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)(η(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 dz(2)) in 5 are considerably destabilized, resulting in more facile oxidative addition; the electron transfer from dz(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, 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