Silacyclopropanes formation

One involves photorearrangement giving the silylalkene mentioned above and the other, the main route, is silylene extrusion, the reverse of the reaction that leads to silacyclopropane formation (52). 

The coordination compound 76 was stable enough for isolation and recording of its NMR spectra, from which a rigid silacyclopropane structure could be deduced. The mechanism of complex formation has also been investigated in detail by matrix techniques. 

The presence of the SiMe2 group strongly hindered the formation of photo-cyclomers 366 and 368 where the bonds were formed between 9,10- and 1 -positions [352,353], Irradiation of di(9-anthryl)dimethylsilane 369 gave a single photocyclomer 370 by intramolecular (4 + 4) photodimerization without the formation of silacyclopropane 371 [354] (Scheme 98). 

This functionalization was limited to the formation of 1,3-substituted oxasilacyclopentanes, as exposure of the in situ-generated silacyclopropane to substoichiometric amounts of copper salts did not produce the complementary 1,2-disubstituted oxasilacyclopentanes. 

From these observations, Woerpel and Cleary proposed a mechanism to account for allylic silane formation (Scheme 7.23).85 Silacyclopropane 94 is formed from cyclohexene silacyclopropane 58 through silylene transfer. Coordination of the Lewis basic benzyl ether to the electrophilic silicon atom86-88 generates pentacoordinate siliconate 95 and increases the nucleophilicity of the apical Si-C bond.89 Electrophilic attack by silylsilver triflate 96 forms silyl anion 97. Intramolecular deprotonation and elimination then affords the silylmethyl allylic silane. 

Woerpel and Clark identified silver phosphate as the optimal catalyst to promote di-ferf-butylsilylene transfer from cyclohexene silacyclopropane to a variety of substituted alkynes (Scheme 7.25).95 While this silver salt exhibited attenuated reactivity as compared to silver triflate or silver trifluoroacetate, it exhibited greater functional group tolerance. Both di- and monosubstituted silacyclopropenes were easily accessed. Terminal alkynes are traditionally difficult substrates for silylene transfer and typically insert a second molecule of the starting acetylene.61,90 93 Consequently, the discovery of silver-mediated silylene transfer represents a significant advance as it enables further manipulation of monosubstituted silacyclopropenes. For enyne substrates, silylene transfer the alkynyl group was solely observed. The chemoselectivity of the formation of 99f was attributed to ring strain as theoretical calculations suggest that silacyclopropenes are less strained than silacyclopropanes.96 97... 

Woerpel and Calad tested for the formation of the silacarbonyl ylide by interrogating the behavior of the electrophilic silver silylenoid intermediate 115 toward a,(3-unsaturated carbonyl compounds (Scheme 7.37).82 They hypothesized that formation of silacarbonyl ylide 131 might trigger a 6jt-electrocyclization to form oxasilacyclopentene 132. As anticipated, exposure of cyclohexene silacyclopropane 58 to substoichiometric amounts of silver trifluoroacetate in the presence of a,(3-unsaturated carbonyl compounds 130 produced oxasilacyclopentenes 132. The reaction tolerated a substitution at the a and/or (3 position and was general for both esters and ketones. 

Toward this end, Woerpel and Nevarez examined the possibility of di-tert-butylsilylene transfer from cyclohexene silacyclopropane 58 to imine 169a (Scheme 7.48).123 Thermolysis produced a mixture of silaaziridine 170a and an imine-dimer byproduct (171). The results by Brook and coworkers suggested that if the temperature of silylene transfer were lowered, isolation of 170a without formation of byproduct 171 would be possible. As anticipated, exposure of cyclohexene silacyclopropane 58 to imine 169a in the presence of substoichiometric amounts of silver triflate produced only 170a. This silaazridine could be purified by bulb-to-bulb distillation to afford the product in 80% yield. Copper salts required... 

Interrogation of the stereochemical course of the mechanism was obtained through submission of allylic ethers 208 and 210 (>95% ee) to reaction conditions (Scheme 7.58). The reaction of a 1 1 mixture of allylic ether 208 produced the allylic silane as a 1 1 mixture of diastereomers. Exposure of 210 to substoichio-metric amounts of copper triflate and cyclohexene silacyclopropane produced cfs-substituted allyl silanes 211 and 212 to reveal that C-Si bond formation occurs on the same face as the C-O bond that is cleaved. The loss of enantiopurity, however, indicates that the rate of allylic transposition is competitive with the insertion process. 

Silver compounds are versatile catalysts for various cycloaddition reactions, including [2 + 1]-, [2 + 2]-, [3 + 2]-, and [4 + 2]-cycloadditions. An example for the silver-catalyzed formation of three-membered rings by [2+ l]-cycloaddi-tion is the silacyclopropanation reaction of mono- and disubstituted alkenes by silylene transfer from the cyclohexene silacyclopropane 432 that was reported recently by Woerpel et /.355,355a (Scheme 127). The reaction tolerates a number of functionalities in the substrate (OBn, OSiR3, BuTlC, etc.,) and is stereospecific with regard to the cisjtrans... 

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. 

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. 

Summary New silacyclopropanes were synthesized quantitatively under mild thermal conditions by reaction of olefins with cyclotrisilane (cyclo-(Ar2Si)3, Ar = Me2NCH2QH4) 1, which transfers all of its three silylene subunits to terminal and strained internal olefins. Thermolysis of silacyclopropanes 3a und 3b indicated these compounds to be in a thermal equilibrium with cyclotrisilane 1 and die corresponding olefin. Silaindane 13 was synthesized by reaction of 1 with styrene via initially formed 2-phenyl-1-silacyclopropane 3d. Reaction of 1 with conjugated dienes such as 2,3-dimethyl-l,3-butadiene, 1,3-cyclohexadiene or anthracene resulted in the formation of the expected 1,4-cycloaddition products in high yield. 

It is tempting to assume, that the facile formation of silylene 2 from cyclotrisilane 1 is due to the effective stabilization of 2 by intramolecular coordination of the dimethylamino group to the silicon centre [10], which should lower the activation energy of a dissociation process from 1 to 2. Reaction of 1 with benzylvinylether resulted in a complex reaction mixture, from which 12 % of vinylsilane 4 was isolated 4 is presumably formed by rearrangement of the unstable oxy-substituted silacyclopropane 3c. 

Stirring 1 for 12 h at 40 °C with excess styrene led quantitatively to silaindane 13 [12], The silacyclopropane 3d was identified as an intermediate in this reaction by its Si NMR shift (5 = -82.5 ppm) [6]. Thus, 13 appears to be formed by initial formation of 3d, which rearranges to intermediate 12. Rearomatization eventually yields 13 (Scheme 2). This pathway resembles the well known mechanism of the reaction of silylenes with conjugated olefins via initial formation of vinylsilacyclopropanes [3]. 

Shimizu et al. claim that formation of the 1,3-disilacyclohexane ring 27 may be understood in terms of a silacyclopropane intermediate 26. The carbanion 25 formed by treatment of 24 with a base should generate silacyclopropane 26 by an intramolecular alkylation reaction. Since 26 should be highly reactive due to the strain energy, it should immediately be attacked at the silicon atom by another nucleophile 25 (Scheme 4) <1996CL1083, 1999ICA231>. 

A 1-methoxy-l-silacyclopropane intermediate 70 has been suggested in order to explain the formation of l-methoxy-3,4-dimethyl-l-silacyclopent-3-ene. Vacuum pyrolysis of l,2-diethyl-l,l,2,2-tetramethoxydisilane in the presence of 2,3-dimethyl-l,3-butadiene, at 400-500°C and 10 torr, in a vertical quartz tube, produces 1-ethyl-l-methoxy-, 1-ethyl-, and l-methoxy-3,4-dimethyl-l-silacyclopent-3-ene. The 1-methoxy-l-silacyclopro-pane intermediate has been suggested in order to explain the formation of l-methoxy-3,4-dimethyl-l-silacyclo-pent-3-ene along with ethyltrimethoxysilane. Product 71 was obtained via the proposed silacyclopropane intermediate 70 (Scheme 18), while the other products proceed via an oxasilacyclopropane intermediate <1997JOM219>. 

Photolysis of di-t-butyldiazidosilane at 254 nm in 3-methylpentane glass at 77 K or in an argon matrix at 10 K led to a highly reactive di-i-butylsilylene intermediate with Amax 480 nm. Subsequent irradiation at 500 nm resulted in formation of l-/-butyl-2,2-dimethyl-l-silacyclopropane (95), the first stable silacyclopropane with an Si—H bond (Scheme 27) (88JA6689). 

The thermal reactions of silacyclopropanes cis- and trans-187a with benzaldehyde give a stereoisomeric mixture of an oxasilacyclopentane product and significant quantities of by-products whereas fhe t-BuOK-catalyzed reactions proceed efficiently under mild conditions wifh inversion of silacyclopropane configuration (Scheme 10.250) [680]. The latter reactions might involve initial formation of a more reactive pentacoordinate sihconate intermediate. The base-catalyzed system is not applicable to insertion of enohzable aldehydes. 

The Pd-catalyzed reaction of silacyclopropanes wifh terminal or electron-deficient alkynes gives siloles along wifh silacyclopentenes (Scheme 10.255) [685]. The mechanism for fhe formation of fhese products might involve oxidative addition of fhe carbon-silicon bond to a Pd(0) complex generated in situ. 

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