Carbon-silicon bonds reductive cleavage

Reductive cleavage of the carbon-silicon bond of acyl silanes, including a-alkoxy and t/./i-dialkoxy derivatives, to give aldehydes may be accomplished by palladium-catalysed hydrogenolysis at ambient temperature and pressure212. 

The silyl halides can now" be prepared in high purity and high yield by the facile hydrogen halide cleavage of the carbon-silicon bond in arylsilanes. " No catalyst is required, and the use of the hazardous intermediate reagent, silane, is avoided. Bromosilane is prepared by the reaction of hydrogen bromide and phenylsilane. The latter is obtained by lithium hydro-aluminate reduction of the commercially available phenyltri-chlorosilane. lodosilane can be prepared in a similar fashion however, mixtures of iodosilane and benzene are difficult to separate. The preferred alternative procedure described below utilizes an isomeric mixture of 2-, 3-, and 4-chlorophenylsilanes as the intermediate. This intermediate is obtained by the chlorination of phenyltrichlorosilane, and is then reduced to the hydride. 

It appears that these reactions all may be explained by invoking a common intermediate phenylsilyl anion (VI) formed from the initial radical anion by reductive cleavage of a silicon-hydrogen or silicon-carbon bond ... 

An application of the synthetic concept to carbon analogs is not possible. LiSPh-elimination and/or LiSPh-elimination after previous H,Li exchange occurred for alkyllithium-intermediates formed by reductive cleavage of C-S bonds. Reaction of this type have been observed for similar compounds [7]. The silicon atom obviously plays a central role in the reactivity of the presented structure types. 

The origin of the persistent radicals which are produced in low yields is of some interest. Simple photochemical silicon-carbon bond cleavage lacks precedent and is not consistent with the failure to observe any alkane or 1-alkene in the photolysis mixture. While the latter could not be expected to survive in the irradiated solution, the former would. A possible route involving photochemical 1,1-reductive elimination is described below. 

Our strategy for the Smij-promoted synthesis of l,2-cis-C-glycosides is illustrated in Figure 3. It was assumed that one-electron transfer to the LUMO of the aryl sulfone group in A would result in concomitant cleavage of the C1 -S bond liberating an anomeric radical intermediate, as previously observed in the reductive lithiation of glycosyl phenyl sulfones." If 5-exo cyclization onto the silicon-tethered alkene/alkyne is substantially favored over a second electron transfer from Smij, then the exocyclic carbon radical B would ultimately be formed. On the other hand, reduction of the Cl-radical by... 

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