Silicon-carbon double-bonded intermediates

Photochemical formation of silaethene intermediates has also been reported (27, 78-82). However, little systematic study had been done until 1975 when we initiated a series of studies on the photolysis of aryl-, alkenyl-, and alkynyl-substituted disilanes. 

In 1972, Sommer and co-workers (78) reported the first example of a silicon-carbon double bonded intermediate generated by the photolysis of pentaphenylmethyldisilane. 

The photolysis of phenylpentamethyldisilane (34) in the presence of ethylene or any of its monosubstituted derivatives always gives the respective 1 1 adducts in moderate yields (83,87). 

Interestingly, in these reactions a stoichiometric amount of the olefin can trap the intermediate A2 as effectively as an excess amount. For instance, irradiation of a benzene solution of 34 with a 20-fold excess of vi-nyltrimethylsilane affords 35 (R = Me3Si) in 37% yield. Under identical conditions 34 reacts with a one molar equivalent of vinyltrimethylsilane to give 35 (R = Me3Si) in 41% yield. This indicates that the intermediate A has a lifetime long enough to allow it to find a partner in a dilute solution. 

With A2 in benzene, 1,1-disubstituted olefins generally afford better yields of the adducts than do monosubstituted olefins. However, with olefins having an electron-withdrawing substituent such as CN or COOMe, yields of the adducts are lower. Thus, photolysis of 34 in the presence of one molar equivalent of methacrylonitrile gives 36 [(R1 = Me, R2 = CN) 

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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. 

The photolysis of tris(trimethylsilyl)phenylsilane results in formation of trimethylsilylphenylsilylene in high yield, together with a small amount of a silicon-carbon double-bonded intermediate, which will be described in detail later. This silylene has a high reactivity toward unsaturated organic substrates such as alkenes and alkynes (44). 

In 1975, we discovered that photolysis of aryldisilanes produces a novel type of the silicon-carbon double-bonded intermediate (83). The structure of this intermediate is quite different from that of the diphenylsilaeth-ene reported by Sommer et al. A transient formation of the reactive intermediate can be confirmed by trapping experiments. Thus, the photolysis of p-tolylpentamethyldisilane with a low-pressure mercury lamp in the presence of methanol- affords 1,4- and 1,6-adducts, 1-methoxydimeth-ylsilyl-4-methyl-5-deutero-6-trimethylsilyl-1,3-cyclohexadiene, and 1-methoxydimethylsilyl-3-deutero-4-methyl-6-trimethylsilyl-l,4-cyclohexa-diene in 27 and 28% yield (Scheme 11). In this photolysis, monodeutero methoxydimethylsilyltoluene to be expected from the reaction of the sila-ethene intermediate, p-CHsC6H4Si(Me)=CH2, with methanol- produces in only 2% yield (84). 

In marked contrast to the reaction of the silaethene intermediates, R Si=CH2, with ketones at high temperature (90, 91), the reaction of all the silicon-carbon double-bonded intermediates generated from the arylpentamethyldisilanes with either enolizable or nonenolizable ketones yields 2-trimethylsilyl(alkoxydimethylsilyl)benzene derivatives (92). Neither silyl enol ethers to be expected from O—H addition of enol form of enolizable ketones to the silicon-carbon double bond nor products from a... 

The reaction of the silicon-carbon double-bonded intermediate generated from phenylpentamethyldisilane with methyllithium and with methylmagnesium bromide leads to the formation of wholly unexpected products (92). Thus, irradiation of 34 in the presence of methyllithium with a low-pressure mercury lamp in diethyl ether followed by hydrolysis produces l,3-bis(trimethylsilyl)benzene (42) and 1,2-bis(trimethylsilyl)-benzene (43) in 60 and 20% yield, respectively, in addition to an 8% yield of phenyltrimethylsilane (Scheme 13). Surprisingly, no disilyl-substituted cyclohexadienes that might be expected from addition of methyllithium to the intermediate followed by hydrolysis are observed. In this photolysis,... 

In sharp contrast to the silicon-carbon double-bonded intermediates of type A, which never afford any volatile products in the absence of a suitable substrate, intermediates of type E may undergo dimerization to give a head-to-tail dimer. Thus, when a solution of l-phenyl-2-vinyltetrameth-yldisilane in dry hexane is photolyzed, cis- and trans-1,1,3,3-tetramethyl-2,4-bis(dimethylphenylsilylmethyl)-l,3-disilacyclobutane can be obtained in 21 and 22% yield, respectively. 

In this case, no product arising from the reaction of the silicon-carbon double-bonded intermediate with methanol can be observed at all. However, on prolonged irradiation of the solution two products, 1,1-dimethyl-2,3-benzo-5-trimethylsilyl-l-silacycIopentene (48) and 1-methoxy-dimethylsilyl-l-trimethylsilyl-2-phenylethane are obtained in 17 and 7% yield, in addition to the (Z)- and (E)-isomers (15 and 12% yield). The formation of the latter compound can best be understood by the transient formation of a silacyclopropane followed by reaction with methanol (98). The mechanism for the production of 48 in the prolonged irradiation of PhCH=CHSiMe2SiMe3 is not fully understood but is tentatively given in Scheme 16. 

The silicon-carbon double-bonded intermediates generated photo-chemically from a-alkenyldisilane derivatives react with both enolizable and nonenolizable ketones to give olefins (98). For instance, the photolysis of a-styrylpentamethyldisilane (49) in the presence of one molar equivalent of acetone gives l-trimethylsilyl-2-phenyl-3-methyl-2-butene in 13% yield as a single product. No silyl enol ether to be expected from the reaction of the intermediate with the enol form of acetone is observed. Similar irradiation of 49 with acetophenone affords (E)- and (Z)-l-trimeth-... 

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... 

This review describes the current status of silenes (silaethylenes, silaethenes), molecules which contain a silicon-carbon double bond. The heart of the material is derived from a computer-based search of the literature which we believe reports all silenes that have been described to date, either as isolated species, chemically trapped species, proposed intermediates (in reactions where some experimental evidence has been provided), or as the result of molecular orbital calculations. Ionized species... 

Our research interest gravitated to organosilicon chemistry when the formation of a silicon carbon double bond (silene) intermediate was found in the decomposition of silyl... 

Besides its synthetic significance, this result is at the same time a valuable proof for the intermediate existence of transient silenes also in the formation of 3-5. Whereas the addition of the organolithium compoimds to the silicon carbon double bond with the observed regiospecifity is obvious, the formation of 6a-6e by interaction of the nucleophilic organolithium derivatives with any precursor of the silenes 2a-2c is hardly conceivable. 

Similarly, when silene 2a is generated in presence of excess tris(trimethylsilyl)silyllithium, the lithium silanide is added across the silicon-carbon double bond to give an organolithiiun intermediate, which undergoes a rearrangement, a l,3-Si,C-trimethylsilyl migration, resulting in formation of a lithium silanide, which is trapped with chlorotrimethylsilane to yield the polysilane 7. The H-silane 8 is obtained as the protonation product after usual hydrolytic work up (Eq. 4-5). 

Compounds containing a silicon-silicon double bond, like those containing a silicon-carbon double bond, can only be isolated if bulky organic residues are present on the molecule. Otherwise they appear only as reaction intermediates. Organodisilenes are very sensitive to oxygen. 

The first 4-silatriafulvene derivative 94 (R = Me) was obtained by Kira s group as a reactive intermediate using the sila-Peterson reaction [52]. By use of the bulky tert-butyldimethylsilyl groups rather than trimethylsilyl (TMS) substituents in 94, the first stable 4-silatriafulvene 95 was synthesized by the same group (Scheme 6.22). An X-ray analysis reveals that 95 has an almost planar skeleton with bond alternation all skeletal carbon and silicon atoms are located almost in a plane and the silicon-carbon double bond length is 1.755 A, which is close to that of tert-butyldimethylsilyl-(trimethylsUyl)adamantylidenesilane 96 (1.741 A) [53]. 

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