Ethers triethylsilane

A neat mixture of the /l-unsaturated ketone (10mmol), triethylsilane (11 mmol), and tris(triphenylphosphine)rhodium(i) chloride (0.01 mmol) was stirred at 50°C for 2h, and the product silyl enol ether was distilled directly (yields 90-98%). 

To a solution of 4-t-butylcyclohexanone (lmmol), tris(triphenylphos-phine)ruthenium(n) chloride (0.05 mmol) and silver trifluoroacetate (0.05 mmol) in toluene (5 ml) was added triethylsilane (1.5 mmol). The mixture was heated under reflux for 20 h, and concentrated under reduced pressure. The residue was diluted with hexane (3 ml), filtered and distilled to give a mixture of triethylsilyl ethers (0.96mmol, 96%), b.p. 70°CI 0.1 mmHg. G.l.c. analysis shows an axial (cis) equatorial (trans) ratio of 5 95—a result comparable to the best LAH results. 

Noell et al. reported the preparation of silica-poly(ether ether ketone) hybrid materials with improved physical properties.155 An amine-end-capped poly(ether ether ketone) was used to react with isocyanatopropyltriethoxysilane in tetrahydrofuran (THF). The triethylsilane-end-capped poly (ether ether ketone) was mixed with tetraethoxysilane (TEOS) in THF. Quantitative amounts of water were introduced into die system, and the mixture was reduxed at 80°C. The entire reaction mixture was allowed to further react in Tedon molds. Tough transparent materials were obtained by diis approach. 

Aldehydes and ketones can be converted to ethers by treatment with an alcohol and triethylsilane in the presence of a strong acid or by hydrogenation in alcoholic acid in the presence of platinum oxide. The process can formally be regarded as addition of ROH to give a hemiacetal RR C(OH)OR", followed by reduction of the OH. In this respect, it is similar to 16-14. In a similar reaction, ketones can be converted to carboxylic esters (reductive acylation of ketones) by treatment with an acyl chloride and triphenyltin hydride. " ... 

Additions of silylated ketene acetals to lactones such as valerolactone in the presence of triphenylmethyl perchlorate in combination with either allyltrimethylsilane 82, trimethylsilyl cyanide 18, or triethylsilane 84b, to afford substituted cyclic ethers in high yields have already been discussed in Section 4.8. Aldehydes or ketones such as cyclohexanone condense in a modified Sakurai-cyclization with the silylated homoallylic alcohol 640 in the presence of TMSOTf 20, via 641, to give unsaturated cyclic spiro ethers 642 and HMDSO 7, whereas the 0,0-diethyllactone acetal 643 gives, with 640, the spiroacetal 644 and ethoxytrimethylsilane 13b [176-181]... 

On reacting aldehydes such as benzaldehyde or cyclohexanecarboxaldehyde 720 with silylated alcohols such as 718 or 721, or with triethylsilane 84b in the presence of TMSOTf 20 at low temperatures, acetal formation and reduction is achieved in one step to afford ethers 719 and 722 in high yields [215] (Scheme 5.74). 

A catalytic amount of TMSOTf 20 (0.1 mmol) is added to a stirred ice-cold solution of 1.2 mmol benzaldehyde and 1 mmol y-benzyloxy-y-butyrolactone 718 in 10 mb CH2CI2. After 1 h at 0°C, 1 mmol triethylsilane 84b is added at 0°C and the mixture is left to warm to room temperature. After 12 h the reaction mixture is diluted with ether, washed with aqueous NaHCOs solution, dried over MgS04, evaporated, and chromatographed on a Si02 column with pentane-ether and ether to give the benzyl ether 719 in 89% yield [215] (Scheme 5.95). 

Esters are reduced by PhSiHs, in the presence of Ph3P (C0)4MnC(0)Me 1847 as catalyst in benzene at room temperature, to give the ethers 1848 [75]. Caprolac-tone gives rise to 69% oxepane [75] (Scheme 12.20). Ethylthio esters such as 1849 and 1851 are reduced in high yields by triethylsilane 84b and 10% Pd/C in ace-... 

Whereas primary amides such as butyric acid amide, on heating to 140-150 °C with triethylsilane 84b and ZnCl2, give, e.g., 78% butyronitrile 1853 and 95% HMDSO 7 [79], the secondary amide benzanilide is readily converted into 90% O-triethylsilyl imino ether 1854 [80] whereas the tertiary amide N,N-diethylacetamide... 

Combined use of Co(acac)2 and DiBAlH also gives selective reduction for a,(3-unsaturated ketones, esters, and amides.112 Another reagent combination that selectively reduces the carbon-carbon double bond is Wilkinson s catalyst and triethylsilane. The initial product is the enol silyl ether.113... 

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... 

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... 

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... 

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

Alkyl iodides, benzyl chlorides, benzyl bromides, and adamantyl bromides and iodides undergo reduction with triethylsilane/palladium chloride.195 The reduction of a /3-chloro ether occurs in excellent yield with this system (Eq. 56).195... 

Examples of the behavior of other substituted vinyl substrates upon exposure to the action of trifluoroacetic acid and triethylsilane are known. For example, -butyl vinyl ether, when reacted at 50° for 10 hours, gives -butyl ethyl ether in 80% yield (Eq. 65).234 In contrast, -butyl vinyl thioether gives only a 5% yield of n-butyl ethyl sulfide product after 2 hours and 15% after 20 horns of reaction.234 It is suggested that this low reactvity is the result of the formation of a very stable sulfur-bridged carbocation intermediate that resists attack by the organosilicon hydride (Eq. 66). 

Treatment of A 8(9)-dehydroestradiol with trifhioroacetic acid and triethylsilane gives estradiol in 96% yield (Eq. 86).239 The 3-methyl ether is similarly reduced to the 3-methyl ether of estradiol in >50% yield.239 The structurally related 18-ethyl and 18-propyl 17-keto compounds experience reduction of the A8(9) function in excess of 70% yield without concomitant reduction of the 17-keto... 

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... 

An interesting hydroiodination reaction occurs when a mixture of cyclohexene and triethylsilane in dichloromethane is treated with a mixture of bis(pyridine) iodonium tetrafluoroborate and tetrafluoroboric acid in diethyl ether (Eq. 125). A 50% yield of iodocyclohexane is produced after one hour at 20°.268... 

The combination Et3SiH/(C6F5)3B reduces acid chlorides to methyl groups (Eq. 138).281,282 If a smaller amount of triethylsilane is used, the same combination reduces aryl acid chlorides to the trimethylsilyl ethers of the benzyl alcohols.281,282... 

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... 

Unsymmetrical ethers may be produced from the acid-promoted reactions of aldehydes and organosilicon hydrides when alcohols are introduced into the reaction medium (Eq. 173).327,328 An orthoester can be used in place of the alcohol in this transformation.327 335 A cyclic version of this conversion is reported.336 Treatment of a mixture of benzaldehyde and a 10 mol% excess of triethylsilane with methanol and sulfuric, trifluoroacetic, or trichloroacetic acid produces benzyl methyl ether in 85-87% yields.328 Changing the alcohol to ethanol, 1-propanol, 2-propanol, or 1-heptanol gives the corresponding unsymmetrical benzyl alkyl ethers in 45-87% yield with little or no side products.328 A notable exception is the tertiary alcohol 2-methyl-2-propanol, which requires 24 hours.328 1-Heptanal gives an 87% yield of //-lie ply I methyl ether with added methanol and a 49% yield of benzyl n-heptyl ether with added benzyl alcohol under similar conditions.328... 

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... 

Superior yields of ethers from aldehydes are obtained by the use of several other electrophilic species. The addition of 5 mol% of trityl perchlorate to a mixture of triethylsilane and 3-phenylpropanal in dichloromethane at 0° produces an 83% yield of bis-(3-phenylpropyl) ether within 10 minutes (Eq. 176),329 Reductive polycondensation of isophthalaldehyde occurs with two equivalents of triethylsilane in the presence of 10 mol% of trityl perchlorate to give 40-72% yields of polyether with average molecular weights ranging from 6,500 to 11,400 daltons (Eq. 177).337 Addition of one equivalent of an alkoxytrimethylsilane to the reaction mixture produces unsymmetrical ethers in good to excellent yields. Thus, a mixture of (ii)-cinnamaldehyde, 3-phenylpropoxytrimethylsilane, and triethylsilane in dichloromethane reacts under the influence of a catalytic amount of trityl perchlorate to give the unsymmetrical ether in 88% yield (Eq. 178).329... 

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... 

Equivalent amounts of aldehydes and alkoxytrimethylsilanes react to form unsymmetrical ethers in near quantitative yields in the presence of either trimethylsilane or triethylsilane and catalytic amounts (ca. 10 mol%) of TMSI in dichloromethane.329,333,334,341 The procedure is particularly convenient experimentally when trimethylsilane is used with TMSI because the catalyst provides its own color indicator for the reduction step (color change from deep violet to vivid red-gold) and the only silicon-containing product following aqueous workup is the volatile hexamethyldisiloxane (bp 99-100°). It is possible to introduce trimethylsilane (bp 7°) either as a previously prepared solution in dichloromethane or by bubbling it directly into the reaction mixture. Cyclohexyloxytrimethylsilane and n-butanal react by this method to give a 93% isolated yield of n-butyl cyclohexyl ether (Eq. 183).334... 

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