Halosilanes hydrolysis

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. 

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... 

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. 

Isocyanates and isothiocyanates both react with water in a manner similar to the halosilanes to give hydrated silica. They also react with primary and secondary amines to give silylamines which, on hydrolysis, afford A-mono- and N,N-disubstituted ureas and thioureas (Scheme 31), which is a useful synthesis of these compounds. ... 

The theme of photosensitizing semiconductor electrodes introduced in Section 57.3.2.5(iii) may be developed with an example from ruthenium—bipyridyl chemistry. The sequence (40) is well known. The effectiveness of the photosensitization should be increased by the covalent attachment of the tris(bipyridyl)ruthenium(II) entity to the semiconductor surface, for example to Sn02. This has been achieved using the versatile halosilane chemistry shown in equation (41). The coimter anion was PFg . Cyclic voltammetry showed that the behaviour of the sjretems Sn02/aqueous [Ru(bipy)3] " and Sn02(Alm)/electroly te were very similar but with a -1-0.05 V shift in E°. The coated electrode gives a photocurrent with a red shift of 10 nm which is twice as large as for the non-coated electrode. Unfortunately the current falls off due to promotion of the hydrolysis of the Aim. 

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. 

In the past few years, numerous experimental results have illustrated the fundamental importance of penta- and hexacoordinate silicon species in reactions at silicon. The implication of pentacoordinate intermediates in substitution reactions at silicon is now well accepted (10, 11) the nucleophilically induced racemization (13,268) and hydrolysis (or alcoholysis) of halosilanes (268, 269), both controlled by entropy factors, take place through expansion of coordination at silicon. Pentacoordinate species with two or three carbon atoms attached to silicon have been isolated (129, 155) and finally, five-coordinate anions with five carbon atoms around silicon have been identified in the gas phase (107). This shows how much the expansion of coordination at silicon is an energetically favorable process. 

These polarographic data—quantitative hydrolysis of halosilanes when a nucleophilic catalyst (HMPA) is present-give rise to a new method for the determination of water concentration in organic solvents (343) that is as sensitive as the Karl Fischer method. 

Silanol functional groups have been long recognized as reactive intermediates in silicon chemistry. They are formed in the hydrolysis of silanes with various silicon functional groups, such as halosilanes, alkoxysilanes, etc., and then transformed into siloxanes by spontaneous or catalytic condensation. Owing to this tendency, only a limited number of organosilanols have so far been synthesized. 

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