Silanone

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

A number of relatively stable silenes with bulky substituents are known at present, but stable silanones have not been isolated till now. Their instability, like that of silenes, is caused by a kinetic factor, according to various calculations. Thus, cyclooligomerization of silanones should proceed with zero activation barrier (Kudo and Nagase, 1985). 

In order to study the influence of substituents on the spectral behaviour and on the thermal and kinetic stability of silanones, two new organic silanones, dimethylsilicate [144a] and silabenzophenone [144b], have been examined. These were obtained in the vacuum pyrolysis of the corresponding epoxides [145a] and [145b] (Khabashesku et al., 1988c). 

It has been found that, among the silanones studied, silabenzophenone seems to be the most thermally stable while dimethyl silicate is the kinetically most stable (Table 11). 

The platinum complex [ Pt(dmpe) 2( X-H)2](OTf)2 can catalyze the generation of silanones from secondary silanols under very mild conditions (at room tempera-... 

Schemes 6-31 Pt(ll)-catalyzed generation of silanones and the proposed mechanism...

With N20, the iminosilanes Me2Si=NSi(CMe3)3 and (Me3C)2Si= NSi(CMe3)3 form unstable [2 + 3] cycloadducts with the participation of the iminosilanes, these cycloadducts react to yield products composed of one molecule of the silanone and one molecule of the iminosilane.1430 For example,30... 

It appeared that if heated at 160°C in a sila-Wittig-type reaction, 26 actually decomposes into the transient silanone Is2Si=0, which immediately dimerizes to 27 and the phosphaalkenes 28 and 28 (Scheme 6).14... 

The adducts 41 from 1 and ketones or thiobenzophenone undergo interesting photochemical cycloreversion to afford a silanone or silanethione intermediate 42 in addition to silene 43 both of these intermediates are trapped by ethanol, as shown in Eq. (14).68 71 In the reaction with the thiobenzophenone adduct 41 (R = Ph, X = S), the intermediate silene 43 (R = Ph) was detected by Si NMR.71... 

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. 

For example, in the case of intramolecularly base-stabilized silanone 4 it was trapped by excess hexamethylcyclotrisiloxane to give the corresponding insertion product 5 but isolated as a trimer 6 in the absence of such a trapping reagent (Scheme 2).17... 

More recently, Belzner et al. reported a new type of oxygen transfer reaction from isocyanates to bis[2-(dimethylaminomethyl)phenyl]silylene (8)18 which was thermally generated from the corresponding cyclotrisilane 7, and they obtained some convincing results of the involvement of silanone 9 (Scheme 3). However, they found that silanone 9 is not stable enough to be isolated. Only cyclic di- and trisiloxanes 10 and 11 (i.e., the cyclic dimer and trimer of the silanone 9) were obtained together with the corresponding isonitrile as other main products when... 

Since the silanone intermediates have been postulated as key intermediates in reports on the synthesis and reactions of organosilicon compounds,27 the isolation of a stable silanone and its characterization should be one of the most fascinating subjects in the future chemistry of low-coordinate organosilicon compounds. 

In contrast to the ready trimerization of the silanone 4 having the same ligands, silanethione 31a was found to be relatively long lived in solution (t 1/2 3 d in CDC13 at 25°C) as a monomeric species, though extreme precautions must be taken to... 

As in the case of silanone 9, the reaction of the silylene bis[2-(dimethylamino-methyl) phenyl]silanediyl (8) with phenyl isothiocyanate was examined.29 In this reaction the expected silanethione 36 was obtained as a single product even in the presence of (Me2SiO)3, no insertion product of 36 into a Si-O bond of (Me2SiO)3 was observed (Scheme 10). 

In contrast to the remarkable progress in the chemistry of silanones and silanethiones, very little is known for the chemistry of their heavier chalcogen... 

Some remarkable chemistry is observed when silenes react with heteroatom systems, in particular carbonyl compounds (]>C=0) and imines Q>C=N—R). The reaction with ketones was first described by Sommer (203), who postulated formation of an intermediate siloxetane which could not be observed and hence was considered to be unstable even at room temperature, decomposing spontaneously to a silanone (normally isolated as its trimer and other oligomers) and the observed alkene [Eq. (14)]. Many efforts have been made to demonstrate the existence of the siloxetane, but it is only very recently that claims have been advanced for the isolation of this species. In one case (86) an alternative formulation for the product obtained has been advanced (204). In a second case (121) involving reaction of a highly hindered silene with cyclopentadienones,... 

Scheme 26. Reaction of 1 with C02 and trapping of the silanone (Me5C5)2Si = O.

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