Silanols disiloxanes

Entry Silane Oxidant Nature Conversion % Selectivity silanol/ disiloxane % Refs. 

Most significant, however, is that the observation of a common intermediate among silanol, disiloxane, and fluorosilane cross-coupling reactions signifies that these systems, which encompass the vast majority of documented organosilicon cross-coupling systems to date, likely react according to a common mechanism. 

Photolytic generation of silene Me3Si(Me)C=SiMe2 in the presence of an excess of D20 or a trace of H20 affords disiloxanes 25, presumably via condensation of the silanol 26 in the case of D20, or by reaction of the silanol 27 with further silene in the case of H20 (Scheme 7) (138, 139). 

An asymmetric variant of this reaction was developed using chiral Pd complex 111 with either silanes or disiloxanes [66-68]. Both relative and absolute stereochemistries were controlled in this system and good yields (60-85%) were obtained after oxidation (Eq. 18). Formation of the silane-containing product was inhibited by the presence of water due to competitive formation of the palladium hydrides and silanols [68]. The use of disiloxanes as reductants, however, provided expedient oxidation to the alcohol products without decreasing the isolated yields enantioselectivity was 5-15% lower in this more robust system [66]. Benzhydryldimethylsilane proved to be a good compromise between high yield and facile oxidation [66]. Palladium com-... 

In contrast, hydrolysis of the methyl derivative, Cp(OC)2Fe-SiMe(H)Cl (Id), yields the corresponding ferrio-silanol, Cp(OC)2Fe-SiMe(H)OH, as a short-lived intermediate identified by means of IR and NMR spectroscopy, which immediately reacts with Id to give the bis(ferrio)disiloxane 6. Conversion of 6 to the fluorine analogue [Cp(0C)2Fe-Si(Me)F]20, characterized by a linear Si-O-Si arrangement [6], is achieved via consecutive H/Cl- and Cl/F-exchange. 

To get further insight into the reactivity of the metallo-silanols, condensation with diverse chlorosilanes has been studied. It offers easy and general access to the ferrio-disiloxanes 15a-d via interaction of 2a or its lithiation product 13 with alkyl-, aryl- or metallo-chlorosilanes in the presence of NEt3 (Eq.(3)). 

From silanes 52 are obtained, in high yield, the corresponding silanols 53, which react further to produce disiloxanes 56 and 58-60. Silanes 54 alkoxysilanes 55 and disilanes 57 give high yields of disiloxanes 56. Ozonolysis of tetraethylsilane yields initially acetaldehyde and trimethylsilyl hydroperoxide 61. The latter is partially converted to bis(triethylsilyl) peroxide 62, which is hydrolyzed to silanol 63 and hydrogen peroxide. The ozonolysis is of first order, both in regard to the silanes, and to ozone. The ozonolysis starts with formation of 64 followed by formation of the trioxide 65, which decomposes to acetaldehyde and hydroperoxide 61 (Scheme 14)79 80. 

Equilibrium constant determination for alkoxysilane hydrolysis. Triethyl-silanol was selected as a model compound for determination of the equilibrium constant for equation (1), since under neutral conditions the condensation to disiloxane was observed to take place only over an extended period of time (i.e. years), eliminating equilibria (2) and (3) as interfering factors. 

From these studies of condensation of mono-, di-, and trifunctional silanols in aqueous and aqueous-organic solutions, a kinetic expression describing the rate of silanol disappearance (condensation of silanol to disiloxane) is obtained ... 

However, most organosilicon compounds are metabolized after ingestion or injection, and their metabolites are eliminated. For example, silameprobamate (27) undergoes to - 1 hydroxylation, following the same pattern as meprobamate itself. The difference in the metabolism of the two compounds is that the silicon compound also undergoes silicon-carbon cleavage to yield a silanol that can be isolated from the urine as a disiloxane (32) see Eq. (1). 

The described metallo-silanols exhibit high stability concerning condensation, however, the OH groups display reasonable reactivity in context with chlorosilanes. These properties can be used to transform the ferrio-silanols 4a, 4b via reaction with trimethylchlorosilane to the ferrio-di- and -trisiloxanes 13a, 13b characterized by a hydrogen substituted a-silicon. In the case of 4b the first siloxane unit is established after one day, the second takes five days (13b). The same reaction pattern can be applied for the generation of the ruthenio-disiloxanes 13c, 13d, resulting fi-om the treatment of the ruthenio-silanols 8c, 8e with Me2Si(R )Cl. 

Treatment of 7 with DMD leads to the short-lived bis-silanol A. However, this intermediate undergoes intramolecular condensation, resulting immediately in the formation of the tungsten complex 8 bearing a disiloxane unit bridging the cyclopentadienyl ligand and the metal center. 8 is... 

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