Silylene precursor photolysis

Photochemical behaviour of compounds 83-86 [33] in the gas phase has been reported, in order to distinguish between silyl radical and silylene formation. Photolysis of the noncyclic precursors 83 and 84 gave products derived from silyl radicals, which come from a direct Si—Si bond homolysis, with a little evidence of silylene formation. In contrast, dimethylsilylene (Mc2Si ) was observed as a direct photoproduct from the cyclic precursors 85 and 86. The reaction sequence including a Sni step shown in Scheme 6.18 for the formation of dimethylsilylene was proposed to explain the different observations for cyclic and noncyclic systems. 

CO complexes of substituted silylenes have been claimed to be observed after photolysis of silylene precursors in CO-doped argon or organic matrices by Arrington [7] and by West [8] and coworkers, respectively. 

There have been a number of routes to silylenes that are related to the simple photolysis of the precursors. The principal photoprocesses of alkylpolysilanes in solution are (a) chain abridgement through ehmination of silylene and (b) chain scission by Si—Si bond homolysis. The photolysis of trisilanes [RR Si(SiMe3)2] or cyclopolysilanes [(RR Si) ] is a well-established method for the generation of silylene (Scheme 14.7). ... 

Both silene isomers 278 and 279 are ideal precursors for the generation of silylene 284, since their interconversion to 284 is spontaneous (in the case of 278) or can be easily induced by irradiation (in the case of 279). There are numerous well-established methods to prepare transient silylenes 279. Three important examples are shown in equation 69, namely the photolytic generation from a trisilane 280153, thermolytic or photolytic decomposition of cyclic silanes 28114,154,155 and degradation of diazidosilanes 282153,156. The photolysis of the diazido silane 282 is an especially clean reaction which has been used in several spectroscopic studies157. The photolysis of w-diazo compounds 283 is the only frequently used reaction path to silenes 284 via a carbene-silene rearrangement8. 

Our group tried to observe the formation of silacyclobutadienes via a photochemical pathway59-61. Photolysis of cyclopropenyl silanes 41 a-1 in hydrocarbon matrices at 77 K could produce the corresponding silylenes 42, which should be suitable silacyclobutadiene precursors (equation 13). In only two cases (41a,d), the photolysis produced the expected silylenes 42, but it was not possible to initiate their photochemical conversion into the desired silacyclobutadienes 43. 

Other recent examples89-92 of the photolysis of trisilanes to prepare silylenes as precursors to disilenes will be found in Section VII. 

When di(t-butyl)silylene 321, generated in a 3-methylpentane glass at 77 K or in an argon matrix at 10 K by photolysis of a precursor bis-azide, was irradiated with 500-nm light, intramolecular C—H insertion occurred yielding the silacyclopropane 322 (equation 26)161. 

Photolysis of phenanthraquinone (PQ) in the presence of disilane precursors such as 7,8-disilabicyclo[2.2.2]octa-2,5-diene using two 500 W tungsten-halogen lamps led to the formation of 227 and 228 as silylene-transfer products (Equation 46) <2001JOM63>. 

Up-to-date, a variety of disilenes have been reported, owing to the improvement of several synthetic methods. For example, the photolysis of cyclic trisilanes (Scheme 5) and the reduction of dihalosilanes or 1,2-dihalodisilanes (Scheme 6) were found to be useful approaches to the corresponding stable silylenes when the precursors are available as stable compounds. 

Silylenes can be generated by photolysis, thermal decomposition of a suitable precursor molecule, by reduction of silylhalides, and by transition metal catalysis. Ever more unique and ingenious methods have been used to prepare such intermediates. Since this section is devoted to reactions at a Si(II) center, the means by which that center is created are not discussed in detail. However, a brief overview of the most common methods for generating such species follows. 

Bis(diisopropylamino)silylene has been generated by photolysis of the precursor (76), and trapped chemically by reaction with triethylvinylsilane and 2,3-dimethylbuta-l,3-diene.Contrary to theoretical prediction, its dimer seems to have a disilene structure and not a bridged structure. 

While many examples of carbene oxidations have been reported, only four papers on the reaction of silylenes 1 with molecular oxygen have been published. The limited number of experimental studies on the oxygenation of silylenes is mainly due to the lack of suitable precursors.The photolysis of matrix-isolated trisilanes produces silylenes in close proximity to disilenes or other products of the precursor decomposition rather than matrix-isolated silylenes.Gas-phase thermolysis of disilanes and other thermal precursors requires very high temperatures, while the photolysis of diazidosilanes requires short-wavelength UV irradiation. In all of these cases, the yields of silylenes are rather poor. 

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