Silenes photochemical isomerization

Conlin148 also studied the pyrolysis of 1-methyl-1-silacyclobutane in the presence of excess butadiene at various temperatures where the decomposition followed first-order kinetics and where the silene isomerized to the isomeric silylene prior to reacting with the butadiene. The value for the preexponential factor A for the silene-to-silylene isomerization was found to be 9.6 0.2 s-1 and the Ewl for the isomerization was 30.4 kcal mol-1 with A// = 28.9 0.7 kcal mol-1 and AS = -18.5 0.9 cal mol-1 deg. More recently, the photochemical ring opening of l,l-dimethyl-2-phenylcyclobut-3-ene and its recyclization was studied. The Eact for cycli-zation was 9.4 kcal mol-1.113... 

A related scrambling of groups in a silene has also been reported by Eaborn (143) to explain the structure of compounds isolated from the thermolysis of tris(trimethylsilyl)fluorodiphenylsilylmethane at 450°C, where Me and Ph groups freely interchange between silicon atoms [Eq. (19)]. A related rearrangement is probably also involved in the photochemical silene-to-silene isomerizations derived from acylpolysilanes described earlier. 

Rearrangements of disilanes to a-silylsilenes are well established and are involved in the exchange of substituents between a silylene center and the adjacent silicon.Pulsed flash pyrolysis of acetylenic disilane (41) gave rise to the acetylenic silene (42), which subsequently rearranged to the cyclic silylene, 1-silacyclopropenylidene (43). Irradiation of the cyclic silylene resulted in the isomerization to the isomeric 42, which itself could be photochemically converted into the allenic silylene (44). Both 42 and 43 also were reported to isomerize on photolysis to the unusual (45), which was characterized spectroscopically (Scheme 14.24). 

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 facile photochemical sigmatropic 1,3-trimethylsilyl shift in polysilylacylsilanes from silicon to oxygen (equation 33) was utilized historically to prepare the first relatively stable silenes3 86 87. Silenes prepared by isomerization of acylpolysilanes bear, due to the synthetic approach, a trimethylsiloxy group at the sp2-hybridized carbon and relatively stable silenes of this type have in addition also at least one trimethylsilyl group at the silicon. These substituents strongly influence the physical properties and the chemical behaviour of these silenes. This is noticeable in many reactions in which these Brook -type silenes behave differently from simple silenes or silenes of the Wiberg type. 

Maier and coworkers have shown that it is possible to induce by irridation using the appropriate absorption band, a 1,2-hydrogen shift in silenes and in silylenes29,158. Thus, it is possible to switch photochemically between silylenes and silenes. Michl, West and coworkers have used this approach to isomerize dimethylsilylene 285 and 1-methylsilene 26 several times (equation 70). Due to the clean formation of 285 from the diazido precursor 286153 it was possible to measure the IR transition moment directions for both 26 and 285156. 

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). 

Intramolecular electron transfer occurs on irradiation of the stilbene derivative (319) in methylene chloride. Irradiation populates a charge transfer state that undergoes ,Z-isomerism and when methanol is added to the system not only is isomerism, observed but also the trisilanyl group in (319) is converted into the silane (320). The photochemical reactivity of the vinyldisi-lanes (321) has been examined and direct irradiation of (321a) in cyclohexane is reported to yield two products identified as the (2+2)-dimers (322) and (323). Irradiation of (321c) in cyclohexane with added methanol afforded the methoxy addition product (324). Both these results are in agreement with the intermediacy of silenes such as (325) and (326) in these experiments and... 

When the substituents on silicon are modified, e.g. by replacing one Me3Si group by a t-Bu or phenyl group, the initially formed silene itself is photochemically active and undergoes complex and novel photochemical rearrangements to yield isomeric silenes,... 

Unlike thermal silene isomerizations and fragmentations, which were studied primarily on transient silenes involved in pyrolytic processes, photochemical investigations can be performed conveniently only on silenes that are either kept in matrix isolation or are stable in solution or as solids. The number of such investigations is quite limited. 

A remarkable photochemical silene-to-silene isomerization was observed108 when a solution of the stable l-tert-butyl-l-(trimethylsilyl)-2-(trimethylsiloxy)-2-(l-adamantyl)-silene was irradiated in the UV. The product, 1-methyl-l-(trimethylsiloxy)-2-(terf-butyldimethylsilyl)-2-(l-adamantyl)silene (70), led to two different dimers. The principal one, a head-to-tail dimer, was isolated as a solid, and its structure was identified unambiguously by an X-ray structure determination. When the tert-butyl group was replaced by a phenyl, the photochemical transformation of the starting silene was much more complicated. 

Finally, a third type of photochemical bond shift in a silene, a 1,5-hydrogen migration, is most likely responsible for the recent observation109 that irradiation of 1,1-bis(trimethylsilyl)-2-mesityl-2-(trimethylsiloxy)silene produces a derivative of benzo-cyclobutene, 71 (equation 114), in a process reminiscent of the thermal isomerization of tetramesityldisilene to the silabenzocyclobutene 25 (see Section II.B.l.d). The 2,6-diethylphenyl analogue behaves in a similar fashion109. 

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