Exclusive Formation of α-Methyleneoxetanes in Ketene−Alkene Cycloadditions. Evidence for Intervention of Both an α-Methyleneoxetane and the Subsequent 1,4-Zwitterion

This paper describes a new mechanistic feature for the Staudinger ketene−alkene cycloaddition reactions to give cyclobutanones. Low-temperature NMR (13C, 19F, and 1H) monitoring of a reaction between bis(trifluoromethyl)ketene (1) and ethyl vinyl ether (2) has shown that the Staudinger reaction proceeds to form initially and exclusively an α-methyleneoxetane (3) by [2 + 2]C  O cycloaddition across the ketene CO bond. The initial intermediate 3 undergoes ring cleavage to produce a 1,4-zwitterion (4), which is converted to the final [2 + 2]C  C-type product, cyclobutanone (5). The key intermediate 3 has been isolated in its pure form and was found to be converted to the final products 5 on warming, via the 1,4-zwitterion 4. The α-methyleneoxetane 3 is so reactive that it reacts with methanol rapidly even at −80 °C via solvolysis to afford an adduct 7. The ion 4 derived from the pure isolated oxetane 3 was intercepted with acetone by a 1,4-dipolar cycloaddition to give a 1,3-dioxane 8. An open-chain α,β-enone (6) has been also obtained from 3. We conclude that the (1 + 2) reaction proceeds in a new three-step mechanism; formation of an α-methyleneoxetane 3, a [2 + 2]-type cycloadduct across the CO bond of ketene, followed by ring cleavage to give the zwitterion 4 and by recombination to form the final product, cyclobutanone 5. The zwitterion 4 is not equilibrating with reactants 1 and 2 but comes from the α-methyleneoxetane 3. Exclusive formation of another oxetane 12 has been observed in a reaction between diphenylketene (9) and methyl isopropenyl ether (11). The selectivity of initial formation of cyclobutanone or oxetane has been generalized with aid of frontier-orbital theory and ab initio calculations.