Samarium reagents carbonyl compounds

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. 

A general approach to P-functional alkanols involves formation of a-substituted samarium reagents and their reaction with carbonyl compounds. Thus, P,P-diiodoalkanols, °. -iodohydrins, " P-phenylselenoalkanols, and P-phenylthio-alkanols are prepared from the corresponding substituted halides. The products with a syn disposition are predominant. 

Certain other reagents like samarium iodide in aqueous THF ", sodium dithionite in aqueous DMF , sodium sulfide in presence of polyethylene glycol " and metallic zinc along with nickel chloride. " Using the latter reagent (Zn/NiClj), a,P-unsaturated carbonyl compounds can be very readily reduced under ultrasound conditions (Scheme 105). 

A few Pd(0)-catalyzed reactions of aUyUc carbonates cannot be classified under Sects. A-O. Cationic Tj -allylpalladium complexes can be reduced by tin(II) chloride to afford a tin-allyl intermediate provided with nucleophilic reactivity (umpolung), which reacts with carbonyl groups (Scheme 62). Tm(II) chloride has been used by Masuyama and co-workers in Pd(0)-catalyzed reduction of aUylic carbonates,allylic cyclic carbon-ates, and even allylic alcohols,followed by reaction with carbonyl compounds. Related results with cyclic carbonates have been reported by Kang and co-workers Diethylzinc and samarium(II) iodide have been used as the reducing reagent. Protona-tion of the allylsamarium intermediate gives overall reduction of the C—O into C—H bond.f ... 

A combination of iodomethyltrimethylsilane and Sm(OTf)2 also works as a methylenating reagent for carbonyl compounds (Scheme 2.28). When this reagent is applied to the reaction with ketones and aldehydes in THF/HMPA at room temperature, the carbonyl compounds are smoothly methylenated this process is known as the samarium-Peterson reaction [69]. 

The two reactions discussed here are new to you—they are reductive processes where an alkali or alkaline earth metal supplies electrons, to convert carbonyl compounds into radical anions, which then couple together on the metal surface. The first is the pinacol coupling reaction of ketones, which, classically, uses magnesium metal as an electron source. The reaction mechanism is shown in Figure 20.8. In recent years, many other metals have been used similarly titanium and samarium reagents have been particularly popular, and some applications are shown in Figure 20.9. 

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. 

---