The rearrangement of cyclopropyl carbenes

Abstract

This work has been divided into two parts. The first part was concerned with experimental studies on the rearrangement of cyclopropyl carbenes, generated by the decomposition of the p-toluene sulphonyl hydrazones of
cyclopropyl aldehydes and ketones at 135 - 140° in an alkaline aprotic medium.

Cyclopropane carboxaldehyde p-toluene sulphonyl hydrazone itself gives rise mainly to cyclobutene. The direction of ring opening has been determined from a study of asymmetrically substituted cyclopropyl carbenes, The substitution pattern in the product cyclobutenes indicated that the least substituted bond of the cyclopropane ring was migrating preferentially during the rearrangement.

Cis-2-methyl cis-3-vinyl cyclopropyl carbene was studied in order to gain some insight into the spin state in which variously substituted cyclopropyl carbenes react. The results indicated that a substantial amount of the reaction was occurring via the triplet state.

Part Two has involved an attempt to simulate the rearrangement theoretically, using the method of CNDO (Complete Neglect of Differential Overlap) for molecular orbital calculations.

Methods were developed for describing the reaction pathway in terms of cartesian coordinates suitable for use with the computer programme for the CNDO calculations.

The CNDO method was evaluated by determining the barriers to rotation in various cyclopropyl compounds. Reasonably satisfactory values were obtained, so the rearrangement itself was studied. Although the results obtained were not in accord with the experimental observations on the preferred direction of ring-opening, a number of useful points emerged, which assisted in the evaluation of the experimental part.

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