Catalysis of Organic..

6Immobilized Metal Complex Catalystsstate of the reaction involves two Co salen centers (9). Given this knowledge,Jacobsen has shown that the design of homogeneous complexes based on Co salendimers (34), dendrimer-immobilized Co salens (35), and cyclic oligomeric Co salens(36) give enhanced catalytic rates as a consequence of facilitating the bimoleculartransition state. Thus, proper design of immobilized Co salen catalysts could allowfor not only enhanced activity compared to the homogeneous complex but alsoincreased turn-over numbers if the catalyst can be recycled. Polymer-supported Cosalens have been reported, having been synthesized via two strategies, (i) graftingreactions of salen ligands onto insoluble supports such as resins (37-38), or (ii)polymerizations of salen monomers (39-43). The first method is often realizedusing a multi-step grafting route, suffering from the coexistence of ill-definedspecies in the polymers and relatively low catalyst loading. Therefore, the secondmethod may be considered advantageous compared to the first, yet it has beenpracticed only with symmetrical salens as monomers. While salens with C 2symmetry are readily available from a synthetic point of view, polymerization orcopolymerization of such monomers introduces the salen cores along the main-chainor as a crosslink of the polymer matrix, respectively, which undesirably hinders theaccessibility and flexibility of the catalytic sites. Therefore, in comparison with theirhomogeneous counterparts, the polymer-bound salen complexes often exhibit poorenantio-control and reduced reactivity. We have derived two soluble polymersupported Co salen catalysts for the hydrolytic kinetic resolution (HKR) of racemicepichlorohydrin that contain side-chains functionalized with pendant Co(III) salencomplexes.The first system is based on a poly(norbornene)-supported Co salen and thesecond system is based on a poly(styrene) backbone. For the poly(norbornene)system, a homopolymers and several different copolymers were prepared (44), withvarying fractions of Co salen side-chains and spacer side-chains (Figure 3,copolymer 1a-c and homopolymer 1d). For the poly(styrene) system, bothhomopolymers of salen-containing monomers and copolymers with styrene (45,46)were prepared (Figure 3, copolymer 2a-c and homopolymer 2d).N NCoO OH 3 C-(CH 2 ) 7OOOOnN NCoO OnmCopolymer 1a: n = 25, m =25Copolymer 1b: n = 25, m = 75Copolymer 1c: n = 10, m = 90Homopolymer 1d: n=20mCopolymer 2a: n = 15, m =14Copolymer 2b: n = 11, m = 40Copolymer 2c: n = 6, m = 48Homopolymer 2d: n= 24Figure 3 Immobilized Co(II) salen complexes are oxidized to Co(III) and utilized inthe hydrolytic kinetic resolution of rac-epichlorohydrin.