Silylcarbene

Two indirect routes to silenes, one derived from silylenes and the other from silylcarbenes, are of some generality and importance. Silylenes (e.g., Me3Si—Si—<]) (53) have been derived from the thermolysis of either methoxy or chloro polysilyl compounds. Thermolysis resulted in the elimination of trimethylmethoxy- or trimethylchlorosilane and yielded the silylene, which, based on products of trapping, clearly had rearranged in part to the isomeric silene [Eq. (5)]. Alternatively the silylene Me2Si has... 

The great advantage of the (phosphino)(phosphonio)carbene 2d over its silyl analogues is that it can be crystallized. As we have seen, these two types of phosphinocarbenes are very similar thus, conclusions from the X-ray analysis of 2d28 can probably be extended to the silylcarbenes. Ball and stick views of the molecule are shown in Fig. 2, and the pertinent geometric parameters are in the legend. 

In contrast to silylcarbenes, the analogous stannylcarbenes 2p are not stable, which explains why they have attracted little interest. Their instability is probably due to the long carbon-tin bond, which does not allow sufficient steric protection of the carbene center. Their reactivity seems to be quite similar to that of stable (phosphino)(silyl)carbenes Cyclopropanation reactions have been reported with methyl acrylate as well as coupling reactions with tert-butyl isonitrile.73... 

Silene (27) can undergo a 1,2-shift to give either methylsilylene (28) or, less favorably, to silylcarbene (29). The thermochemistry and the kinetics of these reactions have been points of major disparity between theory and experiments... 

All silicon-substituted carbenes that have been studied experimentally so far contain a SiR3 group (as opposed to silicon with other coordination numbers) therefore, the expression silylcarbenes will be used throughout this chapter. 

Further methods for the generation of silylcarbenes are available, but their application is usually limited to special types of silylcarbenes. Such methods will be discussed separately in Section ELF. 

A specific silylcarbene may be generated from different precursors. Furthermore, these reactive intermediates may undergo different competing transformations. Therefore, this... 

Silylcarbenes, especially when generated by photochemical or thermal decomposition of diazo compounds or tosylhydrazone alkali salts or a-hydroxy-a-silylsilanes, tend to... 

All silenes generated so far on the silylcarbene route are reactive intermediates themselves, which were characterized by typical subsequent reactions35 such as isomerization and dimerization or by trapping reactions (see below). However, photolysis of (silyl)diazo compounds in inert matrices at low temperature allowed the isolation and spectroscopic (IR, UV) characterization of several silenes (Scheme 2, Table 3). Irradiation of (dia-zomethyl)silanes 7 at X > 360 nm produced both diazirine 8 and silenes 10, but at shorter wavelength (X > 305 nm) the silenes were produced cleanly from both precursors the... 

The following examples in this section illustrate that the silylcarbene-to-silene rearrangement and subsequent silene reactions are common also under nonmatrix conditions. The early research on this topic was reviewed in 197937. 

When two carbene functions are separated by one or more silicon atoms, one can expect them to enter independently the usual inter- or intramolecular reactions. Among the intramolecular reactions, extensions of those which have been discussed in Sections III.B and IILC are particularly appealing, namely silylcarbene-to-silene rearrangement at one or both carbene centers and intramolecular carbene dimerization to form a C,C double bond and thus an unsaturated silaheterocycle. 

A diazosilene is probably also involved in the photochemical or copper-catalyzed decomposition of bis(diazoacetate) 156 in benzene (equation 36). In both cases, dia-zoketene 157 was the only identified product72. Its formation was explained by the silylcarbene-to-acylsilene-to-silylketene sequence outlined in Scheme 5. Efforts to achieve the N2 extrusion from the remaining diazo function by thermolysis in boiling toluene or by prolonged photolysis resulted only in unspecific decomposition. 

The dramatic influence of a methyl group on the reaction pathway is exemplified by carbene 163 which rearranges to a vinylsilane. In the absence of this methyl group, however, the silylcarbene-to-silene rearrangement (equation 16) occurs48,49. 

In a similar manner, 3,3-disubstituted l,2-bis(trimethylsilyl)cyclopropenes rearrange to l,l-bis(trimethylsilyl)allenes, most likely by 1,2-silyl shift of primarily formed (1-silylvinyl)silylcarbenes. According to ab initio calculations, this reaction pathway is energetically more favorable than those including a 2,2-disilylcyclopropylidene or a 2,3-disilylpropylidene90b. 

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