Ionic reduction with triethylsilane

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%). ... 

Ionic hydrogenation has been referred to in CHEC-I <84CHEC-I(4)741>. Logically, this has to find a place in this section, since it represents reaction of a thiophenium ion with a hydride ion derived from triethylsilane. This has been reviewed <85HC(44/i)457>. Reduction of 5-hydroxy-benzo[ >]thiophene with triethylsilane in TEA has given the 2,3-dihydroderivative <91SC959>. 

Reduction of dienes incorporated into steroid structures may lead to different configurations in the products. For example, treatment of 8(9),14(15)-bisdehydroestrone 42 (R = H) for four hours at room temperature with twenty equivalents of trifluoroacetic acid and two equivalents of triethylsilane leads to an ionic hydrogenation product mixture containing the natural 8/1,9a,14a-estrone 43 as a minor component (11%) and the 8a,9/i, l 4/i-isomcr 44 as the major component (83%) (Eq. 92).241 The related methyl ether (42, R = Me) behaves in a similar fashion.241 The yield of natural isomer 46 formed from the methyl ether of A8(9),i4(i5)-bigdehydroestradiol analog 45 increases from 22 to 34%, and that of... 

Synthesis of standards. Friedel-Crafts acylation of 2,3-dimethylthiophene with hexadecanoic acid and a subsequent reduction of the ketone with LiAlD4 yielded the 2,3-dimethyl-5-(l ,l -d2-hexadecyl)thiophene (II, Table II). Experimental details of these reactions have been reported elsewhere (10). Ionic hydrogenation of the 2,3-dimethyl-5-(l ,l -d2-hexadecyl)thiophene in trifluoroacetic acid using triethylsilane and BF3.etherate as catalyst yielded 2,3-dimethyl-5-(l ,l -d2-hexadecyl)thiolane (IV, Table II). For... 

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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