Catalysis of Organic..

4Immobilized Metal Complex Catalystscomplexes on both insoluble (porous silica) and soluble supports (polymers). Co(III)salen complexes represent powerful enantioselective catalysts for epoxide ringopeningreactions (9). In this case, the design of the proper support is of paramountimportance, as the transition state of the reaction involves two Co salen centers, andhence the supported system must be able to accommodate such a transition state.Here we explore our use of different soluble polymeric supports of differingbackbone and side-chain structure and evaluate the role of the support on thecatalytic properties in the hydrolytic kinetic resolution of rac-epichlorohydrin.Results and DiscussionA variety of Pd(II) palladacycle complexes have been reported over the past decadefor applications in Heck, Suzuki and other coupling reactions (10). Theseprecatalysts appeared quite stable under reaction conditions and little evidence wasobserved for the formation of Pd(0), the usual form of active palladium in thesecoupling reactions. For this reason, a new catalytic Heck cycle was hypothesized toaccount for the catalytic activity observed when using these precatalysts a Pd(II)-Pd(IV) cycle, rather than the usual Pd(0)-Pd(II) cycle. Over the last 5-7 years, it hasbeen systematically shown that bidentate palladacycles based on SC, NC and PCligands (Figure 1) decompose to liberate soluble, ligand-free Pd(0) that is the activecatalyst (11-17). However, up until 2004, tridentate palladacycle such as Pd(II)pincer complexes of NCN, SCS, or PCP ligation (Figure 1) had still been thought topotentially be stable complexes that catalyze the Heck coupling by a Pd(II)-Pd(IV)cycle (18-19). Here we share our recent results that conclusively show that SCS (20-21) and PCP (22) Pd(II) pincer complexes decompose to liberate soluble catalyticPd(0) species, and that solid-supported Pd(II) pincers simply represent reusablesoluble precatalyst sources, rather than the previously hypothesized stable recyclablecatalysts (20-23).R 1 YR 2 R 2R 1YXPdXPd XYY= NR 2 , PR 2 , SR, etcR 1 , R 2 = H, alkyl, aryl, etcX= Cl, Br, I, OTf, OAc, etcFigure 1 Homogeneous palladacycle complexes commonly used in Heck couplings.A variety of SCS and PCP Pd(II) pincer complexes were prepared andimmobilized on polymer or silica supports. (Figure 2 shows supported PCPcomplexes on poly(norbornene) and silica). Insoluble supports such as mesoporoussilica and Merrifield resins along with soluble supports such as poly(norbornene)allowed for generalization of our observations, as all immobilized catalysts behavedsimilarly. The application of poisoning tests, kinetics studies, filtration tests, and