Thermal isomerization of 1,2-dialkenylcyclopropanes

Abstract

The etc, ett, and ttt isomers of l,2-di(prop-l'-enyl)cyclopropane isomerize at 165°at comparable rates to give, respectively, c/s,dj,dj-di(prop-l'-enyl)cyclopropane, írans-6,7-dimethylcyclohepta-l,4-diene, and cis-6,7dimethylcyclohepta-1,4-diene. These highly stereoselective and stereospecific reactions are interpreted in terms of a common mechanism involving a one-center epimerization as the rate-limiting step, followed by rapid Cope rearrangement of the trans,cis,trans-and m.ds.fra/w-dipropenylcyclopropanes.Among the archetypal thermal isomerizations, the l cA-divinylcyclopropane (1) to 1,4-cycloheptadiene (2) reaction occupies a prominent interstice between l 2 the electrocyclic conversion of 1,3,5-hexatriene to 1,3cyclohexadiene and the degenerate [3,3] sigmatropic rearrangement of 1,5-hexadiene. Since the initial demonstration of this process by Vogel in I960,2-5 and the first studies exploiting this reaction to demonstrate fluxional isomerism by Doering and Roth,6•7 numerous examples have been investigated.

Dienamine‐Induced Divinylcyclopropane–Cycloheptadiene Rearrangements

“…Their spontaneous [3,3]‐sigmatropic rearrangement was expected to result in the formation of cycloheptadienes 4 . The reaction was previously shown to proceed via a boat like transition state ,,. Hydrolysis of the enamine would then release the catalyst and the desired cycloheptadienals 5 .…”

“…It has been shown that a Z ‐configuration within the substrates results in a significant increase of the activation barrier for the 3,3‐sigmatropic rearrangement of divinylcyclo‐propanes –. Such a rearrangement usually requires higher reaction temperatures not compatible with the temperature sensitive organocatalyst (−)‐ 2 a .…”

Rearrangement Reactions

“…The cis-divinyl cyclopropane to cycloheptadiene rearrangement profits from the release of ring strain and is a well-known variation of the Cope rearrangement [32]. An application in the total synthesis of (±)-gelsemine 16 is depicted in Scheme 5.20 [33].…”

Thermally Forbidden Reactions

“…The thermal conversion of irtfw,y-l,2-divinylcyclopropanes to cyclohepta-1,4-dienes, which occur by way of cw-l,2-divinylcyclopropanes (Ullenius et al, 1972;Baldwin and Ullenius, 1974), may involve one-center epimerizations or, conceivably, two-center epimerizations, at C(l) and C(3), or C(2) and C(3). The point has not yet been subjected to an experimental test.…”