Some Borderline Cases of Cycloaddition

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 dichioroketene to 2,4-hexadiene and 2butene is stereospecific, as required by the (2 + 2a) mechanism, but is followed by rapid base-catalysed rearrangement of the resulting 2,2-dichiorocyclobutanones into four stereoisomeric 2,4-dichiorocyclobutanones and, in the case of the diene adduct, an eventual dehydrochiorination product (H. Knoche, H. Griengi). The dichioroketene—cyclopentadiene adduct, in aqueous acetic acid, yields an analogous rearranged monochioro-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-dietboxyethylene, or with total loss of configuration by a direct, low-temperature (— 78°) reaction of triphenyiphosphite ozonide (A. P. Schaap, G. D. Mendenhall). The singlet oxygen addition does not involve a dipolar transition state, since its competition with Diets—Alder and ene reactions is unaffected by polar solvents. What we mean by borderline cases are those in which either two mechanisms are operating competitively or it requires special effort to determine which of two mechanisms prevails. In cycloaddition we encounter borderline cases of several sorts. Past work has led to several propositions concerning the determination of reaction mechanisms in cycloaddition. (1) Cycloaddition of (2 + 4) electron systems are usually concerted, as allowed by orbital symmetry considerations1, and proceed with total retention of configuration in both reacting components2' 3, (2) In the exception provided by 1,1 -difluoroalkenes, biradjoal formation is specially facilitated by destabilization of the alkene and often by groups which delocalize an odd electron in the intermediate, so that formation of 4-rings with extensive loss configuration4 either competes5'6 with the concerted cyclohexene formation or supersedes it altogether. (3) The ability to form biradicals is much more sensitive to molecular structure than is the ability to react concertedly4. (4) Despite polar contributions to the transition states in both concerted and radical reactions, it is generally easy to distinguish stepwise cycloadditions involving dipolar ion intermediates79 (with kCH3CN/kalkane ratios as high as 1O) from biradical cycloadditions (k11 CN/kalkane < 1O)'. (5) A bifunctional intermediate, whether a biradicaP L 6 or a dipolar ion6'8 if formed in the 'cyclo' conformation with its ends within reacting distance of each other, may close with little or no loss of configuration. This is rare with biradicals but common with dipolar