Some borderline cases of cycloaddition

Abstract

Common cases of cycloaddition include concerted (2 + 4) and stepwise (2 + 2) reactions which latter are usually easily recognizable as involving biradicals or dipolar ions. The addition in situ of dichloroketene to 2,4-hexadiene and 2butene is stereospecific, as required by the (2S+ 2a) mechanism, but is followed by rapid base-catalysed rearrangement of the resulting 2,2-dichlorocyclobutanones into four stereoisomeric 2,4-dichlorocyclobutanones and, in the case of the diene adduct, an eventual dehydrochlorination product (H. Knoche, H. Griengl). The dichloroketene-cyclopentadiene adduct, in aqueous acetic acid, yields an analogous rearranged monochloro-acetate which is intermediate in the hydrolytic rearrangement leading to tropolone (T. Ando). 1,2-Dioxetanes can be formed stereospecifically by addition of singlet oxygen to 1,2-diethoxyethylene, or with total loss of configuration by a direct, low-temperature (-78°) reaction of triphenyl phosphite ozonide (A. P. Schaap, G. D. Mendenhall). The singlet oxygen addition does not involve a dipolar transition state, since its competition with Diels-Alder and ene reactions is unaffected by polar solvents. © 1971, Walter de Gruyter. All rights reserved.