Mechanism of hydride transfer reaction from 4-(dimethylamino)phenyl methane derivatives to 2,3-dichloro-5,6-dicyano-p-benzoquinone

The kinetics of the hydride-transfer reactions from bis[4- (dimethylamino)phenyl] methane (MH₂), bis[4(dimethylamino)phenyl] methoxy methane (MHOMe), Leuco Malachite Green (MGH) and Leuco Crystal Violet (CVH) to 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) have been investigated spectrophotometrically by means of the stopped-flow technique. The negative activation enthalpies were obtained for the hydride-transfer reactions from MH₂, MHOMe and MGH to DDQ in dichloromethane. These negative activation enthalpies suggest that the charge-transfer (CT) complexes are true intermediates in the hydride-transfer reactions from these donors to DDQ, as previously pointed out for MH₂-DDQ system in trichloroethane. The formation of charge-transfer complexes between MH₂, MHOMe, MGH and CVH and p-chloranil (CA) were observed in EPA (ethylether-pentane-ethanol) at low temperatures. The oxidation potential (E(ox)/°) of MHOMe, MGH and CVH were estimated using the linear correlation between the CT transition energy and the E(ox)/°value for the known CT complexes between CA and N,N-dimethylaniline derivatives. The rates were found to be correlated to the E(ox)/°values of the donors.