Reduction with Et3SiH

The EtsSiH/tetracyanoethylene combination reduces acetals and ketals to the corresponding ethers but the yields are mixed.500 The full reduction of benz-aldehyde acetals to the toluene derivatives is realized by the initial reduction with Et3SiH/SnBr2-AcBr followed by Bu3SnH/AIBN (azobis(isobutyronitrile)) or LiAlH4.479 The overall yields are excellent. 

A/b-Acetyl-5-acetylamino-l-methoxytryptamine (13) is prepared with an overall yield of 32% in eight steps from tryptamine (14, Scheme 3) [15]. M),l-Diacetyl-2,3-dihydrotryptamine (15) is obtained from 14 through Nb-acetylation (97%), reduction with Et3SiH/CF3COOH (98%) [16], and subsequent acetylation of N(l)-H (78%). 5-Nitro compound (16, 81%), ob-... 

These reductions of lactols with Et3SiH 84b in combination of BE3 -OEt2, TfOH, or TMSOTf 20 have become standard reactions for synthesis of cyclic ethers [62-69]. Thus even co-hydroxyketones such as 1837 cyclize readily with excess EtsSiH 84b in the presence of TMSOTf 20, in high yields, via the lactols 1838, to give cyclic ethers such as the substituted oxepane 1839 in 90% yield [65] (Scheme 12.18). 

When 46 was treated with Et3SiH in the presence of TFA, 48 was produced, meaning that both the olefin double bond and the sulfur-oxygen bond were reduced. In addition, 47 could also be reduced to 48 under the same conditions. Therefore, it was not clear from these results whether the reduction of the olefin in sulfoxide 46 was directed by the sulfoxide oxygen. 

Ether bond formation via reduction of the oxonium species with Et3SiH. 

Motherwell and Whitehead et al. reported a similar intramolecular reductive aldol reaction of aldehyde-enoate derivatives. The cyclization of 6-oxo-ester 23 was catalyzed by RhCl(PPh3)3 (1 mol%) with Et3SiH (210 mol%) as terminal reductant (Scheme 9) [17,18]. The cyclization proceeded at 50 °C for 18 h to give the aldol product 24 in 81% yield with ds-selectivity (cis trans =... 

Thiol esters undergo smooth reduction to give aldehydes by the Fukuyama hydrosilylation procedure, which is an alternative way to transform carboxylic acids to aldehydes. Upon treatment with Et3SiH and 10% Pd/C, a thioester underwent smooth reduction to give an aldehyde.409,410 For example, to a stirred mixture of thioester and Pd/C in acetone may be added Et3SiH at room temperature under an Ar atmosphere. Stirring is continued until the hydrogenolysis is complete (0.5-1 h) (Scheme 4.117). 

In some instances, treatment of polyfunctional benzylic alcohols with acid in the presence of organosilicon hydrides causes multiple functional group transformations to occur simultaneously. This phenomenon is illustrated by the reduction of the secondary benzylic alcohol function and concomitant loss of the methoxymethyl protecting group of 2-(l-hydroxydecyl)-5-methoxy-l-(methoxy-methyleneoxy)naphthalene upon treatment with Et3SiH/TFA in dichloromethane (Eq. 26).167... 

Interesting variations are observed in the stereoselectivities of these ionic hydrogenations. Reduction of 1,2-dimethylcyclopentene with Et3SiH/TFA near... 

Treatment of A8(9)-dehydro-D-homoestradiol (39, R = H) (or its 3-methyl ether, R = Me) with Et3SiH/TFA followed by saponification of the trifluoroac-etate ester intermediate leads to D-homoestradiol (40) (or its 3-methyl ether) containing 2-15% D-homoequilenol (41) (or its 3-methyl ether).240 By contrast, reduction and saponification of 3,17-diacetyl-A8(9)-dehydro-D-homoestradiol (39, R = AcO) gives a 60% yield of D-homoestradiol without the presence of any D-homoequilenol (Eq. 87).240... 

The full reduction of 1,3-dienes with Et3SiH/TFA occurs in certain systems although the yields are only modest.231 For example, 1,3-cyclohexadiene gives a 65% yield of cyclohexyl trifluoroacetate, presumably by way of cyclohexene (Eq. 88).211 On the contrary, 1,4-cyclohexadiene fails to undergo reaction with 10 equivalents of triethylsilane and 20 equivalents of trifluoroacetic acid even after 24 hours at room temperature (Eq. 89). 

The triethylsilane/Pd2(dba)3 combination is also used for these reductive cycli-zations, although lower yields are reported.247 1,6-Diynes are reductively cyclized to 1,2-dialkylidenecyclopentanes in good yields with Et3SiH/Pd2(dba)3 CHCl3 (Eq. 104).248... 

Similar reactivity is realized with 2-acetylthiophene using triethylsilane with aluminum chloride.259 Treatment of the ethylene glycol acetal of 2-thiophenecar-baldehyde with Et3SiH/TFA results in reduction of the ring and oxidation of the side chain to the silylated carboxylic acid (Eq. 119),260 whereas similar treatment of 2-thiophenecarbaldehyde gives 2-methyltetrahydrothiophene and 2-... 

The reaction of lactones of benzyl alcohols with Et3SiH/TFA results in complete reduction of the alcohol part of the lactone to the methylene group while preserving the carboxylate function (Eq. 148).305... 

The reaction of tert-butyl esters with Et3SiH/TFA results in the reductive deprotection of the ester and formation of isobutane. The yields of the isobutane are not recorded, but the acids are obtained nearly quantitatively (Eq. 150).307 In a similar manner, the lactone shown in Eq. 151 is converted into the acid in good yield.308 In like manner, the reductive deprotection of allyl esters provides the carboxylic acids in high yields.270... 

Certain catalysts promote the reduction of ketones with organosilanes. The reduction of acetophenone with Et3SiH is catalyzed by the diphosphine 65 and gives only a small amount of overreduction to ethylbenzene.377 Aryl alkyl enones and ynones are reduced to the corresponding alcohols with triethoxysilane and the titanium-based catalyst 66.378 Trichlorosilane reduces acetophenone in 90% yield with /V-formylpyrrolidinc catalysis.379... 

Naphthoquinone is reduced to 1,2,3,4-tetrahydronaphthalene with Et3SiH/TFA in 60% yield.393 Quinones can be reduced to hydroquinones in good yields with hydridosiloxanes such as TMDO with iodide present (Eq. 209).314,316,357 The reductive dehydration of a 1,3-diketone leads to an enone (Eq. 210).374... 

When carried out under standard conditions with Et3SiH/TFA, reduction of acrolein leads to a mixture of allyl alcohol, 1-propanol, and di-n-propyl ether in addition to allyl trifluoroacetate and -propyl trifluoroacetate.434 The 1,2-reduction of cinnamaldehyde with triethoxysilane in the presence of fluoride ion provides the corresponding allyl alcohol in good yields (Eq. 261). 

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... 

A single example of the reductive cyclization of allenic carbonyl compounds is reported, which employs a rhodium-based catalyst in conjunction with Et3SiH as terminal reductant.113 This protocol promotes hydrosilylation-cyclization to form both five- and six-membered rings with exceptional levels of yy -diastereocontrol. As revealed... 

Reduction of ethynyl lactols obtained from 51a with Et3SiH-Lewis acid... 

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