Carbinyl radical rearrangement

In reaction (214), the addition of RS to the C(6) position is followed by a very rapid and irreversible cyclopropyl carbinyl radical rearrangement [reaction (215) Carter et al. 2000]. The resulting radical is comparatively long-lived and can be reduced by the thiol [reaction (216)]. Thus in this example, the minor pathway is detected, while the other, being reversible, remains unobserved. 

Although radicals are not nearly so prone to rearrangement as are, for example, carbocations, there are a few such rearrangements which have become identified as characteristic of carbon radicals. These include radical cyclizations, particularly the 5-hexenyl radical cyclization, and radical C-C bond cleavages, particularly the cyclopropylcarbinyl to allyl carbinyl radical rearrangement. In hydrocarbon systems, as organic synthetic chemists have learned how to control rapid chain processes, such rearrangements have become important synthetic tools [176-179]. 

Triethylamine as the electron donor was also used by Mattay and co-workers in tandem fragmentation cyclization reactions of a-cyclopropylketones. The initial electron transfer on the ketone moiety is followed by the fast cyclopropyl-carbinyl-homoallyl rearrangement, yielding a distonic radical anion. With an appropriate unsaturated side chain within the molecule both annealated and spi-rocyclic ring systems are accessable in moderate yields (Scheme 41) [62]. 

In many cases, particularly for the study of fast reactions (ky> key in Scheme 164), the opening of the cyclopropylmethyl radical to the isomeric allyl-carbinyl radical, one thousand times faster than the cyclization of the 5-hexenyl radical, is a useful complementary tool. It has been used mainly by Kochi for the study of cage reactions of alkyl radicals in solution as well for the study of ligand and electron transfer oxidations of alkyl radicals. Furthermore, in the last three studies the knowledge of the fate of the isomeric cyclobutyl intermediate was very useful in distinguishing between a homolytic (no rearrangement of the cyclobutyl radical) and a heterolytic pathway (fast... 

Crimmins, M. T., Wang, Z., and McKerhe, L. A., Rearrangement of cyclobutyl carbinyl radicals total synthesis of the spirovetivane phytoalexin ( )-lubiminol. Tetrahedron Lett., 37, 8703-8706, 1996. 

Extensive studies have been devoted to these radicals in connection with their structure (classical or nonclassical) and their reactivity (homoallylic rearrangement and 1,2-vinyl migration) and the main results have been reviewed.For instance, the classical nature of the allylcarbinyl, cyclopropyl carbinyl, and cyclobutyl radicals now seems well established. 

In contrast, irradiation of 22c (R = CH, R = CH3) produced bicycHc diketone 25c and alkyHdene phthaKde 26c together with cyclobutanol 23c and the unsaturated alcohol 24c. Compound 26 is the result of secondary photoisomerization of 25. The formation of 25 can be explained by a cyclopropyl-carbinyl rearrangement of ketyl radical VII followed by 1,5-biradical-recombination. Such an inner cyclopropane bond fission may be favored by the better stabilization by R (= CH3) of the new radical center. As found in 22d, replacement of R = CH3 by a Ph group simplified the reaction outcome to provide only 25. 

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