Organosilane Reductions

Vinyl and Aryl Halides and Triflates. The organosilane reduction of aryl halides is possible in high yields with triethylsilane and palladium chloride.195 The reaction is equally successful with aryl chlorides, bromides, and iodides. Aryl bromides and iodides, but not chlorides, are reduced with PMHS/Pd(PPh3)4 in moderate to excellent yields.199 This system also reduces vinyl bromides.199 p-Chlorobenzophenone is reduced to benzophenone with yym-tetramethyldisilo-xane and Ni/C in excellent yield (Eq. 59).200 There is a report of the organosilane reduction of aryl and vinyl triflates in very high yields with the combination of Et3SiH/Pd(OAc)2/dppp (l,3-bis(diphenylphosphino)propane) (Eq. 60).201... 

The organosilane reduction of acid chlorides to aldehydes has been accomplished in high yields with the use of the pentacoordinated organosilane 60 (Eq. 137).107 This transformation has been reported to occur with tribenzylsilane and triethylsilane, but yields were not reported.285,286... 

The selective organosilane reduction of ketone functions can be effected in the presence of a number of other functional groups including epoxides,320,366 ketals,86,367 thioketals,368 other ketones,369,370 /1-lactams,371 alkynes,372 esters,79,80,83,84,87,320,373,374 a-bromides,76,80,83 amides80,83,84,86,276,320,375... 

Reductive Esterification. Organosilane reductions of y- or 5-keto acids and esters provide the corresponding lactones as the final products (Eqs. 226 and 227).69,79,402... 

The organosilane reduction of ketones in the presence of alcohols provides an excellent route to unsymmetrical ethers. The reaction of cyclohexanone with ethanol and Et3SiH/TFA gives cyclohexyl ethyl ether in good yield.327,328 The... 

Diketones are reductively cyclized in a TFA-catalyzed reaction. The cycliza-tion of the cage structure shown in Eq. 236 illustrates this ring closure in the formation of an acetal of trifluoroacetaldehyde.409 The organosilane reduction of triketone 69 followed by Jones oxidation gives the cyclic ketoether in fair yield (Eq. 237).410... 

Reductive Animation. An excellent application of organosilane reductions is found in the reductive animation of ketones. This method is a useful alternative... 

Because the organosilane reduction of ketones passes through a positively charged intermediate via the complexation or protonation of the carbonyl oxygen, the presence of suitably placed C=C functions can lead to cyclizations with the hydride of the silane adding to the C=C group. This strategy applies to... 

The organosilane reduction of pentafluorophenyl acrylate in the presence of an imine was shown to lead to /1-lactams in good yields (Eq. 295)476 The conversion of an ethyl ynoate into an E-ethyl enoate in high yield is shown in Eq. 296 477... 

The organosilane reduction of hydrazones to hydrazines is readily accomplished in good yields with Et3SiH/TFA (Eq. 336).560,561 (V-Tosylimines294 are reduced to their A-Boc tosylamino counterparts,294 and are also reduced with (MeO)3SiH/LiOMe in good yields.294 Benzyl-protected hydroxylamines are reduced with PhMe2SiH/TFA.551... 

The asymmetric organosilane reduction of prochiral ketones has been studied as an alternative to the asymmetric hydrogenation approach. A wide variety of chiral ligand systems in combination with transition metals can be employed for this purpose. The majority of these result in good to excellent chemical yields of the corresponding alcohols along with a trend for better ee results with aryl alkyl ketones than with prochiral dialkyl ketones. 

---