Mechanisms of Hydride Abstractions by Quinones
J. Am. Chem. Soc., 2014, 136, 13863-13873 published on 08.09.2014
J. Am. Chem. Soc.
J. Am. Chem. Soc.
The kinetics of the hydride abstractions by 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) from 13 C–H hydride donors (acyclic 1,4-dienes, cyclohexa-1,4-dienes, dihydropyridines), tributylstannane, triphenylstannane, and five borane complexes (amine–boranes, carbene–boranes) have been studied photometrically in dichloromethane solution at 20 °C. Analysis of the resulting second-order rate constants by the correlation log k₂(20 °C) = sɴ(E + N) ( J. Am. Chem. Soc. 2001, 123, 9500) showed that the hydride abstractions from the C–H donors on one side and the Sn–H and B–H hydride donors on the other follow separate correlations, indicating different mechanisms for the two reaction series. The interpretation that the C–H donors transfer hydrogen to the carbonyl oxygen of DDQ while Sn–H and B–H hydride donors transfer hydride to a cyano-substituted carbon of DDQ is supported by quantum-chemical intrinsic reaction coordinate calculations and isotope labeling experiments of the reactions of D₈-cyclohexa-1,4-diene, Bu₃SnD, and pyridine•BD₃ with 2,5-dichloro-ρ-benzoquinone. The second-order rate constants of the reactions of tributylstannane with different quinones correlate linearly with the electrophilicity parameters E of the quinones, which have previously been derived from the reactions of quinones with π-nucleophiles. The fact that the reactions of Bu₃SnH with quinones and benzhydrylium ions are on the same log k₂ vs E (electrophilicity) correlation shows that both reaction series proceed by the same mechanism and illustrates the general significance of the reactivity parameters E, N, and sɴ for predicting rates of polar organic reactions.