Photolysis of silanes

Milligan, D. E., Jacox, M. E. Infrared and ultraviolet spectra of the products of the vacuum-ultraviolet photolysis of silane isolated in an argon matrix. J. Chem. Phys. 

Thermolysis or photolysis of silanes produces new Si—H bonds the reactions involve silylenes ... 

A teehnique that is a convenient source of radieals for study by EPR involves photolysis of a mixture of di-t-butyl peroxide, triethylsilane, and the alkyl bromide corresponding to the radieal to be studied. Photolysis of the peroxide gives t-butoxy radieals, whieh selectively abstract hydrogen from the silane. This reactive silicon radieal in turn abstracts bromine, generating the alkyl radieal at a steady-state eoncentration suitable for EPR study. 

Silicon centered radicals can be generated by transfer to silanes and by photolysis of polysilanes. Rate constants for addition to monomer are several orders of magnitude higher than similar carbon centered radicals.453,43 The radicals have nucleophilic character. 

The total dissociation cross sections of silane and disilane have been taken from Perrin et al. [201]. An uncertainty in the present knowledge of the silane chemistry is the branching ratio of the silane dissociation channels [192]. Here, the branching ratio is taken from Doyle et al. [197], who suggest using the branching ratio determined by Perkins et al. [202] for photolysis, viz., a branching of... 

In contrast to the observed photochemistry of adamantyltris(trimethylsilyl)silane which efficiently yields the appropriate silene compound, similar photolysis of the germanium analogue provided no evidence for the production of germene13. However, photolysis of the germanium compound in CCI4 did result in a Norrish type 1 cleavage of... 

The formation of silyl radicals in the exhaustive photolysis of the silane polymers was indicated by the isolation of disilanes of general structure (HSiRR SiRR H) as shown in Table 1. These materials accumulate in the photolysate and are photostable as they absorb only weakly at the irradiation wavelength (254 nm). Longer chain silanes are presumably continuously degraded under the conditions of the exhaustive irradiation. 

Photolysis [KINETTCMEASUREMENTS] (Vol 14) maleic anhydnde [MALEIC ANHYDRIDE, MALEIC ACID AND FUMARIC ACID] (Vol 15) of molybdenum compounds [MOLYBDENUM AND COMPOUNDS] (Vol 16) of pesticides [SOIL CHEMISTRY OF PESTICIDES] (Vol 22) of silanes [SILICON COMPOUNDS - SILANES] (Vol 22) of titanium compounds [TITANIUM COMPOUNDS - ORGANIC] (V ol 24)... 

All silenes generated so far on the silylcarbene route are reactive intermediates themselves, which were characterized by typical subsequent reactions35 such as isomerization and dimerization or by trapping reactions (see below). However, photolysis of (silyl)diazo compounds in inert matrices at low temperature allowed the isolation and spectroscopic (IR, UV) characterization of several silenes (Scheme 2, Table 3). Irradiation of (dia-zomethyl)silanes 7 at X > 360 nm produced both diazirine 8 and silenes 10, but at shorter wavelength (X > 305 nm) the silenes were produced cleanly from both precursors the... 

In this connection Ishikawa and coworkers studied the photodegradation of poly(disilanylene)phenylenes 203126, and found that irradiation under the same conditions as in the photolysis of the aryldisilanes results in the formation of another type of nonrearranged silene 204 produced together with silane 205 from homolytic scission of a silicon-silicon bond, followed by disproportionation of the resulting silyl radicals 206 to 204 and 205 (equation 51). 

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. 

Tetramethylsilacyclopropene, the first silacyclopropene derivative, was prepared in 1976145. With the help of the matrix-isolation techniques, attempts were made to synthesize the parent silacyclopropene (114). Experiments with this goal culminated in the isolation of 1, 1 -dimethylsilacyclopropene (110) in solid argon146. Sander and coworkers showed that 110 is accessible by photolysis of the corresponding bis(diazomethyl)silane (109) (equation 29). After subsequent irradiation with shorter wavelengths, isomerization into the photostable ethynyldimethylsilane (111) takes place. 

A third photochemical access to silylenes, beside the isomerization of silenes and the photolysis of tri- or cyclopolysilanes, is the irradiation of geminal diazidosilanes, which under nitrogen loss often gives the corresponding divalent silicon species. However, silan-imines are frequent by-products164 or in some cases even the only products (Section IV). 

The photolysis of phenyltris(trimethylsilyl)silane in the presence of excess ethanol gave, as the first product, PhSi(H)OEt—SiMe3. The amount of this product went through a... 

---