Anhydrides Tebbe methylenation

A limitation of the methylenation of carboxylic acid derivatives with Tebbe-type reagents such as 3 and 11 is that highly electrophilic acid anhydrides and acid halides cannot be transformed to the corresponding enol esters or alkenyl halides. The reaction of titanocene-methylidene generated from 11 with add chlorides results in the formation of acylation products via titanium enolates 12 [44] rather than methylenation. Since the titanium enolates 12 are stable and do not isomerize to the more highly substituted enolates, they are useful for regioselective aldol reactions (Scheme 4.11) [44a]. Although similar titanium enolates are also produced... 

Stepwise methylenation of the anhydrides and thioanhydrides was achieved by using appropriate amounts of 30 [70] (Scheme 4.25). Although the same transformation is effected with the Tebbe reagent, the isomerization of dimethylene compounds to the corresponding thiophenes is essentially inevitable. Similar mono-and bis-carbonyl methylenations of imides have also been reported [68]. 

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