Samarium diiodide carbonyl compounds

The Julia olefination involves the addition of a sulfonyl-stabilized carbanion to a carbonyl compound, followed by elimination to form an alkene.277 In the initial versions of the reaction, the elimination was done under reductive conditions. More recently, a modified version that avoids this step was developed. The former version is sometimes referred to as the Julia-Lythgoe olefination, whereas the latter is called the Julia-Kocienski olefination. In the reductive variant, the adduct is usually acylated and then treated with a reducing agent, such as sodium amalgam or samarium diiodide.278... 

Another useful reagent for reduction of a-acetoxyketones and similar compounds is samarium diiodide.233 Sml2 is a strong one-electron reducing agent, and it is believed that the reductive elimination occurs after a net two-electron reduction of the carbonyl group. 

Reduction of multiple bonds with samarium diiodide has been reviewed. Chemo-and stereo-selective reduction of various compounds such as conjugated alkenes, c/,/3-unsaturated carboxylic acids, activated alkynes, carbonyl, azides, nitriles, and nitro compounds, under mild conditions, has been discussed. Recent developments in the use of samarium metal in this field have also been discussed.381... 

Alternatively, similar compounds could be prepared by a lead tetraacetate promoted degradation of serine residues to a glycine acetate and subsequent nucleophiUc displacement with 2-mercaptopyridine (Scheme 8, Route B) [12]. The alkylation step was then achieved by the treatment of a solution of the peptide and carbonyl compound with samarium diiodide in the presence of 1 mol% Nil2. Examples of the modified peptides are illustrated in Scheme 9. Yields of the peptides reached 90% and do not seem to be dependent on the chain length of the peptide. 

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. 

Utimoto, K., Matsubara, S. Samarium diiodide-mediated reaction of organic halides with carbonyl compounds. Yuki Gosei Kagaku KyokaishitOOS, 56, 908-918. 

Samarium diiodide-induced intermolecular coupling of aldehydes and ketones to indoles with an electron-withdrawing group at the 3-position of the indole have also been reported [19]. Carbonyl compounds including acetone, acetophenone, benzophenone, pivaldehyde, and isobutyraldehyde added to the C-2 position of the indole to give the thermodynamically stable trani-dihydroindole derivatives 27. 

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