Radical intermediates ring contraction

The initial transient formed, rearranges in a reaction that involves the ring contraction step in reaction (74). The lifetime of this intermediate is considerably longer than that reported for any other intermediate with a copper(II)-carbon bond in aqueous solution (85-87,101,136), suggesting the stabilized structure featuring the metallocycle. This intermediate decomposes via heterolysis of one of the copper(II)-carbon -bonds followed by homolysis of the second to form the cyclopentyl-methanol radical in reactions (75) and (76), which reacts with Cu + to form the final product cyclopentanecarbaldehyde (89). 

The reaction may include a cyclopropanol intermediate derived from an anion radical, as seen in the reduction of cyclohexenones under Clemmensen conditions to afford ring-contracted cyclopentanones along with cyclohexanone derivatives.Thus the diastereomeric cyclopropanol acetates (12) and (13) can be obtained in different ratios from both (10) and (11) (12/13 = 3 from 10, 12/13 > 100 from 11 Scheme 6). ... 

A sulfuranyl-alkyl biradical has been proposed [13] as intermediate in this ring contraction (Scheme 8). Nevertheless, the question whether trialkyl-sulfuranyl radicals are to be considered stable intermediates [37] or just transition states [38] remains open. 

SCHEME 18.42. Ring contraction through a radical intermediate. 

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