Ring expansion with radical carbonylation

AIBN, (67) forms in high yield because the initial radical abstracts the hydrogen a to the carbonyl (with concomitant elimination of BusSn) more rapidly than it cyclizes. However, blocking this pathway results in smooth ring expansion, as illustrated by the conversion of (66) to (68). The alkene stereochemistry is dictated by the configuration of the precursor (see Scheme 49). 

With the same concept, but using the more reactive Ti(III) cationic radical [Cp2TiCl(THF)2] or a cationic salphen aluminum complex in combination with the cobalt anion [Co(CO)4] , Coates et al. succeeded to make the epoxide or aziridine carbonylative ring expansion reaction catalytic (Scheme 60) [149]. For both substrates, it is proposed a nucleophilic attack of the cobalt anion at the least-substituted carbon atom of the three-membered ring, the latter being activated by the Lewis acidic part of the catalyst. Of note, catalysts 106 and 107 used in this reaction are described as ion pairs rather than M-Co bond containing complexes. 

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