CHEMISTRY OF DIHYDROPYRIDINES .9. HYDRIDE TRANSFER FROM 1,4-DIHYDROPYRIDINES TO SP3-HYBRIDIZED CARBON IN SULFONIUM SALTS AND ACTIVATED HALIDES - STUDIES WITH NAD(P)H MODELS

The reactions of 1, 2, 6-trimethyl-3, 5-bis(methoxycarbonyl)-1, 4-dihydropyridine (2a), 1-methyl-3, 5-bis(methoxycarbonyl)-1, 4-dihydropyridine (2b), and 1-benzyl-3-acetamido-1, 4-dihydropyridine (1) with various sulfonium salts have been investigated. On thermal (60 °C) activation 2a, b react with, for example, methylphenacylphenvlsulforiium tetrafluoroborate (3b) to give the respective pyridinium salts, acetophenone, and phenyl methyl sulfide. There is some competing isomerization of the 1, 4-dihydropyridines to their (nonreactive) 1, 2-dihydro isomers, this being catalyzed by the pyridinium salt formed on reduction of the sulfonium salt. Phenacyl-, acetonyl-, and in some cases benzylsulfonium salts can be reduced. The group to which hydride transfer takes place should be electron deficient. These reductions can be initiated at room temperature with visible light (at room temperature pyridinium salt induced isomerization of the 1, 4-dihydropyridine to its 1, 2 isomer occurs only over a period of several days). The effect of visible light can be enhanced greatly by adding small amounts of dyes to the reaction mixtures. Eosin sodium salt, tetraphenylporphine, and Ruu(2, 2'-bpy)3Cl2 are all capable of increasing the rate of reduction; the last dye is by far the most effective. The results of mechanistic investigations are consistent with the hypothesis that the light-induced reductions are one-electron transfer reactions. In some cases there appears to be a separate thermal mechanism not sensitive to either light or sensitizers. Some sulfonium salts react with dihydropyridines to give the pyridinium salt and sulfide in less than quantitative yield and no reduction products from the sulfonium salt. As determined for the case of the reaction of bromomalonitrile with 1, and presumably also for other reactions, alkylation of an enamine carbon in the dihydropyridine is involved; the resulting iminium salt has been trapped by methanol and characterized. © 1979, American Chemical Society. All rights reserved.