Reduction trichlorosilane reductant

The formation of silicon carbide, SiC (carborundum), is prevented by the addition of a little iron as much of the silicon is added to steel to increase its resistance to attack by acids, the presence of a trace of iron does not matter. (Addition of silicon to bronze is found to increase both the strength and the hardness of the bronze.) Silicon is also manufactured by the reaction between silicon tetrachloride and zinc at 1300 K and by the reduction of trichlorosilane with hydrogen. 

Silicon Tetrachloride. Most commercially available sihcon tetrachloride is made as a by-product of the production of alkylchlorosilanes and trichlorosilane and from the production of semiconductor-grade sihcon by thermal reduction of trichlorosilane. 

At atmospheric pressure, the conversion to trichlorosilane is limited to about 16%. The conversion of SiCl to HSiCl was found to be at equiUbrium. If contact time was greater than 45 s and the mole ratio of hydrogen to siUcon tetrachloride 1 1, then at 14 kPa (2 psi) and 550°C, the HSiCl mole fraction reached 0.7 but substantial formation of H2SiCl2 occurred (62). Enhancements in yield have been reported through preactivating the siUcon mass by removal of oxides (73) and the rapid thermal quench of the effluent gas stream (74). The reduction of siUcon tetrachloride in a plasma has also been reported (75). 

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. 

METHYL GROUPS BY REDUCTION OF AROMATIC CARBOXYLIC ACIDS WITH TRICHLOROSILANE-TRI-n-PROPYLAMINE 2-METHYLBIPHENYL... 

The next step is the hydrogen reduction of the trichlorosilane (Reaction 2 above). The end product is a poly crystalline silicon rod up to 200 mm in diameter and several meters in length. The resulting EGS material is extremely pure with less than 2 ppm of carbon and only a few ppb of boron and residual donors. The Czochralski pulling technique is used to prepare large single crystals of silicon, which are subsequently sliced into wafers for use in electronic devices.1 1... 

FIGURE 4.4 The production of polycrystalline silicon for the eleetronics industry involves several ehemieal steps aimed at the reduetion of impurities. These inelude (1) reaction of metallurgical grade silicon to produce a mixture of chlorosilanes, (2) distillation of trichlorosilane, and (3) reduction of trichlorosilane to polycrystalline silicon. Excerpted by special permission from Chemical Engineering, June 10, 1985. Copyright 1985 by McGraw-Hill, Inc., New York, NY 10020. 

More recently, P-cored derivative (116) was prepared from a straightforward combination of a Heck coupling, to afford an intermediate functionalised stil-bene phosphine oxide (114),a Horner-Wittig reaction yielding the phosphine oxide (115), and finally trichlorosilane reduction (Scheme 31) [89]. Using similar strategies, both the valence isoelectronic N- (117) and C- (118) cored dendrimers have been prepared (Scheme 31). 

In all of these cases, paUadium-catalyzed hydrosilylation proceeds via hydropalla-dation followed by reductive elimination of alkyl- and silyl group from the palladium. In the reaction of o-aUylstyrene (24) with trichlorosilane, which gives hydrosilylation products on the styrene double bond 25 and cycUzed product 26, the hy-dropalladation process is supported by the absence of side products which would result from the intermediate of the silylpaUadation process (Scheme 3-11) [37]. 

Preparation of Phosphines by Reduction.- A procedure for the reduction of phosphine oxides using trichlorosilane in inert... 

Cationic palladium complex 121 reductively coupled enynes (Eq. 20) using trichlorosilane as the stoichiometric reductant [71]. This combination of catalyst and silane afforded silylated methylenecyclopentanes such as 122 in good yield from enynes such as 123. Attempts to develop an enantioselective version of this reaction were not successful [71]. When enediyne 124 was cyclized in the presence of trichlorosilane, the reaction favored enyne cycli-zation 126 by a 3 1 ratio over diyne cyclization to 125 (Eq. 21). In contrast, when the more electron-rich dichloromethylsilane was used as the reductant, diyne cyclization product 125 was preferred in a ratio of 4 1 [71]. Selectivities of up to 10 1 for enyne cyclization were observed, depending on the substrate employed [72],... 

Pu and co-workers incorporated atropisomeric binaphthols in polymer matrixes constituted of binaphthyl units, the macromolecular chiral ligands obtained being successfully used in numerous enantioselective metal-catalyzed reactions,97-99 such as asymmetric addition of dialkylzinc reagents to aldehydes.99 Recently, they also synthesized a stereoregular polymeric BINAP ligand by a Suzuki coupling of the (R)-BINAP oxide, followed by a reduction with trichlorosilane (Figure 10).100... 

The reagent combinations PMHS/ZnCl2,315 PMHS/[Bu2(AcO)Sn]20,316 and PMHS/HCuPPh3317 all promote reduction of aldehydes to the corresponding alcohols in good yields. Trichlorosilane in dimethylformamide reduces aldehydes to alcohols in high yields.318... 

A similar reducing system is created by combining dilithium catecholate and trichlorosilane at —78° in tetrahydrofuran. It is speculated that the relatively unstable pentacoordinate bis(l,2-benzenediolato)hydridosilicate (61) is formed in situ and that it is this species that can reduce aldehydes and ketones, but not esters, to alcohols when they are added to the reaction mixture at 0° (Eq. 168).93 In a like manner, the dilithium salt of 2,2/-dihydroxybiphenyl, which forms a pentacoordinate intermediate that is stable enough to react at room temperature, can also be used to promote the reduction reaction. The alkoxides of aliphatic diols... 

Certain catalysts promote the reduction of ketones with organosilanes. The reduction of acetophenone with Et3SiH is catalyzed by the diphosphine 65 and gives only a small amount of overreduction to ethylbenzene.377 Aryl alkyl enones and ynones are reduced to the corresponding alcohols with triethoxysilane and the titanium-based catalyst 66.378 Trichlorosilane reduces acetophenone in 90% yield with /V-formylpyrrolidinc catalysis.379... 

Various chiral ligands with metal catalysts can be employed in the organosilane reduction of imines to amines. Many of these provide modest success. These include (oxazolino)diphenylphosphinoferrocene ligands with ruthenium,605 (—)-DIOP/Rh(I),606,607 3,3 -BINOL (l,l -bi-2-naphthol) and LiHMDS,608 and (S)-phenyl V-formylprolinamidc with trichlorosilane.609... 

Finally, Cristau and coworkers have reported on a quite efficient preparation of triphenylphosphine oxide (Figure 2.13) by a similar addition-elimination reaction of red phosphorus with iodobenzene in the presence of a Lewis acid catalyst followed by oxidation of an intermediate tetraarylphosphonium salt.42 This approach holds the potential for the preparation of a variety of triarylphosphine oxides without proceeding through the normally used Grignard reagent. Of course, a variety of approaches is available for the efficient reduction of phosphine oxides and quaternary phosphonium salts to the parent phosphine, including the use of lithium aluminum hydride,43 meth-ylpolysiloxane,44 trichlorosilane,45 and hexachlorodisilane.46... 

The isomeric bicyclic phosphines (41) have been obtained by reduction with trichlorosilane of the related isomeric phosphine oxides, the reaction proceeding with... 

Reduction of the diazaphospholines (151) by trichlorosilane yields a mixture of cis- (Ra = H, R3 = Ph) and trans- (R2 = Ph, R3 = H) phosphines, the lack of stereospecificity being attributed to changes in a pentaco-ordinate intermediate. Stereomutation of the starting oxide is brought about by hexachlorodisilane or silicon tetrachloride.118... 

Reduction of esters by trichlorosilane in tetrahydrofuran in the presence of tert-butyl peroxide and under ultraviolet irradiation gave predominantly ethers from esters of primary alcohols, while esters of tertiary alcohols were cleaved to acids and hydrocarbons. Esters of secondary alcohols gave mixtures of ethers and acids/hydrocarbons in varying ratios. 1-Adamantyl trimethylacetate, for example, afforded 50-100% yields of mixtures containing 2-42% of 1-adamantyl neopentyl ether and 58-98% of adamantane and trimethylacetic acid [1033]. 

Next, a few examples of asymmetric reductions with trichlorosilane are presented. An asymmetric reduction of ketones and imines was reported by Matsumura and coworkers by using trichlorosilane as reductant and A-formyl pyrrolidine derivative 36 as ligand (Scheme 28) [101, 102]. 

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