline shift on the DSC heating trace at a scan rate of 20 uC min21. d Softening temperaturemeasured by TMA with a constant applied load of 10 mN at a heating rate of 10 uC min21. e No discernible transition.


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moderate polar solvents such as m-cresol and THF at roomtemperature or upon heating. Comparison of the solubility ofthe previously reported polymers194 with these polymersindicated that polyamides with naphthalene pendent groupsdisplayed an increased solubility in organic solvents. All thepolymers exhibited good thermal stability, their decomposi-tion at argon atmosphere began at about 373–434 uC asindicated by initial decomposition temperature. Temperatureof 10% weight loss was in the range of 441–479 uC and about80 uC higher than those of related polyamides withoutnaphthalene groups.194 Therefore, the results showed that

introduction of naphthalene units into the polymers backboneenhance the thermal stability. Depending on the structure ofthe diamine components, which affect stiffness of the polymerbackbones, the Tg values of the polyamides were in the rangeof 241–268 uC. In comparison with the related aramids withoutnaphthyl groups, these polymers displayed higher Tg thatcould be attributed to the entanglement effects of naphthylpendent groups.

As shown in Scheme 26, the oxidation of compound 4.48 tosulfone-bridged diester compound 4.51 was carried out by theheterogeneous reaction with hydrogen peroxide in formic acid

Scheme 23 Synthesis of binaphthyl-based polyimides 4.45(a–f).190

Scheme 24 Synthesis of binaphthyl-based poly(sulfone ether ester imide)s 4.47(a–c).191


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at room temperature.195 The sulfone-bridged diacid monomer4.52 was prepared from 4.51 according to our reportedprocedure.192 A series of organic-soluble polyamides 4.53(a–f)bearing ether and sulfone linkages were synthesized from 4.52with various aromatic diamines via a direct polycondensationwith triphenyl phosphite and pyridine. The resulting polymershad inherent viscosities ranging from 0.38 to 0.46 dL g21at aconcentration of 0.5 g dL21 in DMAc solvent at 30 uC. All thepolymers showed outstanding solubility and could be easilydissolved in amide-type polar aprotic solvents and evendissolved in less polar solvents (e.g., THF and m-cresol). The

Tgs of these aramids were recorded between 152–185 uC. The10% weight loss temperatures were recorded in the range of276–327 uC in air atmosphere.

Fluorinated polymers have often been used in various high-technology fields, such as surfaces and coatings, photonic andelectronic applications, and gas-separation membranes.196–198

It has been proved that the incorporation of bulky trifluor-omethyl groups into polyamide backbones resulted in greatbenefits for improving polymer solubility, optical transpar-ency, and dielectric performance, which was attributed to thelow polarizability of the C–F bond and the increase in free

Scheme 25 Synthesis of S-BINOL-based diacid monomer and corresponding polyamides 4.50(a–e).192

Scheme 26 Synthesis of SO2-BINOL-based monomer and the corresponding polyamides 4.53(a–f).195


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volume.199–202 Quite recently, a sulfide-bridged bis(etheramine) monomer containing bulky trifluoromethyl andnaphthyl substituents, 1,19-thiobis[2-(4-amino-2-trifluoro-methylphenoxy) naphthalene] (4.54), was synthesized in ourlaboratory by the halogen displacement of 2-chloro-5-nitro-benzotrifluoride with S-BINOL in the presence of potassiumcarbonate, followed by the palladium on activated carbon-catalyzed reduction of bis(ether nitro) intermediate withhydrazine hydrate in refluxing ethanol.203 As shown inScheme 27, a series of fluorinated aramids 4.55(a–h) weresynthesized from 4.54 with various commercially availablearomatic diacids (a–h) via a direct polycondensation methodwith TPP and pyridine. The resulting aramids had inherentviscosities, ranging from 0.61 to 0.93 dL g21. As shown inTable 2, all the polymers exhibited outstanding solubility andcould be easily dissolved in amide-type polar aprotic solvents

(for example, NMP, DMSO, and DMAc), and even dissolved inless polar solvents (for example, pyridine, THF, and chloro-form). Moreover, these polymers could be cast into transpar-ent, flexible and tough films from DMAc solutions. The Tgswere recorded between 209 and 251 uC, and 10% weight losstemperatures were in excess of 470 uC in nitrogen and 450 uCin air atmosphere.

As expected, all the resulting polymers had low refractiveindices because of the presence of trifluoromethyl groups inthe polymer backbone. Among them, 4.55(g) showed thelowest refractive index (1.5614), which could be attributed tobulky hexafluoroisopropylidene groups in the polymer struc-ture. In addition, all the polyamide films showed positivebirefringence (0.0055–0.0097), indicating that the molecularchains were preferentially aligned in the film plane. As shownin Table 3, the low birefringence of these polymers could be

Scheme 27 Synthesis of fluorinated S-BINOL-based polyamides 4.55(a–h).203

Table 2 Solubilityb of fluorinated S-BINOL-based polyamides 4.55(a–h)a203

Polymer codeSolventc

NMP DMSO DMAc DMF Pyridine THF Acetone Chloroform

4.55a ++ ++ + + ¡ ¡ ¡ S4.55b ++ ++ ++ ++ + + ¡ ¡4.55c ++ ++ ++ ++ ¡ ¡ ¡ ¡4.55d ++ ++ ++ ++ + + + ¡4.55e ++ + + + ¡ ¡ S —4.55f ++ ++ ++ ++ + + + +4.55g ++ ++ ++ ++ ++ ++ + +4.55h ++ ++ ++ ++ + ¡ ¡ ¡

a (++) Soluble at room temperature, (+) soluble after heating, (¡) partially soluble, (S) swelling, (2) insoluble. b Solubility: measured at apolymer concentration of 0.05 g mL21. c NMP: N-methyl-2-pyrrolidone; DMAc: N,N-dimethylacetamide; DMF: N,N-dimethylformamide; DMSO:dimethyl sulfoxide; THF: tetrahydrofuran.


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mainly attributed to the naphthyl groups that are orientedperpendicularly to the polymer main chain. These bulkygroups prevent the polymer chains from close packing andhence effectively hinder the in-plane orientation of the mainchains. The dielectric constant values estimated from averagerefractive indices of the resulting polymer films were in therange of 2.68–2.75. The low dielectric constants of thesepolymers could be attributed to the presence of the bulkytrifluoromethyl groups in the diamine moiety, which resultedin less-dense chain packing and increased free volume.

4.2.2 Binaphthyls as bulky pendant groups. As mentionedbefore, binaphthyl-based systems as bulky aromatic pendantgroups can remarkably enhance the solubility of polymerswhile maintaining their thermal stability. Therefore, severalresearch groups have taken advantage of these units toproduce high performance polyamides and polyimides.

In 2003, Banihashemi and Firoozifar204 reported thesynthesis of two binaphthyl-based diamines (4.56 and 4.57)as aromatic monomers. As shown in Scheme 28, two series ofpolyamides (4.58 and 4.59) having 2-naphthyl pendant groupswere also synthesized by direct and indirect polycondensationof these monomers with various aromatic dicarboxylic acids.The resulting polymers had inherent viscosities of 0.43–0.84dL g21 and were obtained in quantitative yield. Moreover, allthe polyamides were readily soluble in a wide range ofsolvents, such as DMF, N,N-dimethylacetamide (DMAc),dimethyl sulfoxide (DMSO), and N-methyl-2-pyrrolidone(NMP), and most of them were soluble even in m-cresol andpyridine.

In 2005, Liaw and co-workers186 reported the synthesis andcharacterization of a binaphthyl-based monomer,2,29-dinaphthylbiphenyl-4,49-diamine (4.60), and its use inthe preparation of polyamides 4.61(a–d) and polyimides4.62(a–c) by the reaction of 4.60 with commercial dicarboxylicacids and dianhydrides (Scheme 29). These polymers showedgood solubility in a variety of solvents such as NMP, DMAc,DMF, pyridine, THF, DMSO, c-butyrolactone, and cyclohex-anone at ambient temperature or upon heating at 70 uC. Thegood solubility of these polymers could be attributed to theeffect of the bulky and noncoplanar naphthyl groups in the

Table 3 Refractive indices and dielectric constants of fluorinated S-BINOL-basedpolyamides 4.55(a–h)203

Polymer codeRefractive indices and birefringence Dielectric

constantse

nTEa nTM

b nAVc Dnd

4.55a 1.5764 1.5679 1.5735 0.0085 2.474.55b 1.5739 1.5666 1.5714 0.0073 2.474.55c 1.5835 1.5754 1.5808 0.0081 2.504.55d 1.5809 1.5740 1.5786 0.0069 2.494.55e 1.5792 1.5695 1.5759 0.0097 2.484.55f 1.5696 1.5632 1.5674 0.0064 2.454.55g 1.5633 1.5578 1.5614 0.0055 2.444.55h 1.5844 1.5753 1.5813 0.0091 2.50

a nTE: the in-plan refractive index at 1310 nm at ambienttemperatures. b nTM: the out-of-plan refractive index at 1310 nm atambient temperatures. c Average refractive index; nAV = (2nTE + nTM)/3. d Birefringence; Dn = nTE 2 nTM. e Optically estimated dielectricconstant; e = nAV

2.

Scheme 28 Synthesis of two series of soluble polyamides 4.58 and 4.59.204


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polymer backbone which could inhibit close packing anddecreased the interchain interactions to enhance solubility.The Tg of the polymers were found to be in the range of 288–318 uC. The temperatures for 10% weight loss were in therange of 533–583 and 505–520 uC for polyimides andpolyamides, respectively. The incorporation with naphthylgroups at the 2,29-position on the biphenyldiamine increasethe size and sharp anisotropy of the 2,29-disubstituted groupof the diamine and affect the molecular packing.205–207

Consequently, the introduction of the bulky naphthalenegroups could improve the processing characteristics of theserelatively intractable polymers. Moreover, the high thermalstability of these polyamides and polyimides was also becauseof the presence of rigid naphthalene groups in the polymerbackbone. The flexible films were obtained by casting polymersolution in DMAc. They had a tensile strength in the range of84–128 MPa, an elongation at break in the range of 6–10%,and a tensile modulus in the range of 2.0–2.6 GPa. In general,these properties could make these polyamides and polyimidesattractive for useful applications such as processable high-temperature engineering plastics due to the bulky andnoncoplanar naphthalene groups in the polymer chain.

As shown in Scheme 30, two series of aramids havingnoncoplanar biphenylene units in the main chain and bulkyphenyl or naphthyl pendant group at 2,29-disubstitutedposition were prepared from the two aromatic dicarboxylicacid monomers, 2,29-diphenylbiphenyl-4,49-dicarboxylic acid(4.63) and 2,29-dinaphthylbiphenyl-4,49-dicarboxylic acid

(4.64), and various aromatic diamines (a–g).208 These poly-amides containing phenyl or naphthyl pendant groups(4.65(a–g) and 4.66(a–g)) showed excellent solubility incommon organic solvents. All the polyamides exhibited typicalamorphous patterns by WAXD diagrams. Also, the amorphousnature of these polymers was reflected in their good solubility.All the aromatic polyamides showed good thermal stabilityand the 10% weight loss temperatures of these polymers innitrogen and in air atmospheres were recorded in the range of495–600 and 475–605 uC, respectively. These results indicatedthat polymers with bulky pendant naphthyl groups (4.66)exhibited higher thermal stability than polymers with phenylpendant groups (4.65). The aromatic polyamides 4.65 and 4.66series showed fluorescence emission maxima around 440–462and 442–459 nm in NMP solution, respectively, with fluores-cence quantum yield ranging from 0.16 to 0.86%. Theincorporation of naphthalene chromophore into the polymerchains resulted in the higher fluorescence quantum yield ofaromatic polyamides 4.66(f–g) compared to 4.65(f–g). In thesolid state, the UV-vis absorption and photoluminescencespectra of polyamides 4.65 and 4.66 series were nearlyidentical and exhibited a maxima absorbance and emissionat 316–349 and 435–530 nm, respectively. The thin films ofaramids without triphenylamine units exhibited high opticaltransparency from UV-vis transmittance measurement withcutoff wavelength in the range of 323–345 nm. The polyamide4.66(d) derived from the triphenylamine-based diaminerevealed excellent electrochromic contrast and coloration

Scheme 29 Synthesis of binaphthyl-based polyamides 4.61(a–d) and polyimides 4.62(a–c).186


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efficiency, changing color from the pale yellowish neutral formto green then to the blue oxidized forms when scanningpotentials positively from 0.00 to 1.30 V.

5 Conclusions

During the last 40 years, significant development has beenmade in synthesis, structure, and properties of binaphthyl-based macromolecules. In the biotic world, chiral recognitionis an essential aspect of structural recognition in complexa-tion. In the early 1970’s, Cram designed systems whichincorporated one or more binaphthyl units into crown ethersfor complexing a variety of amine salts enantioselectively. Thecombination of binaphthyl units and crown ethers providedsystems which were also useful for complexing various ions byintramolecular ion-pairing in which the charges on hosts andguests could be matched. In fact, binaphthyl units play animportant role in the introduction of chiral elements or offunctional groups into hosts. Because of variability of the aryl-aryl dihedral bond angle in direct-linked 1,19-binaphthyl units,the diameter of the hole of such hosts is slightly adjustable.

Moreover, chains attached to the 3,3’-positions of these unitscan extend the chiral barrier and converge on guests one overeach of the two faces. In addition, chains attached to the 5,5’-positions diverge from the binding site and can be used forattaching them to solid supports for chromatographic separa-tions or for controlling solubility of hosts. In indirect-linkedbinaphthyl systems, chirality is not induced because of the freerotations between a definite linkage and two naphthyl rings.However, the complexation of these systems with variouscations or even anions can be very impressive especially due tothe possibility of tuning the diameter of the hole of these hoststhrough using different kinds of linkages between twonaphthyl groups.

On the other hand, the study of chiral-conjugated polymershas become a very exciting field of research and various mainchain chiral-conjugated polymers derived from binaphthylunits have been prepared over the past decade. Thesepolymers can be used to prepare chiral electrodes and sensors,ferroelectric liquid crystals, and non-linear optical materials.In these polymers, the chiral binaphthyl units allow thesematerials to have stable main chain chiral configuration.

Scheme 30 Synthesis of biphenylene-based polyamides 4.65(a–g) and 4.66(a–g).208


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Although most of these polymers contain conjugated linkersincluding phenylene, phenylenevinylene, phenyleneethyny-lene, and thiophene, polybinaphthyls without such linkershave also been synthesized. Researches on developingbinaphthyl-based polymers have not been restricted tochiral-conjugated polymers. In recent years, many researchershave synthesized some non-chiral indirect-linked binaphthyl-based monomers to prepare high performance polyamides,polyimides, and their copolymers. In fact, the formation ofnon-coplanar conformation due to the presence of binaphthylmoieties as bulky main chain units or bulky pendant groupscan disturb interchain packing, decrease glass transitiontemperature, and remarkably increase processability whilemaintaining good thermal stability of the resulting polymers.

Previous studies have usually represented a specializedaspect of binaphthyl-based systems. This could make itdifficult for readers who were not familiar with these materialsto evaluate and compare diverse structures, properties, andapplications. Therefore, in this review, besides synthesisapproaches and structures, we also provided a brief review ofmajor applicative potential of binaphthyl-containing macro-molecules from molecular hosts for chiral recognition andcomplexation with various guests to main chain chiral-conjugated polymers and high performance polymeric materi-als based on our results and some important contributions ofother groups. Although the binaphthyl-based systems coveredhere are at different levels of maturity, there is little doubt thatmany opportunities remain to capitalize on their potentialspecificity and uniqueness. Finally, it is worth noting thatbinaphthyl-based macromolecules offer a very broad andpromising field of research, not only to address the corre-sponding opportunities and challenges for properties andapplications but also to open new perspectives. The number ofarticles published in this field suggests that in the near futurean enormous interest in the preparation of new productsshould arise based on these materials.

Acknowledgements

The research in the authors’ laboratory was supported in partby the Kharazmi University (former Tarbiat MoallemUniversity). This work would not have been possible withoutthe support of many people. We are truly indebted to all of ourcollaborators. Their scientific contributions and dedicationsare reflected in the references.

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Binaphthyl-based macromolecules: a review