The Vinylcyclopropane-CyclopenteneRearrangement

The vinylcyclopropane‐cyclopentene isomerization was discovered in 1959. After the mechanism was studied in detail, this rearrangement was incorporated into many useful synthetic schemes. Various heterocyclic permutations of the vinylcyclopropane system have also been investigated, yielding several synthetic methods based on the rearrangements of cyclopropyl ketones and cyclopropyl imines to dihydrofurans and dihydropyrroles, respectively. This chapter is concerned only with the rearrangements of vinylcyclopropanes to cyclopentenes. Excluded are Cope rearrangements of divinylcyclopropanes and rearrangements of cyclopropyl compounds not leading to cyclopentenes.

The isomerization of 2,2‐dichlorovinylcyclopropane was described in 1959. The rearrangement of the parent hydrocarbon was reported independently by two laboratories a year later. Although no prior mention of this rearrangement exists in the literature, it appears likely that it occured unobserved as early as 1922 during the preparation of vinylcyclopropane by drastic methods. The presence of cyclopentene in the reaction mixtures resulting from the Hofmann elimination of a β‐cyclopropylethylammonium salt is a likely possibility, considering the low temperature at which vinylcyclopropane rearranges. The detection and identification of reaction products by refractive indices only may have prevented recognition of the isomerization at that time.

During the decade following the discovery of the rearrangement, virtually every simple vinylcyclopropane was studied in an attempt to settle the question of biradical versus concerted mechanism. Later there was intense activity in the applications of this rearrangement and many of its analogs to organic synthesis. Studies have also emphasized transition‐metal‐catalyzed rearrangements of vinylcyclopropanes.