Evidence of Carbenoid Intermediates

The great difference in reactivity between Me2CO and iPr2CO can be accounted for by steric factors the more sterically hindered the ketyl radicals are, the more difficult they are to dimerize. Steric hindrance would also destabilize the pinacolate intermediates with respect to rupture of their C-C bond and reverse conversion to the ketyl radicals. 

Mechanistic Analogies Between the McMuny, Wittig, and Clemmensen Reactions 

The results presented in Section 6.4.3 revealed that the McMurry reactions of ster-ically hindered ketones can in fact be viewed as Wittig-like olefination reactions. While these two reactions were for a long time thought to be mechanistically different, formation of carbenoid species by reduction of ketones with low-valent titanium complexes is not really surprising in view of the highly oxophilic and reducing character of titanium. Carbonyl olefinations by means of titanium carbe- 

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