DOI: 10.1002/cssc.200800025

Immobilization of Cobalt(II) Schiff Base Complexes onPolystyrene Resin and a Study of Their Catalytic Activityfor the Aerobic Oxidation of AlcoholsSuman Jain[a, b] and Oliver Reiser*[a]

Dedicated to Prof. W. A. Herrmann on the occasion of his 60th birthday

Introduction

The propensity of cobalt complexes to bind with molecularoxygen[1] and the use of the resulting dioxygen–cobalt com-plexes for oxidation reactions has led to intensive researchover the last few decades.[2] Cobalt Schiff base complexes areparticularly important and have been widely used as efficientand selective homogeneous catalysts to develop a variety ofoxidation methodologies using molecular oxygen as oxidant.[3]

However, the main drawback of these catalysts is the necessityof their separation from the reaction mixture at the end of thereaction. The heterogenization of these catalysts on polymericsupports therefore constitutes a logical approach to combinetheir homogeneous catalytic properties with those of hetero-geneous catalysts, such as facile separation from the reactionmixture and recyclability. The key methods which are hithertoknown for the synthesis of polymer-bound cobalt Schiff basecomplexes involve either in situ synthesis of a polymer-sup-ported Schiff base during copolymerization[4] or immobilizationof Schiff base ligands on functionalized polymers,[5] followedby their complexation. The catalysts and reagents often haveto be employed in excess to achieve high loading onto thepolymeric supports. We report here two efficient approachesfor the immobilization of cobalt Schiff base catalysts that callonly for stoichiometric use of ligands and metal in the ligationprocess. Either stepwise synthesis of polymer-bound Schiffbases followed by subsequent complexation with metal ions,or the direct covalent attachment of preformed homogeneouscobalt complexes to the resins, requiring modifications on theligand periphery of the complex without affecting its coordi-nated metal center, has been successfully achieved.We focused our study on two coupling strategies for the im-

mobilization of cobalt Schiff base complexes on a polymericsupport: 1) the copper-catalyzed[6] [3+2] azide–alkyne cycload-

dition[7] (CuAAC) termed as a click reaction has developed tobe one of the most versatile methods for ligating moleculefragments and is increasingly recognized for the preparation ofimmobilized catalysts ;[8] 2) the metal-free Staudinger ligation[9]

between azides and appropriately substituted carboxylic acidsmediated by triphenyl- or trialkylphosphine, in which a chemi-cally stable amide bond is formed. While this latter approachhas extensively been used for the immobilization of bioactivemolecules,[10] to the best of our knowledge there are no re-ports in the literature on the use of the Staudinger ligation forthe preparation of polymer-bound metal catalysts.

Results and Discussion

Synthesis of Immobilized Cobalt Schiff Base Catalysts

Catalytically relevant cobalt Schiff base complexes comprise ametal/ligand stoichiometry of 1:2. Consequently, immobiliza-tion of such complexes to a polystyrene support by covalentattachment requires either A) immobilization of one ligandmolecule followed by subsequent complex formation byadding metal salt and the second unsupported ligand mole-cule, B) immobilization of both ligand molecules followed bysubsequent complex formation by adding the metal salt re-

The copper-catalyzed [3+2] azide–alkyne cycloaddition and theStaudinger ligation are readily applicable and highly efficient forthe immobilization of cobalt Schiff base complexes onto polystyr-ene resins. Stepwise synthesis of polymer-bound Schiff bases fol-lowed by their subsequent complexation with metal ions weresuccessfully carried out. Direct covalent attachment of preformedhomogeneous cobalt Schiff base complexes to the resins wasalso possible. The catalytic efficiency of the so-prepared poly-

ACHTUNGTRENNUNGstyrene-bound cobalt Schiff bases was studied for the oxidationof alcohols to carbonyl compounds using molecular oxygen asoxidant. The immobilized complexes were highly efficient andeven more reactive than the corresponding homogenous ana-logues, thus affording better yields of oxidized products withinshorter reaction times. The supported catalysts could easily be re-covered from the reaction mixture by simple filtration and reusedfor subsequent experiments with consistent catalytic activity.

[a] Dr. S. Jain, Prof. Dr. O. ReiserInstitut f/r Organische Chemie, Universit2t RegensburgUniversit2tsstrasse 31, 93053 Regensburg (Germany)Fax: (+49)941-9434121E-mail : Oliver.Reiser@chemie.uni-regensburg.de

[b] Dr. S. JainOn leave from Chemical and Biosciences DivisionIndian Institute of Petroleum, Dehradun-248005 (India)

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quiring two ligand molecules in close enough proximity to co-ordinate the same cobalt atom, or C) immobilization of an al-ready assembled [M(LL)2] complex with the support that callsfor a mild ligation strategy, which allows modification of theligand periphery without affecting the metal complex(Figure 1). The efficiency of these strategies is also dependenton the equilibrium of [M(LL)2] complexes attached to the poly-meric support and the corresponding homogeneous ana-logues that are formed in the solution.

Azidomethyl-polystyrene, which is readily accessible fromcommercial Merrifield resin,[11] served as a convenient startingpoint for both the CuAAC and the Staudinger ligation strat-egies investigated. By strategy A (Figure 1), the polystyrene-anchored Schiff base 6 was prepared in a five-step sequence(Scheme 1). Starting from N-Boc-protected tyrosine methylester 1, propargylation gave rise to 2, which upon deprotec-tion followed by its condensation with salicylaldehyde in meth-anol afforded the Schiff base 3 as a yellow solid. [3+2] Cyclo-addition of 3 catalyzed by copper(I) iodide (5 mol%)[6c,12] withazidomethyl-polystyrene resin (4.2 mmolg�1 resin) resulted inthe quantitative formation of 4 as determined by the completedisappearance of the typical IR absorption band of azide(2095 cm�1; 92% yield, determined by mass balance). Theligand loading on the polystyrene support was estimated tobe 1.66 mmolg�1 resin from the nitrogen content determinedby elemental analysis, which compares well to the initial load-ing of the azidomethyl-polystyrene resin.[13] Compound 4 was

treated with the corresponding unsupported Schiff base 5[3c]

and anhydrous cobalt chloride in 1:1:1 molar ratio to yield thegreen-colored polystyrene-bound CoII complex 6, which wasisolated by filtration, washed thoroughly with hot methanol,and dried under vacuum (Scheme 1).The formation of 6 was confirmed by the complete disap-

pearance of the phenolic IR stretching band (2930–3200 cm�1)upon complexation of the ligand with cobalt. Furthermore, theshift of the IR frequency of the C=N group from 1627 cm�1 inthe ligand to 1600 cm�1 in the complex also confirmed thesuccessful formation of 6. For 6 as well as for all other com-plexes subsequently described, the amount of Schiff baseloaded onto the resin was estimated from the nitrogen con-tent as determined by elemental analysis, and the amount ofcobalt loaded onto the resin was determined by complexomet-

Figure 1. Strategies for the immobilization of [M(LL)2] complexes on poly-meric supports.

Scheme 1. Synthesis of polystyrene-bound cobalt complexes 6 and 7(Boc= tert-butyloxycarbonyl, TFA= trifluoroacetic acid, DIPEA=N,N-diisopro-pylethylamine).

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Aerobic Oxidations with Supported Co Catalysts

ric titration[5a] with ethylenediamine tetraacetic acid (EDTA)using xylenol orange as indicator (Table 1).The nitrogen content, which was somewhat higher than ex-

pected for the molecular formula of 6, suggested that somecobalt had been complexed only by the polymer-bound Schiffbase. For comparison, a complex of type 7, in which only poly-mer-bound Schiff base is available for complexation(Scheme 1), was therefore prepared by treatment of 4 withcobalt chloride (strategy B, Figure 1).Following strategy C (Figure 1), that is, direct immobilization

of preformed cobalt Schiff base complex onto the polystyrenesupport by CuAAC, we synthesized the cobalt Schiff base com-plex 8[3e] by treating 5 with anhydrous cobalt chloride in 2:1molar ratio (Scheme 2). Subsequent propargylation and cop-per(I) iodide catalyzed ligation with azidomethyl-polystyrene

resin led to the covalently attached polystyrene-bound CoII cat-alyst 10 as a brown solid. Again, the principle evidence for theformation of immobilized cobalt complex on the polystyrenesupport was found in the disappearance of the typical IR fre-quency at 2095 cm�1 for the azide group. Moreover, a broad IRband in the region of 3250–3300 cm�1 attributed to alkyne CH,in correlation with the data obtained for the complex loading(Table 1), suggests that CuAAC had occurred to a largeextent—but not exclusively—only once on the complex 9.

The Staudinger ligation was investigated as an alternative,metal-free strategy for the immobilization of cobalt Schiff basecatalysts, again following the three general approaches out-lined in Figure 1. Applying strategy A (Figure 1), p-aminobenzo-ic acid (11) was treated with azidomethyl-polystyrene resin inthe presence of an equimolar amount of triphenylphosphineto yield the anchored p-aminobenzoic acid 12 (Scheme 3),which was characterized by the complete disappearance in theIR spectrum of the azide band (2095 cm�1) and appearance of

Scheme 2. Synthesis of polystyrene-bound cobalt complex 10.

Table 1. Compositional characteristics of the PS-bound cobalt Schiff basecomplexes prepared from azidomethyl-PS resin.

Complex Co loading [mmolg�1] Efficiency ofcalcd[a] found[b] complexation [%][c]

6 1.07 0.90 847 0.83 0.75 9010 1.03 0.95 9215 1.36 0.90 6616 1.07 0.78 7315’ 1.36 0.99 73

[a] Calculated for the quantitative conversion of azidomethyl-PS resin(4.2 mmolg�1) to the respective cobalt complex. [b] Determined by com-plexometric titration. [c] Ratio of experimentally found cobalt ions loadedand calculated cobalt ions.

Scheme 3. Synthesis of polystyrene-bound cobalt complex 15.

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O. Reiser and S. Jain

strong amide bands (3328, 3209, 1623, 1602 cm�1). Formationof the Schiff base 13 was readily achieved by condensation of12 with salicylaldehyde, as conveniently monitored by IR spec-troscopy with the shift of the amide carbonyl band from 1623to 1678 cm�1 and the appearance of a new peak at 1598 cm�1

attributed to the C=N stretching vibration of the imine. Theloading of the Schiff base onto the polystyrene support(1.78 mmolg�1 resin) was estimated by the nitrogen contentdetermined by elemental analysis. Finally, treatment of 13 withcobalt acetate and the corresponding unsupported Schiff base14 in 1:1:1 molar ratio in ethanol under heating at reflux af-forded 15 as a mustard yellow solid. Again, the disappearanceof the phenolic stretching band in the IR spectrum was indica-tive of the successful transformation. Direct complexation of13 with cobalt chloride without addition of unsupportedligand (strategy B, Figure 1) was alternatively carried out toyield the supported catalyst, 16 (Scheme 3).The synthesis of 15 could also be achieved by direct immo-

bilization of the known CoII complex 17[14] onto polystyreneusing the Staudinger protocol as described above (15’,Scheme 4). As the typical bands for the carboxyl group (1692

and 3378 cm�1) were still prominently present, it suggestedthat the reaction of the resin occurred to a large extent onlyonce on the complex molecule.On the basis of mass balance, the nitrogen content deter-

mined by elemental analysis and the cobalt content deter-mined by complexometric titration for all three “click” com-plexes (6, 7, and 10) revealed that an overall loading of over80% had been achieved (Table 1). The overall loading obtainedby the Staudinger ligation was somewhat lower but neverthe-

less acceptable (>65%). The larger deviations from the theo-retically expected (100% conversion in all steps of the reactionsequence) nitrogen and cobalt contents are quite likely due toincomplete reaction of N=PPh3 residues that cause significantchanges in the elemental composition and molecular weightof the resin-bound complex even if only few reactive centersare affected. Noteworthy, no excess of reagents had to be em-ployed for both ligation strategies, demonstrating especiallythe high efficiency of CuAAC as a catalytic method for thefunctionalization of polymers.

Catalysis with Immobilized Cobalt Schiff Base Complexes

The development of synthetic methodologies for the oxidationof alcohols to carbonyl compounds using molecular oxygen asoxidant is important both from environmental and economicalviewpoints. In this context, there have been only a few reportsof the aerobic oxidation of alcohols using homogeneouscobalt Schiff base complexes as catalysts,[3b,e] but to the bestof our knowledge there have been no reports on the use ofpolymer-bound cobalt Schiff base complexes as catalysts forthis transformation.The catalytic efficiency of the prepared polystyrene-anch-

ored cobalt Schiff base complexes was studied and comparedwith that of their homogenous analogues for the oxidation ofalcohols 18, both primary and secondary, to the correspondingaldehydes and ketones 19 using molecular oxygen as oxidantand 2-methylpropanal as the reducing agent (Scheme 5). By

this protocol, 2-methylpropionic acid is generated as a by-product which is a disadvantage of this method from theaspect of sustainability. Nevertheless, the by-product is readilyremoved by aqueous workup, allowing the isolation of theproducts 19 in high yield and purity.Simple mixing of the polystyrene-bound cobalt salen com-

plexes (2 mol%) with a stirred solution of alcohols 18 and 2-methylpropanal (1.5 equiv) in acetonitrile under ambient dioxy-gen atmosphere at 50 8C allowed their convenient and high-yielding oxidation to 19. After completion of the reaction asmonitored by TLC analysis, the catalyst was separated by filtra-tion and reused as such for subsequent runs. The filtrate wassubjected to the usual workup to give the corresponding car-bonyl compounds. Importantly, the presence of neither metalnor ligand was detected, indicating that no leaching had oc-curred. Especially, selected product samples were subjected toinductively coupled plasma atomic emission spectroscopy (ICP-

Scheme 4. Synthetic route to 15’.

Scheme 5. Aerobic oxidation of alcohols with PS-bound cobalt Schiff basecomplexes.

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Aerobic Oxidations with Supported Co Catalysts

AES), which confirmed that the products contain less than1 ppm cobalt (detection limit 1 ppm).To evaluate the catalytic efficiency of the two sets of cobalt

Schiff base complexes 6, 7, and 10 and 15, 15’, and 16 relativeto the corresponding non-immobilized analogues, we firststudied the oxidation of benzhydrol (18a) and determined theconversion of the transformation by 1H NMR spectroscopyafter a reaction time of 30 min (Table 2). The polymer-support-

ed cobalt Schiff base complexes were more active, requiringshorter reaction times than their homogeneous analogues. En-hanced catalytic activity as well as selectivity of heterogenizedcobalt Schiff base complexes has been reported before for theoxidation of phenols with hydrogen peroxide.[5c,f]

We next examined the recyclability of the immobilizedcobalt complexes, using again benzhydrol (18a) as a represen-tative substrate (Table 3). In all cases, the catalysts were recov-

ered from the reaction mixture by simple filtration, washedwith dichloromethane, dried, and subjected to subsequent ex-periments (five cycles) without further activation. The yieldsand reaction time remained almost the same in all cases, es-tablishing the recyclability and reusability of the prepared cata-lysts. To check for leaching, we stirred the heterogenized com-

plexes in acetonitrile at 50 8C for 5 h. The polymer was re-moved by filtration, and the filtrates were used for the oxida-tion of benzhydrol (18a) under the described experimentalconditions. In all cases, no oxidation was observed even afterprolonged reaction times.Last but not least, a variety of primary and secondary alco-

hols were oxidized (Table 4) to explore the scope of the cata-lysts. Primary benzyl alcohols afforded the corresponding alde-hydes selectively without any evidence for the formation ofacids owing to further oxidation. However, among the varioussecondary alcohols studied, those with aromatic substituentswere more reactive than alicyclic alcohols. The catalysts pre-pared by immobilization of the preformed cobalt complex (10and 15’) were a little less active than those prepared by com-plexation of the previously immobilized Schiff bases. The pres-ence of 2-methylpropanal was found to be vital for this trans-formation; in its absence, the oxidation of alcohols to carbonylcompounds did not proceed. Likewise, the oxidation of benz-hydrol (18a) to benzophenone did not occur in the absence ofcatalyst as established by a control experiment.

Conclusions

In summary, we have described the first successful applicationof the copper catalyzed [3+2] azide–alkyne cycloaddition andthe metal-free Staudinger ligation for grafting cobalt Schiffbase complexes onto polystyrene supports. Either stepwisesynthesis of polymer-bound Schiff bases followed by their sub-sequent complexation with cobalt ions or direct immobiliza-tion of already prepared homogeneous complexes onto thesupport was successfully demonstrated. While it can be arguedwhether the use of azides on a large scale is practical and sus-tainable, the unique features of the coupling strategies pre-sented here for the functionalization of the polymer resin, thatis, its simplicity in use, versatility, and high efficiency with re-spect to overall catalyst loading and economic use of reagents,appear to be attractive for the preparation of immobilized cat-alysts. The so-obtained cobalt Schiff base complexes could beapplied in multiple cycles without observable leaching of themetal or ligand or loss of activity in the aerobic oxidation of al-cohols to give the corresponding aldehydes or ketones in ex-cellent yields.

Experimental Section

Materials and techniques: Merrifield polymer (4.3 mmol Cl pergram) was purchased from Fluka. l-(�)-Tyrosine, salicylaldehyde, p-aminobenzoic acid, propargyl bromide (80 wt% solution in tolu-ene), and alcohols were commercially available and used as ob-tained. N,N-Diisopropylethylamine (DIPEA) was purchased from Al-drich and flushed with nitrogen before use. CoCl2 and Co ACHTUNGTRENNUNG(OAc)2were heated at 110 8C for 3–4 h under vacuum prior to use. IRspectra were recorded on an Excalibur RTS 3000 FTIR Spectrome-ter. 1H and 13C NMR spectra were recorded on a Bruker Avance 300Spectrometer in CDCl3 with CHCl3 as a standard (dH=7.27 ppm,dC=77 ppm). Elemental analysis (Heraeus Elementar Vario III) andmass spectrometry (Finnigan ThermoQuest TSQ 7000) measure-ments were performed by the Central Analytical Laboratory (Uni-

Table 2. Comparison of the catalytic activity of PS-bound Co-salen com-plexes with the corresponding homogeneous complexes.[a]

Entry Catalyst Conv. [%][b]

1 6 522 7 503 10 404 15 425 16 446 15’ 427 Co-salen 7 188 Co-salen 15 25

[a] Benzhydrol (18a ; 1 mmol), 2-methylpropanal (1.5 mmol), dioxygen(1 atm), catalyst (2 mol%), CH3CN, 50 8C. [b] Determined by 1H NMR spec-troscopy after a reaction time of 30 min.

Table 3. Results of recycling experiments of PS-bound Co-salen complex-es.[a]

Entry Catalyst t [h] Yield [%][b]

1 6 1.0 97–982 7 1.0 96–983 10 2.0 96–974 15’ 1.25 96–985 16 1.25 97–986 15 1.5 96–98

[a] Five consecutive runs were performed with each catalyst: Benzhydrol(18a ; 1 mmol), 2-methylpropanal (1.5 mmol), dioxygen (1 atm), catalyst(2 mol%), CH3CN, 50 8C; after the indicated reaction time, the catalystwas recovered, filtrated, washed once with CH2Cl2, and reused in the nextcycle. [b] Range of yields obtained in five runs performed with each cata-lyst.

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O. Reiser and S. Jain

versitOt Regensburg), and inductively coupled plasma-atomic emis-sion spectroscopy (ICP-AES, PS-3000UV) measurements were con-ducted by Leeman Labs.

Analysis of cobalt loading:[5a] The cobalt(II) ions in the polymer-anchored complexes were leached out with hot 10n H2SO4, andtheir amount was determined by complexometric titration withEDTA using xylenol orange as indicator.

Preparation of azidomethyl-polystyrene:[11] Merrifield resin (2.5 g,4.3 mmolg�1) in DMSO (25 mL) was shaken at 50 8C with NaN3

(0.65 g, 10 mmol) for 2 days. After cooling to room temperature,the suspension was filtered and the resin was washed with MeOH(5P15 mL) and CH2Cl2 (5P15 mL) to give azidomethyl-polystyreneresin (2.4 g yield). IR: n=2095 cm�1. The loading of the azidogroup per gram of resin (4.2 mmolg�1) was estimated from the ni-trogen content determined by elemental analysis.

l-3-(4-Propargyloxyphenyl)-2-(N-tert-butylcarboxy)aminopropionicacid methyl ester (2): K2CO3 (9.10 g, 1.5 equiv) and KI (0.72 g, 0.1eq) were added to a stirred solution of l-N-tert-butylcarboxytyro-sine methyl ester (1; 13.10 g, 45 mmol) in acetone (150 mL) atroom temperature. Propargyl bromide (80 wt% solution in toluene;6 mL, 45 mmol) was then added dropwise during 0.5 h, and the re-sulting mixture was refluxed for 12 h. After cooling to room tem-perature, the mixture was filtered and the filtrate was concentratedunder reduced pressure. The residue thus obtained was dissolved

in ethyl acetate, washed with water twice, and dried over anhy-drous MgSO4. Evaporation of the solvent under reduced pressureyielded 2 (14.50 g, 98%). 1H NMR (300 Hz, CDCl3): d=1.35 (s, 9H),2.51 (t, J=2.46 Hz, 1H), 3.02–3.07 (m, 2H), 3.70 (s, 3H), 4.62 (dd,1H, J=2.19 and 4.24 Hz), 4.66 (d, J=2.19 and 6.22 Hz, 2H), 4.90 (brs, 1H, NH), 6.90 (d, J=8.7 Hz, 2H), 7.05 ppm (d, J=8.7 Hz, 2H).

Schiff base 3 : Ester 2 (14.00 g, 42 mmol) was added to a stirred so-lution of trifluoroacetic acid (TFA; 20 mL) in CH2Cl2 (50 mL), andthe mixture was heated at reflux for 10 h. Then, the mixture wasdiluted with dichloromethane and basified with 1n NaOH. The or-ganic layer was washed with water, dried over anhydrous MgSO4,and evaporated under vacuum to yield l-(4-propargyloxy)tyrosinemethyl ester (9.4 g, 96%). IR: n=3394, 3248, 2922, 2853, 1716,1608 cm�1; 1H NMR (300 Hz, CDCl3): d=2.51 (t, J=2.46 Hz, 1H),2.82 (dd, J=4.62 and 9.0 Hz, 1H), 3.03 (dd, J=4.62 and 5.21 Hz),3.68 (m, 1H), 3.71 (d, J=2.19 Hz, 2H), 6.92 (d, J=8.7 Hz, 2H), 7.12(d, J=8.7 Hz, 2H), 7.26 ppm (s, 2H). Then, salicylaldehyde (2.44 g,20 mmol) was added to a stirred solution of l-(4-propargyloxy)tyro-sine methyl ester (4.60 g, 20 mmol) in methanol (25 mL), and theresulting mixture was stirred at room temperature for 12 h. Theprecipitated yellow Schiff base was separated from the reactionmixture by filtration and recrystallized with methanol to give 3(6.50 g, 97%). IR: n=3272, 2996, 2923, 1732, 1633, 1512 cm�1;1H NMR (300 Hz, CDCl3): d=2.46 (t, J=2.46 Hz, 1H), 3.06 (dd, J=4.67 and 9.05 Hz), 3.30 (dd, J=4.67 and 4.93 Hz), 3.70 (s, 3H), 4.12

Table 4. Application of PS-bound CoII Schiff base complexes for the oxidation of alcohols to aldehydes or ketones.[a]

Entry Substrate Catalyst t [h] Yield [%] Entry Substrate Catalyst t [h] Yield [%]

1 6 1.0 98 31 6 4.0 922 7 1.0 98 32 7 4.5 953 10 2.0 97 33 10 5.5 904 15 1.25 98 34 15 5.5 945 16 1.25 98 35 16 5.0 956 15’ 2.0 98 36 15’ 5.5 93

7 6 1.75 97 37 6 0.75 988 7 2.0 96 38 7 0.5 989 10 2.75 95 39 10 1.25 9810 15 1.75 97 40 15 0.75 9811 16 2.0 97 41 16 0.75 9712 15’ 2.25 96 42 15’ 1.0 97

13 6 2.5 98 43 6 3.5 9714 7 2.75 98 44 7 3.5 9715 10 3.5 96 45 10 5.0 9616 15 2.0 97 46 15 4.5 9617 16 2.0 97 47 16 4.5 9518 15’ 2.5 95 48 15’ 4.5 95

19 6 2.25 98 49 6 3.5 9620 7 2.5 97 50 7 3.5 9521 10 3.0 97 51 10 4.5 9422 15 2.5 98 52 15 4.0 9623 16 2.5 97 53 16 4.5 9724 15’ 2.75 98 54 15’ 4.0 95

25 6 3.0 9826 7 3.5 9827 10 4.25 9728 15 3.5 9829 16 3.5 9730 15’ 3.5 97

[a] Alcohol 18 (1 mmol), 2-methylpropanal (1.5 mmol), dioxygen (1 atm), catalyst (2 mol%), CH3CN, 50 8C.

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(dd, J=9.0 and 4.93 Hz), 4.62 (d, J=2.46 Hz, 2H), 6.84 (d, J=8.5 Hz,2H), 7.06 (d, J=8.7 Hz, 2H), 7.95 ppm (s, 1H); 13C NMR (CDCl3): d=39.14, 52.48, 55.82, 73.26, 75.48, 77.45, 78.51, 114.99, 117.12,118.40, 129,71, 130.61, 131.82, 132.89, 156.48, 161.0, 167.0,171.30 ppm.

PS-supported Schiff base 4 : A suspension of azidomethyl-polystyr-ene resin (1 g, 4.2 mmol N3 per gram resin), CuI (5 mol%), DIPEA(3.5 mL, 20 mmol), and 3 (1.50 g, 4.5 mmol) was heated at reflux inCH2Cl2 (30 mL) for 24 h, after which time the typical IR band forazide (2095 cm�1) had completely disappeared. The precipitate wasfiltered, washed with methanol, and dried under vacuum to yield 4(2.20 g, 92%). IR: n=2930 (br), 1737, 1627, 1509 cm�1; elementalanalysis (%) calcd for quantitative conversion: N 9.83; found: N9.33.

PS-supported CoII Schiff base 6 : A mixture of 4 (1.20 g, 2 mmol),methyl salicylidine-N-methyl-3-(4’-hydroxyphenyl)propionate (5[3c] ;0.60 g, 2 mmol), and anhydrous CoCl2 (0.29 g, 2.25 mmol) in dryacetonitrile (15 mL) was stirred for 24 h at room temperatureunder nitrogen atmosphere. The green precipitate was separatedby filtration, thoroughly washed with acetonitrile and dichlorome-thane, and dried under vacuum to yield 6 (1.42 g, 71%). IR: n=3335 (br), 1742, 1600, 1510 cm�1; elemental analysis (%) calcd forquantitative conversion: N 7.49; found: N 7.88.

PS-supported CoII Schiff base 7: A mixture of 4 (1.20 g, 2 mmol)and anhydrous CoCl2 (0.15 g, 1.2 mmol) in dry acetonitrile (15 mL)was stirred for 24 h at room temperature under nitrogen atmos-phere. The so-obtained green precipitate was separated by filtra-tion, thoroughly washed with acetonitrile and dichloromethane,and dried under vacuum to yield 7 (1.18 g, 94%). IR: n=3042,2924, 1738, 1608, 1510 cm�1; elemental analysis (%) calcd for quan-titative conversion: N 9.29; found: N 8.76.

Bispropargylated CoII Schiff base 9 : Complex 8[3e] (3.27 g, 5.0 mmol)was added to a stirred mixture of K2CO3 (20 mmol, 2.76 g) in ace-tone (30 mL). Propargyl bromide (80 wt% in toluene, 15 mL,12.5 mmol) was then added dropwise during 15 min, and the re-sulting mixture was heated at reflux for 12 h. After cooling toroom temperature, the solution was filtered, the filtrate was con-centrated under reduced pressure, and the residue was driedunder vacuum to give 9 (3.58 g, 98%). IR: n=3283 (br), 2954,1740, 1599, 1509 cm�1; ESMS (m/z): 731 [MH+] .

PS-supported CoII Schiff base complex 10 : A suspension of azido-methyl-polystyrene resin (0.5 g), CuI (5 mol%), DIPEA (1.75 mL,10 mmol), and complex 9 (1.64 g, 2.25 mmol) was stirred in di-chloromethane (15 mL) under reflux for 24 h, after which time thetypical IR band for azide (2095 cm�1) had completely disappeared.The precipitate was filtered, washed thoroughly with acetonitrileand dichloromethane, and dried under vacuum to yield 10 (1.76 g,90%). IR: n=3280 (br), 2923, 1737, 1609, 1510 cm�1; elementalanalysis (%) calcd for quantitative conversion: N 7.21; found: N8.30.

PS-supported Schiff base 13 : A mixture of azidomethyl-polystyreneresin (1.0 g, 4.2 mmol) and triphenylphosphine (1.17 g, 4.5 mmol)in benzene (30 mL) was heated at reflux for 3 h, and then p-amino-benzoic acid (11; 0.62 g, 4.5 mmol) was added to the solution.Heating under reflux was continued for 24 h. The precipitate wasfiltered off, washed with dichloromethane, and dried undervacuum to yield 12 (1.32 g, 94%). Subsequently, a mixture of 12(1.30 g, 4 mmol), salicylaldehyde (1.03 g, 4 mmol), and triethyla-mine (14 mL, 10 mmol) in methanol (25 mL) was stirred at roomtemperature for 3 h. The precipitate was filtered and washed thor-

oughly with hot methanol to yield 13 (1.96 g, 87%). IR: n=3209,2995, 1678, 1598, 1435 cm�1; elemental analysis (%) calcd for quan-titative conversion: N 6.41; found: N 4.99.

PS-supported CoII Schiff base complex 15 : Supported Schiff base13 (1.13 g, 2 mmol) was added to a stirred solution containing an-hydrous Co ACHTUNGTRENNUNG(OAc)2 (0.39 g, 2.25 mmol) and p-salicylidine aminoben-zoic acid (14 ; 0.48 g, 2 mmol) in ethanol. The resulting mixture washeated at reflux for 12 h. The mustard yellow colored precipitatewas separated by filtration, washed thoroughly with hot methanoluntil the filtrate became colorless, and dried under vacuum toyield 15 (1.28 g, 74%). IR: n=3378 (br), 3232, 1692, 1659, 1618,1594, 1545 cm�1; elemental analysis (%) calcd for quantitative con-version: N 5.71; found: 4.52.

PS-supported CoII Schiff base complex 16 : A mixture of 13 (1.12 g,2 mmol) and anhydrous CoCl2 (0.15 g, 1.2 mmol) in dry acetonitrile(15 mL) was stirred for 24 h at room temperature under nitrogenatmosphere. The green precipitate was separated by filtration,thoroughly washed with acetonitrile and dichloromethane, anddried under vacuum to yield 16 (1.08 g, 92%). IR: n=3058, 1699,1600, 1532 cm�1; elemental analysis (%) calcd for quantitative con-version: N 5.99; found: 3.44.

PS-supported CoII Schiff base complex 15’: Complex 17[14] (2.42 g,4.5 mmol) was added to a mixture containing azidomethyl-poly-styrene resin (1.0 g) and triphenylphosphine (1.17 g, 4.5 mmol) inbenzene (30 mL). After heating at reflux for 24 h, the mixture wascooled to room temperature and the mustard yellow colored pre-cipitate was filtered, washed thoroughly with hot methanol, anddried under vacuum to yield 15’ (2.74 g, 89%). IR: n=3378 (br),1692, 1659, 1618, 1594, 1383 cm�1; elemental analysis (%) calcd forquantitative conversion: N 5.71; found: 4.60.

General procedure for the aerobic oxidation of alcohols to carbonylcompounds: The PS-supported CoII Schiff base complex (2 mol%)was added to a stirred solution of alcohol (1 mmol) and 2-methyl-propanal (1.5 mmol), and the mixture was heated at 50 8C underdioxygen atmosphere. The progress of the reaction was monitoredby TLC (SiO2). At the end of the reaction, the catalyst was removedby filtration and the solvent was evaporated under reduced pres-sure. The residue thus obtained was dissolved in ethyl acetate andwashed with saturated sodium bicarbonate solution (3P10 mL)and brine solution (2P10 mL). The organic layer was dried over an-hydrous MgSO4 and concentrated under vacuum. The crude prod-uct was purified by column chromatography (SiO2, hexane/ethylacetate 6:4). The yields of the carbonyl compounds and their reac-tion times are presented in Tables 2–4.

Acknowledgements

This work was supported by the Humboldt Foundation (postdoc-toral fellowship to S.J.), the International DoktorandenkollegNANOCAT (Elitenetzwerk Bayern), and the Fonds der ChemischenIndustrie.

Keywords: click chemistry · cobalt · oxidation · Schiff bases ·supported catalysts

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Received: February 2, 2008Published online on April 24, 2008

ChemSusChem 2008, 1, 534 – 541 ; 2008 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.chemsuschem.org 541

Aerobic Oxidations with Supported Co Catalysts