Silylenes photochemical

The use of siliranes as thermal sources of silylenes has long been known. At least in some cases, they can also yield silylenes photochemically. This has been demonstrated for di-t-butylsilylene56 and di-l-adamantylsilylene (16)57 as shown in equations 25 and 26. 

In a similar way, a set of disilanyl and polysilanyl complexes has recently been synthesized and exposed to photochemical deoligomerization reactions [137]. The photolytically obtained reactive silylene complexes have been identified by trapping experiments [138, 139]. 

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

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... 

When silylenes are generated photochemically in hydrocarbon matrices in the presence of electron-pair donors, they may form Lewis acid-base complexes that act as intermediates in the silylene dimerization to disilenes.3233 In a typical example, Mes2Si(SiMe3)2 was photolyzed in 3-meth-ylpentane (3-MP) matrix containing 5% of 2-methyltetrahydrofuran. At 77 K, dimesitylsilylene (Amax 577 nm) was formed. When the matrix was... 

Earlier, it had been shown that 25 reacts with CH3OH under photolysis to give similar silylene trapping products this reaction may involve photochemical dissociation of 25 into silylenes.23a... 

Secondary Photochemical Processes. While the nature of the primary photochemical step may be described as still uncertain, the nature of the subsequent secondary steps is best characterized as obscure. A previous trapping study during exhaustive irradiation (30) demonstrated that silylenes are formed somewhere along the line and implicated silyl radicals as well since the formation of Si-H bonds was observed, presumably by hydrogen atom abstraction. 

The photochemical cleavage of Si-Si bonds of cyclotetrasilanes has been reported to generate several reactive intermediates. For example, Nagai and co-workers reported that silylene and cyclotrisilane are generated during the photolysis of a cyclotetrasilane with a folded structure.73 Shizuka, Nagai, West, and co-workers reported that the photolysis of planar cyclotetrasilanes gives two molecules of disilene.74... 

Photochemical transformations of cyclic and short chain polysilane oligomers have been intensively investigated (39). Irradiation of these materials in the presence of trapping reagents, such as silanes or alcohols, has suggested that substituted silylenes and silyl radicals are primary reactive intermediates. The former have been... 

In summary, the production of substituted silylenes and silyl radicals upon exhaustive irradiation at 254 nm of polysilane high polymers suggested that the polymer photochemistry resembled that previously reported for short chain acyclic and cyclic oligomers (39). More recent experiments, however, have suggested that the photochemical mechanism for the degradation of the high polymers is more complex than first envisioned (vide infra) (48). 

Regarding this proposal, it should be noted that while 1,1-eliminations on Si-Si-C units to generate silylenes are well known thermal processes (54) the photochemical variant seems not to have been described. The rearrangement of silylsilylenes (4) to disilenes is known to be rapid (55), and silyl radical addition at the least hindered site would produce the observed persistent radical. Preliminary evidence for the operation of 1,1-photoelimination processes in the polysilane high polymers has been obtained, in that the exhaustive irradiation at 248 nm of poly(cyclohexylmethylsilane) (PCHMS) produces —10-15% volatile products which contain trialkylsilyl terminal groups. For example, the following products were produced and identified by GC— MS (R=cyclohexyl,R = methyl) H(RR Si)2H (49%), H(RR Si)3H (19%), R2R SiH (2%), R 2RSiRR SiH (5%) and R2R SiRR SiH (7%). 

On the basis of the results described, there seem to be at least three processes which are responsible for the molecular weight reduction in substituted silane high polymers upon irradiation in solution (i) Chain abridgement by silylene extrusion which occurs only at short wavelengths (ii) chain scission by silicon-silicon bond homolysis and (iii) chain scission by 1,1-photochemical reductive elimination. 

An attempt to investigate the possible photochemical rearrangement of silacyc-lobutenylidene (132) to give silacyclobutadiene produced seven silylene species in the... 

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. 

Photochemical irradiation of (i-Pr3Si)3SiH (14) with light of 254 nm in either 2,2,4-trimethylpentane or pentane leads to the elimination of f-Pr3SiH and the generation of bis(triisopropylsilyl)silylene (/-Pr3Si)2Si (15). Silylene 15 can also be generated by the thermolysis of the same precursor 14 at 225 °C in 2,2,4-trimethyl-pentane (Scheme 14.11). Reactions of 15 include the precedented insertion into an Si H bond, and additions to the ti bonds of olefins, alkynes, and dienes. 

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). 

The mechanism of the photochemical degradation of catenated silicon derivatives has received considerable attention (25). Substituted cyclic derivatives photochemically extrude a silylene fragment which can be intercepted by appropriate trapping reagents (e.g., trialkylsilanes or 2,3-dimethyl butadiene). This extrusion results in the formation of the corresponding ring contracted cyclopolysilane. The process continues upon additional irradiation until a cyclotetrasilane results which then undergoes... 

Silylenes, the silicon analogs of carbenes, are important intermediates in many thermal and photochemical reactions of organosilicon compounds [168]. Some have been isolated [169]. The chemistry is characterized both by high electrophilicity and nucleo-philicity for the same reasons as discussed above in the case of silyl cations. 

Disilanyl enol ethers (22) are the main product of the reaction of alkyl ketones with photochemically generated phenyltrimethylsilylsilylene (Scheme 30) (77JOM(135)C45). They could result from silylene insertion into the enol O—H bond, particularly as many have been isolated from dodecamethylcyclohexasilane and alkyl ketones or aldehydes on photolysis (Scheme 31) (78CL609). 

The photolysis of tris(trimethylsilyl)phenylsilane in the presence of a series of alkynes alforded the silacyclopropene through silylene addition to the triple bond. Those obtained from monosubstituted alkynes underwent photochemical isomerization to the disilanyl-alkyne through a 1,2-hydrogen shift (Scheme 48) (80JOM(190)117). Disubstituted alkynes form silirenes that can be isolated by preparative GLC. 

Levin et al.55 observed, in line with the qualitative results of Gillette et al. 6 that less bulkily substituted silylene 13 is stabilized when coordinated by Lewis bases. Dimethylsilylene 13 was generated photochemically from 32 in cyclohexane solutions at 296 K, and it was shown by means of UV spectroscopy that the lifetime of this silylene is 0.4 fjs under the conditions... 

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