Samarium diiodide-mediated carbanion

Samarium Diiodide-Mediated Carbanion Fragmentation Studies... 

The mechanism for the transformation of 5 to 4 was not addressed. However, it seems plausible that samarium diiodide accomplishes a reduction of the carbon-chlorine bond to give a transient, resonance-stabilized carbon radical which then adds to a Smni-activated ketone carbonyl or combines with a ketyl radical. Although some intramolecular samarium(n)-promoted Barbier reactions do appear to proceed through the intermediacy of an organo-samarium intermediate (i.e. a Smm carbanion),10 ibis probable that a -elimination pathway would lead to a rapid destruction of intermediate 5 if such a species were formed in this reaction. Nevertheless, the facile transformation of intermediate 5 to 4, attended by the formation of the strained four-membered ring of paeoniflorigenin, constitutes a very elegant example of an intramolecular samarium-mediated Barbier reaction. 

Intramolecular addition of vinyl radicals to olefins as a method for heterocycle synthesis has been examined. The vinyl radicals can be conveniently generated from vinyl bromides and samarium(II) diiodide [95JOC7424], The intermediate radical after cyclization undergoes a further electron transfer from samarium to furnish a carbanion which is quenched at the end of the reaction. A samarium(II) diiodide mediated aryl radical cyclization onto a dihydrofuran has been reported [95T8555],... 

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