INVERSE ELECTRON DEMAND DIELS-ALDER REACTIONS - CYCLOADDITION OF ENOLETHERS AND ENAMINES WITH 4-SUBSTITUTED 6-NITROBENZOFUROXANS AND A NITROETHYLENE MODEL - AN AB-INITIO AND SEMIEMPIRICAL THEORETICAL-STUDY

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 ele ctron demand Diels-Alder reactions are under frontier orbital control. A one-step reaction pathway involving a dissymmetrical transition sta te is favored relative to a two-step mechanism in which the intermedia te has essentially a diradical character. Study of reactions of a seri es of 4-substituted g-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 reac tions as a function of the nature of reactants is discussed.