Silicon, halosilane

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). 

An alternative method for preparing phosphinous amides makes a profit on the high affinity between silicon and halogen atoms. This is the driving force of the reactions between halophosphanes and Ar-(trimethylsilyl)anilines, AT-(tri-methylsilyl)amides or AT-(trimethylsilyl)ureas and thioureas, as represented in the Scheme 6. In these processes the desired P-N bond and an halosilane are simultaneously formed [53,58-60]. 

Silicon substituents can be introduced into alkenes and alkynes by hydrosilation.70 This reaction, in contrast to hydroboration, does not occur spontaneously, but it can be carried out in the presence of catalysts such as H2PtCl6, hexachloroplatinic acid. Other catalysts are also available.71 Halosilanes are more reactive than trialkylsilanes.72... 

While trifluoro and other halosilanes function by increased electrophilicity at silicon, nucleophilic reactivity of allylic silanes can be enhanced by formation of anionic adducts (silicates). Reaction of allylic silanes with aldehydes and ketones can... 

An important reaction of silicon compounds containing electronegative substituents, such as the halogens or a Group 15 or 16 element, is hydrolysis, which often occurs very readily even with atmospheric moisture. The primary reaction in silicone production is the hydrolysis of halosilanes, usually the readily available chlorosilanes, to give silanols, which then undergo acid-catalyzed condensation with loss of water, as shown in Scheme 1. 

The presence of bulky substituents such as Bul on silicon greatly reduces the tendency towards siloxane formation, and with two or more such groups present the precautions suggested in points 1, 2, and 3 can often be relaxed. For example, the hydrolysis of highly sterically hindered halosilanes may require rather severe conditions. 

Although halosilanes undergo smooth nucleophilic substitution at the silicon, they are rather inert to redox reactions. Therefore, chlorosilanes are usually used as trapping agents of anionic intermediates generated by electroreduction of organic compounds. However, in the absence of other reactive substrates halosilanes are reduced electrochemically to form Si-Si bonds. Indeed, there are a number of reports in the literature of the cathodic reduction of chlorosilanes (Sect. 4.2). 

The properties and applications of five commercially important groups of silanes those containing Si-H (hydride functional silanes), Si-X (halosilanes), Si-C (organosilanes), Si-OSi (siloxanes), and Si-OR (silicon esters) are thoroughly discussed by Arkles.14 As the role of functional siloxanes grows, so does the importance of the hydrosilylation reactions in this field. A broad introduction to hydrosilylation processes as applied to the synthesis of silicone precursors and to the functionalization of siloxanes is provided in the earlier editions of COMC, and,... 

ALKYLHALOSILANES, ALKYLSILANES HALOSILANES, ORGANOSILYL PERCHLORATES SILANES, SILICONE GREASE, SILICONE LIQUID, SILICONE OIL SILYLHYDRAZINES, TRIALKYLSILYLOXY ORGANOLEAD DERIVATIVES... 

In conclusion, all these observations suggest the presence of a five-coordinate silicon intermediate both in nucleophilic displacements at silicon and in the racemization of halosilanes as previously suggested (5, 59). The ability to form a pentacoordinate silicon adduct is not controlled by the electronegativity of the substituents at silicon, but by the tendency of the Si—X bond to be stretched under the influence of a nucleophile ... 

A reaction related to the nucleophile-catalysed hydrolysis of silanes is the nucleophile-catalysed racemization of silanes. The racemization of a range of halosilanes was found to have an order with respect to nucleophile varying from 1 to 346,47. The reactions have entropies of activation which are large and negative, and enthalpies of activation which are small and sometimes negative. Both racemization and hydrolysis are slowed by increasing steric crowding at silicon.48... 

Recently, Hengge and coworkers reported that the use of a silicon carbide and a hydrogen electrode instead of sacrificial anodes was effective for the electrochemical coupling of halosilanes without formation of metal chlorides (equation 65)94. 

Cyclic siloxanes are important precursors in the silicon industry, being formally dimers or trimers of silanone (R2Si=0), a known intermediate. Cyclic siloxanes have been synthesized by four routes, the conventional methods being the condensation of silanediol or the hydrolysis of species such as halosilanes or aminosilanes (Scheme l)16-20. Alternatively, oxidation of disilene by triplet oxygen (equation l)21-27 or oxidation of oxadisiliranes by singlet oxygen (equation 2)28-31 may be utilized. 

Recently Bassindale et al. (288) discovered a new mechanism of halo-silane racemization in the presence of nucleoophiles. They performed dynamic NMR studies of the inversion at silicon catalyzed by HMPA and N-trimethylsilylimidazole, using as a model a halosilane having di-astereotopic methyl groups. Diastereotopic separation of the H- and 13C-NMR signals is apparent in both the substrate and the silylonium complex. Thus, inversion at the Si center leading to coalescence may be... 

Halosilane. A halogen derivative of a silane a compound of silicon containing at least one Si—X bond, where X represents a halogen. 

Activation parameters at coalescence temperature show that the coordinate interaction in these compounds is not a function of the electronegativity of X but is controlled by the ability of the nitrogen atom to stretch the Si—X bond. The tendency of the silicon atom to increase its valency decreases in the order X = OCOR, Br, Cl > SR F > OR, H This sequence corresponds directly to the rate of racemization of halosilanes and to the substitution of R3SiX with inversion of configuration Although no intramolecular coordination was observed in solutions of acetoxysilanes (CH3) Si(OCOR)4 by the Si NMR method the shape of the H NMR spectra of these compounds with chiral silicon atom points to Si -0 interaction... 

PhjPO solutions of various halosilanes (e.g., SiCl, MeSiClj, Me SiClj, MCjSiCl) show significant downfield P shifts relative to the free base. The coordination shift of approximately 8-9 ppm indicates bonding interaction between phosphoryl oxygen and silicon. An effect of added MeCN on the chemical shift of PhjPO— McjSiCl solutions has been interpreted in terms of a coordination interaction between the nitrile group and silicon 23,324)... 

The reaction between halosilanes and ammonia or primary or secondary amines is the most widely used synthetic route to compounds containing 8i-N bonds. 8uch reactions are greatly controlled by steric effects if large groups are present on either the silicon or the amine, then less than full substitution may occur. For example, ammonia reacts with chlorosilanes to give a variety of products depending on the size of the chlorosilane (equation 32). 

Because of the commercial importance of halosilanes, particularly the dihalo and trihalo derivatives, in the production of silicone polymers, the reaction in which M = Si has received considerable attention. For example, the direct process produces MegSiCl and MeSiClg as part of a complex mixture including the valuable Me2SiCl2, which is diflScult to separate because of similar boiling points. Fortunately, the equilibration of alkyl groups on silicon is not statistical, a fact that permits a very useful application of the redistribution reaction ... 

Here we describe the results of the reaction of the diketoamine ligand 1 with several halosilanes, which furnish the novel silicon dimers 6-11. 

As a general mle, unless an anion-stabilizing group, such as phenyl, or a heteroatom such as sulfur is present, the alkylsilane is not readily deprotonated. The a-halosilane can be deprotonated but, unlike the readily available chloromethyltrimethylsilane, there are few general methods to this approach. Al-kyllithium reagents add to vinylsilanes ( ) to produce the carbanion (287). Silyl derivatives with heteroatoms, such as sulfur, selenium, silicon or tin, in the a-position (288) may be transmetallated (Scheme 41). Besides the difficulty in synthesizing the anion, alkene formation lacks specificity for simple di- and tri-alkyl-substituted alkenes. As a result, the Peterson reaction of an a-silyl carbanion with a carbonyl has found the greatest utility in the synthesis of methylene derivatives, (as discussed in Section 3.1.3), heterosubstituted alkenes and a,p-unsaturated esters, aldehydes and nitriles. 

Organoalkoxysilanes are utilized in different application sectors, because under application conditions no acidic cleavage products are formed as is the case with organo-halosilanes. In addition intentional hydrolysis, e.g. in silicone chemistry, is generally easier to control than in the case of organohalosilanes. 

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