Palladium Complexes of Chelating Carbenes for Catalytic Heck Reactions and 1,4-conjugate Addition Reactions

Abstract

Activities of a series of modular palladium Chugaev-type carbene complexes were investigated in catalytic Heck and 1,4-conjugate addition reactions. Significant variations of catalytic activity with ligand structure were observed in the Heck reaction. A methyl hydrazine-derived palladium dicarbene dibromide complex was recognized as the most effective precatalyst for the Heck reaction. The best precatalyst selected mediated the Heck reaction of electron-poor aryl chlorides and a range of aryl bromides with styrene in high yields at 120 C. Heck reactions performed under air showed limited air tolerance. The modular nature of the palladium Chugaev-type carbene complexes also allowed optimization of the catalytic 1,4-conjugate addition reaction of organoboranes. A methyl hydrazine-derived palladium dicarbene dichloride complex was identified as the most promising precatalyst for the addition of phenylboronic acid to acyclic and cyclic enones at 40�C. Monitoring the 1,4-conjugate addition reactions revealed the need to optimize the reaction time, because the yield of the product decreased in most cases as the reaction was allowed to proceed for longer durations.