Silylenes photochemical generation

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... 

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). 

K. L. Bobbitt, Photochemical Generation of Germylenes and Silylenes Mechanism of Germylene and Silylene Addition to 1,3-Dienes, Doctoral Dissertation, Washington University, St. Louis, December 1990. 

The purpose of this review is to summarize the recent results, obtained mainly in our laboratory, on photochemical generation and reactions of the silylenes and silicon-carbon double-bonded intermediates. 

Photochemically generated silylene species may react either with themselves to form poly silane polymers, or with a trapping agent added to intercept the species. The fate of the silylene and the structure of reaction products depend on the relative rates of these two processes, which may proceed simultaneously. 

Photochemically generated silylenes react with enolizable ketones to give silyl enol ethers. Irradiation of 20 with a low-pressure mercury lamp in the presence of acetone, diethyl ketone, or cyclohexanone gives the re-... 

The reaction of 3 with t-butyJ chloride proceeded under clean formation of covalent 6 (Eq. 2). The satisfactory yields and selectivity of these transformations are in marked contrast to the reactions of photochemically generated trimethylsilylphenylsilylene 7 with chloroalkanes, which afforded, depending on the substrate, the silylene insertion product 8 and/or the hydrogen chloride abstraction product 9 in low yield (Eq. 3) [9],... 

The study of the reactivity of trialkylsilyl radicals in solution has been placed on a firmer foundation by the measurement of absolute rate constants for some reactions of triethylsilyl radicals (generated by the reactions of tert-butoxyl radicals with triethylsilane), with some organic halides and benzil/ These data show for example, the greater reactivity of Et3Si in halogen abstraction than that of trialkyltin radicals. Kinetic isotope effects (kn/fco) for the insertion of photochemically generated dimethylsilylene and methylphenylsilylene into Si-H and 0-H single bonds are about 1.3 and 2.1 - 2.3, respectively. The preferred mechanism for insertion of silylenes into O-H bonds is shown in equation (1). Other workers haye shown that dimethylsilylene inserts preferentially into O-H bonds of alcohols compared with S-H bonds in silanes or Si-O bonds in alkoxy-silanes. ... 

The silylene, Me2Si, can be generated catalytically from a photochemically generated [GpFe(GO)(SiMe3)] species and pentamethyldisilane (Scheme 14). ... 

Kira M, Iwamoto T, Maruyama T, Kuzuguchi T, Yin D, Kabuto C, Sakurai H (2002) Hexakis (trialkylsilyl)cyclolrisilanes and photochemical generation of bis(trialkylsilyl)silylenes. J Chem Soc Dalton Trans 1539... 

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... 

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%). 

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. 

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... 

Corriu et al.29 presented several attempts to generate intramolecularly coordinated silylenes either chemically or photochemically. One approach used to generate a silylene employs the metal-mediated reductive dehalo-genation of dihalosilanes 112 and 115, respectively. Trapping experiments... 

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