Conversion of Cyclobutane to Bicyclobutane by Base-Catalyzed 1,3-Dehydrohalogenation Reaction: A Mechanistic Study¹

The kinetics of the t-BuOK-t-BuOH-induced 1,3-dehydrohalogenation of the geometrical isomers of cyclobutanes 2–5 to give the corresponding bicyclobutanes was investigated. Substitution of H by Cl or Me on the carbon bearing the leaving group (substrates 1–3) caused rate reduction by a factor of 2 only. The cyano-activated substrates with Cl as a leaving group (substrates 1 and 3) underwent syn-anti isomerization under the reaction conditions. Substrate 2, which due to local symmetry lacks the geometrical syn and anti isomerism, was found to undergo³H incorporation during the course of the reaction. In the presence of crown ether, elimination rate constants were significantly enhanced compared to those for isomerization. Negligible isomerization was detected in the reactions of the bromo derivatives 4s and 4a as well as in those of the two carbonyl-activated substrates 5a and 5s. The effect of added crown ether on the elimination rate constants for the last two substrates was relatively small. The leaving group element effect (k^Br/k^cl) was found to be 71 for the pair 4s/3s and 30 for the pair 4a/3a. On the basis of the analysis of the element effect and supporting data it was concluded that the elimination from the cyano-activated substrates occurs from the hydrogen-bonded carbanion whereas an (ElcB)₁mechanism was assigned to the reactions of the carbonyl-activated substrates. The results obtained in this study combined with literature data suggest a low probability for a concerted 1,3-elimination reaction in the cyclobutane-bicyclobutane system.