Silicon silanone

Unstable compounds with double-bonded silicon and germanium atoms (silenes, silanones, germanones, germathiones)... 

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. 

Silicon-Oxygen Double Bond Compounds (Silanones)... 

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 rearrangement of silicon compounds 97 does not involve the silanone 100 as an intermediate (Scheme 24). 

Silanone, disilene oxidation, 821 Silicon peroxides see Silyl peroxides Silicotungstate compound, olefin epoxidation,... 

Recent advances in the chemistry of silicon-heteroatom multiple bonds 1065 B. Theoretical Calculations 1. Silanones and silanethiones... 

When a toluene solution of a mixture of cyclotrisilane 34 and cyclohexyl isocyanate (or f-butyl isocyante) was heated at 70 °C, cyclic di- and trisiloxanes 37 and 38, i.e. the cyclic dimer and trimer of the silanone 36, were obtained together with the corresponding isonitrile RN=C. The formation of 37 as well as 38 was completely suppressed in the presence of hexamethylcyclotrisiloxane (19 D3) instead, quantitative conversion of 35 into 39, the formal insertion product of the silanone 36 into the Si—O bond of D3, occurred (Scheme 14). Since neither cyclodisiloxane 37 nor cyclotrisiloxane 38 reacted with D3 under the reaction conditions, the possibility that 37 or 38 is the precursor of 39 was ruled out. Whereas the oxidation of 35 with cyclohexyl and t-butyl isocyanates proceeded with exclusive formation of 37 and 38 (as the silicon-containing compounds) the reaction of 35 with phenyl isocyanate resulted in the formation of 37 in low yield. Furthermore, in this case the presence of D3 did not totally suppress the formation of 37. According to the authors, these results indicate that the oxidation of 35 with cyclohexyl and f-butyl isocyanates appears to use other reaction channels than that with phenyl isocyanate. 

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 . 

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. 

For comparison, we present below the G2(MP2)-calculated heat effects AH (OK) of the Si = O bond rupture for a number of silanone molecules containing different substituents on the silicon atom. The calculations were carried out for the molecules of C2v symmetry, with the spatial geometry of the starting silylene molecules corresponding to the situation where the SiOSiO atoms in the (H3Si-0)2Si and (F3Si-0)2Si molecules are trans-positioned, while the HOSiO atoms in the (HO)2Si molecule are cis to each other, and this symmetry is retained upon the formation of the silanone group ... 

Thus, the Si = O bond strength in the silanone group increases with decreasing electronegativity of the substituents at the silicon atom. The bond strength in the surface (=Si-0)2Si = O center proved to be intermediate between those obtained for the smallest-size molecular clusters F2Si = O and (HO)2Si = O (with the trara-arranged O, H, Si, and O atoms). The influence of the spatial position of the /1-substituent (in this case, H atom) on the properties of silicon atom is discussed in detail in Section 5. 

A comparison of the tziso(29Sia) values for the Si(OH)3 (—470 G (calculation), frans-configurated H atoms), SiF3 (— 502 G (calculation) and —500 G (experiment, [43]), and Si(0-Si=)3 (—480G (experiment)) radicals indicates that the order of their influence on the properties of the silicon atom is the following -0-1 (trans) < -0-Si=<-F. This also follows from the values of the Si = O bond strength calculated for the silanone molecules with different substituents (Section 4.3). 

In defects of this type (diamagnetic), the Si atom is linked to two lattice oxygen atoms. SC is related to several diamagnetic defects in which the silicon atom appears in different oxidation states. Currently, silanone (=Si-0-)2Si = O [18,19,52] and siladioxirane (=Si-0-)2Si<02 groups [18,73,74] are identified in this series. 

It was established that the reaction of the methyl radical with the silanone group occurs via two steps first it adds to the silicon atom to form the oxy radical, and then the radical is isomerized via a shift of the H atom to oxygen (reaction 2 in Table 7.11). The estimated activation energy of this monomolecular reaction was 16kcal/mol. This suggests that the addition of other alkyl radicals (reactions 3 and 4) to the silanone groups also occurs via steps. The mechanism of the addition of an H(D) atom to the silanone group remains unclear. 

Silanone groups easily undergo the addition to unsaturated molecules (reactions 6-9) to form the corresponding cyclic structures [86,87], According to the results of quantum-chemical calculations, in these reactions, comparatively stable ( 10kcal/mol) intermolecular complexes are formed, which then yield the final products. The silicon atom of the silanone group mainly participates in complex formation. 

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