Reduction silane reductant

A traditional method for such reductions involves the use of a reducing metal such as zinc or tin in acidic solution. Examples are the procedures for preparing l,2,3,4-tetrahydrocarbazole[l] or ethyl 2,3-dihydroindole-2-carbox-ylate[2] (Entry 3, Table 15.1), Reduction can also be carried out with acid-stable hydride donors such as acetoxyborane[4] or NaBHjCN in TFA[5] or HOAc[6]. Borane is an effective reductant of the indole ring when it can complex with a dialkylamino substituent in such a way that it can be delivered intramolecularly[7]. Both NaBH -HOAc and NaBHjCN-HOAc can lead to N-ethylation as well as reduction[8]. This reaction can be prevented by the use of NaBHjCN with temperature control. At 20"C only reduction occurs, but if the temperature is raised to 50°C N-ethylation occurs[9]. Silanes cun also be used as hydride donors under acidic conditions[10]. Even indoles with EW substituents, such as ethyl indole-2-carboxylate, can be reduced[ll,l2]. 

Sihca is reduced to siUcon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous siUcon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum haUdes, siUca can be converted to silane in high yields by reaction with hydrogen (15). SiUcon itself is not hydrogenated under these conditions. The formation of siUcon by reduction of siUca with carbon is important in the technical preparation of the element and its alloys and in the preparation of siUcon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and siUcate. At 800—900°C, siUca is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce siUca to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

Lithium hydride is perhaps the most usehil of the other metal hydrides. The principal limitation is poor solubiUty, which essentially limits reaction media to such solvents as dioxane and dibutyl ether. Sodium hydride, which is too insoluble to function efficiently in solvents, is an effective reducing agent for the production of silane when dissolved in a LiCl—KCl eutectic at 348°C (63—65). Magnesium hydride has also been shown to be effective in the reduction of chloro- and fluorosilanes in solvent systems (66) and eutectic melts (67). 

The versatility of lithium aluminum hydride permits synthesis of alkyl, alkenyl, and arylsilanes. Silanes containing functional groups, such as chloro, amino, and alkoxyl in the organic substituents, can also be prepared. Mixed compounds containing both SiCl and SiH cannot be prepared from organopolyhalosilanes using lithium aluminum hydride. Reduction is invariably complete. 

Reduction/Reaction with Hydrogen. Tetraduorosilane reacts with hydrogen only above 2000°C. Tetrachlorosilane can be reduced by hydrogen at 1200°C. Tetraio do silane can be reduced to sihcon at 1000°C (165). Reduction of tetraduorosilane with potassium metal to sihcon was the first method used to prepare sihcon (see Silicon and silicon alloys). The reduction of sihcon tetrachloride by ziac metal led to the first semiconductor-grade sihcon (166,167). 

Reduction of halosilanes with hydtides leads to the formation of hydtide functional silanes and is considered in that section. 

The production of sihcon tetrachloride by these methods was abandoned worldwide in the early 1980s. Industrial tetrachlorosilane derives from two processes associated with trichlorosilane, the direct reaction of hydrogen chloride on sihcon primarily produced as an intermediate for fumed sihca production, and as a by-product in the disproportionation reaction of trichlorosilane to silane utilized in microelectronics. Substantial quantities of tetrachlorosilane are produced as a by-product in the production of zirconium tetrachloride, but this source has decreased in the 1990s owing to reduction in demand for zirconium in nuclear facihties (see Nuclearreactors). The price of tetrachlorosilane varies between l/kg and 25/kg, depending on grade and container. 

A McMurry coupling of (176, X = O Y = /5H) provides ( )-9,ll-dehydroesterone methyl ether [1670-49-1] (177) in 56% yield. 9,11-Dehydroestrone methyl ether (177) can be converted to estrone methyl ether by stereoselective reduction of the C —double bond with triethyi silane in triduoroacetic acid. In turn, estrone methyl ether can be converted to estradiol methyl ether by sodium borohydride reduction of the C17 ketone (199,200). 

In an effort to identify a more stereoselective route to dihydroagarofuran (15), trimethylsilylated alkyne 17 was utilized as a substrate for radical cyclization (Scheme 2). Treatment of 17 with a catalytic amount of AIBN and tri-n-butyltin hydride (1.25 equiv) furnishes a mixture of stereoisomeric vinyl silanes 18 (72% combined yield) along with an uncyclized reduction product (13% yield). The production of stereoisomeric vinyl silanes in this cyclization is inconsequential because both are converted to the same alkene 19 upon protodesiiyiation. Finally, a diastereoselective di-imide reduction of the double bond in 19 furnishes dihydroagaro-... 

The oxidative addition of silanes (with silicon-hydrogen bonds) to coordinatively unsaturated metal complexes is one of the most elegant methods for the formation of metal-silicon bonds. Under this heading normally reactions are considered which yield stable silyl metal hydrides. However, in some cases the oxidative addition is accompanied by a subsequent reductive elimination of, e.g., hydrogen, and only the products of the elimination step can be isolated. Such reactions are considered in this section as well. 

The reaction of thiyl radicals with silicon hydrides (Reaction 8) is the key step of the so-called polariiy-reversal catalysis in the radical chain reduction. The reaction is strongly endothermic and reversible with alkyl-substituted silanes (Reaction 8). For example, the rate constants fcsH arid fcgiH for the couple triethylsilane/ 1-adamantanethiol are 3.2 x 10 and 5.2xlO M s respectively. 

Substitution at the SiH moiety has been carried out with alkylthio groups, such as MeS and i-PrS. Tn s(alkylthio)silanes, (RSlsSiH, are radical-based reducing agents which can effect the reduction of bromides, iodides, xanthates, phenylselenides, and isocyanides in toluene, using AIBN as the initiator at 85... 

The low reactivity of alkyl and/or phenyl substituted organosilanes in reduction processes can be ameliorated in the presence of a catalytic amount of alkanethiols. The reaction mechanism is reported in Scheme 5 and shows that alkyl radicals abstract hydrogen from thiols and the resulting thiyl radical abstracts hydrogen from the silane. This procedure, which was coined polarity-reversal catalysis, has been applied to dehalogenation, deoxygenation, and desulfurization reactions.For example, 1-bromoadamantane is quantitatively reduced with 2 equiv of triethylsilane in the presence of a catalytic amount of ferf-dodecanethiol. 

Homolytic aromatic substitution often requires high temperatures, high concentrations of initiator, long reaction times and typically occurs in moderate yields.Such reactions are often conducted under reducing conditions with (TMSlsSiH, even though the reactions are not reductions and often finish with oxidative rearomatization. Reaction (68) shows an example where a solution containing silane (2 equiv) and AIBN (2 equiv) is slowly added (8h) in heated pyridine containing 2-bromopyridine (1 equiv) The synthesis of 2,3 -bipyridine 75 presumably occurs via the formation of cyclohexadienyl radicals 74 and its rearomatization by disproportionation with the alkyl radical from AIBN. ... 

When double bonds are reduced by lithium in ammonia or amines, the mechanism is similar to that of the Birch reduction (15-14). ° The reduction with trifluoro-acetic acid and EtsSiH has an ionic mechanism, with H coming in from the acid and H from the silane. In accord with this mechanism, the reaction can be applied only to those alkenes that when protonated can form a tertiary carbocation or one stabilized in some other way (e.g., by a OR substitution). It has been shown, by the detection of CIDNP, that reduction of a-methylstyrene by hydridopenta-carbonylmanganese(I) HMn(CO)5 involves free-radical addition. ... 

Silanes And Base. In the presence of bases, certain silanes can selectively reduce carbonyls. Epoxy-ketones are reduced to epoxy-alcohols, for example with (MeO)3SiH and LiOMe. ° Controlling temperature and solvent leads to different ratios of syn- and anti- products.Silanes reduce ketones in the presence of BF3-OEt2 ° and transition metal compounds catalyze this reduction. ... 

Reduction of the fill factor in order to improve the intake behavior when working in an open mixer has an additional positive effect on the silanization efficiency. A combination of both measures, silanization in an open mixer and reduction of the fill factor, leads to further improvement. 

The efficiency of the silanization reaction is increased by aU measures enhancing devolatilization of ethanol from the silica compound in the mixer. One possible measure is the reduction of the ethanol concentration in the void space of the mixer, thus increasing the driving force for mass... 

Use of highly dispersible silica together with silanes and high vinyl S-SBR has met all critical magic triangle requirements like good dry and wet traction, reduction in tread wear, and low RR. 

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