Triethylsilane, reducing agent

The reduction is general for a variety of substituted benzophenones Such substituents as CH3 OH, OCH3, F, Br. N(CH3)2, NO2. COOH, COOCH3, NHCOC Hreaction conditions and do not alter the course of the reduction Diarylmethanols are reduced to diarylmethanes under the same conditions and probably are the intermediates in the reduction of ketones [26] Triethylsilane also can be used as a reducing agent in trifluoroacetic acid medium [27J This reagent is used for the reduction of benzoic acid and some other carboxylic acids under mild condiUons (equation 14) Some acids (phthalic, sue cinic, and 4-nitrobenzoic) are not reduced under these conditions [27]... 

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... 

Reductive Amidation of Aldehydes. The reductive amidation of aldehydes using an organosilane as the reducing agent has been realized. Benzaldehyde reacts over a 74-hour period with triethylsilane and acetonitrile in 75% aqueous sulfuric acid at room temperature to produce an 80% isolated yield of N-benzylacetamide (Eq. 169).313 Octanal fails to react under the same conditions.313 Reductive amidation of aldehydes also occurs with the reagent combination Et3SiH/TFA/primary amide (Eq. 170).326... 

The use of trimethylsilyl-based electrophilic catalysts with organosilicon hydrides also promotes the conversion of aldehydes into ethers and avoids the need to employ the potentially hazardous trityl perchlorate salt.314,334,338 One reagent pair that is particularly effective in the reductive conversion of aldehydes into symmetrical ethers is a catalytic amount of trimethylsilyl triflate combined with either trimethylsilane, triethylsilane, PMHS,334 or 1,1,3,3-tetramethyldisiloxane (TMDO, 64) as the reducing agent (Eq. 179).314 Either... 

Reduction of the ketone carbonyl of cis-1, 2,3,4,4a,9b-hexahydro-8-hydroxydi-benzofuran-3-one with trifluoroacetic acid and triethylsilane at 0° produces a mixture of the a- and /3-isomers of the C3 alcohol with an u / ratio of 1 4 (Eq. 211).394 This result can be compared with the isomer ratio of 100 1 that results when sodium borohydride is used as the reducing agent.394 The same cis pair of alcohol isomers is formed in 77% combined yield, but in a reversed ratio of a /3 = 4 1, when the less saturated tetrahydrodibenzofuran analog is used as the substrate (Eq. 212).394... 

A reduction of symmetrically substituted dihydropyridines 358 to the corresponding hexahydro derivatives 359 can be executed at 50 °C under the action of a mild reducing agent —triethylsilane [307]. At room temperature, on the other hand, the reaction stops at the formation of tetrahydro derivatives 360 (Scheme 3.95). Both processes are stereoselective. 

A convenient two-step method for tetrahydropyran synthesis from tetrahydropyran-2-ones can be achieved by using a titanocene complex in the presence of a stoichiometric reducing agent, followed by treatment with Amberlyst 15 and triethylsilane <1998JOC2360>. 

A related (tandem) SNlreduction will be introduced in Figure 17.72. There a benzyl(tri-ethylsilyl) ether undergoes reduction by triethylsilane as a reducing agent and trifluoroacetic acid as a Bronsted acid. 

Fig. 17.72. Ionic hydrogena-tion/hydrogenolysis of an aromatic ketone (meto-nitroace-tophenone). CF3C00H causes a reversible protonation of the ketone to the ion A. The reducing agent triethylsilane then transfers a hydride ion onto A to form a benzylic alcohol. This alcohol presumably is silylated, protonated, and converted into the benzyl cation B. A second hydride transfer yields the final product.

A selective reducing agent for replacement of halogens by hydrogen, triphenylstannane (triphenyltin hydride), reduces aryl bromides to arenes in high yields (Table 4). Triethylsilane (triethylsilicon hydride) converts bromobenzene to benzene in the presence of 10% palladium on carbon in 79% yield (Table 4). ... 

An acid promoted hydrolysis is usually slower than a similar base promoted hydrolysis. In some cases, as with a z-butyl ester, however, acid catalyzed hydrolyses are preferred. With the Z-butyl ester, sometimes a reducing agent such as formic acid or triethylsilane is added to scavenge the carbocation. 

Stereoselective reduction of bicyclic ketals. A new route to 6- and 8-membercd cyclic ethers involves stereoselective reduction of the bicyclic ketals such as 2. Depending on the reducing agent, the bicyclic ketal 2 can be reduced to either the cis- or trans-cyclic ether 3. Thus reduction of 2 and 4 with triethylsilane and TiCl4 at -78° yields the ci.s-disubstituted cyclic ethers 3 and 5 with high stereoselectivity. Reduction of 2 and 4... 

As with the similarly polarized boron and aluminium hydrides, hydrido-silanes can transfer formal hydride ions to electropositive carbon centres. Unlike the first two reducing agents, hydridosilanes require additional activation of the carbon centre by Lewis or protic acids before such hydride transfer can take place. This overall process is known as ionic hydrogenation (4). The reagent system of triethylsilane and boron trifluoride etherate has provided an extremely selective method for the reductive deoxygenation of lactols (5), derived in turn from DIBAL reduction (6) of the corresponding y- or 6-lactones ... 

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