Cyclotrisilanes

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. 

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... 

Comparable compounds have recently been described by Weidenbruch et al., starting from cyclotrisilanes as a source for di-organylsilylenes [10]. Whether these complexes are formed by electron donation from the nitrogen lone pairs to silicon with subsequent electron transfer from silicon to the bipyridyl system or by a [4+1]cycloaddition process involving the diazadiene fragment is still uncertain. 

Although homonuclear cyclosilanes have been known for a long time, it is only recently that the smallest members of this series, the cyclotrisilanes, have become available [1], Cyclotrisilanes 1 are obtained by reductive halogen elimination from ort/zo-disubstituted diaryldichlorosilanes [2, 3] or, in particular cases, from hexaalkyl-l,3-dichlorotrisilanes [4]. 

The differences between thermally and photochemically induced reactions of 2 are illustrated by the photolyses of cyclotrisilane 2 in the presence of disparate isocyanides. While in the thermally induced process ring-enlarged molecules can be isolated, the photolytic process results in the formation of 2,4-disila- and 3,4-disilacyclobutanediimines. A plausible explanation for the formation of these... 

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... 

Trisilacyclopentanes (67) under electron impact appear mostly to lose neutral olefin and form cyclotrisilane radical cation (68) <82CC1366>. Elowever, 1,2,3-trisilacyclopent-4-enes <93JA3lll> and 1,2,3,4-tetrasilacyclopentanes <93JA1591> produced no (68). 

Strained cyclotrisilane derivatives, which are silicon analogues of cyclopropane, are prepared by the reductive coupling of overcrowded dichlorodiarylsilanes using lithium naphthalenide (Scheme 14.9). 

Insertion into Si H and Si Si Bonds. Silylenes, generated by thermolysis of cyclotrisilanes, inserted into the Si—Cl or Si—H bonds of monosilane to yield a variety of disilanes, which could be further functionalized. In contrast to carbenes, the insertion of silylenes into C—H bonds has not been observed. However, the insertion into Si—H bonds has been studied extensively. The occurrence of direct insertion has been indicated by formation of nongeminate homocoupling products. ... 

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