Reaction of phosphinylated nitrosoalkenes with electron-rich heterocycles. Electrophilic aromatic substitution vs. cycloaddition

The behavior of phosphinyl nitrosoalkenes with indole, pyrrole and 2,5-dimethylpyrrole is described. The reaction of nitrosoalkenes with indole leads to the formation of 3-substituted indoles. While a concerted asynchronous [4 + 2] cycloaddition process may explain the formation of 3-substituted indole when a methyl group is present at the 3-position of nitrosoalkene, the presence of a 3-methoxycarbonyl group at the same position of nitrosoalkene increases its electrophilic character, and both mechanisms, an electrophilic aromatic substitution and a [4 + 2] cycloaddition process, are predicted to be competitive, although thermodynamically the cycloaddition is favoured. Phosphinyl nitrosoalkenes react with pyrrole leading to the corresponding 2-substituted pyrroles, while the treatment of 2,5-dimethylpyrrole with these nitrosoalkenes gives rise to the formation of bicyclic 1,2-oxazines. The mechanism of the reaction of phosphinyl nitrosoalkenes with pyrrole and 2,5-dimethylppyrrole may be explained by an initial hetero-Diels-Alder cycloaddition in both cases, but only subsequent rearomatization in the case of pyrrole. Theoretical studies show very good agreement with the experimental findings and the proposed mechanisms.