Catalysis of Organic..

Jones et al. 31. On the Use of ImmobilizedMetal Complex Catalysts in Organic SynthesisChristopher W. Jones, 1,2 Michael Holbach, 2 John Richardson, 1William Sommer, 2 Marcus Weck, 2 Kunquan Yu, 1 and Xiaolai Zheng 1,21 Georgia Institute of Technology, School of Chemical & Biomolecular Engineering,2 School of Chemistry and Biochemistry, Atlanta, GA 30332 USAAbstractcjones@chbe.gatech.eduHeterogenization of homogeneous metal complex catalysts represents one way toimprove the total turnover number for expensive or toxic catalysts. Two case studiesin catalyst immobilization are presented here. Immobilization of Pd(II) SCS andPCP pincer complexes for use in Heck coupling reactions does not lead to stable,recyclable catalysts, as all catalysis is shown to be associated with leached palladiumspecies. In contrast, when immobilizing Co(II) salen complexes for kineticresolutions of epoxides, immobilization can lead to enhanced catalytic properties,including improved reaction rates while still obtaining excellent enantioselectivityand catalyst recyclability.IntroductionSupported metal complexes (1) have been studied for many years due to theirpotential for combining the best attributes of both homogeneous and heterogeneouscatalysis – high reaction rates and selectivities coupled with easy catalyst recovery.Unfortunately, in many cases, immobilized metal complex catalysts display thedisadvantages of each class of catalysts, poor recyclability due to catalyst leaching,low reaction rates due to diffusional limitations, and poor selectivities due to thepresence of multiple types of active sites. Indeed, although hundreds of differentmetal complexes have been immobilized on virtually every type of known catalystsupport, supported metal complex catalysts still are relatively poorly understoodcompared to the more typical homogeneous (e.g. soluble metal complexes) andheterogeneous (e.g. supported metals) catalysts that dominate commercial processes.To this end, we have undertaken a detailed, long-term investigation of twofamilies of supported metal complex catalysts, supported Pd pincer complexes foruse in C-C couplings such as Heck and Suzuki reactions and supported Co salencomplexes for epoxide ring-opening reactions. These two catalyst systems representinteresting targets for detailed study. Pd(II) pincer palladacycles have been proposedin the literature to be well-defined, stable Pd(II) catalysts that are active in Heck orother coupling reactions (2-7), potentially via a controversial Pd(II)-Pd(IV) catalyticcycle (8). Here we summarize our studies of supported Pd(II) PCP and SCS pincer