Carbene Review

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Stable Carbenes

Didier Bourissou, Olivier Guerret, Francois P. Gabba, and Guy Bertrand*,

Laboratoire dHeterochimie Fondamentale et Appliquee (UPRES-A CNRS 5069), 118 route de Narbonne, Universite Paul Sabatier (Bat 2R1),F-31062 Toulouse Cedex 04, France, Centre de Recherche Rhone-Alpes, Elf-Atochem, BP 63, F-69493 Pierre Benite, France, and Department of

Chemistry, Texas A&M University, College Station, Texas 77843-3255

Received March 2, 1999

Contents

I. Introduction 39

II. Influence of Substituents on the ElectronicStructure and Stability of Carbenes

39

II.1. Ground-State Carbene Spin Multiplicity 39

II.1.1. Electronic Effects 41

II.1.2. Steric Effects 43

II.2. Concluding Remarks 43

III. Persistent Triplet Carbenes 43

III.1. Synthesis and Stability 43III.2. Reactivity 44III.3. Concluding Remarks 46

IV. Stable Singlet Carbenes: Synthesis andStructural Data

46

IV.1. (X,X)-Carbenes: Diaminocarbenes and OtherAminocarbenes

46

IV.2. (Z,Z)-Carbenes: Diborylcarbenes 49IV.3. (X,Z)-Carbenes 50

IV.3.1. Phosphinosilyl- andPhosphinophosphoniocarbenes

50

IV.3.2. Sulfinylcarbenes 54V. Reactivity of Stable Singlet Carbenes 55

V.1. 1,2-Migration Reactions 55V.2. Carbene Dimerization and Related Reactions 57V.2.1. Carbene Dimerization 57V.2.2. CarbeneCarbenoid Coupling Reactions 58

V.3. Addition to Multiple Bonds 61V.3.1. Addition to CarbonCarbon Double

Bonds61

V.3.2 Addition to Carbonyl Derivatives 62V.3.3. Addition to Carbon-Heteroatom Triple

Bonds62

V.3.4. Addition to Cumulenes 64V.3.5. Addition to 1,3-Dipoles 64

V.4. Insertion Reactions 64V.4.1. Insertion into Unpolarized CH Bonds 64V.4.2. Insertion into Polarized XH Bonds 65V.4.3. Insertion into Other Bonds 66

V.5. CarbeneLewis Acid Adducts 66V.5.1. Protonation of Carbenes 66V.5.2. CarbeneGroup 13 Element Adducts 66V.5.3. CarbeneGroup 14 Element Adducts 68V.5.4. CarbeneGroup 15 Element Adducts 70V.5.5. CarbeneGroup 16 Element Adducts 70V.5.6. CarbeneGroup 17 Element Adducts 70

V.6. CarbeneLewis Base Adducts 71

V.7. CarbeneMetal Adducts 72V.7.1. CarbeneAlkali Metal Adducts 72V.7.2. CarbeneAlkaline Earth Metal Adducts 73V.7.3. CarbeneTransition Metal Adducts 73V.7.4. CarbeneGroup 11 Metal Adducts 82V.7.5. CarbeneGroup 12 Metal Adducts 83V.7.6. CarbeneRare Earth Metal Adducts 84

VI. Conclusion 85VII. References 86

I. Introduction

B eginning as chemical curiosities w ith the pioneer-ing work of Curtius,1a St a u din g e r,1b and even earlierefforts,1c carbenes have played an important role astransient intermediates over the last f ive decades. 2

Intr oduced by D oering into organic chemistry in th e1950s3 and by Fischer into organometallic chemistryin 1964,4 these fascinating species are involved inmany reactions of high synthetic interest .

In the last 10 year s,5 our understanding of carbenechemistry ha s a dvanced dra mat ically w ith the prepa-ra tion of persistent tr iplet dia rylcarbenesI 6 a n d t h eisolation of heteroatom-substituted singlet carbenes(II -XIV)7 (Figure 1). This review summa rizes t hisfruitful part of carbene chemistry , which only star tedin the 1980s.8

The influence of t he subst ituents on th e ground-sta te spin multiplicity a nd sta bility of car benes w illbe discussed first. S ubsequently, th e results concern-ing persistent triplet carbenes I will be presented.The next chapter will be devoted to the synthesis,structura l, a nd spectroscopic feat ures of singlet car-benesII-XIV. Last , their reactivity w ill be presenteda n d comp a re d wit h t h a t observe d for t ra n sie n t ca r-benes.

II. Influence of Substituents on the ElectronicStructure and Stability of Carbenes

II.1. Ground-State Carbene Spin Multiplicity

Ca rben e s a re n e u t ra l comp ou n ds fe a t u rin g a di-va le n t ca rbo n a t o m wit h o n ly six e le ct ro n s in i t sva lence shell. Considering a prototype carbene -C -,t h e ca rbo n a t om ca n be e it h e r l in ea r or ben t , e a chgeometry describable by a certa in degree of hybrid-ization. The linear geometry implies an sp-hybridized

39Chem. Rev. 2000, 100,3991

10.1021/cr940472u CCC: $35.00 2000 American Chemical SocietyPublished on Web 12/22/1999


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ca rbe n e ce n t er wit h t w o n o n bon din g de g en e ra t eorbitals (pxa n d py). B ending th e molecule breaks thisdegeneracy and the carbon atom adopts an sp 2-typeh y b r id iz a t i on : t h e p y or b it a l r e ma i n s a l m os t u n -changed (it is usually called p), while the orbita l tha tst a rt s a s p u re pxorbita l is sta bilized since it a cquiressome s character (it is therefore called ) (Figu re 2).The linear geometry is an extreme case; most car-

be n e s a re be n t a n d t h e ir f ro n t ie r o rbit a ls wil l besystematically called a n d p.

As depicted in F igure 3, four electronic configura -tions can be envisa ged. The tw o nonbonding electr onsca n be in t w o dif fe ren t orbit a ls w it h p a ra l lel sp in s(triplet state); hence, the molecule is correctly de-scribed by t h e 1p1 configurat ion (3B 1 s t a t e ) . I ncontrast , for singlet carbenes, the two nonbondingelectrons can be paired in the same or porbital.Therefore, there are two different 1A1 s t a t e s, t h e 2

be in g g e n e ra l ly mo re st a ble t h a n t h e p2. L a s t , a nexcited singlet sta te wit h 1p1 configuration can alsobe envisaged (1B 1 sta te).

The ground-state spin multiplicity is a fundamen-ta l feat ure of car benes tha t dictat es their reactivity.9

Indeed, singlet carbenes feature a filled and a vacantorbita l, and therefore, they should possess an am bi-philic char acter. On t he other ha nd, triplet carbeneshave two singly occupied orbitals and are generallyre g a rded a s dira dica ls .

The carbene ground-sta te mult iplicity is rela ted tot h e re la t ive e n e rg y o f t h e a n d p orbitals. Thes in g let g r ou n d s t a t e i s f a v or ed b y a l a r g e -pseparation; Hoffmann determined that a value of atleast 2 eV is necessary to impose a singlet groundstate, whereas a value below 1.5 eV leads to a triplet

Didier Bourissou was born in Nice in 1972. He studied at the EcoleNormale Superieure in Paris from 1992 to 1994 and then received hisPh.D. degree from the Universite de Toulouse in 1998. He was a researchassociate with F. Mathey at the Ecole Polytechnique in Palaiseau and iscurrently Charge de Recherche at the Laboratoire d'HeterochimieFondamentale et Appliquee at the Universite Paul Sabatier. He is presentlyworking on new types of carbenes and other electron-deficient species.

Olivier Guerret was born in 1971 in Montpellier. He entered the Ecole

Polytechnique (Palaiseau) in 1991 and then joined Guy Bertrands teamin 1994 working on his Ph.D. degree, which he received in 1997. He iscurrently working for Elf Atochem in the Centre de Recherche RhoneAlpes and his research is mainly focused on the design of new organiccatalysts for living radical polymerization.

Francois Gabba was born in 1968 in Montpellier (France). Before joiningthe research group of A. H. Cowley at the University of Texas at Austin,he studied chemistry at the Universite de Bordeaux (France). In 1992and 1993, he fulfilled his French National Duties by taking part in a Franco-American cooperation under the supervision of G. Bertrand and A. H.Cowley. He completed his Ph.D. degree in 1994 and then was awardedan Alexander von Humboldt Fellowship to join the research group of H.Schmidbaur at the Technische Universitat Munchen (Germany). After twoyears of this collaboration, he was awarded a Marie Curie Fellowship ofthe European Commission and was generously offered to stay as a

Habilitand in the Bavarian Institute. Since 1998 he has been an AssistantProfessor at Texas A&M University where he is presently working ondiverse aspects of p-block element chemistry.

Guy Bertrand was born in Limoges in 1952. He graduated (ingenieurENSCM) from the Universite de Montpellier and moved to the Universitede Toulouse as an Attache de Recherche CNRS in 1975. From 1988 to1998 he was Directeur de Recherche at the Laboratoire de Chimie deCoordination du CNRS and is now the Director of the LaboratoiredHeterochimie Fondamentale et Appliquee at the Universite Paul Sabatier.His research spans a wide range of topics at the border between organicand inorganic chemistry. He received the International Council on MainGroup Chemistry Award in 1993, the Humboldt Award in 1994, and themedaille dargent of the CNRS in 1998 and was elected MembreCorrespondant of the French Academy of Sciences in 1996.

40 Chemical Reviews, 2000, Vol. 100, No. 1 Bourissou et al.


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ground state.10 There are some similarit ies w ith t hecrystal-field theory (strong-field low-spin and weak-field high-spin configura tions).

Given these statements, the influence of the sub-stituents on the carbene ground-state multiplicity can

be easily analyzed in terms of electronic and stericeffects.

II.1.1. Electronic Effects

II.1.1.1. Inductive Effects. The in fluence of th esubstituents electronegativity on the carbene mul-tiplicity was recognized relat ively early on 11,12 a n dreexam ined recently. 13 It is now well established that-electron-wit hdra wing substituents favor t he singletve rsu s t h e t r ip le t s t a t e . In p a rt icu la r , Ha rriso n e t

a l. 11a,c showed that the ground state goes from triplett o sin g le t w h e n t h e su bst it u e n t s a re ch a n g e d fromelectropositive lithium to hy drogen a nd t o electrone-gative fluorine (although mesomeric effects surelya lso pla y a role for t he lat ter element ) (Figur e 4). Thiseffect is easily rationalized on the basis of pertubationorbita l diagra ms (C 2vsym metry). Indeed,-electron-wit hdra w ing substituents inductively stabilize the nonbonding orbita l by increasing its s cha ra cter andleave the porbita l unchan ged. The -pgap is thusincreased and t he singlet sta te is favored (Figure 5a).In con t ra st , -electr on-donat ing subst ituent s inducea s ma l l -p ga p w h i ch f a v or s t h e t r i p le t s t a t e(Figure 5b).

Figure 1. Triplet carbenes I and singlet carbenes II -XIV.

Figure 2. Relat ionship between t he carbene bond a ngleand the na ture of the front ier orbita ls .

Figure 3. Electronic configurations of carbenes.

Stable Carbenes Chemical Reviews, 2000, Vol. 100, No. 1 41


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II.1.1.2. Mesomeric Effects. Although inductiveeffects dictate the ground-state multiplicity of a fewcarbenes such as the triplet Li-C -Li, 11c mesomeric

effects can pla y a m ore significant role.14 Substituentsinteracting w ith t he carbene center can be classifiedin t o t wo t yp e s, n a me ly X ( fo r -electron-donatinggroups such a s-F ,-C l, -B r,-I ,-NR 2,-P R2,-OR ,-S R , -S R 3, . . . ) a n d Z ( fo r -electron-withdrawingg ro u p s su ch a s -COR, -C N , C F 3, -B R 2, -S iR 3,-P R 3+, ...). Therefore, th e singlet car benes consideredin t h is re vie w ca n be cla ssi f ie d a cco rdin g t o t h e irsubstit uents: the highly bent (X,X)-carbenes and th e

linear or quasi-linear (Z,Z)-and (X,Z)-carbenes. In allcases, the mesomeric effects consist of the interactionof the carbon orbita ls (s, por px, py) and appropriat ep or orbita ls of the t wo carbene substit uents. Theseinteractions are clear ly illustra ted using pertur bat ionorbita l dia gra ms (Figure 6).

(X,X)-Ca rbenes a re predicted t o be bent singletcarbenes. 12,13 The energy of the vacant p orbital isincreased by interaction with the sym metric combi-na tion of the subst ituent lone pairs (b1). Since the orbit a l re ma in s a lmo st u n ch a n g ed, t h e -pg a p isincreased and t he singlet sta te is favored (Figure 6a).No t e t h a t t h e orbital and the nonsymmetric com-bination of the substituent lone pairs (a 2) are close

in energy, their relat ive posit ion depending on theelectronegativity of X compared to that of carbon.Donation of the X-substituent lone pairs results in a

Figure4. Influence of the subst ituents electronegativityon the ground state carbene spin multiplicity.

Figure 5. P ertubat ion orbita l diagra ms showing the influence of the induct ive effects .

Figure 6. Pertubation orbital diagrams showing the influence of the mesomeric effects.



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polarized four-electron three-center -system. The

C -X bonds acquire some multiple bond character,w hich implies tha t (X,X)-carbenes a re best describedby the superposit ion of two zwitterionic structureswit h a n e g a t ive ch a rg e a t t h e ca rbe n e ce n t er. Th emo st re p re se n t a t ive ca rbe n e s o f t h is t yp e a re t h etra nsient dimethoxy-15 an d diha locarbenes16 a n d t h est a ble dia min o ca rbe n es t h a t wil l be de scribed insection IV.1.

Most of t he (Z,Z)-carbenes a re predicted t o be linearsinglet carbenes.12,13 For this type of compound, thesymme t ric combin a t ion of t h e su bst it u e n t va ca n to rbit a ls in t e ra ct s wit h t h e py orbital, which is per-pendicular to the valence plane (Figure 6b). Thisinteraction does not a ffect the p xorbita l. Therefore,

the (px,p y) degeneracy is broken making these car-benes ha ve a singlet ground sta te even th ough th eya r e l in ea r . 12 N ot e t h a t t h i s s u b s t it u t i on p a t t er nresults in a polar ized tw o-electr on thr ee-cent er-sys-t e m. He re a lso , t h e C -Z bonds ha ve some multiplebond chara cter; t hese (Z,Z)-carbenes a re best de-scribed by t h e su pe rposit ion of t wo zwit t e rion icstructures featuring a posit ive char ge at t he car bonat om. Some of th e most st udied carbenes of this t ypear e the tra nsient dicarbomethoxycar benes17 a n d t h emasked diborylcarbenes (see section IV.2).

La st, t he qua si-linear (X,Z) ca rbenes combine bothtypes of electronic int eraction (Figure 6c). The Xsu bst i t u e n t lo n e p a ir in t e ra ct s wit h t h e p y orbital,

wh ile the Z substituent vacant orbita l interacts witht h e px o rbi t a l . Th e se su bst i t u en t e ffect s a re bot hs t a b il iz in g a n d b ot h f a v or t h e s in g le t s t a t e (t h ev a ca n t py orbital is destabilized, while the filled p xorbital is stabilized). These two interactions resultin a p o la rize d a l le n e -t yp e syst e m wit h XC a n d CZmult iple bonds. G ood exam ples of this t ype of carbenear e given by the tr ans ient ha logenocarboethoxycar -benes 18 an d by the st able phosphinosilyl- an d phos-phinophosphoniocarbenes (see section IV.3.1).

II.1.2. Steric Effects

B ulky substituents clear ly kinetically st abilize allty pes of ca rbenes. Moreover, if electronic effects a re

n e g lig ible , t h e st e ric e f fe ct s ma y a lso dict a t e t h eground-sta te spin multiplicity.Sin ce t h e e le ct ro n ic st a bi l iza t io n of t h e t r ip le t

re la t ive t o t h e sin glet s t a t e is a t a ma x imu m wh e nthe carbene frontier orbita ls a re degenerat e (Figure2), a linea r geometr y w ill favor the tr iplet st a te. Thisis illustrated by the influence of the carbene bonda n g le o n t h e g rou n d-st a t e sp in mu lt ipl ici t y of t h eparent car bene: below 90 the energy of the singletmethylene drops below tha t of the triplet sta te.14 I nthe same way, increasing the steric bulk of carbenesu bst i t u e n t s bro a de n s t h e ca rben e bo n d a n g le a n dtherefore favors the t riplet st at e.19 Dimethylcarbenehas a bent singlet ground state (111), 20 wh ile t h e

di (tert-butyl)-21a and diada mant ylcarbenes21b are t rip-

lets. In the lat ter compounds the two bulky substit-uents impose wide carbene bond a ngles (143 an d152, respectively). Not surprisingly, cyclopenty li-dene22a h a s a s i ng le t g r ou n d s t a t e d u e t o a n g u la rconstra int , as well a s cyclopropenylidene,22b,c whichcombin es bot h a n g u la r con st ra in t a n d a ro ma t icit y .

II.2. Concluding Remarks

U n d ou b t ed ly , t h e b es t w a y t o s t a b i li ze t r i pl etcarbenes kinetically consists of protecting t he highlyreactive carbene center by bulky substituents.

It is clear from t he above discussion tha t it is m ucheasier to design a substitution pattern for stabilizing

singlet ra ther tha n triplet carbenes. In fact , as ear lyas 1960, P auling 12b re a l ize d t h a t t h e i deal substitu-ents to sta bilize singlet carbenes should preserve theelectroneutra lity of t he carbene center. This can beachieved in three different w ay s (Figure 7), each ofwhich has been studied experimentally.

(1) Two -donor -a t t ra ct o r su bst it u e n t s , i.e., apush,push mesomeric-pull,pull inductive substit u-tion patt ern. G ood examples are dia minocar benes inwhich the carbene electron deficiency is reduced bythe donat ion of the tw o nitrogen lone pairs w hile thecarbene lone pair is st abilized by t he inductive effectof tw o electronegat ive nitrogen at oms.

(2) Two -attractor -donor substituents, i.e., apull,pull mesomeric-push,push inductive substitu-tion pattern. Diborylcarbenes are good representa-tives.

(3) A -donor and a -acceptor substituent, i.e., ap u sh ,pu ll me some ric su bst it u t ion p a t t e rn ; in t h iscase, the inductive effects ar e not of primary impor-ta nce. This cat egory is illustr a ted by phosphinosilyl-a nd phosphinophosphoniocar benes.

III. Persistent Triplet Carbenes

III.1. Synthesis and Stability

Apart from a n early study by Zimmerman n,23

untilthe w ork of Tomioka et a l.,6 no at tempts to genera tetriplet carbenes that would be stable under normalconditions had been reported. All the results obtainedin this f ield have been recently reviewed;6 h e re wewill only recall the main achievements.

As discussed in the previous section, the use ofbulky substituents is the only wa y to sta bilize tripletcarbenes; accordingly, st erically hindered diar ylcar-benes, and especially polyhalogenated and polym-ethylat ed diphenylcar benes, ha ve been prepared. Arecent result24 su g g est s t h a t t h e t r i f lu orome t h ylgroup on the phenyl r ing could also be used efficientlyto stabilize diarylcarbenes. Compounds Ia-l h a v e

Figure 7. Electronic effects of t he subst ituents for dia mino-, phosphinosilyl-, a nd diboryl-carbenes.



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classically been prepar ed in low-temperature ma tri-ce s by p h ot o lysis of t h e ir dia zo p recursors 1a-l(T a ble 1) a n d ch a ra ct e rize d by UV a n d ESR sp e c-troscopy.25

Their stability at room temperature is quantified

by their half-life t ime in benzene measured by thelaser flash photolysis (LFP) technique33 (Ta ble 1). Thevalues obtained for t he unsubstituted I a,26 the poly-fluorinatedIb,27 the polychlorinated Ic,d,23,28,29 a n dthe polybrominated Ih-j31,34 diphenyl derivativesclearly demonstra te tha t t he main fa ctor is the stericbulk of the a ryl substituents, especially a t t he o r t h o posit ions. Moreover, the results observed for poly-chlorinatedIc,d23,28,29 and polymethylatedIe-g23,30,35diphenylcarbenes provide evidence for a buttressingeffect 36 of the metasubstituents. U p to now, t he moststa ble triplet carbene isI j. I t is indefinit ively sta bleat 130 K a nd ha s a ha lf-life of 16 s at room t emper-at ure in fluid solution. It is of par ticular interest t ha t

i t is a lso in de fin it ively st a ble in t h e cryst a l s t a t e a troom tempera ture.31,34b

III.2. Reactivity

Zimmerma nn, P lat z, and Tomioka ha ve studied thereactivity of triplet carbenesI b-l. The ph otolysis ofthe dia zo precursors a t r oom t empera ture generallyleads to a highly complex mixture. However, cleanreactions could be obtained by a strict control of thereaction conditions.

In degassed benzene, the polychlorinated diphen-ylcarbene Ic dimerizes37 to give th e corresponding

tetrakis(aryl)ethylene {Ic}2 in 70-80% yield (eq1).23,28,29 I t i s n o t e w o r t h y t h a t , s o f a r , o n l y s m a l l

am ounts of car bene dimers ha ve been obta ined withthe polybromina ted diphenylcarbenesIh-j, probably

because the carbene center is too hindered.31 Withthe polymethyla ted diphenylcarbenes Ie-g, dimer-ization as well as intramolecular hydrogen abstrac-tion followed by electrocyclizat ion of the result ingt ra n sie n t o-xylylenes2e-g occurs (Scheme 1).23,30,35aThe ra tio {Ie-g}2/benz ocyclobut enes3e-g highlightsthe influence of the but tressing effect on t he reactiv-ity of the carbene center. Ind eed, a s the ort h o methylg rou p s a re bu t t re ssed more e ffect ive ly, t h e y a rebrought much closer to the carbene atom and, there-fore, intramolecular H-abstraction is favored overdimerization (Scheme 1).30,35a

I rra dia t io n o f 1k a t ro o m t e mp e ra t u re g ive s t h ephenylindan4, presuma bly as t he result of a stepwise

Table 1. Half-Life Times of Triplet Carbenes Ia-l at Room Temperature in Benzene Solution



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insertion of the carbene center into a C -H bond of atert-butyl group at an ortho position (eq 2).30,35b

The reactivity of diarylcarbenes I b-j t owa rd va ri-ous trapping agent s ha s also been studied. P hotolysisof t h e dia zo p re cu rsors 1b-j in t h e p rese n ce ofoxygen gives t he corr esponding dia ryl ketone oxides5b-j, which have been characterized by UV27,29-32,35ba n d e v e n N M R 30b spectroscopy in the case of 5g.

P hotolysis of1b-g in methanol results in the almostexclusive formation of the methyl ethers 6b-g,27-30,32obviously produced by insertion of the carbene intothe solvent O-H bond (Scheme 2).38

Other hydrogen donors and especially 1,4-cyclo-hexadiene have been demonstrated to be effectivequenchers for triplet carbenes, presumably by hy-drogen-abst ra ction reactions. In the ca se of1c a nd1h-j,23,27-29,31 t h e t ra n sie n t dia ryl ra dica ls 7c,h-jha ve been spectr oscopically cha ra cterized.28 The sol-vent ha s a dr a ma tic effect on the product distr ibutiona s sh own in t h e ca se o fIc (Scheme 3).29

On the other ha nd, the (9-triptycyl)(R-naphthyl)-carbene Il r ea c t s w i t h t h e (E)--deuterio-R-methyl-

styrene affording the corresponding cyclopropanes11l in ca. 60% yield (eq 3).32 Du rin g t h is re a ct ion ,

t h e st e re o ch e mist ry o f t h e st a rt in g o le f in wa s n o tretained in the product. Therefore, the formation of11l certainly results from a stepwise addit ion reac-t i on v ia t h e t r a n s i en t b ir a d i ca l 10l, t h e l os s of

Scheme 1

Scheme 2



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stereochemistry result ing from rotat ion a bout t he C-C bonds of10l. So far , there is only one postula tedexam ple of t hree-membered ring closure by ra dicalcoupling occurring with retention of configuration.39

III.3. Concluding Remarks

The crystallographic characterizat ion of a tripletcarbene has y et to be achieved, and t he synthesis of

a t r ipl et ca r b en e t h a t w ou ld b e s t a b le i n r oomtemperature solution remains an exciting challenge.Th e se sp ecie s wo u ld n ot on ly be of fu n da me n t a linterest but could a lso find numerous a pplicat ions.For example, Tomioka et al. recently chara cterizedt h e t r isca rbe n e Im in a 2-me t h ylt et ra h ydrofu ra nma t rix .40 The fine-str ucture E SR spectr um r ecorded

a t 5 K i s d u e t o a s p i n -se pt e t sp ecies, a n d n on ot i cea b l e p e a k s d u e t o l ow e r s pi n s t a t e s w e r eobserved up t o 90 K. This result d emonstr at es tha tdia rylca rbe n es ca n be e ff icien t ly con n e ct e d by atopological linker, and polycarbene derivatives such

a s Im a re u n do u bt e dly o f in t e re st in t h e f ie ld o fpurely organic magnetic materials.41

IV. Stable Singlet Carbenes: Synthesis andStructural Data

As shown in section II, carbene centers are highlysensit ive to electronic interactions with their sub-stituents. Accordingly, the singlet carbenes consid-ered in this review have been classified as (X,X)-,(Z,Z)-, and (X,Z)-carbenes. The nature of the stabili-za tion is perceptible in th e stru cture of the molecules,as well as in t heir reactivity. In th e present section,t h e s y n t h e si s a n d t h e m a i n s t r u c t ur a l f ea t u r e s o feach type of carbene will be successively presented.Sp ecia l a t t e n t ion wil l be g iven t o t h e n a t u re o f t h ebonding sy stems.

IV.1. (X,X)-Carbenes: Diaminocarbenes and OtherAminocarbenes

In t h e e a rly 1960s, Wa n zlick re a l ize d t h a t t h esta bility of carbenes could be dra ma tically enhan cedby t h e p re se n ce o f a min o su bst i t u en t s a n d t r ie d t oprepar e th e 1,3-diphenylimida zolidin-2-ylidene

II afrom 12aby therma l elimina tion of chloroform (eq4).42 At t ha t t ime, only th e dimeric electron-rich olefin

{II a}2 was isolated and cross-coupling experimentsdid not support an equilibrium between {II a}2a n dt h e t w o ca r b en e u n it s II a.43a -c However, recent

results by Denk et a l. were interpreted as providingsome evidence for t his equ ilibrium.43d In 1970, Wan-zlick and co-w orkers demonstra ted tha t imida zoliumsa lt s13a,b could be deprotona ted by pota ssium tert-butoxide to afford the corresponding imidazol-2-ylidenes IIIa,b, w h i c h w e r e t r a p p e d b u t n o t i s o -lated. 44 Following this principle, almost two decadeslater Arduengo et al. prepared A Sta b l e C rysta l l i n e Carbene. 45 Compound IIIc w a s o b t a i n e d i n n e a rqua ntita t ive y ield by deprotona tion of t he 1,3-di-1-adamantylimidazolium chloride13c wit h sodium orpota ssium hydr ide in the presence of cat alyt ic amountsof either t-B uOK or t he dimeth yl sulfoxide anion (eq5). The colorless crys ta ls ofIIIc a re t h e rma lly st a ble

a n d m el t a t 240-241 C wit hout decomposit ion.Int erestingly, Herrma nn a nd co-w orkers showed thatt h e de p ro t o n a t io n o ccu rs mu ch fa st e r wit h l iqu idammonia as the solvent (homogeneous phase), andoxygen, nitrogen, and phosphorus N-functionalizedIIId-f a s w e l l a s c h i r a l IIIg an d bis-imida zol-2-

Scheme 3



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ylidenes IIIh h a ve bee n p rep a re d fol lowin g t h isprocedure.46 In 1993, Kuhn a nd co-w orkers developed

a new an d versat ile approach to the alkyl-substitut edN-heterocyclic carbenes IIIi-k.47 This original syn-thetic strategy relied on the reduction of imidazol-2(3H)-thiones14i-kwith potassium in boiling THF(eq 6). La st , E nders et a l. reported in 1995 tha t t he

1,2,4-triazol-5-ylidene IVa could be obta ined in qua n-titative yield from the corresponding 5-methoxy-1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazole 15a by ther-mal elimination (80 C) of methanol in vacuo (0.1mba r) (e q 7).48 Compound IVa be ca me t h e f irst

carbene to be commercially ava ilable.Fo llowin g t h e se syn t h e t ic ro u t es, a n u mber of

sta ble am inocarbenes have been isolated: imidazo-lidin-2-ylidenesII,49 tetrahydropyrimid-2-ylidene II ,50imidazol-2-ylidenesIII,45-47,51-54 1,2,4-triazol-5-ylidenesIV,48 1,3-thiazol-2-ylidenes V,55 a s w e l l a s a c y c l i cdiamino- VI,56,57 aminooxy- VII ,58 and am inothiocar-benes VIII58 (Table 2). For all of these compounds,t h e ca rbe n e ce n t er bea rs t w o -donor substituents,of which a t lea st one is a n a mino group. The superior-donor a bility a nd t herefore the superior sta bilizingeffect of amino versus alkoxy groups has been evi-denced experimentally. Indeed, t he bis(dimethyl-am ino)carbene Me 2N-C -NMe 2 VIc can be observedby NMR spectroscopy a t room temperature, 57 wh ilethe dimethoxycarbene MeO-C -OMe has only beencharacterized in matrices at low temperature (life-t ime in solution a t room t empera ture: 2 ms).15

The car bene car bon ofII-VIII resona tes at ra therlow field in 13C NMR spectroscopy ( ) 205-300ppm) (Ta ble 2) compar ed t o the corr espondin g ca rbonatom of the cationic precursors () 135-180 ppm).More precisely, th e 13C chemical shift of the carbenecenter is in the ra nge 205-220 ppm for the unsa tur -

ated heterocyclic carbenes II I-IV w h i l e i t i s a p -proxima tely 15-25 ppm downfield for the correspond-ing saturated carbenesII, t he benzimida zol-2-ylideneIIIr be in g a t t h e bo rde r be t we e n bo t h t yp e s (231p p m ) . L a s t , t h e c a r b e n e c e n t e r o f a l l t h e a c y c l i caminocarbenes VI-VIII reson a t e s a t e ven lowe rfields (235-300 ppm).

The solid-sta te structure of derivat ives II b,c, II -Ib,c,k-n,p,q, IVa, Va, VIa, a n d VIIa h a s b e e nelucida ted by single-cryst a l X-ra y diffra ction studies(Table 2). The bond an gle observed at the carbenecenter (100-110) is in g o o d a g re e me n t wit h t h a texpected for s inglet ca rbenes of this ty pe. The la rgervalue observed in t he a cyclic diaminocar beneVI a56

(121.0) probably results from severe steric effects.Th e n it ro ge n a t o ms o f t h e a min o g rou p a re a lwa y sin a plana r environment, an d the N-C bond lengthsar e ra ther sh ort (1.32-1.37 ). It is notew orthy t ha tsimila r s t ru ct u ra l da t a a re o bse rve d fo r t h e ir imi-nium salt precursors, the N -C bond lengths beingonly a little shorter (1.28-1.33 ). These data as awh ole indicat e tha t t he CN bonds have some multiplebond character, which results from the donation ofthe nitrogen lone pairs to the carbene vacant orbital.Th is is con f irmed by t h e la rg e ba rriers t o ro t a t io na bo ut t h e N-C bond determined for VIa a n d VIIb(13 and a t lea st 21 kca l/mol, respectively) by va ria ble-tempera ture solution NMR experiments.56,58 There-fore, for many purposes, diaminocarbenes are best

described by resona nce forms B a n d C, w h i ch m a ybe summarized by structure D (Figur e 8). For a mino-thio Va,55 VIIIa,58 and aminooxycarbenesVIIa,58 t h eS -C a n d O-C bonds have very lit t le -character, 59a n d t h er ef or e, t h e b es t r ep r es en t a t i on f or t h es emonoaminocarbenes is provided by resonance formB.

Several a b init io studies ha ve been performed forthe parent compounds II *-VI* (Table 3).60-66 Th ecalculat ed structura l data for the singlet ground stat eare in very good agreement with those determinedexperimenta lly. Indeed, the nitr ogen a toms a re pre-dicted to deviat e only slightly from plan ar ity a nd theN-C bond lengths are short (1.33-1.38 ). In cyclicaminocarbenesII*-V*, the value of the carbene bonda n g l e i s p r ed ict e d t o b e i n t h e r a n g e 9 8-105. Aso me wh a t la rg e r va lu e is o bt a in e d fo r t h e p a re n tacyclic diaminocar beneVI*(111-113). These d at an o t o n ly h ig h lig h t t h e in f lu e n ce o f r in g e f fe ct s inderivatives II -V, bu t a lso co rro bo ra t e t h e ro le o fsteric effects inVIa, for which a larger carbene bondangle has been experimentally observed (121.0).

In th e triplet st at e for I I*,III*, a n dVI*, the N-Cbon ds a re sig nif ica n t ly e lon g a t e d (by 6.2 p m ona ve ra g e ) , t h e g e o me t ry a t t h e n i t ro g e n a t o ms be -come s p yra mida l , a n d re su lt s cla ssica l ly60 i n a nopening up of the car bene bond a ngle. These dat a asa wh o le in dica t e t h a t t h e do n a t ion of t h e n i t rog e nlone pairs is now negligible. Interestingly, the cal-

Figure 8. Resonance structures for aminocarbenes.



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culated bond angle for the triplet VI*60 (130.8 ) issimilar to that of the triplet methylene (130.2). 13

This value can ha rdly be reached in the triplet st at eof the five-membered ring carbenes II * a n d III*,

w h i c h m i g h t , i n p a r t , e x p l a i n w h y t h e c a l c u l a t e dsinglet -triplet gap is higher for II * a n d III* (69.4a nd 84.5 kcal/mol, respectively) tha n for th e a cyclicdiaminocarbeneVI* (58.5 kca l/mol).60 This lar ge gap

Table 2. Pertinent Structural Data (Bond L engths in and Bond Angles in Deg) and Chemical Shifts (in ppm) forthe Aminocarbenes II-VIII



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will have further consequences on the behavior ofthese diam inoca rbenes towa rd dim erizat ion (see sec-tion V.2.1).

The stability of singlet carbenes II -VIII ma in lyresults from electronic effects [mesomeric (+M ) a swe ll a s in du ct ive (-I) effects] , even though sterichindra nce certa inly plays a n importa nt contribution.49b

In a ddition, the a romatic chara cter of the 6--electronfive-membered ring carbenes II I-V h a s re ce n t lyga ined credence, despite being somew ha t ill-definedan d being a su bject of contr oversy .61,65 At the outset,Dixon and Arduengo62 explained the extraordinaryst a bil i t y o f t h e se ca rbe n e s a s e sse n t ia l ly re su lt in gfrom th e inductive effect of the neighboring nit rogenat oms. The nitrogen lone pairs an d t he CdC doublebond were su pposed to ensure enough kinetic sta bil-ity because of their high electron density, a notionthat was supported by a variety of different experi-me n t a l t e ch n iqu es.52,67,68 A s u b s eq u en t s t u d y b yCioslowski69 e ve n ca me t o t h e co n clu sio n t h a t t h e-donation by the nitrogen lone pairs plays only aminor r ole. The end of the controversy came in 1996w hen Apeloig61a a n d Fre n kin g 61b independently in-v es t i ga t e d t h e i m por t a n c e o f t h e a r o m a t i ci t y i ncarbenes II I . Accordin g t o st ru ct u ra l , t h e rmody-n a mic, a n d ma g n et ic cri t e ria a s we ll a s f ro m t h e p o p u la t io n s a n d io n iza t io n p o t e n t ia ls , i t wa s co n -cluded that cyclic electron delocalization does indeedoccur in the imida zol-2-ylidenes II I . This has been

confirmed by inner-shell electron energy loss spec-troscopy.65 Although this aromatic character is lesspronounced tha n in the imida zolium sa lt precursors13, it brings a n a dditional sta bilization of ca. 25 kcal/mol.61a However, aromaticity is not the major stabi-lizing effect for ca rbenes of type III . More importa nti s t h e i n t e r a c t i o n o f t h e c a r b e n e c e n t e r w i t h t h e-donating -a t t ra ct in g a min o su bst it u e n t s . Th isexplains w hy a minoca rbenes of typesIIa ndVI-VIIIcan a lso be isolat ed.

IV.2. (Z,Z)-Carbenes: Diborylcarbenes

No diborylcarbenes have yet been isolated. How-ever, as shown below, the borylm ethyleneboran es17

a n d 19 ca n b e con si der ed a s t h eir m a s kedanalogues,7b,c a nd th erefore, t hese (Z,Z)-ty pe carbeneshave been included in the present discussion.

In t h e e a rly 1980s,70 Be rn dt e t a l . p re p a re d t h eboriranylideneboranes 17a-d by re du ct io n o f t h ecorresponding 1,1-bis(chloroboryl)ethylenes 16a-d(eq 8).

The X-ra y diffr a ction stud y performed on17d71 a ndthe ab init io calculat ions performed for the modelmethyleneborane17*72 were in excellent agreementand demonstrated that compounds17 had a nonclas-sical structure. They feature both - a n d -three-center -tw o-electron (3c,2e) bonds, as depicted inTable 4 (the circle designates the 3c,2e -bond and

the dotted t ria ngle represents the 3c,2e-bond). Thenonclassical bridging structure of17 results n ot onlyfrom -intera ction 73 (hyperconjuga tion) betw eent h e st ra in e d e n docyclic C -B s i n g l e b o n d a n d t h ehighly electrophilic dicoordinate boron atom, but alsofrom a - interaction between the BC double bondan d the electrophilic tr icoordina te boron a tom.

On the basis of variable-temperature multinuclearNMR spectr oscopy, B erndt et a l. noticed quickly tha tcompounds 17 exhibit a topomeric equilibrium a t

Table 3. Calculated Geometrical Parameters (BondLengths in and Bond Angles in Deg) for the ParentAminocarbenes II*-VI* (S for Singlet, T for triplet)

st a t e NC : XC : (N CN ) lev el of t heor y r ef

II *S 1.337 1.337 105.0 H F /TZ2P 60S 1.357 1.357 103.4 MP 2/6-31G (d) 61aT 1.411 1.411 112.6 H F/ T Z 2P 60

III*S 1.352 1.352 101.9 TC S C F /D Z 62S 1.345 1.345 101.6 H F /TZ2P 60S 1.371 1.371 99.4 MP 2(F C)/6-31G ** 63S 1.350 1.350 101.5 C AS SC F /ANO 64S 1.373 1.373 100.9 MP 2/6-31G (d) 61T 1.419 1.419 112.2 H F/ T Z 2P 60T 1.402 1.402 106.8 CA SSCF / A N O 64

IV*S 1.377 1.353 98.5 MP 2/6-31G ** 66

V*S 1.355 1 .722 103.4 M P 2(F C)/6-31G ** 6 3S 1.331 1.331 113.1 H F /TZ2P 60S 1.346 1.346 111.5 MP 2/6-31G (d) 61aT 1.391 1.391 130.8 H F/ T Z 2P 60

Table 4. Experimental and Calculated StructuralData for 17d and 17*, respectively (Distances in ,Angles in Deg)



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room t e mpe ra t u re in solu t ion (e q 9).70 F r om t h e

coa lescence temperat ures (-47, -30, an d -11 C for1H , 13C , a n d 11B N M R, r es pe ct i ve ly ) a n d t h e a c -

companying chemical shift differences, the energyba rrie r o f t h is re a ct io n wa s e st ima t e d t o be 11.4-12.5 kcal/mol. This va lue is in good a greement w ithth a t predict ed by a b initio calcula tions (15.3 kcal/molfor t h e mo de l comp ou n d 17** a t the HF /6-31Glevel).72b,c Moreover, Schleyer et a l. showed t ha t t hetopoisomerism proceeds via th e tra nsition sta te 18**an d thr ough the intermedia te cyclic di(boryl)carbeneIX**, wh ich is predicted t o be only 10.2 kcal/molhigher in energy than 17** (eq 10).72c These r esults

as a whole suggested that the borira nylidenebora nes17 could a ct a s ma sked cyclic diborylcarbenes IX.

Mo re re ce n t ly, Be rn dt e t a l . re p o rt e d t h a t fa ci letopomeriza tions a lso occur for a cyclic compounds19(eq 11).74 The corresponding free a ctivation entha l-

pies were determined experimenta lly for 19b(22.0kca l/mol) a nd 19c (23.0 kca l/mol). The in t erm edia cyof acyclic diborylcarbenes X in these 1,2-migrationrea ctions w a s supported by MP 2/6-31G* calcula tions.Indeed, the model diborylcarbene X*(R )R ) M e)w a s predicted t o be 23.1 kcal/mol higher in energytha n the related borylmethylenebora ne19*, a n d t h ecorresponding energy barrier from X* t o 19* w a sassumed to be low (0.1-0.4 k ca l/m ol).74,75

Opposite electronic effects a re involved in diboryl-ca rbe n es a n d dia min oca rbe n es. In comp ou n ds oftypesIX a n dX, the ca rbene center str ongly interactsw i t h t w o -donor -acceptor substituents [carbenesof the (Z,Z) series]. The carbene lone pair, which isin a p orbital, is stabilized by delocalizat ion w itht h e p a ra l le l p o rbit a l o f t h e t wo bo ro n a t o ms, a n dt h u s, t h e R 2B C B R 2 skeleton is perfectly plana r, t heB CB fragm ent being linear in unconstrained acyclic

syst e ms. Mo reover, t h e va ca n t ca rben e p orbit a l ,which lies in this plane, interacts with only one ofthe tw o B-R bonds of each of the boryl substituents(Figu re 9). A preference for one of th e tw o equ iva lentbonds is t ypically 73 observed for electron-deficientce n t ers t h a t s t ro n gly in t e ra ct wit h n e ig h borin g bonds. These effects ar e reflected in the optimizedstructure of X*:74 t h e B -C bonds (1.44 ) ar e shorta n d e qu a l , t h e CB CH 3angles ar e alterna tively smalla n d la rg e (1110 a n d 127) , a n d t h e B -C H 3 bondsa sso cia t e d wit h t h e sma ll a n g le s a re sig n if ica n t lyelongated (1.60 compared to 1.58 ). In cyclic dibor-ylcarbenes such a sIX*,72b,c boron-carbon bondingi s a l so e v id en t f r om t h e s h or t B -C bonds (1.42com p a r ed t o 1. 71 f o r t h e B -C H 2 bo n ds) a n d-orbita l populat ions (C, 1.37; B , 0.29) (Figur e 9).

Although diborylcarbenes still have to be isolatedin t his form, borylmethylenebora nes17 a n d 19 ca nbe used as their synthetic equivalents as experimen-tally demonstrated by trapping reactions (see sectionsV.2.2 and V.6).

IV.3. (X,Z)-Carbenes

IV.3.1. Phosphinosilyl- and Phosphinophosphoniocarbenes

In con t ra st t o t h e o t h er st a ble ca rbe n es, i t is t h emost classical route to transient carbenes, namely,the decomposit ion of diazo compounds, which hasbeen used t o prepar e st able phosphinocarbenes. 5-P hosphorus-substituted diazo derivat ives ha ve beenknown for a long time, 76 but t he synthesis of the firstR-diazophosphine was only reported in 1985. 77 Th e[bis(diisopropylamino)phosphino](trimethylsilyl)diazo-me t h a n e 20a w a s ob t a in ed b y t r ea t m en t of t h eli t h iu m sa l t o f t r ime t h ylsi lyldia zo me t h a n e wit h 1equiv of bis(diisopropylam ino)chlorophosphine. D ini-trogen elimination occurs by photolysis (300 nm)78

o r t h e rmo lysis (250 C u n de r va cu u m),8c a n d t h ecorresponding phosphinosilylcarbene XIa w a s i s o-lated as a red oily material in 80%yield (Scheme 4).I t is s t a ble for we e ks a t ro om t e mpe ra t u re a n d ca neven be purified by flash dist illat ion under va cuum(10-2 Torr) a t 75-80 C. A number of phosphinosi-

Figure9. Influence of the electronic structure of diboryl-carbenes on their geometry.



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l y lca r b en es h a v e b ee n p r ep a r ed u s in g t h e s a m eprocedure, 7d-g b u t o n l y a f e w o f t h e m XIa-g a r esta ble at room tempera tur e. During th e course of ourstudy, we realized that , as expected, bulky substit-

uents kinetically stabilize phosphinocarbenes, butin t ere st in g ly, t h e st a bi l it y of ca rbe n es is o f t en in -versely proportional to tha t of t he diazo precursors(Ta ble 5).79 The silyl group at the car bene center can

Table 5. Stability of the Carbenes XI and XII and of Their Diazo Precursors 20 and 21

Scheme 4



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be replaced by an isoelectronic a nd isovalent phos-phonio substituent without dramatic modificat ion.Indeed, the stable phosphinophosphoniocarbenes XI-Ia80 a n d XIIb81 were synthesized from the corres-ponding diazo precursors 21aa n d 21b in 76% a n d85% yields, respectively (Scheme 4).

In solution, multinuclear NMR spectroscopy is byfar t he most informative technique for a na lyzing the

str ucture an d bonding of phosphinocar benes. In fact,prior to the synthesis and single-crystal X-ray analy-sis of the phosphinosilylcarbeneXIg82 an d phosphi-nophosphoniocarbeneXIIa,80 the only s pectr oscopicevidence for the formation of carbenes came fromNMR. T a ble 6 su mma rize s a l l p e rt in e n t ch e mica lshifts and coupling constants for the known phos-phinocarbenes XIa-g a n d XIIa,b an d t heir respec-tive d iazo precursors 20a-ga n d 21a,b. The phos-phinosilylcarbenes XI are all characterized by high-field chemical sh ifts for phosphorus (-24 to -50 ppm)and silicon (-3 t o -21 ppm) and low-field chemicalshifts for carbon (78-143 ppm) with large couplingconsta nts to phosphorus (147-203 Hz). Cla ssica lshielding arguments indicate an electron-rich phos-phorus atom or equally an increase in coordinationnumber. The silicon a tom seems a lso to be electron-rich , wh ile t h e ca rbo n h a s a ch e mica l sh if t in t h era nge expected for a multiply bonded species. Thevalues of coupling constants are difficult to rational-ize as it is not possible to predict the influence oforbital, spin-dipolar, Fermi contact, nor higher orderqu a n t u m me ch a n ica l con t ribu t ion s t o t h e ir ma g n i-tude. However, classical interpretat ion of the NMRda t a in dica t e s t h a t t h e P -C bond of phosphinosilyl-carbenes has some multiple bond character. The silyl-and phosphoniophosphinocarbenes XI a n d XII a r e

very similar, a s deduced from the similar ity of theirNMR spectroscopic features (Table 6).

The exact nature of the bonding system in phos-phinocarbenes was clarified to some extent by theX-ra y a na lyses performed on t he phosphinophospho-niocarbeneXIIa80 an d more recently on the phosphi-nosilylcarbene XIg.82 Ball and stick views ofXIIa a re

sh own in F ig ure 10, a n d t h e p e rt in en t g e ome t ricpara meters ar e in the legend. No intera ction wit h thetrifluorometha nesulfonate an ion is observed, con-firming the ionic character of XIIa. The shortness oft h e P 1-C1 bond [1.548(4) ] and th e pla na r geom-e t ry a rou n d P 1 in dica t e a s t ro n g in t era ct ion of t h ephosphorus lone pair w ith t he ca rbene vacant orbital.In o t h e r wo rds, t h e p h o sp h o ru s-ca rbo n bo n d h a ssome multiple bond chara cter. Moreover, since th eP 1-C 1-P2 framework is bent [P1-C 1-P 2 a n g le )165.1(4)-164.1(4) ], phosphin ophosphonioca rben esXIIa re best representat ed by the phospha vinylylideform B (Figur e 11). B ecause of a dis order a ssociat edwith the phosphonio part of the molecule [sof) 0.62-(1)] , t h e va lu e s o f t h e P2-C 1 a n d P 2 a -C1 bo n dlengths [1.605(5) and 1.615(5) ] could not be verya ccu ra t e . An ywa y, t h e P 2-C1 bond length is far tooshort for a phosphorus-ca rbon sin g le bo n d a n d ismore in the range observed for phosphorus ylides. 83

This datum indicates an interaction of the carbenelone pair with the -acceptor phosphonio substituent(form D).

Similar conclusions can be drawn from the X-raycrystal structure analysis of the phosphinosilylcar-bene XIg(Figure 12),82 t h e si lyl g ro u p p la yin g t h esa me ro le a s t h e p h o sp h o n io g ro u p o f XIIa. TheSi2N2C3N1C2P1C1Si1 skeleton is planar (maximum

Table 6. Pertinent Chemical Shifts (in ppm) and Coupling Constants (in Hz) for the Phosphinocarbenes XI andXII and their Diazo Precursors 20 and 21



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de via t ion from t h e be st p la n e : 0 .03 ), t h e P 1C1bond length [1.532(3) ] is short, and the P1C1Si1fram ework is bent [152.6(3) ]. The presence of astrongly polar ized P +C - fragment (form B) is sug-

g est e d by t h e sh ort Si1C1 [1.795 comp a re d wit h1.86-1.88 for Si1-C H 3] a n d PN bo n d dist a n ce s[1.664(2) ].

Calculations perfomed on model compounds (Fig-ure 13) not only corrobora te the conclusions of t heexperimental analyses concerning the bonding sys-tem in phosphinocarbenes, but also give more insightinto the r ole of th e carbene an d phosphorus substit-uents.

The first study by Hegarty and co-workers 84 con-clu ded t h a t t h e p a re n t p h osp h in oca rben e H 2P C HXIg* h a s a s in g le t g r ou n d s t a t e , w i t h a s in g le t-tr iplet gap of 3 kcal/mol, while la ter on H offman n a ndKuhler85 predicted a somew hat larger gap (6.7 kcal/

mol) u sin g a more sop h ist ica t e d level of t h e ory.Tog et h e r, t h e p la n a r g e ome t ry of t h e sin g le t s t a t eand the shortness of the PC bond (1.616 ) confirmthe strong interaction of the phosphorus lone pair

wit h t h e va ca n t orbit a l of t h e ca rbe n e ce n t er. Th ephosphavinylylide form B definitively describes thisspecies (Figure 11). Interestingly, the structure Afeaturing a pyramidal phosphorus center is neithera minimum nor a saddle point on the potential energysurface. Moreover, the5-phosphaa cetylene structur eC is t h e t ra n si t ion st ru ct u re corre spon din g t o t h ei n ve r si on a t t h e ca r b en e ce nt e r w i t h a n e ne r gybarr ier of only 10.3 kcal/mol. This sma ll bar rierprobably explains why carbeneXIIa has difficulty inma in t a in in g o n e discre t e fo rm in t h e so l id st a t e .Indeed, the tw o units observed in the X-ra y a na lysis80

simply result from a n inversion a t th e centra l car bon,followed by a 180 r otat ion ar ound th e P2-C1 bond.In co n t ra st wit h t h e sin g le t s t a t e , t h e ca lcu la t io n sassociated with the triplet state predict a pyramidalgeometry at phosphorus and a long P -C bond (1.782). Moreover, the PCH bond angle is wider (133.9)tha n t ha t of the singlet sta te (123.5 ), a s classicallypredicted for most carbenes.14

Dixon et al. theoretically investigated the role ofsilicon substitution at carbon and of amino groupsa t phosphorus (Figur e 13).86 In the singlet sta te, theS iH 3 substituent induces a w idening of the car benea n g l e (XIh*, 131.2). This effect , which has beenpreviously reported for other silylcar benes,87 cor-robora tes t he prediction by Schoeller 12a and Pa uling12b

tha t car benes substit uted by elements less electrone-gat ive than ca rbon preferentially a dopt linear st ruc-t u re s. Mo re o ve r, t h e sh o rt n e ss o f t h e C -Si bo n d(1.843 ), which is in the range typical for silicon-ca rba n ion bon d len g t h s,88 in dica t e s a s ig n if ica n tback-donation of the carbene lone pair into the *orbit a ls of t h e si lyl g rou p (Z-t yp e su bst i t u e n t ).89

Replacement of the hydr ogen a toms by a mino groupsat phosphorus (XIi*) led to a shortening of the P -Ca n d C -S i b o n d s , w h i l e t h e P -C -Si f ra g me n t be -comes almost linear. These data might indicate t ha t

t h e ca rbe n e lon e p a ir in t e ra ct s w it h t h e * orbita lsof both the silyl and the phosphino groups (negativehyperconjugation).90,91 However, the electron localiza-tion function (ELF) analysis82 of bot h t h e be n t a n dlin e a r forms of XIi* le a d s t o a r a t h e r d if fe re ntconclusion (Figure 14). For the ground-state bentform (Figure 14a), the lone pair on the carbon atomi s d ir ect e d a w a y f r om b ot h t h e p hos ph or u s a n dsilicon, indicating t ha t neither t he triple bond (C) northe cumulene (D) structure is the best formulationfor phosphinosilylcarbenes. Since the PC double bondis clearly evident, XIi* h a s t o b e r e g a r d e d a s t h ephosphavinylylideB. Interestingly, even the linearform ofXIi* (Figure 14b) featu res the t ypica l pat ternfor a P C d ouble bond (purple) [for compa rison, Fig ure14c show s the E LF plot for H CtP which possesses agenuine triple bond (purple)]. The stretched shapeof the light green isosurface is an indication of SiC

Figure 10. B all a nd s t ick view of the tw o units of XIIa.Selected bond distances () and bond angles (deg) are asfollows: P2-N3 1.635(4), P 2-N4 1.641(4), P 2a -N3 1.622-(5), P2a -N4 1.638(5), P2-C 1 l.605(5), P2a -C 1 1.616(5),P 1-C 1 1.548(4), P1-N1 1.632(3), P 1-N2 1.635(3); P 2-C 1-P 1 165.1(4), P 2a -C 1-P 1 164.1(4), C 1-P 1-N1 126.3-(2), C1-P 1-N2 126.7(2), N1-P 1-N2 107.0(2).

Figure 11. P ossible representa tions for phosphinocarbenes.



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double bond character, and since this isosurface isp e rp e n dicu la r t o t h a t a t t r ibu t e d t o t h e PC do u blebond, the linear form ofXIi* is best described by thecumulenic st ructure D.

Not e t h a t t h e t r iplet s t a t e o f bot h ca rben e s XIh*a n d XIi* is higher in energy, by 5.6 and 13.9 kcal/mol, r espectively. 86 Their geometries are quite com-p a ra ble t o t h a t of t h e p a re n t t r ip le t p h o sp h in oca r-bene XIg*, i n di ca t i n g a w e a k er i n fl u en ce of t h ecarbene substituents.

Recently, Nyulaszi et al. studied the influence of avar iety of substituents on the electronic structure ofphosphinocarbenes.92 Strongly bent geometries arepredicted, except for t he phosphinoborylca rbene H 2-P C B H

2 which has a linear allene-type structure.

The stud y by Alhrichs an d co-workers deals wit hthe phosphinophosphonioca rbeneXIIa itself (Figur e13).93 The optimized geometry is very simila r to tha tobserved in the solid state. The 3P ce n t e r is in atrigonal pla na r environment. The at omic charges (P 1,+1.1; C ,-0.9; P 2,+1.1) indicat e tha t the very shortP 1C bond [experimen ta l 1.548(4) ; theoret ical 1.557] is a double bond reinforced by Coulombic attrac-tion.93 Interestingly, the 4P -C bond length is alsoshort [experiment a l 1.605(5)-1.615(5) ; th eoretica l1.698 ], in t h e ra n g e e xp ect e d for p h osp h oru sylides,83 which indicates a degree of back-donationof the carbene lone pa ir int o the low -lying orbita ls oft h e 4-phosphorus center.

In su mma ry, t h e p h osp h in o g rou p s a ct a s X su b-stituents w hile the silyl and phosphonio groups ar eboth a ble to act a s Z substituents due to the aptitudeof phosphorus an d silicon for hyperva lency.95 In otherwords, the phosphorus lone pair of the phosphinosubstituent interacts with t he car bene vacant orbitalan d there is an addit ional w eak interaction betw eenthe car bene lone pair a nd low-lying * orbitals a t the

silyl or phosphonio groups.However, when compared to nitrogen, phosphorusis much more reluctant to achieve a plana r configu-r a t i o n w i t h s p2 hybridization.59,96-98 The ensuingsmaller stabilizing effect of phosphorus compared ton it rog en is i l lust ra t e d by t h e sma ll s in g le t-tripletgap predicted for the phosphinocarbenesXIg-i (5.6-13.9 kca l/mol)86 com pa r e d t o t h a t ca l cu la t e d f ora cyclic as w ell as cyclic diaminocar benes (58.5-84.5kca l/mol ).61,62 This means t ha t th e commitment of thelone pair to donation into the vacant orbital on thedivalent carbon atom is less definitive for phosphorustha n for nitrogen, and thus, t he phosphinocarbenesreta in more of a divalent-carbon behavior.

IV.3.2. SulfinylcarbenesIn the 1980s, S eppelt et al. prepar ed th e (trifluo-

roethylidyne)sulfur trifluoride XIII8a and the [(pen-tafluorothio)methylidyne]sulfur trifluorideXIV8b bygas-phase dehydrofluorina tion of 22 a n d 23,99,100respectively, with K OH a t 70-80 C (eqs 12 and 13).

Both compoundsXIII a n dXIV are colorless gasesof ra t h e r low t h e rma l st a bi li t y (XIII bp -15 to-10 C , m p -122 C, de c -5 0 C ; XIV mp -130 C,dec -78 C). Their IR spectra exhibit strong absorp-tion bands in th e expected range for C S triple bondstretching (XIII01 1740 cm -1; XIV8b 1717 cm -1); theCS bond lengths determined experimentally a s w ellas theoretically (Table 7) are 9% an d 20% shorter

Figure 12. ORTEP view ofXIg. Selected bond d ista nces() an d bond an gles (deg) ar e as follows: P 1-N1 1.664(2),P 1-N2 1.665(2), P1-Cl 1.532(3), C1-Si1 1.795(3); N1-P 1-N2 87.01(11), N1-P 1-C1 130.85(15), N2-P 1-C 1142.12(14), P 1-C 1-Si l 152.6(3).

Figure 13. Ca lculated geometric parameters for s ingletand triplet phosphinocarbenes XI a n d XII (bond lengthsin ; bond angles in deg).

Figure 14. E LF plots, isosurfaces 0.85: (a) bent form ofXIi*; (b) linear form of XIi*; (c) PtC H .



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than comparable double and single bonds, respec-t ive ly. On t h e ba sis o f t h e se cri t e ria , t h e a u t h o rsclaimed the isolation of the first compounds featuringa sulfur-carbon triple bond.8a,b However, t he geom-etry of the bonding system (linear or bent X -C -Sfram ework) ha s a t tr acted m uch at tention (Table 7).All the studies concluded that there was a nonclas-sical structur e for th e CS t riple bond.102,103 The X-ra yda t a fo r XIII8b,103 indicate a significant bending in

t h e cr y s t a l li n e s t a t e (p ha s e I , 171. 4 ; p ha s e I I ,162.9), which is even accentuated in the gas phase,a s de du ce d fro m a g a s e le ct ro n dif fra ct ion st u dy(155 ).101 A similar trend is observed for XIV: t h eX-r a y d if fr a c t ion s t u dy y i el d s a l in ea r s t r u ct u r eimposed by th e cubic cryst al symmetry ,8b whereas thegas electron diffra ction st udy r evealed a bent str uc-tur e (159).104 These conflict ing results were para l-leled by a b in i t io ca lcu la t ion s for XIII : a l in ea rgeometr y w a s first predicted a t t he HF /3-3-21105 a n dH F /D Z+D(C,S)106 levels, but using a larger basis setan d including electron correla tion,101 bending an glesfro m 174 t o 148 we re o bt a in e d. In a n y ca se , t h ebending potential is very flat , even at considerablebe n din g a n g le s ( t h e ba rrie r t o l in e a ri t y h a s be e nestimat ed to be only 0.35 kcal/mol a t the MP 2/6-31G *//H F /6-31G * lev el).101 Therefore, weak lat t iceforces can certa inly dictat e the value of the CCS bondangle within certain limits.

How ca n this w eak bending potentia l be plausiblyex pl a i ne d? Th e p ol a r i t y of t h e C S b on d cl ea r l yemerges from t he calculated cha rge distribution forXIII(C -0.7S +1.6)101 and from the surprisingly high 13CNMR ch e mica l sh if t for t h e ce n t ra l ca rbo n a t o m(XIII, 30.4 ppm compared to 67.5 ppm for CF 3C H 2-S F 5).102 Therefore, the 5-sulfavinylylide form B(Figure 15) is certainly of importance. This conclusionwa s corro bora t e d by a cryst a l s t ru ct u re de t ermin a -tion forXIV8b (although the electron density distribu-

tion has been only poorly evaluat ed in this case dueto crysta l disorder). A full theoretical underst an dingof the bending potentia l is hindered by the fact t ha tt h e 3-sulfinylcarbene form C cannot be accuratelyinvestigated. However, the singlet sta te of the F3S -C -C F 3 carbene was predicted to be separated fromthe corresponding a cety lenic-type gr ound st at e (formA) by a barr ier of only 12 kcal/mol. The a uthorsconcluded tha t t he energetically close lying, cert ainlystrongly bent, carbene stat e causes th e weak bendingpotential.7h

In summary, bending about the carbon is so easyt h a t t h e ca r b on-sulfur tr iple-bonded compoundsXIII a nd XIV ma y be con sidere d a s ma ske d su lf i-nylcarbenes (form C); t h is i s i n l in e w i t h t h ei rrea ctivity (see sections V.1, V.2.1, a nd V.4.3).

V. Reactivity of Stable Singlet Carbenes

Whereas tr iplet ca rbenes exhibit ra dical-like reac-tivity, singlet carbenes are expected to show nucleo-philic a s w ell as electr ophilic beha vior beca use of thelone pair and vacant orbital. Among the most typicalreactions of transient singlet carbenes are the rear-rangements resulting from 1,2-shifts, dimerizations,[1+2]-cycloa ddit ions t o car bon-carbon double bonds,an d insertions into C-H bonds. Many other rea ctionsin volvin g t ra n sien t ca rben e s h a ve bee n observe d,

including the formation of ylides with Lewis bases.H e r e , w e w i l l c o m p a r e , a s o f t e n a s p o s s i b l e , t h ereactivity of the different types of stable carbeneswith that of their transient congeners.

V.1. 1,2-Migration Reactions

It is now well established that 1,2-migration (eq14) is a fundamental reaction for singlet carbenes andoccurs via a unimolecular concerted mechanism .25,107

Table 7. Experimental and Calculated Structural Data for XIII and XIV (Distances in , Angles in Deg)

XI I I XI V

d(CS)


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Alternative intermolecular pathways, such as thoseinvolving car bene-olefin -complexes,108,109 have re-

cently been ruled out.107e

In aromatic carbenes II I-V, 1,2-hydrogen migra-t ion s ca n n ot p rocee d t h ro u gh a n in t ra mo lecula rp ro ce ss in t h e p la n e o f t h e r in g . T h is me ch a n ismwould impose the crossing of two orbitals with thesame symmetry, a s shown in the correlat ion diagra m(Figure 16a). An alternative possibility, which hasbeen theoretically studied,60,110 involves the interac-tion of the N-H bond w ith t he out of plan e porbita lof the carbene. This process induces a deformationof t h e r in g a n d t h us t h e l os s of t h e el ect r on icdelocalization of the nitrogen lone pairs (Figure 16b).He in e ma n n e t a l . p re dict e d t h a t e ve n t h o u g h t h erea ction would be exoth ermic (-26.1 kca l/mol a t th e

RH F/MP 2DSQ level60

a n d -29 kcal/mol a t th e DF T/B3LYP level110a ), t he carbene III* should be kineti-ca l ly s t a ble t o wa rd 1,2-sh ift s , s in ce t h e a ct iva t ione n e rg y o f t h is re a rra n g e me n t is h ig h (+46.8 kcal/mol,60 +39.8 kca l/mol110a ). R ece nt l y , M a i er et a l .obtain ed simila r r esults for t he 1,3-thia zol-2-ylidenesystem V*(H ) -34.0 kca l/mol, Eq ) +42.3 kcal/mol a t t he M P 2(fc)/6-31G (d) level).110b

Our group has recently reported an example of aforma l 1,2-silyl shift in the 1,2,4-tr iaz ol-5-ylideneseriesIV bu t h a s p roved t h a t t h is re su lt e d from a nintermolecular process.111 Indeed, deprotona tion ofN-silyltriazolium salt s 24b-e w i t h v a r i o u s b a s e sr ea d i ly occu r r ed a t 0 C t o l ea d , a f t er w o r ku p, t o

triazoles25b-e (42-81% yields) (eq 15). Although

a ll a t t e mpt s t o sp ect ro scop ica l ly ch a ra ct e rize ca r-benes IVb-e fa iled, despite monitoring the depro-tonat ion reaction at -78 C, the tra nsient forma tionof IVb-e wa s u n a mbig u ou sly e st a blish e d by t ra p -p in g wit h a la rg e e x ce ss o f be n za lde h yde.112 Th eintermolecular n at ure of the 1,2-migra tion w as provedby t he deprotonat ion of a 1/1 mixtur e of tria zoliumsa lt s 24b a n d24e, bearing different substituents a tboth nitrogen atoms, which led to a mixture of the

four rearrangement products 25b-e. I n t h e s a m ewa y, sta rt ing from a 1/1 mixture of tria zolium salt s24c a n d 24d, t h e sa me p ro du ct s 25b-e were ob-t a in e d (e q 16). Of cou rse i t wa s ch ecke d t h a t in

solution no exchange reactions of the silyl groupsoccurred between the triazolium salts 24ba n d 24eand between 24c a n d24d nor between the tria zoles25b-e. We h a ve sh own t h a t in fa ct a n u cleop h il icat ta ck of a ca rbene of typeIV on the silyl gr oup of ast a rt in g ca t ion 24 resu lt ed in t h e forma t ion of t h eC- and N-silyl-substituted tria zolium sa lt 26, a long

wit h t r ia zo le 27. Then, a nucleophilic a t ta ck of thenitrogen of 27 a t t h e N-silyl group of the cation 26afforded the rearrangement product 25 and regener-a t e d t h e st a rt in g sa l t 24 (eq 17).

Therefore, 1,2-migrations can occur for aromaticcarbenes such asIVb-e113 but only via intermolecu-lar processes. This rearrangement involves both thecarbene and an electrophilic partner. This is remi-n isce n t of t h e sp ecif ic beh a vior of t h e st a ble a mi-nocarbenes II -VIII t o wa rd dimeriza t ion , in wh ichthe protonated form is involved (see next section).

A 1,2-fluorine migration has been postulated tora t io n a lize t h e fo rma t io n o f FS -C F 2-C F 3 i n t h ema t rix irra dia t ion of t h e su lfe n ylca rben e XIII .7h

Figure 16. (a) Correlation diagram for the in-plane 1,2-H-migration inIII*. (b) Energy diagr am for the calculat edout-of-plan e 1,2-H-migra tion in III*.



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However, the presumably initially formed alkylidenesulfur difluoride has not been characterized (eq 18).

No 1,2-migra tion involving sta ble phosphinocar -benes ha s yet been observed. Note that , in contra st ,the tra nsient bis(phosphino)carbene 28re a rra n g e sinto the phosphaa lkene 29114 (eq 19).

V.2. Carbene Dimerization and Related Reactions

V.2.1. Carbene Dimerization

I n t h e C a r t e r a n d G o d da r d f or m u la t i on ,115 t h estrength of the CdC double bond resulting from thedimerization of singlet carbenes should correspondto that of a can onical CdC double bond (usually thatof ethene) minus tw ice the singlet-triplet energy ga pfo r t h e ca rbe n e . Fo r e x a mp le , t h e sin g le t -tripletsplit t ing in the parent imidazol-2-ylidene III* h a sbeen calcula ted to be ca. 85 kcal/mol;60 accordingly,one expects t he CC bond st rength in the dimer t o beapproximately only [172 - (2 85)] 2 k ca l/mol. Inthe sa me wa y, the calculated value for t he energy ofdimerization of E nders-type ca rbeneI V* is only 9.5

kca l/m ol.66

These remarkably small values, at leastp a rt ia l ly du e t o t h e lo ss o f a ro ma t ici t y in t h e Ar-du e n g o a n d En de rs ca rbe n e dime rs, h ig h lig h t t h ediff icu lt y wit h wh ich su ch ca rben e s dime rize . Incon t ra st , in t h e ca se o f t h e p a re n t dia min o ca rbe n eVI*, He in e ma n n a n d T h ie l60 found a dimerizationenergy of about 45 kcal/mol. This poses an otherquestion: Wha t is th e value of the energy barrier fort h e dimeriza t ion ? No wa da ys, t h e dimeriza t ion ofsin glet ca rbe n es is bel ieve d t o fol low a n on lea stm ot ion pa t h w a y t h a t i nv ol ves t h e a t t a ck of t h eoccupied in-plane lone pair of one singlet carbenece n t er on t h e ou t -of-p la n e va ca n t p-orbit a l of asecond carbene (Figure 17).115,116 Calculations regard-

i n g t h e d im er i za t i on p a t h i n di ca t e a s ig n if ica n tba rrier of about 19.4 kcal/mol for th e car beneI V*,66an d Alder estima ted the G* for t he dimeriza tion of

t h e bis(N-piperidinyl)carbene VIb t o b e >25 kcal/mol.57 These large va lues are not sur prising since thedimerization reaction involves the carbene vacantorbita l w hich is very high in energy due t o electrondonation by t he nitrogen lone pairs.

However, besides the work by Wanzlick, 42 severalexam ples of a minocar bene dimerizations ha ve beenreported.49b,55,57,117 Not e wo rt h y is t h e isola t ion byArduengo55 of both the thiazol-2-ylidene Va a n d ofits dimer {Va}2(eq 20), t he s pectr oscopic char a cter-ization by Alder an d B lake of the bis(N-piperidinyl)-carbene VIb and of the tetrakis(N-piperidinyl)ethene{VIb}257 (e q 21), a n d t h e syn t h e sis by T a t o n a n dCh e n of bis(ca rben e ) IIIs a n d t e t r a a z a f u lv a l en e{IIIt}2, de p e n din g o n t h e le n g t h o f t h e me t h yle n ebridges (eqs 22 and 23). 117a

In t e rest in g ly, Ardu en g o observe d t h a t Va w a ssta ble with respect to dimerization in th e absence ofa B ron sted or Lewis acid ca ta lyst .55 Similarly, in thea bse n ce o f a n a cid ca t a lyst , d ime riza t io n o fVIb i se xt re me ly slow a n d is f irst orde r in t h e ca rbe n e.57

Therefore, t he observed forma l dimerization ofVaa ndVIb does not involve the coupling of tw o ca rbenesbut the nucleophilic at ta ck of one carbene upon itsconjugate acid, followed by proton elimination, asa lre a dy su g g est e d by Ch e n a n d J orda n 118 (Scheme5).

Figure 17. Schematic representation of the dimerizationof s inglet carbenes by a nonleast m otion pat hwa y.



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No t e t h a t t h e fo rma t io n o f t h e t e t ra a za fu lva le n e{IIIt}2 might involve a genuine intramolecular dimer-ization of singlet carbenes, which w ould be fa voredby the entropy factor 117a (eq 23). However, a mech-an ism involving one car bene and a n imida zolium sa ltcan certa inly not be excluded either.

In ma rked contra st w ith a minocarbenes, which arereluctant to dimerize, the formation of the E olefin

{XIII}2 is the domina nt reaction of the (trifluoroet-hylidyn e)sulfur tr ifluorideXIII .102,103 Surprisingly, akin e t ic st u dy of t h is re a ct ion sh ows t h a t t h e disa p -pearance of XIII follows first-order kinetics over arange of 1 order of magnitude in concentrat ion, attempera tures betw een -25 and +5 C . Seppelt et al.concluded that the rate-determining step has to bethe transformation of F 3C CtS F 3XIII (form A) intothe carbene form C (F3C -C -S F 3) (H* ) 12 kcal/mol) before the la t t er reacts further 103 (eq 24). This

conclusion clearly deserves further studies.La st, no dimerizat ions leading t o olefins have been

observed for stable phosphinocarbenes XI a n d XII .H o w ev er , i t s h ou ld b e n o t ed t h a t t r a n s i en t (n otspectr oscopically observed) phosphinoca rbenes ha vebeen reported to undergo head-to-tail dimerizationsl ea d i n g t o 15,35-diphosphetes119,120 s u c h a s 30120(Scheme 6). H ere also, the mecha nism is not clear .

V.2.2. CarbeneCarbenoid Coupling Reactions

For carbenoids, w e will not only consider the higheran alogues of ca rbenes (silylenes, germy lenes, stan -nylenes, plumbylenes, phosphinidenes), but also spe-cies wh ich can behave as carbenoids, such as imino-phosphines an d isocyanides.

Coupling rea ctions of imida zol-2-ylidenesI II w i t hall t he group 14 car benoids ha ve been r eported, w hilediborylcarbenes IX and phosphinocarbenes XIh a v eonly been reacted w ith germylenes and sta nnylenes.The different electronic structure of the three typesof carbene I II , IX, a n d XI is w e ll i l lu st ra t e d by t h ecom p a r is on of t h e g eom et r y a n d s t a b il it y of t h eg ermylen e a n d st a n n yle n e a ddu ct s .

Reaction of the imidazol-2-ylidene IIIl wi t h g e r-ma n iu m diiodide a f fords t h e ca rbe n e-germyleneadduct 32 in 65%y ield (eq 25).121 The st a bility of32is s t r ikin g (mp 210-214 C ), s in ce m os t of t h egermaethenes 122 a r e on ly s hor t -l iv ed i nt er -mediates.1 22 a, b 1H a n d 13C N MR d a t a s uppor t a

structure for 32 in wh ich t h e n e wly fo rme d C -G ebo n d is n o t a t ru e do u ble bo n d bu t ra t h e r h ig h lypolarized C +-G e-. This is confirmed by the crystal-log ra p h ic da t a : t h e g e rma n iu m cen t e r is d ist in ct lyin a p yra mida l e n viron me n t a n d t h e C -Ge bond isvery long (2.102 ). This geometry has to be com-pared with t hat of the germa ethene 35,122c wh e re t h egerman ium an d carbon atoms exhibit trigonal-plana rcoordination and a short (1.803 ) weakly twisted(5.9 ) C-G e bond. Therefore, it is clear th a t 32 is bestdescribed as a Lewis base-Lewis acid adduct .

B erndts car beneIXa reacts w ith germylenes123 a troom temperature to afford the corresponding ger-maethenes36a,b as the sole products (eq 26). 124 Th e

Scheme 5

Scheme 6



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C -G e bond dist an ce (1.827 ) is very sim ilar to tha t

found in 35, bu t t h e a vera g e t wist a n g le a t t h e GedC bon d is close t o 36 . Mo reover, a l t h o u g h t h e G ea t o m i s i n a p r a ct i ca l l y p l a n a r e nv ir on m en t , t h ecarbon at om is slightly pyra midal. The X-ra y st ruc-ture analysis as well as the NMR data suggest somesignificance for the ylide resona nce formula of t ypeC --G e+, expected from the interaction between anelectrophilic carbene and a nucleophilic germylene.Th u s, t h e p ola riza t ion of t h e Ge C bon d in 36 isstronger tha n tha t of35 or sta ndard germa ethenes122and opposite to tha t of 32.

Phosphinosilylcarbene XId a lso re a ct s wit h g e r-manium(II) compounds affording the C-germylphos-phaalkenes 40a,b in 78% and 46% yields, respec-tively (eq 27).125 I t i s r e a s on a b le t o p os t u la t e t h e

primar y forma tion of the germa ethenes38a,b, whichwould undergo a subsequent 1,3-shift of the dimethy-la min o g ro u p fro m p h o sp h o ru s t o t h e g e rma n iu mat om to produce derivat ives 40a,b. This reaction ishighly chemiselective since w e only observed th emig ra t ion of t h e sma llest p h osp h oru s su bst it u e n t .The instability of 38a,b is not surprising whateverthe polarit y of the GedC bond, sin ce the phosphoruscenter is n ot efficient enough t o sta bilize a n a djacentposit ive charge and destabilizes a negative charge.

I n t e re st i n gl y , i t i s cl ea r t h a t b eca u s e of t h ei rnucleophilicity, aminocarbenes react with germylenesthrough HOMOcarbene-L U M Ogermylene intera ctions. Incontrast, since the diborylcarbenes are electrophilic,

t h ey r ea c t w i t h g er m y len es v ia H O MOgermylene-L U M Ocarbene in t era ct ion s. In t h e ca se of p h osp h i-nocarbenes, it is difficult to determine whether theHOMO or the LU MO of the carbene is involved (videinfra).

Similar reactions have been observed by reactingthe imidazol-2-ylideneIIIi,126 diborylcarbene IXa,127and phosphinocarbeneXId125 with stannylenes (eqs25-27). The result ing ad ducts 33a,b a n d 37h a v e

geometries and therefore electronic structures com-para ble to those of their germa nium a na logues, while39a,b ar e unstable and r earra nge as observed for38.

Int erestingly , imid a zol-2-ylidenes II I a l s o g iv estable adducts with silylenes 128a an d plumbylenes.128b

As observed in the germanium and t in adducts, theC-Si a n d C -P b bonds of31a nd34, respectively, a restrongly polarized and can hardly be considered asdouble bonds.

Imidazol-2-ylidenes II I a l s o g iv e a d d uct s w i t hpnictinidenes. In fact , carbenes II I a re sufficientlynucleophilic to depolymerize cyclopolyphosphines[(PPh)5 a n d ( P C F 3)4] and cyclopolyarsines [(AsPh)6and (AsC 6F 5)4] t o p rodu ce a ddu ct s of t h e t yp e ca r-bene-PnR42 (eq 28).129 The strong electron-releas-

ing chara cteristics of the imidazole ring render th esepnictaalkenes strongly polarized, as shown by the



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very high field 31P NMR chemical shift (-23 ppm ),t h e s m a l l C -P n-C angles (97-102), and the longcarbene-P n b on d s (on l y 4% s h or t e r t h a n P n -Csingle bonds), which allows a free rotat ion at roomtemperature. Further support for the carbene-phos-phinidene bonding description stemmed from cyclicvoltam metry experiments.130 Indeed, an acetonitrilesolution of42b exhibits a n irreversible one-electr onoxidation a t -0.08 V (vs S CE ). Su ch a low oxidat ion

potential is consistent with an electron-rich phos-phorus center since the one-electron oxidation po-tentials of typical phosphaalkenes fall in the range1.07-2.94 V.131 Therefore, the pnictinogen at ompossesses two available lone pairs, as confirmed byt h e r e a ct i on of 42b w it h B H 3TH F . I n d eed , a nexclusive formation of the diborane adduct 43 w a sobserved (eq 28).130

Du e t o t h e e xist en ce o f t wo e n erg e t ica l ly closeoccupied MO s and n, the orbital sequence HOMO/LU MO of iminophosphines44can be /*, inducingan alkene-like reactivity, but also n/*, leading to abehavior ana logous to tha t of car benes.132 Indeed, thephosphinocarbeneXId reacts wit h iminophosphines

44 leading t o the phospha alkenes46 (23-87%yield)(eq 29).133 T h e se re su lt s a re st r ict ly a n a lo g o u s t o

t h o se observe d in t h e re a ct ion of XId w i t h g er -mylenes and stannylenes, and therefore, it is quitelikely that a carbene-carbenoid coupling-ty pe rea c-tion occurs, leading to t he t ra nsient (methylene)-(imino)phosphoranes 45, w hich would subsequentlyu n derg o a 1 ,3-mig ra t ion of a d imet h yla min o su b-stituent from the 3-phosphorus to t he electr ophilic5-phosphorus a tom. These rea ctions ar e also highlychemiselective since we only observed t he migra tion

of the sma llest phosphorus substit uent. Here a gain,it is difficult to know whether the phosphinocarbeneXId reacts via it s lone pair or its potentia lly ava ilableva ca n t orbit a l .

Tra nsient carbenes are known to react w ith isoni-t ri les t o g ive t h e corre spon din g ket e n imin e s (e q30).134 tert-Bu t yl iso cya n ide is on e o f t h e very ra re

r e a g e n t s t h a t r e a c t s w i t h a l m o s t a l l o f t h e s t a b l ephosphinocarbenesXIa ndXII .79,81,135,136 For example,

i t r e a c t s w i t h XIa, e v e n a t -78 C , a f f or d in g t h eketenimine 47a, wh ich wa s isola t e d a f t e r t re a t me n twit h e le men t a l su lfu r a s i t s t h iox op h osp h ora n ylanalogue in 90%yield (eq 31).135

P entafluorophenyl isocya nide also reacts with thephosphinocarbeneXIa but gives the heterocycle 49(eq 32).137 Most probably, the initially formed keten-

imine47b rea rra nges by a 1,3-shift of a d iisopropyl-amino group from phosphorus to carbon leading to48, which subsequently undergoes an electrocycliza-tion. Note tha t the 1,3-migrat ion process is strict lyanalogous to that observed in the reaction of phos-phinoca rbenes w ith germylenes, sta nnylenes (eq 27),and iminophosphines (eq 29). Moreover, the sulfe-

nylcarbene XIII g ive s a s imila r re a ct io n wh ich a f-fords 51, probably via the transient ketenimine 50(eq 33).103

The great reactivity of isonitriles toward the car-

benes XI, XII , a n d XIII can be easily explained int e rms o f s t e ric fa ct ors : t h e re a ct ive sit e o f RNdC :is comp a ra t ively u n h in dere d. H owe ver, t h e mostimporta nt q uestion is to know w hether isonitriles actas Lew is acids towa rd carbenes or if they act as Lewisba ses. Interest ingly, a ccording to G ru tzm a cher138 a ndOkazaki,139 the stanna-and silaketenimines52a a n d52b, re spe ct ively, r e sult f rom t h e re a ct ion of t h estannylene and silylene, act ing as Lewis acids, withthe isocya nide, acting as a Lewis ba se (eq 34). Similarcon clu sion s h a ve bee n dra w n for ma t r ix-isola t e dsilaketenes R 2SiCO,140,141 result ing from the intera c-tion between carbon monoxide and silylenes. Notetha t the imidazol-2-ylidene IIIc-carbon monoxide



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adduct has been reported,142a but more recently, thisresult ha s been refuted.142b

V.3. Addition to Multiple Bonds

V.3.1. Addition to CarbonCarbon Double Bonds

B ot h sin glet a n d t r ip le t t r a n sie n t ca rbe n es r e a ctwit h o le f in s t o g ive cyclo p ro p a n e s, a l t h o u g h by atotally different mechanism, as is a ppar ent from thestereochemistry of the reaction (Scheme 7).143,144

On t h e o t h e r h a n d, i t h a s lo n g be e n kn o wn t h a tnucleophilic carbenes in which the singlet state isstabilized by interaction of the vacant porbital w ith

the lone pair of a heteroatom subst ituent do not rea ctwit h e le ct ro n -rich a lke n es bu t wit h e le ct ro ph ilicones.145

1,2,4-Triazol-5-ylideneIVa reacts with diethyl fu-m a r a t e a n d a l s o d ie t hy l m a l ea t e , n o t g iv in g t h ecorresponding cyclopropane 53, but the methylene-tria zoline derivat ive55 (Scheme 8a).48 According to

E n d e r s e t a l . ,48 a [1+2]-cycloa ddition first occurs,leading to the tr an sient cyclopropane53. Then a rin gopening would lead to the zwitterionic derivative 54,which would undergo a 1,2-H shift 146 [AM1 calcula-tions predict a strongly negative reaction enthalpy(H ) -18 kca l/mol) for th e rea rra ngement of53 t o55].48 Ho we ve r, i t is qu it e cle a r t h a t a me ch a n ismdirectly leadin g to th e zwit terionic species54 readilyexplains the experimenta l results (Scheme 8a).

The phosphinosilylcar benesXIa,fa lso rea ct wit hdimet h yl fu ma ra t e , bu t n o t wit h dimet h yl ma lea t e ,to give the corresponding cyclopropanes 56a,fw i t hretention of the stereochemistry about the doublebond (Scheme 8b).135 The difference in reactivitybe t we e n cis a n d t ra n s o le f in s t o wa rd ca rbe n e s h a sa lre a dy be e n n o t e d by Mo ss e t a l . wit h me t h o x y-(phenyl)carbene. 147a

In fact , carbenes XIa,f undergo cyclopropanationreactions wit h a va riety of monosubstit uted electron-poor alkenes such as methyl acrylate,135 perfluoro-

Scheme 8

Scheme 7



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alkyl-substit uted olefins or sty rene derivat ives.148 Allthese reactions occur at room t empera ture, a nd t hecorresponding cyclopropa nes 57a,f-61a,f a r e o b -tained in high yields (Scheme 9).

Int erestingly, in each case, only one diast ereomerresults. The NMR data for57a,f-61a,far e consistentwith a syn-attack149 of the phosphinocarbenesXIa,f,an d this h as been confirmed by a single-crysta l X-ra ydiffraction study of58f.148 So far, this stereoselectiv-i t y is n ot we ll u n de rst ood, e sp ecia l ly s in ce st e riceffects should favor a trans attack [the bis(amino)-phosphino group being more sterically demandingthan the trimethylsilyl group].

The singlet nature of XIa,f w a s w i t h o u t d e b a t efrom the calculations a nd th e spectr oscopic da ta , buti t w a s of imp ort a n ce t o brin g e viden ce t h a t a con -certed mechanism 150 w as involved in th e cyclopropa-n a t io n re a ct ion s, h e n ce e st a blish in g t h e g en u in ecarbene natur e ofXIa,f. Recently , using pure cis andtrans monodeuterated styrene isomers,147b w e h a v eobserved the stereospecific format ion of the corre-sponding cyclopropanes 62(cis) a n d 62(trans), re -spectively (S cheme 10).148 The concerted na tur e of th e

cyclopropana tion react ions ha s been corrobora ted bythe results observed by reactingXIa,fwit h a va riet yof carbon-heteroat om double and triple bonds (seesections V.3.2 and V.3.3).

No cyclopropana tion rea ctions ha ve been reportedw i th B e rn dt a n d S eppel t ca r ben es IX,X a n dXIII,XIV, r espectively.

V.3.2 Addition to Carbonyl Derivatives

Electrophilic tra nsient carbenes a re know n to reactwith carbonyl derivatives through the oxygen lone

p a ir t o g ive ca rbon yl yl ide s 63 (Scheme 11).151a,152

These 1,3-dipolar species ar e usually tra pped by[3+2]-cycloa ddit ion rea ctions or can even be isolat ed;a sma ll a mo u n t of t h e corre spon din g o xira n e s issometimes obtained.151

The phosphinosilylcarbeneXIa does not react wit ha l iph a t ic ke t on e s. Ho we ver, i t re a dily a n d clea n lya dds t o t h e more e le ct ro ph il ic ben za ldeh yde a n dcinna ma ldehyde, affording th e oxiranes64a a nd64b,re spe ct ively, a s on ly on e dia st e re ome r.135 Thesere sult s s t ro n gly su g g est a con ce rt ed me ch a n ism,since the formation of a zwit terionic intermediate,s uch a s 65, w ou ld r es ul t i n t h e f or m a t ion of ap h osp h oryl a lken e via ox yg en a t om a t t a ck a t t h e

phosphorus center (Scheme 12).In marked contrast, the 1,2,4-triazol-5-ylideneIVa

re a ct s wit h p a ra fo rma lde h yde t o a f fo rd t h e co rre -sponding a cyl anion equivalent 66, w h ich h a s be eniso la t e d a s i t s p ro t o n a t e d fo rm 67 i n u p t o 6 5 %yield153a (Scheme 13). It is noteworthy that when onlya catalyt ic amount ofIVa is used, a formoin conden-sation reaction occurs and hydroxyacetaldehyde isobtained in 59%yield (Scheme 13). In this reaction,the h eterocyclic car bene acts as a nucleophilic cat a-lyst , wh ich is re min isce n t o f t h e t h ia zo liu m sa l t -catalyzed benzoin condensation reactions describedb y U k a i154 a n d Bre slo w.155

Thiazolium 68, imidazolium 69, a n d t r i a z o l i u m

sa lt s 70 a re we ll-kn own ca t a lyst s for va riou s C -Ccoupling reactions in basic media. Major examplesare the benzoin condensation of aldehydes to R-hy-droxyketones, the Micha el-St etter rea ction y ielding1,4-dicarbonyl derivatives, and the formoin conden-sa t io n re a ct io n s a f fo rdin g C2 t o C6 ca rbo h ydra t e s(Scheme 14).153

E v en a s y m m et r ic b en zoi n con d en s a t ion a n dMichael-St etter rea ctions ha ve been reported usingthe optically a ctive tria zolium salt 71(Scheme 15).156

I t i s i n t e r e s t i n g t o n o t e t h a t , a s e a r l y a s 1 9 5 8 ,Breslow155 recognized the role of heterocyclic car-be n e s a s ke y ca t a lyst s in e n zyma t ic re a ct io n s fo rwh ich t h ia min e p yro p h o sp h a t e is a co fa ct o r. Ca s-tells157 and Myles158 recently demonstrated that the

carbene dimers can also be the catalyt ic species inthe benzoin condensation catalyzed by thiazoliumsalts plus base.

V.3.3. Addition to Carbon-Heteroatom Triple Bonds

Transient electrophilic carbenes are known to reactwith nitriles to give tran sient 152 or even st able nitr ileylides 72 (eq 35).159 No reaction of transient nucleo-philic car benes w ith nitriles ha s been reported.

Scheme 9

Scheme 10

Scheme 11



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Th e f irst e xa mp le o f a z irin e fo rma t ion from acarbene and a nitrile has been observed by a ddit ionof the phosphinocarbene XIf to benzonitrile.160 I t isq u i t e l ik el y t h a t t h e f or m a t i on of t h e a z i r in e 73

results from a concerted [1+2]-cycloaddit ion. A st ep-wise mechanism, involving the init ial nucleophilicat ta ck of the carbene at the carbon at om of the nitrile,would have led to the 1,3-dipole 75, wh ich ca n a lsobe regarded as t he azabetaine 75or th e vinyl nitrene75 (Scheme 16). The ring closure of vinyl nitrenes

to produce azirines is known,161 but it ha s been showntha t these unsat ura ted species are efficiently tra ppedby p h o sp h in e s t o g ive p h o sp h a ze n e a ddu ct s ; 162,163

t h e re fo re , in t h e ca se o f t h e vin yl n i t re n e 75, a n

Scheme 12

Scheme 13

Scheme 14

Scheme 15

Schem

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