Elucidating reactivity differences in palladium-catalyzed coupling processes: the chemistry of palladium hydrides.
This communication describes a series of studies directed at obtaining a better understanding of the Heck reaction. For the first time, the postulated palladium-hydride intermediate (L2PdHX) in the catalytic cycle of the Heck arylation has been identified. In addition, this study establishes that the base-mediated Pd(0)-regeneration step (L2PdHX --> PdL2) of the cycle can be kinetically slow and thermodynamically unfavorable and that the process is remarkably sensitive to the structure of L (PCy3 vs P(t-Bu)3). Finally, this investigation demonstrates that, for certain catalyst systems, slow rates of Heck arylation can be correlated with reluctant reductive elimination of L2PdHX, furnishing a possible rationalization for Brønsted-base (Cs2CO3 vs Cy2NMe) and ligand (PCy3 vs P(t-Bu)3) effects that have been observed.
Hills
Ivory D
ID
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Fu
Gregory C
GC
D000475:Alkenes / Q000737:chemistry*; D001555:Benzene Derivatives / Q000737:chemistry*; D002384:Catalysis; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D010165:Palladium / Q000737:chemistry*
D000475:Alkenes; D001555:Benzene Derivatives; C032391:palladium hydride; D010165:Palladium