Samarium diiodide mechanism

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. 

The acetate function of 98 was then cleaved by treatment with samarium diiodide in methanol in high yield (81 %) [44], A potential mechanism for this transformation is shown in Scheme 3.18. Reduction of the ketone function forms a samarium ketyl radical (103). Transfer of a second electron forms a carbanion (104) which undergoes p-elimination of acetate to generate the samarium enolate 105. Protonation and tautomerization then affords the observed product 107. 

The proposed mechanism includes a reductive epoxide opening, trapping of the intermediate radical by a second equivalent of the chromium(II) reagent, and subsequent (3-elimination of a chromium oxide species to yield the alkene. The highly potent electron-transfer reagent samarium diiodide has also been used for deoxygenations, as shown in Scheme 12.3 [8]. 

Scheme 10 is representative of the mechanism of these coupling reactions involving a captodatively stabhzed glycyl radical 15 from the initial reduction of the pyridyl sulfide group by the divalent lanthanide reagent. Further reduction of this carbon radical by a second equivalent of samarium diiodide leads to a Sm(lII) enolate intermediate 16 of unknown geometry, which ultimately reacts with the carbonyl compound to give 17. 

Samarium diiodide is a one electron reductant that is capable of reducing both alkyl halides and carbonyl compounds. The rate of the reduction depends on the nature of the substrate and the reaction conditions. The mechanism of the addition of alkyl halides to carbonyls was extensively studied. In case of the samarium Grignard processes, it was concluded that the reaction proceeds through an organosamarium intermediate. However, the mechanism of the samarium Barbier processes is not fully understood and there is no unambiguous evidence in favor of any of the possible pathways. 

A large number of interesting rearrangements was described for which mechanisms including cyclic intermediates were proposed. Samarium diiodide-induced rearrangement of aniline derivatives 90a and 90b led to //wo-substitution products 92a and 92b, probably via the intermediates 91160 (equation 37). 

Some low valent metal salts are good reagents for the reduction of azides through a SET mechanism. CtCh in acidic aqueous solutions was initially used. However, samarium diiodide turns out to be a more convenient reagent under neutral conditions in organic solvents. Simultaneous reduction-ring expansion of some azidoketones allows to prepare maaolactams (Scheme 8.39). ... 

---