The vinylcyclopropane rearrangement or vinylcyclopropane-cyclopentene rearrangement is a ring expansion reaction, converting a vinyl-substituted cyclopropane ring into a cyclopentene ring.
Due to its ability to form cyclopentene rings the vinylcyclopropane rearrangement has served several times as a key reaction in complex natural product synthesis. Other unusual cyclic products include cyclobutenes and bicyclic species such as the cycloheptene shown below.
Experimental and computational investigations show that mechanistically, the vinylcyclopropane rearrangement can be thought of as either a diradical-mediated two-step and/or orbital-symmetry-controlled pericyclic process. The amount by which each of the two mechanisms is operative is highly dependent on the substrate.
Origins and history In 1959, a young research chemist with Humble Oil and Refining (Esso, now Exxon) named Norman P. Neureiter was instructed to find new uses for the excess butadiene produced from one of the refinery processes. Discussions about carbene chemistry with one of the company's most respectable consultants at the time, William von Eggers Doering, then a professor at Yale, led the young Ph.D. graduate from Northwestern University to follow a recent procedure combining both, carbenes and butadiene. In particular the procedure described the reaction of 1,3-butadiene with carbenes generated from the action of base on chloroform or bromoform, which had been studied previously by Doering. Neureiter then took the resulting 1,1-dichloro-2,2-dimethylcyclopropane and under pyrolysis conditions (above 400 °C) discovered a rearrangement to 4,4-dichlorocyclopentene which today is considered to be the first thermal vinylcyclopropane-cyclopentene rearrangement in history.
The corresponding all-carbon version of the reaction was independently reported by Emanuel Vogel and Overberger & Borchert just one year after the Neureiter publication appeared. Doering, although interacting with Humble Oil and Refining - and therefore also with Neureiter - as a consultant, in a 1963 publication stated the following : "Credit for discovery that vinylcyclopropane rearranges to cyclopentene is due to Overberger and Borchert, and Vogel et al., who appear to have developed several examples of the rearrangement independently." The development of further vinylcyclopropane rearrangement variants didn't take long as demonstrated by Atkinson & Rees in 1967, Lwowski in 1968. and Paladini & Chuche in 1971.
The classical vinylcyclopropane rearrangement was discovered well after two of its heteroatom variants had already been reported. Although it is believed that the vinylcyclopropane rearrangement must have occurred during Nikolay Demyanov's preparation of vinylcyclopropane by Hofmann elimination at elevated temperatures in 1922, the cyclopropylimine-pyrroline rearrangement by Cloke in 1929 and Wilson's cyclopropylcarbaldehyde-2,3-dihydrofuran rearrangement in 1947 are really the only examples of vinylcyclopropane-like rearrangements.
This last reaction type is also known as the Cloke–Wilson Rearrangement
Mechanism The mechanistic discussion on whether the vinylcyclopropane rearrangement proceeds through a diradical-mediated two-step or a fully concerted orbital-symmetry-controlled mechanism has been going on for more than half a century. Kinetic data together with the secondary kinetic isotope effects observed at the vinyl terminus of the vinylcyclopropane suggest a concerted mechanism whereas product distribution indicates a stepwise-diradical mechanism. In the 1960s, shortly after the rearrangement was discovered, it was established that the activation energy for the vinylcyclopropane rearrangement is around 50 kcal/mol. The kinetic data obtained for this rearrangement were consistent with a concerted mechanism where cleavage of the cyclopropyl carbon-carbon bond was rate-limiting. Albeit a concerted mechanism seemed likely it was shortly recognized that the activation energy to break the carbon-carbon bond in unsubstituted cyclopropane was with 63 kcal/mol exactly 13 kcal/mol higher in energy than the parent activation energy, a difference remarkably similar to the resonance energy of the allyl radical. Immediately people started to appreciate the possibility for a diradical intermediate arising from homolytic cleavage of the weak C1-C2-cyclopropane bond under thermal conditions.
The discussion on whether the vinylcyclopropane rearrangement proceeds via a fully concerted or a two-step, non-concerted mechanism was given further careful consideration when Woodward and Hoffmann used the vinylcyclopropane rearrangement to exemplify [1,3]-sigmatropic concerted alkyl shifts in 1969. They hypothesized that if a concerted mechanism was operative the consequences of orbital-symmetry controlled factors would only allow the formation of certain products. According to their analysis of a vinylcyclopropane substituted with three R groups the antarafacial [1,3]-shift of bond 1,2 to C-5, with retention at C-2, leading to the ar cyclopentene and the suprafacial [1,3]-shift of bond 1,2 to C-5, with inversion at C-2, leading to cyclopentene si are symmetry allowed whereas the suprafacial [1,3]-shift of bond 1,2 to C-5, with retention at C-2, leading to cyclopentene sr and the antarafacial [1,3]-shift of bond 1,2 to C-5, with inversion at C-2, leading to the ai cyclopentene are symmetry-forbidden. Woodward and Hoffmann based their analysis solely on the principles of the conservation of orbital symmetry theory without however making any mechanistic or stereochemical prediction.
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