Photolysis cyclotrisilane

Disilenes are very reactive toward a variety of unsaturated bonds, such as C=C, C=C, C=N, C=N, C=0, C=S, N=N, and N=0, giving interesting four-membered ring compounds that are otherwise difficult to synthesize.3b Reactions using stable tetraaryl- or dialkyldiaryldisilenes and marginally stable tetra-t-butyldisilene (21), which is generated by photolysis of the corresponding cyclotrisilane, are summarized in Schemes 10 and 11, respectively. Some of them merit further comments. 

The cyclotrisilane is one of a few cyclotrisilanes and is, upon photolysis, a useful source of silylenes and disilenes. 

Co-photolysis of 2,5-di(trifluoromethyl)-l,3,4-oxadiazole with cyclotrisilane, which under these conditions decomposes to afford tetra-(/-butyl)disilene and di-(/-butyl)silylene, provides dihydrodioxadiazadisilocine and trihydro-oxazatrisiline derivatives (Scheme 13) <1996JOM355>. 

The photochemical cleavage of Si-Si bonds of cyclotetrasilanes has been reported to generate several reactive intermediates. For example, Nagai and co-workers reported that silylene and cyclotrisilane are generated during the photolysis of a cyclotetrasilane with a folded structure.73 Shizuka, Nagai, West, and co-workers reported that the photolysis of planar cyclotetrasilanes gives two molecules of disilene.74... 

Cyclotrisilanes are easily converted to a mixture of silylene and disilene. Reactions of the silylene (l-Bu)2Si and the disilene l-Bu2Si=Si(Bu-l)2 produced by photolysis of the cyclotrisilane 293 with cyclopentadiene, furan, and thiophene have been reported recently152, as were their reactions with 2,2 -bipyridyl, pyridine-aldimine and ketoimines153. 

The preparative application of photolytic ring contraction reactions of cyclotetrasi-lanes, which also lead to the formation of cyclotrisilanes, is largely impeded by the limited stability of the cyclotrisilane moiety. The photolysis of octaisopropylcyclote-trasilane, for instance, affords the corresponding cyclotrisilane only as an intermediate, which can be detected by UV spectroscopy. Further silylene extrusion gives rise to the... 

Functionalized cyclotrisilane derivatives bearing oxygen- or nitrogen-containing substituents are also known. The synthesis of the aryl-alkoxy cyclotrisilane trimesityl-tris(2,6-diisopropylphenoxy)cyclotrisilane can be easily accomplished by the photolysis of 2-mesityl-2-(2,6-diisopropylphenoxy)-l,l,l,3,3,3-hexamethyltrisilane via several silylene extrusion and addition steps (equation 7)40. 

Further cycloaddition reactions of silylenes generated by the photolysis of cyclotrisilanes have been published since Weidenbruch and coworkers summarized these reactions in an excellent review. Different siliranes were prepared by [2+1]-cycloaddition of di-t-butylsilylene to various alkenes and dienes (Scheme 6)46. Quite interesting results are obtained from the photolysis of hexa-i-butylcyclotrisilane in the presence of unsaturated five-membered ring compounds47 (Scheme 7). With cyclopentadiene and furane, [4 + 2]-cycloaddition of the photolytically generated disilene occurs only as a side reaction. Furthermore, [2 + 1]-cycloaddition of the intermediately formed silylene is highly favored and siliranes are primarily obtained. A totally different course is observed for the reaction in the presence of thiophene. The disilene abstracts the sulfur atom with the formation of the 1,2-disilathiirane as the major product with an extremely short Si—Si distance of 230.49 pm. 

The reactions of the tetraaryldisilene 9 and those of the marginally stable tetra-tert-butyldisilene 41 are the most thoroughly investigated. The latter compound, together with di-fe/T-butylsilylene 42, is obtained most simply by photolysis of the cyclotrisilane 4065 (equation 7)66. 

The first structurally confirmed [2 + 4] adduct of a disilene and a 1,3-diene was compound 89, obtained from cyclopentadiene and 4174. The formation of the tricyclic compound 95 from furan and the cyclotrisilane 40 is probably initiated by a [2 + 4] cycloaddition of 41 to the five-membered ring to afford 94, which then undergoes a [2 + 1] addition at the newly formed double bond with the silylene 42 formed concomitantly in the photolysis of 40 (equation 16)74. 

Photolysis of cyclotrisilane 72, in the presence of five-membered ring compounds, produces l,l-di- /t-butyl-2,2-dimethyl-l-silacyclopropene 37 when reacted with cyclopentadiene (Equation 16), o o-3,3,6,6,7,7-hexa-/i t/-butyl-3,6,7-trisilacyclo-8-oxatricyclo[3.2.1.0 ]octane 38 and l,l-di-fem-butyl-2,2-dimethyl-l-silacyclopropane 39 when reacted with furan (Equation 17), and 2,2,6,6-tetra-/i m-butyl-2,6-disilabicyclo[3.1.01]hex-3-ene 40 when reacted with thiophene (Equation 18), among other products <19950M5695>. These products were characterized by H, and Si NMR, and for the furan derivative 38, also by X-ray diffraction studies. 

Photolysis of cyclotrisilane 72 in the presence of l,6-bis(trimethylsilyl)-(Z)-3-hexene-l,5-diyne resulted in the formation of yellow crystals of bis(silirene) 101 with an olefinic double bond linkage in 76% yield (Equation 26) <20020M636>. 

Photolysis of cyclotrisilane (100) in the presence of simple alkenes or 1,3-dienes gave silacyclopropanes in modest yield (three examples). For example, reaction of cyclotrisilane with 2-methylbutadiene gave l,l-di-/-buty 1-2-isoprenyl silacyclopropane exclusively, via addition of the intermediate silylene to the less-hindered double bond (Scheme 33) <94CC1233>. 

In addition, cyclotrisilane derivatives can be subjected to photolysis to generate organylsilylenes. For instance, the photolysis of hexakis-t-butylcyclotrisilane (369) in the presence of substituted acetylenes, e.g. (370) affords 1,1,4,4-tetrakis-t-butyl-2,3,5,6-tetraorganyl-l,4-disilacyclohexa-2,5-diene (371) (equation 167)190. Alternatively, thermolysis of disilanes in the presence of alkynes is a viable route to disilacyclohexane derivatives. Thus, thermolysis of l,2-dimethoxy-l,l,2,2-tetramethyldisilane (338) in the presence of diphenylacetylene (370) in a pressure vessel—via reactive intermediates and,... 

The photolysis of cyclic trisilanes constitutes an important source of disilenes, one of the newly accessible and exciting classes of organosilicon compounds (the other major source is from the dimerization of arylsilylenes, usually derived from the photolysis of linear trisilanes). The chemistry of cyclotrisilanes has recently been reviewed7. All cyclotrisilanes studied to date yield a disilene and a silylene when photolyzed at 254 nm64"72 (equation 42). 

Similar photolysis of trisilanes may be used to generate other stable disilenes. Stable disilenes can also be synthesized by photoinduced fragmentation of cyclotrisilanes, and, in some cases, by dehalogenation of dihalo-diorganosilanes. ... 

Reaction of di-fe/t-butylsilylene 148, formed in situ by photolysis of the corresponding cyclotrisilane, with 3,6-bis(trifluoromethyl)-l,2,4,5-tetrazine 147 formed 2,5-bis(trifluoromethyl)-l-/i t/-butyl-l,2,4-diazasilole 149 via a [4-f 1] cycloaddition reaction (Scheme 39) <1996JOM(521)355>. 

The photolysis of cyclotrisilane 137 in the presence of furan gives an unusual tricyclic product 138 (among other products), presumably by reaction of furan with both the disilene Bu 2Si=SiBu 2 and the silylene Bu 2Si generated by the photolysis of 137 (Equation 10) <19950M5695>. 

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