Cyclopropylcarbinyl radicals, fragmentation

Fragmentation of cyclopropylcarbinyl radicals has been incorporated into several synthetic schemes.357 For example, 2-dienyl-l,l-(dimethoxycarbonyl)-cyclopropanes undergo ring expansion to cyclopentenes. 

The first is the fragmentation-recombination pathway (see mechanism a. Scheme 4), with acrylate and an acrylate-derived radical as the intermediate state (Scheme 6). This possibility has been suggested only recently, and is based on the reported inhibition of 2-methyleneglutarate mutase by acrylate [47], The second suggested mechanism is the addition-elimination pathway (see reaction c. Scheme 4), with a substituted cyclopropylcarbinyl radical as the intermediate (Scheme 6) [6, 26],... 

The presence of a C=C double bond in the migrating group of the but-3-enyl radical introduces the possibility of the addition-elimination mechanism (path b, Scheme 7), where the appropriate intermediate is the previously discussed cyclopropylcarbinyl radical (5). We find a significant preference ca 100 kJ mof ) for the addition-elimination pathway compared with the fragmentation-recombination pathway. Thus it is more favorable, in the gas phase at least, for the migrating HC=CH2 group to stay bonded to the remaining framework rather than to become detached from it. The cyclopropylcarbinyl radical intermediate involved in the addition-elimination mechanism is predicted to lie in a well of depth 30 kJ mof. ... 

Boto, A, Betancor, C., and Suarez, E., Sequential alkoxy radical fragmentation-cyclopropylcarbinyl rearrangement. Synthesis of highly functionalized eleven-membered rings. Tetrahedron Lett., 35, 5509, 1994. 

Mattay et al. used triethylamine as electron donor in tandem fragmentation/cyclization reactions of a-cyclopropylketones (49) [48]. The initial electron transfer to the ketone moiety is followed by subsequent cyclopropylcarbinyl-homoallyl rearrangement yielding a distonic radical anion (50). With an appropriate unsaturated side chain attached to the molecule both annelated and spirocyclic ring systems are accessible in moderate yields. Scheme 26 shows some representative examples. 

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