Mechanism, cyclopropane ring rearrangement

The unimolecular mechanism involves formation of a protonated cyclopropane ring first, which avoids the formahon of a primary carbenium ion until after skeletal rearrangement has taken place. Such reachon intermediates were first... 

This result can be rationalized in terms of the vinyl migration mechanism by noting that rearrangement will occur to give the more stable of the two possible 1,3-diradicals. The cyclopropane ring in the final product will then incorporate this terminus. 

Scheme 3.32 shows a homobenzylic rearrangement where the carbanion interacts with the aromatic ring. The carbanion appears to insert between the ring and the carbon containing the two methyl groups. The mechanism of the rearrangement is proven by isolation of the cyclopropane structure when a para phenyl group is present. 

Hansson, T, R. Bergman, O. Sterner, and B. Wickberg The Mechanism of the Thermal Rearrangement of the Marasmane Sesquiterpene (+) Isovelleral. Cyclopropane Ring Closure via an Intramolecular Ene Reaction. J. Chem. Soc., Chem. Commun., 1260 (1990). 

The regiochemistry of the di-TT-methane rearrangement is best imderstood by referring to the biradical mechanism of Eq. 16.41. Consider an unsymmetrically substituted system of the sort shown in Eq. 16.47. The second step, the cleavage of the cyclopropylcarbinyl moiety, occurs so as to put the better radical stabilizing substituents on the radical center. Thus, these substituents become part of the cyclopropane ring in the product. 

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