Boron trifluoride triethylsilane

Triethylsilane/boron trifluoride etherate in chloroform at room temperature reduces only the benzylic 12-hydroxy group of the polyfunctional compound 36 to form ( )-homochelidonine 37 in 92% yield (Eq. 44).138... 

ETHERS m-Chloroperbenzoic acid. Thallium(l) ethoxide. Triethylsilane-Boron trifluoride. 

Tricyclo[3.2.1.02,71octanes, 225 Tricyclo-eka-santalol, 24 Triethylaluminum, 415 Triethylammonium hydrofluoride, 415-416 Triethylborane, 416-417 Triethyl orthoacetate, 417 Triethyloxonium tetrafluoroborate, 417 Triethylsilane-Boron trifluoride, 418 Trifluoroacetic acid, 418 Trifluoroacetyl chloride, 419... 

However, this sequence can be reversed. - Thus, the activated cyclopropane can be de-protonated by lithium diisopropylamide, reacted with an appropriate ketone and opened by various methods such as treatment with acid or desilylation with fluoride. Using this reaction sequence, y-lactones 52 with various substituents can be obtained by the intramolecular attack of the ketone oxygen on the siloxy-substituted carbon followed by oxidation with pyridinium chlorochromate. The cyclic hemiacetal intermediates 53 can be converted to the tetrahyd-rofuran derivatives 55 by deoxygenation with triethylsilane/boron trifluoride. 

Triethyl phosphite, 170, 373,501 Triethyl phosphonocrotonate, 337 Triethylsilane-Boron trifluoride, 501 Triethylsilane-Trifluoroacetic acid, 502... 

Similarly, and in contrast to the behavior of its secondary isomer, 2-adaman-tanol, 1-adamantanol undergoes smooth, quantitative reduction to adamantane in less than an hour at room temperature in dichloromethane solution containing triethylsilane under the catalysis of either free boron trifluoride129 or boron trifluoride etherate (Eq. 13).143... 

Benzyl Alcohols. Benzyl alcohols of nearly all kinds undergo reduction when treated with acid in the presence of organosilicon hydrides. The most obvious exception to this is the behavior of benzyl alcohol itself. It resists reduction by the action of trifluoroacetic acid and triethylsilane, even after extended reaction times.26 Reducing systems consisting of triethylsilane and sulfuric acid/acetic acid or p-toluenesullonic acid/acetic acid mixtures also fail to reduce benzyl alcohol to toluene.134 As previously mentioned, substitution of boron trifluoride for trifluoroacetic acid results in the formation of modest yields of toluene, but only when a very large excess of the silane is used in order to capture the benzyl cation intermediate and suppress Friedel-Crafts oligomerization processes.129,143... 

A variety of para-substituted 2-phenyl-2-butanols undergo quick and efficient reductions to the corresponding 2-phenylbutanes when they are dissolved in dichloromethane and a 2-10% excess of phenylmethylneopentylsilane and boron trifluoride is introduced at 0° (Eq. 30).126 Several reactions deserve mention. For example, when R = CF3, use of trifluoroacetic acid produces no hydrocarbon product, even after two hours of reaction time. In contrast, addition of boron trifluoride catalyst provides an 80% yield of product after only two minutes. When R = MeO, both trifluoroacetic acid and boron trifluoride produce a quantitative yield of the hydrocarbon within two minutes. However, when R = NO2, attempts to promote the reduction with either trifluoroacetic acid or even methanesulfonic acid fail even after reaction periods of up to eight hours, only recovered starting alcohol is obtained. Use of boron trifluoride provides a quantitative conversion into 2-(/ -nitrophenyl)butane after only ten minutes. It is significant that the normally easily reducible nitro group survives these conditions entirely intact.126129 Triethylsilane may be used as the silane.143... 

Treatment with triethylsilane and boron trifluoride etherate allows a variety of methyl (i-hydroxy-/3-ary lpropionates to be reduced to methyl ft -ary lpropionates in yields of 85-100% as part of a synthetic sequence leading to the preparation of indanones (Eq. 31).170 Small amounts of dehydration products formed simultaneously are reduced to the methyl -arylpropionates by mild catalytic hydrogenation.170... 

Treatment of l-[2-(2-methoxy-5-isopropylphenyl)-l-hydroxyethyl]-2,6,6-tri-methylcyclohexene with triethylsilane and boron trifluoride etherate in dichloro-methane at —10° leads to its reduction to 2-(2,6,6-trimethyl-l-cyclohexenyl)-l-(2-methoxy-5-isopropylphenyl)ethane in 69% yield (Eq. 36).174... 

Highly diasteroselective and chemoselective reductions may be performed on the hydroxy functions of (r/6-arene)-tricarbonylchromium complexes. Treatment of the chromium-complexed benzylic alcohol 29 with triethylsilane and boron trifluoride etherate in dichloromethane at —78° to 0° gives only diastereomer 30 in 75% yield (Eq. 40).181 In a similar fashion, treatment of the complexed exo-allyl-endo-benzylic alcohol 31 with an excess of Et3SiH/TFA in dichloromethane at room temperature under nitrogen produces only the endo-aflyl product 32 in 92% yield after 1.5 hours (Eq. 41). It is noteworthy that no reduction of the isolated double bond occurs.182... 

The combination of boron trifluoride etherate and triethylsilane can cause the reduction of tertiary fluoride centers even in polyfunctional compounds (Eq. 55).194... 

Partial reduction of polyarenes has been reported. Use of boron trifluoride hydrate (BF3 OH2) as the acid in conjunction with triethylsilane causes the reduction of certain activated aromatic systems 217,262 Thus, treatment of anthracene with a 4-6 molar excess of BE3 OH2 and a 30% molar excess of triethylsilane gives 9,10-dihydroanthracene in 89% yield after 1 hour at room temperature (Eq. 120). Naphthacene gives the analogously reduced product in 88% yield under the same conditions. These conditions also result in the formation of tetralin from 1-hydroxynaphthalene (52%, 4 hours), 2-hydroxy naphthalene (37%, 7 hours), 1-methoxynaphthalene (37%, 10 hours), 2-methoxynaphthalene (26%, 10 hours), and 1-naphthalenethiol (13%, 6 hours). Naphthalene, phenanthrene, 1-methylnaphthalene, 2-naphthalenethiol, phenol, anisole, toluene, and benzene all resist reduction under these conditions.217 Use of deuterated triethylsilane to reduce 1-methoxynaphthalene gives tetralin-l,l,3-yielding information on the mechanism of these reductions.262 2-Mercaptonaphthalenes are reduced to 2,3,4,5-tetrahydronaphthalenes in poor to modest yields.217 263... 

The reduction of aldehydes with the combination Et3SiH/BF3 OEt2 gives both the alcohol and the symmetrical ether,70 as do the Et3SiH/TFA (and other acids) combinations.313 Addition of boron trifluoride etherate to a mixture of 1-octanal and triethylsilane leads to the formation of di-n-octyl ether in 66% yield and //-octyl alcohol in 34% yield (Eq. 155).74... 

They offer the advantage that reductions can be effected under conditions that permit the conversion of substrates that may be adversely sensitive to the presence of strong Brpnsted acids. For example, in the presence of a 10% excess of triethylsilane, addition of one-half equivalent of boron trifluoride etherate to octanal results, within one hour, in the formation of a 66% yield of dioctyl ether after a basic hydrolytic workup. Benzaldehyde provides a 75% yield of dibenzyl ether under the same reaction conditions. The remainder of the mass is found as the respective alcohol.70 Zinc chloride is also capable of catalyzing this reaction. With its use, simple alkyl aldehydes are converted into the symmetrical ethers in about 50% yields.330... 

Reductive Thiolation. Treatment of aldehydes with triethylsilane, thiols, and boron trifluoride monohydrate 217 yields sulfides in a one-flask process. For example, this method gives a 97% yield of benzyl isopropyl sulfide from benzaldehyde and 2-propanethiol (Eq. 204).365... 

Treatment of a pentacyclic la, I I -(2-oxethano) thioketal steroid with excess Et3SiH/TFA causes reduction of the carbon-carbon double bonds as well as the 17-carbonyl group to give a single reaction product (Eq. 213).368 Other work utilizes trifluoroacetic acid, triethylsilane, and anisole in the presence of a catalytic amount of boron trifluoride etherate to reduce the acetyl carbonyl of a 3-acetyl-2-azetidinone derivative with a dr of 8 1 (Eq. 214).395... 

In a similar way, a mixture consisting of 2% boron trifluoride etherate in trifluoroacetic acid and triethylsilane brings about the regioselective reduction of the acyclic carbonyl group of the diketovinyl chloride shown in Eq. 215 in high yield (>94%), but with formation of approximately equal amounts of the two possible diastereomers formed from the creation of a new chiral center.396... 

Triethylsilane in the presence of boron trifluoride [772] or trifluoroacetic acid [777] also reduced the aldehyde group to a methyl group. 

For the reduction of aliphatic ketones to hydrocarbons several methods are available reduction with triethylsilane and boron trifluoride [772], Clemmensen reduction [160, 758] (p. 28), Wolff-Kizhner reduction [280, 281, 759] (p. 34), reduction of p-toluenesulfonylhydrazones with sodium borohydride [785], sodium cyanoborohydride [57i] or borane [786] (p. 134), desulfurization of dithioketals (jaeicaipioles) [799,823] (pp. 130,131) and electroreduction [824]. 

A more useful way of reducing esters to ethers is a two-step procedure applied to the reduction of lactones to cyclic ethers. First the lactone is treated with diisobutylaluminum hydride in toluene at —78°, and the product - a lactol - is subjected to the action of triethylsilane and boron trifluoride etherate at —20° to —70°. y-Phenyl-y-butyrolactone was thus transformed to 2-phenyltetrahydrofuran in 75% yield, and 5-lactone of 3-methyl-5-phenyl-5-hydroxy-2-pentenoic acid to 4-methyl-2-phenyl-2,3-dihydropyran in 72% yield [1034]. 

Some reductions that are not possible with triethylsilane and trifluoreacetic acid (5, 695 6, 616) can be effected with triethylsilane and boron trifluoride hydrate. Although benzene, naphthalene, and phenanthrene are not reduced, anthracene and naphthacene are reduced to tetrahydro derivatives in high yield. 1- or 2-Hydroxynaphthalene is reduced to tetralin in moderate yield. ... 

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