Silanones reactivity

As a result of comparing the properties of silanethione with silanone and formaldehyde, Nagase and Kudo obtained an important finding that silicon is much less reluctant to form double bonds with sulfur than with oxygen.12,13 Thus, silanethione is more stable and less reactive than silanone. They concluded that the maj or obstacle to the successful isolation of silanethione is its relatively high reactivity. 

In the previous reviews,1 la,d f syntheses of many examples of transient silicon-oxygen double bond compounds such as MeHSi=0, Me2Si=0, H2Si=0 (2), (H0)HSi=0 (silanoic acid), and (H0)2Si=0 (silicic acid) have been described, and they are reportedly isolated as stable species in the low temperature matrices. However, the stabilization of this extremely reactive double bond species is very difficult, and no stable example of silanone (RR/Si=0) has been isolated until now even by the methods of thermodynamic or kinetic stabilization. 

The discussion continues regarding the role of silanone and cyclodisiloxanes as reactive intermediates in the formation of Si-O-Si bond.25 In studies of the reaction of dimethyldichlorosilane, phenylmethyldichlorosilane, or diphenyldichlorosilane with dimethyl sulfoxide in the presence of 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane, Weber and co-workers obtained products of the insertion of diorganosiloxy unit into the cyclic siloxane, accompanied... 

Among the silicon-chalcogen double-bond compounds, the silicon-sulfur doubly-bonded compounds (silanethiones) are considered to be easier to synthesize, since it has been predicted by the theoretical calculations that a silicon-sulfur double bond is thermodynamically and kinetically more stable than a silicon-oxygen double bond (silanone)13,14. According to the calculations, the lower polarization of Si=S compared to Si=0 should lead to a lower reactivity of Si=S. In addition, H2Si=S (1) is calculated to be by 8.9 kcal mol-1 more stable than its divalent isomer, H(HS)Si , whereas H2Si=0 (2) is by 2.4 kcal mol-1 less stable than H(HO)Si . 

The photochemical reactivity goes up with increasing number of methyl groups at the double bond and decreasing ionization potential28. Key intermediates in both the photochemical and the thermal oxidation of silenes 12, 7 and 13 are the siladioxetanes 14. These species are labile even in low-temperature matrices and could not be identified spectroscopically. Evidence for their formation comes from the observed oxidation products such as complexes 15 between silanones and formaldehyde and formylsilanols 16. 

In another method of surface activation to form RSi, the greater part of the reactive centers consists of the (=Si-0-)2Si groups, in which the silicon atom is also bonded to only two lattice oxygen atoms. The concentration of these groups is 1013 cm-2. In this case, the silanone groups can be obtained by the oxidation of silylene centers (see Section 4.2). 

High-temperature heating of the hydrated silicon samples is accompanied by the decomposition of hydroxyl groups and formation of the reactive cyclic structures (=Si-0-)2Si < 02> Si(-0-Si=)2. Their concentration is equal to 10l3cm-2 [4,72]. These groups can also form through the combination (condensation) of two silanone groups ... 

In agreement with the competitive kinetic measurements, dichlorosilanes react faster than dialkoxysilanes. Bulkier substituents slow the process compared to less encumbering substituents at Si (Table 2). For triethylsilane, used as silanone trap, it was found that Si-H bond is also reactive... 

The pathway of the reaction of 1 with the above heterocumulenes is discussed as follows (see Scheme 2) The first intermediate is a highly reactive [2+1] cycloaddition product or its ring-opened isomer, which easily loses CO or in the case of isothiocyanates the corresponding isonitrile RNC to give the silanone or silathione 2. The formation of CO was proved in the reaction of 1 with CO2 by reaction with the iron complex (HsC5)Fe(SMe2)3 BF4 [7]. In the reaction of 1 with isothiocyanates the formation of RNC was proved using NMR spectroscopy. The intermediates of the type Cp 2Si=X (X = O, S) are not stable and stabilized via different routes to the final products. In the reaction of 1 with CO2 the double bond species Cp 2Si=0 is transformed by CO2 to the formal [2+2]... 

Silylenes 1 are highly reactive homologues of the carbenes, and we have been interested to compare the reactivity and primary products of the oxidation of these divalent species. In principal one can expect two different primary adducts of a silylene and molecular oxygen the formal "end-on" adducts silanone 0-oxide 2 or "side-on" adducts dioxasilirane 3 (Scheme 1). It was shown by numerous matrix studies [8-12], experiments in solution using time resolved spectroscopy [13-16], and a preparative scale synthesis in solution [17] that triplet as well as singlet carbenes yield carbonyl 0-oxides as the primary oxidation products, while dioxiranes are products of secondary photolysis. Ando et al. reported on the synthesis of the silanone 0-oxide 2e by the reaction of dimesitylsilylene le and O2 in solid argon [1]. This is so far the only experimental evidence for a silanone O-oxide. 

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