Silenes photochemical

West and coworkers studied the photolysis of several adducts of disilenes with ketones, i.e. 1,2-disiloxetanes83. Based on the products obtained when the photolysis was carried out in ethanol as a trapping agent, it appears that the heterocyclic disiloxetane 179 decomposed to the silanone 180 and the silene 181, each trapped by ethanol to give the adducts 182 and 183, respectively (Scheme 29). In the absence of a trapping agent the silene photochemically rearranged to 184. A related 1,2-disilathietane 185 showed similar behavior (Scheme 29184. 

During recent years, fascinating developments have occurred in the area of r 2-silene complexes, which opened up to totally new chemistry. Some of the highlights will be presented in this section. The first investigations of coordination compounds of silenes were carried out by means of matrix isolation techniques at very low temperatures. In particular, photochemical methods proved to be most effective... 

A few other routes to new silenes involving photochemical processes have been reported, most of which have the structure RR Si=CH2110-112 and are listed in Table I. Photolysis of l,l-dimethyl-2-phenyl-l-silacyclo-but-2-ene 22 in a glass at 77 K led to the siladiene 23, which absorbed at 338 nm,113 as shown in Eq. (17) ... 

The trapping of silacyclopentadienes has also been reported recently.115 Using the pyrolysis of 27, or photolysis or pyrolysis of 28, the formation of the silylene 29 was inferred. Further photolysis or thermolysis converted the silylene into silene 30, which could be photochemically isomer-... 

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... 

Jones218 has described an unusual photochemically initiated rearrangement of a silene-anthracene adduct to a silene which is part of an eight-membered ring (Eq. 60). Photolysis of the adduct 190 was believed to form the silaallylic diradical 191, whose canonical form 192 affords the... 

The adducts 41 from 1 and ketones or thiobenzophenone undergo interesting photochemical cycloreversion to afford a silanone or silanethione intermediate 42 in addition to silene 43 both of these intermediates are trapped by ethanol, as shown in Eq. (14).68 71 In the reaction with the thiobenzophenone adduct 41 (R = Ph, X = S), the intermediate silene 43 (R = Ph) was detected by Si NMR.71... 

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... 

Several di- or polysilyl systems have been found to be useful precursors for the photochemical generation of silenes. Vinyldisilanes cleanly yield silylmethylsilenes (133), while alkynyldisilanes yield mixtures of silylated silaallenes and silacyclopropenes [Eq. (8)] (119,136). Aryldisilanes when photolyzed form species presumed to be silenes, but showing unusual chemical behavior (see below) [Eq. (9)] (97-102). 

Finally, research in our group has shown that a wide variety of polysilylacylsilanes consistently undergo very clean photochemical 1,3-rearrangements of silyl groups from silicon to oxygen and yield silenes, some of which are remarkably long-lived, and two of which have been crystallized [Eq. (10)] (104,122-124). 

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. 

The photochemical interconversion of silylenes and silenes is an important link between these two classes of compounds. It was first established that irradiation of the parent silene (38) with irradiation with light of X = 254 nm resulted in the formation of methylsilylene (39) (Scheme 14.22). The reaction is reversible by using light of X = 320 nm. Ultraviolet absorption of silylene 39 strongly depends on the matrix... 

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). 

Sakurai and coworkers75 generated the five-membered silene 44 by a photochemical 1,3-silyl shift in the cyclic divinyldisilane 45 (Scheme 13). Since the silene 44 is constrained to be planar, no bond rotation is possible during the reaction. Contrary to the previous observations i.e. a simple two-step or a concerted four-centered mechanism, alcohols add to 44 nonstereospecifically, although in the cyclic silene bond rotation is prohibited. 

Steinmetz and coworkers carried out mechanistic studies on the far-UV photochemical ring opening of l-silacyclobut-2-ene 80. The intermediates were trapped by alcohols to give 84-87 and by methoxytrimethylsilane to give 88 and 8955. The main reaction is the formation of 1-silabuta-1,3-diene 81, while the formation of silene 2, probably via the carbene 90, is a minor reaction (equation 19). The mechanism suggested was supported by deuterium labelling studies and ab initio calculations. 

The photochemical reactions of 2-silabicyclo[2.2.2]octanes 110 and 112 have been investigated69. Photolysis of 112 in the presence of methanol gives silyl methyl ether 113 as the main product, while 114 is only a minor byproduct (equation 26). This suggests that silene formation is a minor process and the main reaction proceeds via diradical intermediate similar to 115. In agreement, interconvertion of E/Z 110, probably via diradical 115, is found during photolysis along with extensive polymerization (equation 27). 

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. 

Disilanyl substituted naphthalenes exhibit unusual photochemical reactivity131. 1,4-Bis(pentamethyldisilanyl)naphthalene 219 yields compound 220 in both the absence and presence of methanol, possibly via a biradical 221. Noteworthy is the 1,8-silyl migration from position 1 to 8 of the naphthalene ring. In the presence of methanol, compound 222 is formed via an initial silene 223, which then rearranges via 224 (equation 55). In a homogenous solution of methanol/benzene (1 1.5) only 225 is formed, probably by direct reaction of the photoexcited disilane with methanol, before migration of a trimethylsilyl... 

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 photochemical approach via a silylene-to-silene rearrangement was followed by Fink and coworkers in their synthesis of silacyclobutadienes in a 3-methylpentane matrix at low temperatures161. Irradiation of the cyclopropenyltrisilane 294 gives the relatively stable cyclopropenylsilylene 295. 295 can be efficiently converted to silacyclobutadiene 296 by irradiation into the visible absorption band of the silylene (equation 72)161 162. 

Silenes are formed by rearrangement of silylcarbenes. If polysilylated diazomethanes 298-300 are employed, a selective migration of a silyl group to the carbene centre occurs and silenes 301, 92 and 302 are formed (equations 74-76)164. The outcome of trapping reactions is independent of the mode of silene generation photochemical and pyrolytic methods give the same results. 

The photochemical decomposition of bis(silyldiazomethyl)tetrasilane 320 produces one silene group to give 321 followed by intramolecular [2 + 3] silene-diazo cycloaddition via 322 to give the bicyclic compound 323 as final product, while thermal decomposition gives a bis-silene 324 which then undergoes head-to-tail dimerization168,169 to... 

The stereospecificity of methanol addition to neopentylsilenes has been investigated by Jones and Bates68. The mild thermal retro-Diels-Alder reaction (at ca 200 °C) of E and Z anthracene [4 + 2] cycloadducts 110 liberates stereospecifically the corresponding silenes 111, which are trapped by methanol. The ratio of the diastereomeric products 396a/396b coincides with the E/Z ratio of the precursors 110 (equation 117). In photochemical reactions of similar silene precursors, alcohols were used also to probe the decomposition mechanism69. 

In view of the evident reactivity of Brook -type silenes toward carbonyl compounds and the fact that they are generated photochemically from acylpolysilanes, the question arises whether the silenes do react with their precursors. The very reactive silene 507... 

With benzophenone the reactions are stereoselective. Equation 190 outlines the reaction mechanism for the case of acetone (R1 = R2 = Me). The disilanes rearrange via a concerted suprafacial 1,3-silyl shift under the photochemical conditions to produce the silenes diastereospecifically. 

These results are most likely interpreted in terms of the photochemical cycloreversion of the thiadisiletane 106 leading to the formation of the silene 109 and silanethione 110, though both of them were not isolated. 

The same authors investigated the photochemical and thermal oxidation of 2-silapropene 12, silaisobutene 7 and 1,1,2-trimethylsilene 13 in O2-doped argon matrices (equation 5)28. These silenes are easily photooxidized in matrices containing more than 1% 62, but only trimethylsilene 13 exhibits thermal reactivity toward oxygen at temperatures as low as 20-40 K. 

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