LARGE RATE ACCELERATIONS IN THE STILLE REACTION WITH TRI-2-FURYLPHOSPHINE AND TRIPHENYLARSINE AS PALLADIUM LIGANDS - MECHANISTIC AND SYNTHETIC IMPLICATIONS

The effect of changing the palladium ligands on the rates of typical Stille cross-coupling reactions was studied. Large rate enhancements (typically 10(2)-10(3) over triphenylphosphine-based catalysts) were observed with tri-2-furylphosphine (TFP) and triphenylarsine, which are recommended as the new ligands of choice in the palladium-catalyzed coupling between olefinic stannanes and electrophiles. On the basis of the evidence presented, the transmetalation, which is the rate-determining step in the catalytic cycle, is postulated to involve a pi-complex between the metal and the stannane double bond. In general, ligands that readily dissociate from Pd(II) and allow ready formation of this pi-complex are the ones that produce the fastest coupling rates. The utility of the new ligands is demonstrated with several synthetic examples.