Competing [4 + 2] and [3 + 2] cycloaddition in the reactions of nucleophilic olefins with ethyl 3-(toluene-p-sulphonylazo)but-2-enoate

Abstract

The title azo-olefin (4) has been isolated as an unstable oil and its cycloaddition reactions with several enol ethers, an enamine, and two alkyl-substituted indoles have been studied. With ethyl vinyl ether the product is the tetrahydropyridazine (5) but with 2-methoxypropene the product is a pyrrole derivative, (7a). Reaction with α-methoxystyrene similarly gives the pyrrole (7b). 2,3-Dihydrofuran adds to give a tetrahydropyridazine, (8), whereas the product from 4,5-dihydro-2-methylfuran is formulated as a dihydropyrrole, (10). 2-Methyl-1-pyrrolidin-1-ylpropene and 1,3-dimethylindole give mixtures of products arising from both [4 + 2] and [3 + 2] cycloaddition, with N-methyltetrahydrocarbazole, only the product (13) of [3 + 2] cycloaddition is detected. The cycloaddition to ethyl vinyl ether is not accelerated in a more polar solvent, but the addition to 2-methoxypropene is 9.5 times faster in [²H₃]acetonitrile than in tetrachloromethane at 298 K.

These reactions are interpreted as involving (i) concerted [4 + 2] cycloadditions which are subject to retardation by substituents on the nucleophilic olefins, and (ii)[3 + 2] additions through highly unsymmetrical transition states.