Photolysis, silenes from

Using nanosecond laser flash photolysis techniques, Leigh80 observed transient absorption spectra which he attributed to the silenes derived from photolysis of various methylphenyldisilylbenzenes. Thus the silenes 52,53, and 54 were found to absorb at 425,460, and 490 nm, respectively, in isooctane, and 55 was also found to absorb at 490 nm.75 In other studies, the silene Ph2Si=CH2 derived by laser flash photolysis was found to absorb at 323 nm.111... 

In the past decade, Ishikawa et al. have investigated the photochemistry of aryldisilanes.66 73 76 84 Lately, dimers derived from these unusual silenes have been observed from the first time.78 Thus, photolysis of 1,4-bis(penta-methyldisilyl)benzene 76 in hexane presumably gave rise to the silene 77 but, on workup employing crystallization, two head-to-head stereoiso-meric dimers 78 and 79 were obtained in about 45% yield in a 1 1 ratio. These were said to be formed from the silenecyclohexadienes 77 by the two alternative pathways shown in Scheme 12. Related dimers were also... 

Unlike pentamethylphenyldisilane, the photolysis of compound 5 in the presence of methanol in hexane gave no products arising from the reaction of a silene with methanol, but polymeric substances were obtained as main products, in addition to small amounts of bis-(trimethylsiloxy)phenylsilane (6) (4%) and bis(trimethylsiloxy)-phenylmethoxysilane (7) (5%). 

When similar photolysis of 11 in the presence of MeOD was carried out, again the product whose NMR reveals the resonance due to the Si-H proton was observed. The relative ratio of the Si-H and CH3-0 protons was identical with those of the products obtained in the presence of non-deuterated methanol. The formation of the methoxysilyl group can be understood by the addition of methanol across the silicon-carbon double bonds. H NMR spectra of all photoproducts obtained from the photolyses of 11 in the presence of methanol reveal no resonances attributed to the cyclohexadienyl ring protons. This indicates that the photochemical degradation of the polymer 11 gives no rearranged silene intermediates, but produces... 

Two families of silenes warrant special comment. The first arises from the photolysis of a number of aryldisilanes as investigated by Kumada et al. (97-102), who reported that a rearrangement occurs to give products formulated as silenes, as shown in Eq. (16). 

Another family of silenes, those derived from the photolysis of acyldi- or polysilanes at A > 360 nm, also show somewhat unusual behavior compared to simpler silenes. These silenes exhibit great stability which in some cases has allowed isolation of solid silenes, and which has allowed acquisition of much physical data relating to silicon-carbon double bonds as mentioned earlier. 

Silyl Radicals (R3Si). Much is known about the chemistry of silyl radicals. They can be produced from thermolysis, photolysis and electron transfer reactions. With the exception of disproportionation and degradation to silenes, most of the reactions known for the carbon radicals are also known for silyl radicals. 

Silaethylenes (silenes) were extensively reviewed in 1979 and the first example stable enough to be bottled was prepared two years later. It results from the photolysis of the ketone (56) and is stable in the absence of air and other reagents. It melts at 92-95 °C and shows a ai NMR spectrum with the two Me3Si groups on the Si=C multiple bond nonequivalent, indicating restricted rotation. The IR absorption at 1135 cm-1 typifies a silaethylene (Scheme 85) (81CC191). 

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... 

Bis(trimethylsilyl)diazomethane (25) represents an excellent source for silene 2641. It appears that carbene 3c, which is expected from the photochemical or thermal decomposition of 25, escapes most trapping efforts due to rapid isomerization to silene 26 (equation 7). Photolysis of 25 in benzene solution yields 27 and 28 in a combined yield of 64% and disilazane 29 (10%) all these products are likely to be derived from 26. Similarly, photolysis in the presence of methanol or I)20 traps the silene quantitatively (to give 31 and 32). 

The laser flash photolysis of gaseous silacyclobutanes 2224 and 2325 and 1,3-disilacyclobutane 24 produced the transient silenes 25 (from 22 and 24) and 26 (from 23) as the major primary product. The silenes 25 and 26 were identified by their UV spectra with Xmax s=ss 260 nm. Rate constants for the decay processes of the transient silenes were also measured. 

Silenes of the family Me3SiR1Si=C(OSiMe3)Ad-l 137 undergo a complex silene-to-silene photoisomerization reaction90,94,96. When silenes 137 are generated by photolysis of acylsilanes 138, the isomeric silenes 139 and 140 are formed in a subsequent reaction. The reaction was followed by UV and NMR spectroscopy. The disappearance of 138 cleanly follows first-order kinetics and the overall kinetics were consistent with the transformation 138 -> 137 -> 139. 137 as well as 139 were characterized by NMR spectroscopy and, in addition, the structure of 137 was established by trapping with methanol. The identity of 139 and 140 was confirmed by the isolation of their head-to-tail dimers from which crystals, suitable for X-ray analyses, were isolated (equation 34)90. 

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. 

Thus, the alkyl-substituted silenes of the family (Me3Si)2Si=C(OSiMe3)R give with triphenylimine the unstable 514 which is converted completely, faster upon photolysis or more slowly in the dark, into the silaazetidine 515 (equation 174). For the adamantylsilene 150 the complete conversion from the acylpolysilane to 515 (R = 1-Ad) requires 5 days and proceeds in an overall yield of 94%248. The mesitylsilene 349 forms no [4 + 2] cycloadduct, and the only product of the reaction with triphenylimine detected after 24 h is the silaazetidine 515 (R = Mes)248. The imine component also influences the product distribution of the reaction. For example, no [4+2] cycloadducts are formed in the reaction of silenes (Me3Si)2Si=C(OSiMe3)R with A-fluorenylidineaniline and only silaazetidines have been detected248. 

A siloxetane 526 as an intermediate from a [2 + 2] cycloaddition of silene 241 with acetone has been formulated by lshikawa134. It extrudes a silanone equivalent to give the vinyl ether 527. The second regioisomeric silene 242 generated together with 241 by photolysis of 240 undergoes an ene reaction instead (equation 179)134. 

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