Inverse Electron Demand Diels−Alder Reactions: Cycloaddition of Enol Ethers and Enamines with 4-Substituted 6-Nitrobenzofuroxans and a Nitroethylene Model. An <i>ab Initio</i> and Semiempirical Theoretical Study

Abstract

The stereochemistry of the cycloaddition of nitroethylene, as a model of 6-nitrobenzofuroxans, with ethenol and vinylamine has been studied at the ab initio DFT level. MO analysis reveals that these inverse electron demand Diels−Alder reactions are under frontier orbital control. A one-step reaction pathway involving a dissymmetrical transition state is favored relative to a two-step mechanism in which the intermediate has essentially a diradical character. Study of reactions of a series of 4-substituted 6-nitrobenzofuroxans with various enol ethers and enamines at the semiempirical AM1 level shows that in these instances a stepwise mechanism involving a short-lived diradical intermediate is likely to occur. Relative rates and stereoselectivities of these reactions as a function of the nature of reactants is discussed.