NAD(P)+–NAD(P)H Models. 64. The Quantitative Elucidation of the Mechanistic Aspects in the Silica Gel-Catalyzed Reduction of α,β-Unsaturated Carbonyl Compounds by a Model of NAD(P)H

Eight α,β-unsaturated carbonyl compounds, 1-(2′- or 4′-substituted phenyl)-3-phenyl-2-propen-1-ones (2′- or 4′-substituted chalcones), were subjected to the reduction by an NAD(P)H model in the presence of silica gel in benzene. All of these afforded quantitatively the corresponding saturated carbonyl compounds, whereas no 1,2-reduction took place. For 2′-methoxy, 4′-methyl, 4′-bromo, and 4′-nitro derivatives as well as the unsubstituted carbonyl compound, the apparent reactivity of the substrate increases with the increase in electronwithdrawing ability of the substituent, whereas the degree of the adsorption onto the surface of silica gel decreases. It is also apparent that the 4′-hydroxy derivative exerts lower reactivity than the 2′-hydroxy derivative in spite of much more efficient adsorption of the former. In order to uniformly interpret the apparent results, the data obtained were quantitatively analyzed in terms of kinetics as well as equilibrium constants for the adsorption of the substrates onto the surface of silica gel. It has been revealed that the silanol group on the surface of silica gel acts as a general-acid catalyst in the reduction.