Reduction conjugate, with silanes

However, a very common strategy in phosphine synthesis continues to be a final stage reduction of phosphine oxides with silane reagents, of which trichlorosilane is the most popular. Among a considerable range of new monophosphines obtained by trichlorosilane reduction, usually in the final step of the synthesis, are the phosphorus core conjugated triaryl-dendrimer unit... 

In order to explain this DKR, it was proposed that a copper enolate intermediate was formed on asymmetric conjugate reduction of the enone. Subsequent a-bond metathesis with silane yielded the silyl enol ether and regenerated the copper hydride catalyst. Under basic conditions, a rapid... 

Tetralkyl-silanes, -germanes, and -stannanes, as well as disilanes and distannanes, have been successfully used as alkyl radical precursors. The use of alkyl radicals is closely parallel to that presented above starting from ketals for the substitutive alkylation of aromatics and the conjugate addition to alkenes and alkynes (see Equations 4.5 and 4.28). Notice, however, that the final result depends on the sensitizer-trap used. Thus, N-methylacridinium perchlorate is reductively alkylated by hexamethyldisilane but undergoes reductive dimerization with the corresponding digermanes and distannanes. "... 

Palladium-Catalyzed Conjugate Reduction of a,s-Unsaturated Carbonyl Compounds with Diphenyl silane and Zinc Chloride Cocatalyst o,s-D1hydro-B-Ionone... 

The reaction of conjugated enones and dienones with trimethyl- and triethyl-silane in the presence of TiCU, followed by aqueous work-up produces the corresponding saturated ketones. This Lewis acid catalysis is particularly useful for conjugated reduction of sterically hindered systems (Scheme 46). " a,p-Unsaturated esters are not reduced under these conditions. 

Conjugated dienes like 1,3-dienes give 1 1 adducts of various types usually 1,2- and 1,4-addition products, and the regio- and stereo-isomers of each type, but also 1 2 adducts, and dimerization and reduction products. The product distribution depends on the kinds of dienes, silanes and particularly on the kind of catalyst. Of these, chromium hexacarbonyl is specific for the formation of (Z)-crotylsilanes from butadienes. Similar (Z)-selective hydrosilylation is observed with HSiCb and [Pd(PPh3)4] catalyst. The reaction pattern of 1,3-butadiene is summarized in Scheme 8. 

Silane hydrides can be used for the reduction of carbonyls and alkenes. Reaction of methylcyclohexene with a mixture of triethylsilane (EtsSiH) and trifluoroacetic acid (CF3CO2H) reduced the alkene moiety to give methylcyclohexane in 72% yield. 2 Under the same conditions, however, 1-pentene was not reduced. More commonly, this reagent is used for reduction of conjugated carbonyls, probably via formation of a silyl enolate (secs. 9.2, 9.3.B) as in the reduction of cyclohexenone to cyclohexanone in 85% yield with Ph2SiH2. Addition of transition metals such as ZnCl2, or copper salts to the silane facilitates the reduc-tion,594 as in the conversion of 576 to 577 in 96% yield. ... 

The creation of an asymmetric center by C-H bond formation is a very common process which can involve several types of reactions. Hydrogenation of prochiral olefins is often used with the rhodium catalysts of the Wilkinson type (5). These catalysts were shown to be inactive for ketone or imine reduction except in some cases (15), It was then interesting to develop an alternate method for asymmetric synthesis of chiral alcohols or amines. Since it was found that RhCl(PPh3)3 was able to catalyze silane additions to ketones (16,17) or imines (18), preparation of chiral alcohols or amines by asymmetric hydrosilylation could be envisaged (Figure 2). The 1,4-addition of silanes to conjugated... 

The formal conjugate addition of a hydride to a,f(-unsaturated carbonyl compounds with a subsequent aldol reaction of the in situ formed enolate has been frequently employed in organic synthesis. A broad range of procedures have been developed using various metals (e.g., rhodium, cobalt, iridium, mthenium, copper) and different reductants (typically silanes, boranes, or elemental hydrogen) [37]. 

A first example of a domino conjugate reduction/C-C bond formation was reported by Revis and Hilty in 1987. In this reaction, methyl methacrylate 52 was treated with acetone and trimethyl silane in the presence of RhCl3-3H20 as catalyst to afford 3-trimethylsiloxy-2,2,3-trimethylbutyrate 53 in excellent yield (Scheme 9.12) [21]. 

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