Silylene generation

The silicon analogue of carbene is now called silylene and it was discovered by the thermally-induced alpha elimination of methoxydisilanes. The most common method of silylene generation is ... 

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. ... 

This section presents reactions that proceed via the formation of silylene-Lewis base complexes. The material is organized according to the method used for silylene generation. 

Ando and coworkers23 reacted C(,o with bis(2,6-diisopropylphenyl)silylene generated in situ by the photolysis of the corresponding trisilane. The dark brown powder obtained (58%) exhibits 29Si signal at 8 — 72.74 which, together with other experimental evidence, supports the silirane structure (8a) rather than the bridged annulene structure (8b). 

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. 

Monitoring the reaction by 111-NMR spectroscopy clearly indicates the formation of the 1,2-disilacyclobutenes by a two-step addition-insertion process. Primarily, the silylene generated by thermal decomposition of the cyclotrisilane adds to the C—C triple bond yielding the silacyclopropene. Further insertion of a second silylene into the Si—C bond of the silacyclopropene finally affords the corresponding 1,2-disilacyclobutene. 

Similar results were obtained with Me(MeO)Si and Me(Me2N)Si, consistent with a stepwise mechanism in which initial (2+l)cycloaddition is followed by rearrangement, as proposed earlier by Lei and coworkers20. Under the experimental conditions, the temperatures required for silylene generation were 400 °C for [MeSi(OMe)2]2, 420 °C for [MeSi(NMe2)2]2 and 500°C for [MeSiCl2]220. 

In 1981, West et al. synthesized the first stable disilene 1 via the dimerization of the corresponding silylene generated by the photolysis of a trisilane and characterized the structure by conventional spectroscopies [Eq. (2)].5 Availability of 1 and other stable disilenes has stimulated theoretical and experimental studies of various aspects of disilenes such as their bonding and structure, spectroscopic properties, reactivities, applications to the synthesis of novel types of organosilicon compounds, etc. 

On the other hand, the photolysis of bis(l,l,2,3,3-pentamethyl-l,2,3-tri-silacyclopentyl) (7) under the same photolysis conditions as above reveals that the silylene species is expelled in a different manner in two distinct steps. In contrast to 5, the photolysis of 7 proceeds to give only one kind of silylene species in each step. In the first step, compound 8, to be expected from the insertion of the silylene generated by a 1,2-shift of a tri-silacyclopentane ring into diethylmethylsilane, is formed. In the second step, compound 8 thus formed undergoes further photolysis yielding an-... 

Z ) Some other methods of silylene generation are by the photolysis of polysilanes. 

The two silylene-generating processes are believed to occur in the singlet excited state in pericyclic fashion, while the radical-pair-forming homolytic cleavage process is believed to occur in the triplet state (Michl and Balaji,... 

Silicone Polymers - Laser flash photolysis studies on poly(silylenes) generates radical cations along with silyl radicals and polysiloxane composities for the space shuttle have been found to be stable to far UV light exposure. Linear polysiloxanes have been found to be more unstable than branched or crosslinked polymers while the transparency of poly(methylphenylsilane) increases with light exposure. Photooxidised polysiloxanes doped with iodine are converted into semiconductors. ... 

The overall chemical fate of a silylene, SiX2, in a reacting system is summarized in equation (68). This scheme is a modification of the one proposed by Atwell and Weyenberg. The silylene generated from its precursor may interact with three different types of species in a system, (i) It may dimerize or polymerize if such reactions are of high efficiency, and if the instantaneous concentration of the silylene is rather high. The dimers generally behave as reactive... 

The photolysis of trisilanes, RR Si(SiMe3)2, or of cyclosilanes (RR Si) , is a well-established method for silylene generation Here we will describe only selected examples, concentrating especially on findings which bear on the mechanism of silylene elimination. 

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