Gas-Phase and Computational Study of Identical Nickel- and Palladium-Mediated Organic Transformations Where Mechanisms Proceeding via M^II or M^IV Oxidation States Are Determined by Ancillary Ligands

Gas-phase studies utilizing ion-molecule reactions, supported by computational chemistry, demonstrate that the reaction of the enolate complexes [(CH₂CO₂-C,O)M(CH₃)]^- (M = Ni (5a), Pd (5b)) with allyl acetate proceed via oxidative addition to give M^IV species [(CH₂CO₂-C,O)M(CH₃)(¹-CH₂-CH=CH₂)(O₂CCH₃-O,O′)]^- (6) that reductively eliminate 1-butene, to form [(CH₂CO₂-C,O)M(O₂CCH₃-O,O′)]^- (4). The mechanism contrasts with the M^II-mediated pathway for the analogous reaction of [(phen)M(CH₃)]⁺ (1a,b) (phen = 1,10-phenanthroline). The different pathways demonstrate the marked effect of electron-rich metal centers in enabling higher oxidation state pathways. Due to the presence of two alkyl groups, the metal-occupied d orbitals (particularly d_z²) in 5 are considerably destabilized, resulting in more facile oxidative addition; the electron transfer from d_z² to the C=C π∗ orbital is the key interaction leading to oxidative addition of allyl acetate to M^II. Upon collision-induced dissociation, 4 undergoes decarboxylation to form 5. These results provide support for the current exploration of roles for Ni^IV and Pd^IV in organic synthesis.