Silylcarbenes reactions

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 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. 

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. 

Phenyl(triphenylsilyl)carbene has also been trapped without the interference of a silylcarbene-to-silene rearrangement84. It undergoes 0,H insertion with alcohols and is oxidized to the ketone by DMSO the latter reaction is likely to include an S-oxide ylide (equation 56). 

Silenes are formed by rearrangement of silylcarbenes. If polysilylated diazomethanes 298-300 are employed, a selective migration of a silyl group to the carbene centre occurs and silenes 301, 92 and 302 are formed (equations 74-76)164. The outcome of trapping reactions is independent of the mode of silene generation photochemical and pyrolytic methods give the same results. 

The photolysis of silyldiazoalkanes, species which are much stabler than their all-carbon analogs, leads to the formation of silylcarbenes. The subsequent behavior of the silyl-carbenes depends greatly on the nature of the groups attached to the diazo-carbon atom, as well as to the silicon atom. Early results have been reviewed2. In Scheme 43 several representative reactions are listed. 

The reaction types that lead to transient silenes in solution are some of those already considered (Chart 7) 4 + 2 and 2 + 2 cycloreversions, 1,2-shift in a silylcarbene, sigmatro-pic shifts and 1,2-elimination. Among these, 1,2-elimination is of particular importance. 

The photochemical reactions that the silyldiazoalkanes undergo involve silylcarbene chemistry. In line with reactions described for the more conventional carbenes, a variety of processes are common to both types. Thus irradiation brings about 1,2-alkyl shifts, for example, when 515 is photolysed in alcohols to afford a carbene, which transforms into a silene. Four products 516-519 are obtained in good yield. The formation of 519... 

The X-ray structure of this complex was reported by Bottomley and coworkers [88]. Remarkably, the electronic structure of such a complex was predicted before its preparation [72], Reduction of trichloromethyltrimethyl-silane by iron(II) tetraarylporphyrins in the presence of a reducing agent also leads to the carbide complex [89]. This surprising result can be explained by the involvement of an unstable a-silylcarbene ferroporphyrin complex. A het-erometallic /i2-carbido complex was isolated from the reaction of a dichloro carbene iron porphyrin and pentacarbonylrhenate (Eq. 13) [90]. The X-ray structure of this trinuclear complex, [(TPP)Fe = C = Re(CO)4Re(CO)5], shows a 1,3-dimetalla-allene system. 

Stoichiometric reactions of the Grubbs catalyst with vinylsilanes give predominantly silylstyrene and ruthenium methylidene but traces of styrene derived from the opposite regioselectivity was also detected. Unfortunately, ruthenium silylcarbene complex was not detected, equations 19a and 19b. 

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