Silicon-carbon double-bonded intermediates reactions

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. 

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 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-... 

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... 

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]. 

Hydrosilylation.—This reaction is catalysed by the usual homogeneous catalysts. In some cases the mechanism involves insertion of the alkene into a metal-hydrogen bond, as in hydrosilylation of butadiene in the presence of PdL(PPh3)2, with L = p-benzoquinone or maleic anhydride. In other cases concerted addition of the silicon hydride to the carbon-carbon double bond is indicated, as in hydrosilylations catalysed by rhodium(i) catalysts such as RhCl(PPh3)3. In the reaction of silanes with hex-l-ene in the presence of this catalyst, rates depend on the stability of the intermediate adduct RhClH(SiR3)(PPh3)2 such an adduct was isolated in one case. Hydrosilylation of ethylene by trimethylsilicon hydride... 

On the basis of the fact that (R)-BMPP coordinated to the metal center can induce asymmetric addition of methyldichlorosilane across the carbon-carbon double bond of 2-substituted propenes to afford an enantiomeric excess of (R)-2-substituted propylmethyldichlorosilanes, the following processes should be involved in these reactions (a) insertion of the metal center into the silicon-hydrogen bond (oxidative addition of the hydrosilane) (b) addition of the resulting hydridometal moiety to the coordinated olefin preferentially from its re face (in a cis manner) to convert the olefin into an alkyl-metal species and (c) transfer of the silyl group from the metal center to the alkyl carbon to form the product. Since process (b) most likely involves diastereomeric transition states or intermediates, the overall asymmetric bias onto the R configuration at the chiral carbon would have already been determined prior to process (c). A schematic view of such a process is given in Scheme 1. 

Carbonyl olefmation reactions allow two carbons to come together joined by a double bond. In this case, the electrophilic side will come from an aldehyde or ketone. If the nucleophilic side is simply an alkylmagne-sium or an alkyllithium, the secondary or tertiary alcohol intermediate may be dehydrated, but in many cases there is a choice of sites for the new double bond and it may arise elsewhere than the newly joined carbons. This variability may be prevented and the double bond formed specifically if the nucleophilic carbon bears a group with high oxygen affinity that will leave with the oxygen. Phosphorus and silicon are excellent in this role, as demonstrated in the next several examples. 

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