Silylenes methanol

Scheme 33). In the presence of methanol, 128a-c were formed as the products resulting from protonation of the complex and subsequent addition of the methoxide ion. Weidenbruch et a/.84 also investigated the reactions of a bulky silylene with isocyanides, but no formation of a Lewis-base analogous to 28-30 was observed. 

A key report investigated a variety of substrates in their reaction with silicon in an effort to find evidence for silylene intermediates during the silicon direct process reaction. When silicon, copper and methanol were reacted as described above but in the presence of alkenes, alkyldimethoxysilanes and (MeO SiH were formed95-97. The use of allyl propyl ether instead of alkenes gave allyldimethoxysilane, with 38% selectivity. These results and the reaction of silicon with MeCl in the presence of butadiene to give silacyclopent-3-enes indicates intermediate formation of silylenes. 

Trapping reactions were carried out for 36a-c, leading in most cases to silylene trapping products with liberation of the isocyanide. Thus, for instance, reaction with KfjSill yielded the disilane, along with 35a-c (Scheme 6). Reaction of 36a-c with methanol also led mainly to the silylene trapping product, but in one case the complex itself (36a) was trapped in small yield, producing the imine. 

When finely divided silica is treated with methanol and then pyrolyzed, it becomes activated toward chemisorption of various gases341-343. Recent careful spectroscopic studies by Radzig and coworkers344-347 and by Razskazovskii et aZ.348,349 establish beyond reasonable doubt that the principal reactive sites are divalent silicons, (=Si—0)2 Si , silylene centers 350, which participate in a rich chemistry. 

Similarly, reaction of silicon, methanol and ethylene in an autoclave at 433 K produces ethylmethoxysilanes, EtSi(H)(OMe)2 and EtSi(OMe)3, as well as HSi(OMe)3 and Si(OMe)4. And, although 2-propanol does not react with silicon, a mixture of methanol and 2-propanol reacted with silicon to yield i-PrOSi(H)(OMe)2359. Taken together, these results strongly suggest the intermediacy of silylene-like species in these reactions. 

As discussed in the previous section, thermal dissociation of disilenes into the corresponding silylenes may occur if the BDE of the disilenes is small. As shown in review OW, a facile thermal dissociation of disilene 27 into silylene 127 occurs at 50 °C [Eq. (49)],61,91 The formation of silylene 127 is evidenced by its trapping by methanol, triethylsilane, and 2,3-dimethyl-1,3-butadiene. The activation enthalpy and entropy for the dissociation of (Z)-27 to 127 are 25.5kcalmol-1 and 7.8 cal mol-1 K-1 respectively.91 The activation free energy for the dissociation at 323 K (22.9 kcal mol-1) is much smaller than that for the Z-to-E isomerization of 26 (27.8 kcal mol-1), indicating that the E,Z-isomerization of 27 should occur via the pathway (2) in Eq. (47) rather than pathway (1) in Eq. (48). 

Tetraalkyldisilene 63 with a lattice framework in <7/-form dissociates into the corresponding silylene 128 [Eq. (50)],92 The treatment of (4S, 6S, 4/S, 6 S )-63 (dl-63) with methanol for 6 days at rt affords a racemic mixture of methanol adducts of silylene 128. Silylene 128 is also trapped by bis(trimethylsilyl)acetylene. 

Stable diaminosilylene 132 (R = /- Bu) does not dimerize to the corresponding tetraaminodisilene but undergoes an insertion reaction giving 131, which further dimerizes to diaminodisilyldisilene 62. Disilene 62 is stable in the solid state but equilibrates with stable silylene 132 (R = /-Bu) via 131 in solution [Eq. (52)].33,34 Intermediacy of 131 is evidenced by the reaction of crystals of 62 with methanol giving a methanol adduct of 131, 133. 

The evidence that ,Z-isomerization of 92-95 proceeds by Si=Si bond rotation and not a mechanism involving silylene intermediates, produced by cleavage of the Si=Si bond followed by recombination, rests upon the fact that no trapping products consistent with the intermediacy of the corresponding diarylsilylenes could be detected upon heating the disilenes in the presence of known silylene traps such as methanol, triethylsilane or 2,3-dimethyl-l,3-butadiene. In fact, one tetraaryldisilene has been shown to isomerize by this mechanism, the 1,2-dimesityl-l,2-bis(2,4,6-tris[bis(trimethylsilyl)methyl]phenyl derivatives (E)- and (Z)-97a (equation 70)142,143. Arrhenius parameters for the thermal dissociation of (E)- and (Z)-97a to diarylsilylene 98 are listed in equation 70. 

Silylene 5 is remarkably stable, in sharp contrast to previous silylenes. It was purified by vacuum distillation at 85 °C (1 Torr) and survives heating in toluene solution to 150 °C for many months. The pure compound decomposes only at its melting point, 220 °C. Compound 5 is also less reactive than usual silylenes. It is inert to triethylsilane, diphenylacetylene, or 2,3-dimethylbutadiene, all of which react rapidly with conventional silylenes. Moreover, it does not form acid-base complexes with Lewis bases such as THF and pyridine, although normal silylenes do [19]. However, 5 does react with methanol, water, and dioxygen. 

A mixture of reactive intermediates, including l,l-dimethyl-3,3-bis(trimethylsilyl)-Tsilaallene and dimethylsilylene, along with l,l-dimethyl-2,3-bis(trimethylsilyl)-l-silacyclopropene 86 were formed and detected from the direct irradiation of [(trimethylsilyl)ethynyl]pentamethyldisilane in hydrocarbon solution (Equation 21). These species were detected and identified using laser flash photolysis. They were trapped as their methanol adducts in steady-state irradiation experiments. Steady-state irradiation in the presence of methanol affords MeOH-addition products which are consistent with the formation of the silaallene, silacyclopropene, and silylene along with bis(trimethylsilyl) acetylene as the major product <1997JA466>. 

A number of polysilane homo- and copolymers were also irradiated in the presence of trapping reagents such as triethylsilane (TES), methanol, and n-propyl alcohol. The results of the exhaustive irradiation at 254 nm in the presence of TES are shown in Table I. In each case, the adduct of the dialkyl-substituted silylene and TES was the dominant product. Also produced concurrently in significant yields were the corresponding disilanes. 

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>. 

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