Theoretical study on the regioselectivity of nucleophilic attack in silyl-substituted (diphosphino) (η³-allyl)palladium Cations

A density functional study has been performed to explain the regioselectivity of nucleophilic attack in silyl-substituted (diphosphino)(η3-allyl)palladium cations. Optimized geometries of model cations 4b,d,e (bearing at both ends of the allylic system Me and SiH3, t-Bu and SiMe3, and Ph and SiMe3 groups, respectively) show that for unsymmetrical allyl ligands the shortest Pd-C(terminal) bond is the one corresponding to the carbon atom directly bonded to silicon, electronic factors being determinant. Energy barriers for the attack of an ammonia molecule at each one of the terminal allyl carbon atoms of 4b,d,e have been calculated, electronic and steric effects being discussed. When the silicon group is the bulkiest one, both steric and electronic factors favor the attack at the carbon atom remote from silicon (γ-carbon). When the silicon substituent is the least sterically demanding, the attack at carbon bonded to silicon (α-carbon) is sterically more favorable, but electronic effects favor the attack at the γ-carbon.