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—<i>C</i>,<i>O</i>)M(CH₃)]⁻ (M = Ni (<strong>5a</strong>), Pd (<strong>5b</strong>)) with allyl acetate proceed via oxidative addition to give M^IV species [(CH₂CO₂—<i>C</i>,<i>O</i>)M(CH₃)(η¹-CH₂—CH═CH₂)(O₂CCH₃—<i>O</i>,<i>O</i>′)]⁻ (<strong>6</strong>) that reductively eliminate 1-butene, to form [(CH₂CO₂—<i>C</i>,<i>O</i>)M(O₂CCH₃—<i>O</i>,<i>O</i>′)]⁻ (<strong>4</strong>). The mechanism contrasts with the M^II-mediated pathway for the analogous reaction of [(phen)M(CH₃)]⁺ (<strong>1a,b</strong>) (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_<i>z</i>²) in <strong>5</strong> are considerably destabilized, resulting in more facile oxidative addition; the electron transfer from d_<i>z</i>² to the C═C π* orbital is the key interaction leading to oxidative addition of allyl acetate to M^II. Upon collision-induced dissociation, <strong>4</strong> undergoes decarboxylation to form <strong>5</strong>. These results provide support for the current exploration of roles for Ni^IV and Pd^IV in organic synthesis.