Substrate dependent reaction channels of the Wolff-Kishner reduction reaction: A theoretical study

Wolff-Kishner reduction reactions were investigated by DFT calculations for the first time. B3LYP/6-311+G(d,p) SCRF=(PCM, solvent = 1,2-ethanediol) optimizations were carried out. To investigate the role of the base catalyst, the base-free reaction was examined by the use of acetone, hydrazine (H2N-NH2) and (H2O)(8). A ready reaction channel of acetone -> acetone hydrazine (Me2C=N-NH2) was obtained. The channel involves two likely proton-transfer routes. However, it was found that the base-free reaction was unlikely at the N2 extrusion step from the isopropyl diimine intermediate (Me2C(H)-N=N-H). Two base-catalyzed reactions were investigated by models of the ketone, H2N-NH2 and OH-(H2O)(7). Here, ketones are acetone and acetophenone. While routes of the ketone -> hydrazone -> diimine are similar, those from the diimines are different. From the isopropyl diimine, the N-2 extrusion and the C-H bond formation takes place concomitantly. The concomitance leads to the propane product concertedly. From the (1-phenyl) ethyl substituted diimine, a carbanion intermediate is formed. The para carbon of the phenyl ring of the anion is subject to the protonation, which leads to a 3-ethylidene-1,4-cyclohexadiene intermediate. Its [1,5]-hydrogen migration gives the ethylbenzene product. For both ketone substrates, the diimines undergoing E2 reactions were found to be key intermediates.