A DFT study on the mechanism and regio selectivity of the tandem O-nitroso aldol/Michael reaction of nitrosobenzene and cyclohexenone

The tandem nitroso aldol/Michael reaction between nitrosobenzene and cyclohexenone with pyrrolidine-based catalyst has been recently reported to obtain O-nitroso Diels-Alder bicyclic products. We present here a theoretical study for the novel reaction to rationalize the experimental findings of the regioselectivity and bicyclic products of the reaction. By performing density functional theory calculations, we have identified the detailed mechanism of the title reaction and the pivotal factors controlling the regioselectivity of the reaction. Two regioselective channels (O- and N-selective) for the aldol/Michael reaction have been characterized in detail. The calculated results indicate that both the aldol reaction and the next Michael reaction for the O-selective channel are much more favorable in energy than the corresponding N-selective channel. Theoretical results account well for the regioselectivity and the formal nitroso Diets-Alder adducts observed in the recent experiment. (C) 2007 Elsevier B.V. All rights reserved.