Organosilanes

Stereoselective acylahon of racemic hydroxyalkylsilanes 102 was achieved using a crude papain preparation and, surprisingly, 4-phenylpentanoic acid as the acyl donor. The ees of the products 103 and the recovered substrates varied from 30 to 99% (Equation 49).  

In turn, a lipase-promoted acylahon of prochiral substrates 104, using lipases from Candida cylinracea and Chromobacterium viscosum and methyl isobutyrate or acetoxime isobutyrate as the acyl donors, gave the product 105 in yields up to 80% and with ees up to 75% (Equahon 50).  

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A pletliora of different SA systems have been reported in tire literature. Examples include organosilanes on hydroxylated surfaces, alkanetliiols on gold, silver, copper and platinum, dialkyl disulphides on gold, alcohols and amines on platinum and carboxyl acids on aluminium oxide and silver. Some examples and references can be found in [123]. More recently also phosphonic and phosphoric esters on aluminium oxides have been reported [124, 125]. Only a small selection out of tliis number of SA systems can be presented here and properties such as kinetics, tliennal, chemical and mechanical stability are briefly presented for alkanetliiols on gold as an example. 

Organosilanes, such as trichlorosilanes or trimethylsilanes, can establish SA monolayers on hydroxylated surfaces. Apart from their (covalent) binding to the surface these molecules can also establish a covalent intennolecular network, resulting in an enlranced mechanical stability of the films (figure C2.4.11). In 1980, work was published on the fonnation of SAMs of octadecyltrichlorosilane (OTS) 11171. Subsequently, the use of this material was extended to the fonnation of multilayers 11341. 

Hybrid Circuits. The use of parylenes as a hybrid circuit coating is based on much the same rationale as its use in circuit boards. A significant distinction Hes in obtaining adhesion to the ceramic substrate material, the success of which determines the eventual performance of the coated part. Adhesion to the ceramic must be achieved using adhesion promoters, such as the organosilanes. 

Uses. The largest use of lithium metal is in the production of organometaUic alkyl and aryl lithium compounds by reactions of lithium dispersions with the corresponding organohaHdes. Lithium metal is also used in organic syntheses for preparations of alkoxides and organosilanes, as weU as for reductions. Other uses for the metal include fabricated lithium battery components and manufacture of lithium alloys. It is also used for production of lithium hydride and lithium nitride. 

The properties and applications of commercially important hydride functional silanes, ie, compounds having a Si—H bond halosilanes, ie, compounds having a Si—X bond and organosilanes, ie, compounds having a Si—C bond, are discussed hereia. Compounds having Si—OSi bonds are called sdoxanes or sihcones. Those having a Si—OR bond are called siUcon esters. Sdoxanes and siUcon esters are discussed elsewhere ia the Eniyclopedia (see Silicon COMPOUNDS, SILICON ESTERS SILICON COMPOUNDS, SILICONES). 

Physical Properties. The physical properties of organosilanes are determined largely by the properties of the sihcon atom (Table 2). Because sihcon is larger and less electronegative than either carbon or hydrogen, the polarity of the Si—H bond is opposite to that of the C—H bond (Table 3). This... 

Table 2. Properties of Hydride Functional Organosilanes, Siloxanes, and Silazanes ...

Mechanistically the rate-determining step is nucleophilic attack involving the hydroxide ion and the more positive siUcon atom in the Si—H bond. This attack has been related to the Lewis acid strength of the corresponding silane, ie, to the abiUty to act as an acceptor for a given attacking base. Similar inductive and steric effects apply for acid hydrolysis of organosilanes (106). 

Organic amines, eg, pyridine and piperidine, have also been used successfully as catalysts in the reactions of organosilanes with alcohols and silanols. The reactions of organosilanes with organosilanols lead to formation of siloxane bonds. Nickel, zinc, and tin also exhibit a catalytic effect. 

Peracylation of polymethyUiydrosiloxane to produce a cross-linked or cross-linkable material is achieved by reaction with acetic acid and is cataly2ed by anhydrous 2inc chloride (112). This reaction can be extended to monomeric organosilanes under similar conditions. 

A convenient synthesis of organochlorosilanes from organosilanes is achieved by reaction with inorganic chlorides of Hg, Pt, V, Cr, Mo, Pd, Se, Bi, Fe, Sn, Cu, and even C. The last compounds, tin tetrachloride, copper(II) chloride, and, under catalytic conditions, carbon tetrachloride (117,118), are most widely used. 

Geminal polyhahdes also react with organosilanes under peroxide catalysis. For example, triethylsilane affords triethylchlorosilane in good yield upon reaction with carbon tetrachloride in the presence of benzoyl peroxide (bpo) at 80°C (94,100,102). 

Acid hahdes, eg, ben2oyl chloride, acetyl chloride, and ben2oyl bromide, have been used to prepare Si—Cl and Si—Br compounds from organosilanes. Acetyl chloride proceeds to higher yield when cataly2ed by aluminum chloride. 

Alkylation and arylation of organosilanes occur readily with alkyl and aryl alkaU metal compounds. Yields from these reactions are good but are iafluenced by steric requirements on both silane and metal compounds. There is Httie iaductive effect by the organic groups attached to siUcon, as measured by the yield of products (126,127). These reactions proceed more readily ia tetrahydrofuran and ethyl ether than ia ligroin or petroleum ether, where R and are alkyl or aryl and M is Li, Na, or K. 

A catalyst, usually acid, is required to promote chemoselective and regioselective reduction under mild conditions. A variety of organosilanes can be used, but triethylsilane ia the presence of trifiuoroacetic acid is the most frequendy reported. Use of this reagent enables reduction of alkenes to alkanes. Branched alkenes are reduced more readily than unbranched ones. Selective hydrogenation of branched dienes is also possible. 

Direct Process. The preparation of organosilanes by the direct process, first reported in 1945, is the primary method used commercially (142,143). Organosilanes in the United States, France, Germany, Japan, and the CIS are prepared by this method, including CH SiHCl, (CH2)2SiHCl, and C2H SiHCl2. Those materials are utilized as polymers and reactive intermediates. The synthesis involves the reaction of alkyl haUdes, eg, methyl and ethyl chloride, with siUcon metal or siUcon alloys in a fluidized bed at 250—450°C ... 

Table 5. Yields of Organosilanes via Reduction with LiAlH and LiH ...

Organochlorosilanes containing Si—H disproportionate in the presence of aluminum chloride without addition of more organosilane. Organic groups can be replaced by hydrogen (157). For example, tetraphenylsdane [1048-08-4] can be made from phenylmethylsilane [766-08-5]. 

Addition to Olefins. OrganohydrosHanes can also be prepared by addition of halosHanes and organosilanes containing multiple Si—H bonds to olefins. These reactions are catalyzed by platinum, platinum salts, peroxides, ultraviolet light, or ionizing radiation. 

Sodium and magnesium do not react with tetrachlorosilane at room temperature, but do so at elevated temperatures and ia the presence of polar aprotic solvents at moderately elevated temperatures. The Wurtz-Fittig coupling of organosilanes to form disilanes (168) and polysdanes (169) is usually accomphshed usiag molten sodium ia toluene or xylene. 

Silicone Fluids. Sihcone fluids are used in a wide variety of appHcations, including damping fluids, dielectric fluids, poHshes, cosmetic and personal care additives, textile finishes, hydraiflic fluids, paint additives, and heat-transfer oils. Polydimethylsiloxane oils are manufactured by the equihbrium polymerisation of cycHc or linear dimethyl silicone precursors. Trifunctional organosilane end groups, typically trimethylsilyl (M), are used, and the ratio of end group to chain units (D), ie, M/D, controls the ultimate average molecular weight and viscosity (112). Low viscosity fluids,... 

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