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

Abstract

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 MIV 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 MII-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 dz2) in 5 are considerably destabilized, resulting in more facile oxidative addition; the electron transfer from dz2 to the C=C π∗ orbital is the key interaction leading to oxidative addition of allyl acetate to MII. Upon collision-induced dissociation, 4 undergoes decarboxylation to form 5. These results provide support for the current exploration of roles for NiIV and PdIV in organic synthesis. Show more