Gas-phase and computational study of identical nickel- and palladium-mediated organic transformations where mechanisms proceeding via MII or MIV 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)(η¹-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.