Ene-adducts from 1,4-dihydropyridines and α,β-unsaturated nitriles: asynchronous transition states displaying aromatic features

Abstract

Hydride transfer reactions involving 1,4-dihydropyridines play a central role in bioorganic chemistry as they represent an important share of redox metabolism. For this class of reactions, direct hydride transfer is the commonly accepted mechanism; however, an Alder–Ene-like pathway has been proposed as a plausible alternative. The reaction between 1,4-ditrimethylsilyl-1,4-dihydropyridine and α,β-unsaturated nitriles is a solid candidate for this latter pathway. In this work, we perform high level ab initio and density functional theory computations to characterize the mechanism of this reaction, taking into account diverse reaction paths, and evaluating the effect of solvent polarity and variations in the chemical structure. Our analysis explains the stereochemical aspects of the reaction, characterizing the up to now unresolved spatial configurations of the predominant products, and may contribute to the understanding of enzymatic reactions involving NADP(H). The reactions are found to proceed in an asynchronous fashion, with transition states that display significant aromatic features. With this observation in mind, Alder–Ene and direct hydride transfer pathways can be understood as two extremes of a continuous mechanistic spectrum for this kind of reaction, with the analyzed systems located approximately equidistant from both ends.