Reductions with trialkylsilanes

Similarly, trimethylsilyl inflate can be used as a catalyst for the alkylation of 2 methoxy 1,3-oxazolidines [104] or 1-acetoxyadamantane [105] with allylsilane and for the reduction of acetals to ethers with trialkylsilanes [106]... 

Diastereoselective reduction of ketones. Trialkylsilanes can reduce aldehydes and ketones in the presence of a catalytic amount of tetrabutylammonium fluoride in HMPT (cf. reductions with activated hydrosilanes, 11, 554). Under these conditions esters and nitriles are not reduced. The reaction is stereoselective. Thus 2-methylcyclohexanone is reduced selectively to ds-2-methylcyclohexanol the cis-selectivity depends on the bulkiness of the hydrosilane, being highest (95%) with triphenylsilane. 

Simple saturation of the ring can be achieved using ionic hydrogenation ," i.e. a combination of a trialkylsilane and acid, usually trifluoroacetic the reduction proceeds via a sequence of proton then hydride additions" and consequently requires electron-releasing substituents to facilitate the first step. 2,5-Dihydro-products accompany tetrahydrothiophenes as products of reductions with zinc and trifluoroacetic acid."" ... 

The reduction of a,p-unsaturated aldehydes or ketones with trialkylsilanes has proved a valuable reaction for the regioselective preparation of silyl enol ethers. Wilkinson s catalyst [(Ph3P)3RhCl] is suitable to promote the reaction and subsequent hydrolysis provides the saturated carbonyl compound, as illustrated by the... 

Boron trifluoride has been found to be an excellent catalyst for the reduction of aldehydes and ketones with trialkylsilanes." ... 

I 6 PSiP Transition-Metal Pincer Complexes Synthesis, Bond Activation, and Catalysis Table 6.2 Reduction of CO with trialkylsilanes. ... 

Ethers have also been prepared by the reductive dimerization of two molecules of an aldehyde or ketone (e.g., cyclohexanone dicyclohexyl ether). This was accomplished by treatment of the substrate with a trialkylsilane and a catalyst. ... 

Trialkylsilanes are generally more effective than dialkyl- or monoalkylsilanes in minimizing isomerizations. The reduction of 2-bromododecane to dodecane proceeds under aluminum chloride catalysis in 82% yield using n-butylsilane and in 87% yield with tri-n-butylsilane.186 However, similar treatment of bromo-cycloheptane with triethylsilane yields a mixture of 39% cycloheptane and 26% methylcyclohexane. The same substrate yields 65% methylcyclohexane and less than 1% cycloheptane when n -butylsilane is the reducing agent.186... 

Diphenylsilane catalyzed by various salts promotes the 1,2-reduction of cinnamaldehyde.318 Cesium fluoride catalysis is particularly effective.320 It is possible to stop these reactions at the silyl ether stage.73,320 The 1,2-reduction of citral is accomplished in high yield with diphenylsilane and Wilkinson s catalyst (Eq. 262) 435 Interestingly, the trialkylsilanes, ethyldimethylsilane and triethylsilane, give high yields of the 1,4-reduction product whereas diisopropylsilane gives a 1 1 mixture of 1,2- and 1,4-reduction (Eq. 263)435... 

The reduction of alcohols with a trialkylsilane in the presence of a protic acid can be complicated by skeletal rearrangement and alkene formation as a result of carbonium ion formation. This problem is significantly reduced when using BF3 as the acid (Eq. 85) [140]. Peptide isosteres (Eq. 86) can be prepared by the alkylative ehmina-tion of y-oxygenated-a,y3-unsaturated carboxylates by use of BF3 complexes of alkyl-copper reagents [141]. 

A very convenient and versatile new reduction method, which has been intensively studied for the reduction of thiophenes (vide infra), makes use of the combination of a trialkylsilane and an acid, usually trifluoroacetic acid (TFA). This so-called ionic hydrogenation provides a novel reduction of furans to tetrahydrofurans (Scheme 3). Although Bolestova et al. find that the conditions are critical in order to avoid polymerization, the yields of tetrahydrofurans can be quite good. The reduction is accelerated by the presence of boron trifluoride etherate but fails with carbonyl-substituted furans, such as methyl pyro-mucate. ... 

An extremely powerful and efficient new reduction of thiophenes to tetrahydrothiophenes involves ionic hydrogenation with a trialkylsilane and an acid. Typically, a mixture of triethylsilane and trifluo-roacetic acid is employed (equations 26 and 27). This reduction is reasonably general, although thiophene itself is poorly reduced (equation 26) and 2,5-diphenylthiophene is inert to these conditions. Not surprisingly, 2-benzoylthiophene is converted to 2-benzyltetrahydrothiophene (75%). ... 

Carbonyl replacement dominates the observed photoreactions of Co complexes, and with CO-like ligands, total substitution is possible. Table 1 lists examples. The complex CofCOjNO is unusual in that the primary photoprocess is a linear to bent NO transformation, followed by an associative displacement of CO from this shortlived intermediate. Photolysis in CHClj leads to incorporation of chloride in the complex if PPhj is present, then Co(PPh3)2Cl2 is formed " if NO is present then [Co(NO)2C1 j is generated Trialkylsilane exchange in RjSiCo(CO) (Table 1) proceeds via oxidative addition of RjSiH to the intermediate formed by CO loss, followed by reductive elimination of R SiH. This is true for surface-confined analogues as well d. 

The Barton deoxygenation (or Barton-McCombie deoxygenation) is a two-step reaction sequence for the reduction of an alcohol to an alkane. The alcohol is first converted to a methyl xanthate or thioimidazoyl carbamate. Then, the xanthate or ihioimidazoyl carbamate is reduced with a tin hydride reagent under radical conditions to afford the alkane. Trialkylsilanes have also been used as the hydride source. Reviews (a) McCombie, S. W. In Comprehensive Organic Synthesis Trost, B. M. Fleming, I., Eds. Pergamon Press Oxford, U. K., 1991 Vol. 8, Chapter 4.2 Reduction of Saturated Alcohols and Amines to Alkanes, pp. 818-824. (b) Crich, D. Quintero, L. Chem. Rev. 1989, 89, 1413-1432. 

Although trialkylsilanes are obtained by the lithium aluminum hydride-reduction of trialkylhalosilanes, they are conveniently prepared by the alkylation of trichlorosilane with Grignard reagents (eq (63)) [58]. 

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