Silacyclopropanes photolysis

The suggestion that a silacyclopropane intermediate is formed on photolysis of styryl-disilanes (Scheme 46) (76JA7424) prompts the use of disilanylalkynes as precursors to silirenes. The silirene (36) is formed by a 1,2-silyl migration as the major product using (42), but the silapropa-1,2-diene (43) also results. Both products can be trapped as methanol or acetone derivatives (Scheme 47) (77JA3879, 79JOM( 179)377). 

When di(t-butyl)silylene 321, generated in a 3-methylpentane glass at 77 K or in an argon matrix at 10 K by photolysis of a precursor bis-azide, was irradiated with 500-nm light, intramolecular C—H insertion occurred yielding the silacyclopropane 322 (equation 26)161. 

Ando and coworkers found that the silylene Dip2Si 382 (Dip = 2,6-di-/-propylphenyl), formed on photolysis of the trisilane 382 in the presence of Cgo, gave an adduct assigned the structure of the silacyclopropane 384 which evidently arose from addition of the silylene across the C=C between two six-membered rings of the fullerene198. A segment of the structure of 384 is shown in equation 47. 

If a low-pressure mercury lamp with a Vycor filter is used for the photolysis of 10 in the presence of olefins, silacyclopropanes are obtained (52,... 

Under this photolysis condition, the rates of photoisomerization of the initially formed silacyclopropanes to the silylalkenes are rather slow. Irradiation of a hexane solution of 10 in the presence of 1-butene followed by treatment of the photolysis mixture with dry methanol after irradiation was stopped, affords 2-(methylphenylmethoxysilyl)butane in 27% yield, along with a small amount of silylalkenes. Similar photolysis of 10 in the presence of internal olefins or cyclic olefins gives the respective silacyclopropanes as the main products, together with the photorearranged silylalkenes as minor ones. These silacyclopropanes cannot be isolated by distillation or by GLC because of their extreme kinetic instability, but the formation of the silacyclopropanes can be determined by proton NMR spectroscopy (52). 

Photochemically generated trimethylsilylphenylsilylene also adds to the carbon-carbon double bonds of many types of olefins (54). Thus, the photolysis of a hexane solution of tris(trimethylsilyl)phenylsilane (20) in the presence of isobutene by irradiation with a low-pressure mercury lamp produces, after subsequent treatment of the photolysis mixture with methanol, fert-butylphenyI(trimethylsilyl)methoxysilane in 52% yield, as the sole insertion product. Direct evidence for the formation of 1-trimeth-ylsilyl-l-phenyl-2,2-dimethyl-l-silacyclopropane in this photolysis can be obtained by NMR spectroscopic analysis of the reaction mixture. 

The photolysis of silacyclopropanes 21 and 22 by irradiation with a high-pressure mercury lamp proceeds simultaneously by two different routes, one leading to the formation of a 1-alkenyl substituted silane via a 1,2-hydrogen shift which has never been observed in the photolysis of the silacyclopropanes produced from methylphenylsilylene with olefins, and the other involving the usual 1,3-hydrogen shift. The photochemical reaction of 21 is shown in Eqs. (27) and (28) as a typical example. 

Photolysis of cyclotrisilane 72, in the presence of five-membered ring compounds, produces l,l-di- /t-butyl-2,2-dimethyl-l-silacyclopropene 37 when reacted with cyclopentadiene (Equation 16), o o-3,3,6,6,7,7-hexa-/i t/-butyl-3,6,7-trisilacyclo-8-oxatricyclo[3.2.1.0 ]octane 38 and l,l-di-fem-butyl-2,2-dimethyl-l-silacyclopropane 39 when reacted with furan (Equation 17), and 2,2,6,6-tetra-/i m-butyl-2,6-disilabicyclo[3.1.01]hex-3-ene 40 when reacted with thiophene (Equation 18), among other products <19950M5695>. These products were characterized by H, and Si NMR, and for the furan derivative 38, also by X-ray diffraction studies. 

Photolysis of 2-phenylheptamethyltrisilane (97) generated methylphenylsilylene, which readily added to terminal, internal, and cyclic alkenes. Under the best conditions, irradiation of this trisilane with a low-pressure mercury lamp in the presence of alkenes that lacked methyl substituents yielded silacyclopropanes in 27-50% yield (Scheme 29). These silacyclopropane were detected by H NMR and were characterized as their methanolysis products <78JOM(i52)i55>. 

Photolysis of tris(trimethylsilyl)mesitylene (98) in the presence of allyl ethyl ether at 10°C afforded a thermally stable silacyclopropane (99) which could not be isolated in pure form. Treatment of the photolysis mixture with methanol gave 1-allyl-1-mesityl-1-methoxytrimethyldisilane in 40% yield (Scheme 30). Similar reactions of less highly substituted silylenes gave much poorer yields of the methanolysis products <83JOM(248)25l>. 

Photolysis of cyclotrisilane (100) in the presence of simple alkenes or 1,3-dienes gave silacyclopropanes in modest yield (three examples). For example, reaction of cyclotrisilane with 2-methylbutadiene gave l,l-di-/-buty 1-2-isoprenyl silacyclopropane exclusively, via addition of the intermediate silylene to the less-hindered double bond (Scheme 33) <94CC1233>. 

Fullerene silacyclopropane derivative (102) was prepared in 58% yield by photolysis of trisilane (101) in the presence of C60. The spectral data for the product was consistent with a silacyclopropane of C2v symmetry, which would arise from addition of the silylene across the six-ring-six-ring junction (Scheme 34) <93JA1605>. 

In a slightly modified manner, tris(TMS)-phenylsilane (318) is converted into phenyl-TMS-silylene under conditions of photolysis, which can add to allyl ethyl ether to give rise to 1-phenyl-l-TMS-2-ethoxymethyl-l-silacyclopropane (-silirane) (319) (equation 145)169. 

If an aryl group is attached to the sp2-hybridized carbon remote from silicon, different behavior is observed. Reversible photoisomerization of cis to trans alkenes occurs, and on further photolysis insertion-rearrangement reactions lead to silacyclopropanes and silaindanes45 (equation 27). The generality of these rearrangements has not yet been explored in depth. 

Photolysis of di(t-butyl)silacyclopropanes [derived from di(t-butyl)silylene and cis-and rraws-2-butene] regenerated the silylene and 2-butene147 (equation 96). 

C-Labelling showed that the thermal conversion of the phenyl trimethylsilyl diazo-compound (168) into (169) involved isomerization of a phenyl carbene into a cycloheptatrienylidene, silacyclopropanes were not involved. It was shown that the P-naphthyl carbene obtained by rearrangement of 4,5-benzocycloheptatrienylidene enters the singlet-triplet manifold as a singlet. Evidence has been reported that aryl carbenes can show both electrophilic and nucleophilic properties in their intramolecular ring expansion. Low-temperature photolysis of the sodium salt of the toluene-p-sulphonyl hydrazone (170) gave, by intramolecular carbene addition, the dibenzobicyclo[4,l,0]heptatriene (171) which was trapped by buta-l,3-diene. ... 

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