Silylcarbenes photochemical

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

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. 

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. 

Photolysis of l-methyl-2,3,4,5-tetraphenylsilacyclopentadienyldiazomethane (55) with an excess of methanol gave the diazirine 56 and the methanol adducts 59, 60 and 61 (equation 14)38. Products 59 and 60 appear to be derived from the methanolysis of silabenzene (46b), which could arise by a ring expansion of silacyclopentadienylmethyl-carbene (49b). On the other hand, compound 61 is the evident product of methanolysis of 5-silafulvene (57) formed by a methyl shift in 49b. Diazirine 56 was also found to produce silylcarbene under photochemical and thermal conditions and led to the same products 59,60 and 61. 

In contrast to the carbene and carbenoid chemistry of simple diazoacetic esters, that of a-silyl-a-diazoacetic esters has not yet been developed systematically [1]. Irradiation of ethyl diazo(trimethylsilyl)acetate in an alcohol affords products derived from 0-H insertion of the carbene intermediate, Wolff rearrangement, and carbene- silene rearrangement [2]. In contrast, photolysis of ethyl diazo(pentamethyldisilanyl)acetate in an inert solvent yields exclusively a ketene derived from a carbene->silene->ketene rearrangement [3], Photochemically generated ethoxycarbonyltrimethyl-silylcarbene cyclopropanates alkenes and undergoes insertion into aliphatic C-H bonds [4]. Copper-catalyzed and photochemically induced cyclopropenation of an alkyne with methyl diazo(trimethylsilyl)acetate has also been reported [5]. 

Ando and coworkers122 described an even more complex system in which the initially formed silylcarbene suffered 1,2-silyl migration to yield a silene, which then further rearranged by migration of an ethoxy group to silicon to yield a ketene (equation 79). Studies in a matrix demonstrated that this latter step was a photochemical process. 

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... 

By far the best source for 3a is (trimethylsilyl)diazomethane (19). It has already been mentioned that gas-phase pyrolysis of 19 alone yields products which are derived from intramolecular carbene reactions such as 1,3-C,H insertion and silylcarbene-to-silene rearrangement (see equation 20). Also, copyrolysis of 19 with alcohols or benzaldehyde allowed one to trap the silene but not the carbene 33 (see equation 5). Furthermore, solution photolysis of 19 in the presence of alcohols or amines did not give the X,H insertion products of the carbene but rather trapping products of the silene . On the other hand, photochemically generated carbene 3a did undergo some typical intermolec-ular carbene reactions such as cyclopropanation of alkenes (ethylene, frani-but-2-ene, but not 2,3-dimethylbut-2-ene, tetrafluoroethene and hexafluoropropene), and insertion into Si—H and methyl-C—H bonds (equation 39). The formal carbene dimer, trans-1,2-bis(trimethylsilyl)ethene, was a by-product in all photolyses in the presence of alkenes it is generally assumed that such carbene dimers result from reaction of the carbene with excess diazo compound. 

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