Towards an Understanding of the Polar Diels–Alder Reactions of Nitrosoalkenes with Enamines: A Theoretical Study

The polar Diels–Alder reactions of nitrosoalkenes with enamines have been studied using DFT methods at the B3LYP/6‐31G* level of theory. These Diels–Alder reactions are characterized by a nucleophilic attack of the enamine at the conjugated position of the nitrosoalkene with concomitant ring‐closure. The reactions present a total regioselectivity and a large endo selectivity. The analysis, based on the global electrophilicity of the reagents in the ground state, the natural bond orbital (NBO), and the topological analysis of the electron localization function (ELF) in the endo transition state (TS) and “halfway” along the IRC between the TS and cycloadduct, correctly explain the polar nature of these cycloaddition reactions. The large electrophilic character of nitrosoethylenes together with the large nucleophilic character of enamines are responsible for the large acceleration found in these polar Diels–Alder reactions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)