Journal of the American Chemical Society, Vol.136, No.3, 986-998, 2014
A Computational Mechanistic Study of an Unprecedented Heck-Type Relay Reaction: Insight into the Origins of Regio- and Enantioselectivities
Density functional theory (DFT) calculations (B3LYP and M06) have been utilized to study a newly reported Heck-type reaction that uses an allylic or alkenyl alcohol as substrate and palladium as catalyst in the form of a chelate with a chiral pyridine oxazoline (PyrOx) ligand. The reaction not only controls the regio- and enantioselectivities of arylation of the C=C bond, but also forms the carbonyl functionality up to four bonds away from the aryl substituent via tandem C=C bond migration and enol-to-keto conversion. Computations performed on representative reaction systems allow us to propose a detailed mechanism with several key steps. Initial oxidation of palladium(0) by aryldiazonium generates active arylpalladium(II) species that bind the C=C bond of an allylic or alkenyl alcohol. The activated C=C bond inserts into the palladium aryl moiety to attain aryl substitution and a chiral carbon center, and the resulting complex undergoes beta-hydride elimination to give a new C=C bond that can repeat the insertion/elimination process to move down the carbon chain to form an enol that tautomerizes to a highly stable carbonyl final product. The calculations reveal that the C=C bond migratory insertion step determines both the regioselectivity and the enantioselectivity of arylation, with the former arising mainly from the electronic effect of the hydroxyl group on the charge distribution over the C=C bond and the latter originating from a combination of steric repulsion, trans influence, and C-H/pi dispersion interactions.