On the question of free radical intermediacy in cycloaddition reactions of diazocarbonyl compounds to olefins under copper(II) chelate catalysis

Abstract The Copper(II) chelate catalyzed thermal cycloaddition of ethyl diazoacetate, dimethyl diazomalonate, ethyl-2-diazo-3-oxobutyrate, and ethyl diazopyruvate, representatives of diazocarbonyl additions that yield cyclopropanes and dihydrofurans, respectively, to olefins, has been examined in terms of the possible participation of radical species in intermediary stages. To this purpose, the title diazocarbonyl compounds were exposed to 1-methyl-1-cyclopropylethylene and 1,1-dicyclopropylethylene under catalytic conditions. The absence of cyclopropylcarbinyl radical to butenyl radical rearrangement products in cyclopropanations as well as in those reactions that furnish heterocycles suggests that intermediates of free radical nature may not be involved in the cycloaddition process. In turn, the strong electron donor capability of cyclopropyl substituents is interpreted as allusive of a non synchronous 1,3-dipolar cycloaddition of transient metal carbenes to the olefin with possible charge polarization.