Esters triethylsilane

Keywords a,p-Enone esters, triethylsilane, Indium(lll) chloride, trifluoroacetic acid (TEA), toluene, room temperature, 1,4-hydrosilylation, silyl enol ether, stereoselectivity... 

Carboxamides and esters of arenecarboxylic acids are obtainable directly by reacting arenediazosulfones (Ar — N2 —S02 —Ar ) with CO and amines or alcohols, respectively, in the presence of Pd catalysts (Kamigata et al., 1989). Aromatic aldehydes are formed if the reaction is carried out in the presence of triethylsilane (Kikukawa et al., 1984). In an analogous way, arenediazonium salts can be transformed into ketones (ArCO —R R = CH3, C2H5, or C6H5) in the presence of stan-nanes, R4Sn (Kikukawa et al., 1982). 

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. " ... 

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-... 

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... 

The tertiary alcohol m,m,/ra ,v-perhydro-9h-phcnalcnol (7) is converted stereospecifically and in high yield (92%) to /ran.v,/ran.v,/ran.v-pcrhydrophcnalcnc (10) when treated with either triethylsilane or triphenylsilane and trifluoroacetic acid in dichloromethane (Eq. 15). Studies indicate that the reaction path follows the cation rearrangement 8 9 and that the trans trifluoroacetate ester related to... 

Triethylsilane and diethylsilane are somewhat more effective than triphenyl-silane at increasing the amount of reduced product 17.130 Yields of 17 in excess of 90% may be obtained. The remainder of the product is butenyl ester 16. Hydrogenolysis of the cyclopropyl rings does not occur under these conditions. A better yield of 17 is obtained when the reaction is carried out at —15° than at room... 

The triethylsilane/trifluoroacetic acid reagent system reduces alkenes to alkanes in poor to excellent yields depending largely on the ability of the alkene to form carbocations upon protonation. Under these conditions the more substituted olefins are reduced in better yields and styrene double bonds are reduced in high yields.127,202,207,221-228 The reduction of 1,2-dimethylcyclohexene with this reagent gives a mixture of cis- and trans- 1,2-dimethylcyclohexane.229 The formation of the trifluoroacetate esters is a side reaction.205,230... 

Surprisingly, a-cyanoacrylic acid is reported to react spontaneously with triethylsilane in the absence of any additional acid to give a quantitative yield of the triethylsilyl ester of a-cyanopropionic acid.236 Ethyl a-cyanoacrylate requires the presence of trifluoroacetic acid to undergo reduction to ethyl 2-cyanopropionate.236 Many of these reductions are highly stereoselective. For example, treatment of... 

Treatment of progesterone with trifluoroacetic acid and triethylsilane in dichloromethane followed by saponification of the mixture of the trifluoroacetate ester intermediates of 5-/3-pregnane-3a,20/3-diol and 5-j6-pregnane-3a,20a-diol and Jones oxidation yields 5-/3-pregnanedione in 65% yield (Eq. 81).238... 

Hydrogenation of the carbon-carbon double bond occurs without alteration of the ester function when citronellyl acetate is treated with 2.5 equivalents of trifluoroacetic acid and two equivalents of triethylsilane in 2-nitropropane.205 The reduced product is obtained in 90% yield after 22 hours at room temperature in the presence of one equivalent of added lithium perchlorate (Eq. 82). The yields are lower in the absence of this added salt. Similar reduction of an unsaturated phenolic chroman derivative occurs to give an 85% yield of product with only the carbon-carbon double bond reduced (Eq. 83).205... 

Homoconjugation results in enhanced reactivity of substrates toward ionic hydrogenation. Bicyclo[2.2.1]hepta-2,5-diene forms a mixture of the trifluoroac-etate esters of bicyclo[2.2.1]hepten-2-ol, tricyclo[2.2.1.02 6]heptan-3-ol, and bicyclo[2.2.1]heptan-2-ol in a 62 20 17 ratio on treatment with 10 equivalents of triethylsilane and 20 equivalents of trifluoroacetic acid for 24 hours at room temperature (Eq. 96), 230... 

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]. 

Reduction of the keto group in naphtho derivative 115 with sodium borohydride results in 69% of the alcohol 116 (Scheme 23, Section 2.1.3.3 (1999PHA645)). Further triethylsilane reduction gives 117 in 67% yield. Synthesis of a series of pyrrolo-benzazepine and pyrrolo-benzothiazepine acetic acids (Scheme 77, Section 5.1.1 (1994MI385)) includes reduction of ketoesters 380 into corresponding hydroxyl esters, subsequent deoxygenation with iodine/PPhs and hydrolysis. 

Alternatively, the imide-acid chloride is reacted with methanol to give the imide ester which, after borohydride reduction and triethylsilane/trifluoroacetic acid treatment, furnishes the bicyclic lactam 6 as a racemate. The latter is acylated with either propanoyl chloride or 3-phcnylpropanoyl chloride and the resulting amides 7 deprotonated and alkylated with (bro-momethyljbenzene or iodomethane, respectively, to give the major alkylation products 8 with d.r. >98 2 and in 65% yield3. 

Dichloro-2,2-difluoroethylene, 105 (Diethylamino)sulfur trifluoride, 110 Reduction reactions (see also Deoxygenation, Reductive. . . ) of acetals and ketals Dibromoalane, 237 Diisobutylaluminum hydride, 237 Triethylsilane-Tin(IV) chloride, 237 of acetates and other esters to alkanes Nickel boride, 197 Triphenylsilane, 334 of acyl halides to alcohols Sodium cyanoborohydride-Tin(II) chloride, 280... 

J. Nicholas Kirwan, B. P. Roberts, and C. R. Willis, Deoxygenation of alcohols by the reactions of their xanthate esters with triethylsilane An alternative to tributyltin hydride in the Barton-McCombie reaction, Tetrahedron Lett., 31 (1990) 5093-5096. 

The dimethyl esters were reduced with an excess of triethylsilane in trifluoroacetic acid72 giving essentially equimolar quantities of each of the required protected kainoid analogues 112 to 115 and their corresponding C-4 epimers 116 to 119 in reasonable overall yields after chromatographic separation (Scheme 48 and Table 9). It is interesting to note here that isomers 112 to 115 were all found to be less polar than their corresponding C-4 epimers 116 to 119. 

A reduction with triethylsilane, which is a pure SN1 reaction, is familiar from Figure 17.62, namely as the second step of a two-step reduction of a carboxylic acid ester to a simple ether via an oc-acetoxyether. 

The main product in hydrosilation of a,P-unsaturated ketones and aldehydes catalyzed by chloro-platinic acid, platinum on alumina, or metallic nickel is the corresponding silyl enol ether. With nickel catalyst, product distribution is highly dependent on the enone structure. Hydridosilanes add to a, -unsaturated esters, producing the corresponding silyl enolate as well as carbon silylated products. The course of addition depends on substrate structure and the hydridosilane utilized. Thus, triethylsilane undergoes 1,4-addition to methyl acrylate in the presence of chloroplatinic acid, while trichlorosilane with either chloroplatinic acid or Pt/C gives the -silyl ester (Scheme 65). ... 

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