Kinetic Analysis of the Rearrangement of a Conformationally Constrained .alpha.-Cyclopropylbenzyl Radical
摘要:
Modulation of the cyclopropylmethyl (CPM)/ homoallyl radical equilibrium by phenyl substitution at the radical center is exploited in determining the kinetic versus thermodynamic preference for bond scission in a bicyclic CPM radical. Exocyclic ring opening is determined to be 28.7 times faster than the respective endocyclic process. This method will be of general use for determining the regioselectivity of radical rearrangements.
Benzylic Stabilization as a Mechanistic Tool for Studying Radical Rearrangements
作者:Hariharan Venkatesan、Marc M. Greenberg
DOI:10.1021/jo00109a043
日期:1995.2
The kinetic parameters for the rearrangement of, and the relative energies of radicals involved in the equilibria of, a bicyclic cyclopropylmethyl/homoallyl system were determined. Benzylic stabilization of the cyclopropylmethyl radical facilitates characterization of all four ring opening and closing processes. Kinetically disfavored endocyclic ring opening produces the thermodynamically favored homoallyl radical(10). Cyclohexenyl radical 10 is approximately 1.7 kcal/mol lower in energy than 9. The regioselectivity of ring opening of 8 is independent of any assumptions involving trapping rate constants of radicals. The regioselectivity for ring opening of 8 (k(1)/k(2) = 30.2 at 298 K) is similar to that of bicyclo[3.1.0]hexan-1-yl (1). These experiments suggest that benzylic stabilization does not significantly perturb the position of the transition state for ring opening. Therefore, phenyl substitution of a bicyclic cyclopropylmethyl radical should be a useful tool for analyzing the effects of substituents elsewhere in the system.
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