Catalysis of Organic..

10Immobilized Metal Complex Catalystsproduction, it is anticipated that both the polymeric systems could be utilized in amembrane reactor to achieve high turnover numbers.Experimental SectionImmobilized Pd(II) pincer complexes were prepared as described previously (20-22).Heck couplings of iodoarenes and acrylates were carried out in DMF using tertiaryamines as base and reaction kinetics were monitored as previously reported (20-22).Poly(norbornene)-supported Co(III)-Salen complexes (44) and poly(styrene)-supported Co(III)-Salen complexes (45,46) were synthesized via newly developedprocedures. In particular, a new, high-yielding, one-pot synthesis of nonsymmetricalsalens was developed (46). Hydrolytic kinetic resolutions were carriedout at room temperature and the products were characterized by chiral GC.ConclusionsIn this work we presented two case studies of immobilized metal complex catalystsin organic synthesis. It was demonstrated that immobilization of homogeneouscomplexes is neither a general solution to the problem of achieving high turnoverswhen using expensive or toxic metals, nor does immobilization always result incompromised catalytic properties. In the study of Pd(II) pincer complexes in Heckcouplings, we show that the precatalyst decomposes to liberate soluble palladiumspecies that are the true catalysts. In contrast, we show that immobilization ofCo(III) salen complexes can lead to improved catalytic rates and excellentenantioselectivities in the HKR of epoxides. Thus, in one case, immobilization leadsto the same or worse catalytic properties compared to the homogeneous case,whereas in the other, proper immobilization gives a better overall catalyst.AcknowledgementsThe U.S. DOE Office of Basic Energy Sciences is acknowledged for financialsupport through Catalysis Science Contract No. DE-FG02-03ER15459. The authorsthank Profs Peter J. Ludovice and C. David Sherrill of Georgia Tech and Prof.Robert J. Davis at the University of Virginia for helpful discussions.References1. C. Reyes, S. Tanielyan and R. Augustine, Chemical Industries (CRC Press),104, (Catal. Org. React.), 59-63 (2005).2. M. Ohff, A. Ohff, M. vanderBoom and D. Milstein, J. Am. Chem. Soc. 119,11687 (1997).3. F. Miyazaki, K. Yamaguchi and M. Shibasaki, Tetrahedron Lett. 40, 7379(1999).4. D. Morales-Morales, R. Redon, C. Yung and C. M. Jensen, Chem. Commun.1619 (2000).