Silacyclopropenes formation

When silylene transfer was attempted to alkynes substituted with halides or sulfones, however, silacyclopropene formation was not observed.101 Instead, acetylenic silanes 113 were observed (Scheme 7.32). Treatment of silacyclopropene 112c (or 112d) with acetophenone and substoichiometric amounts of Cul also induced alkyne formation. 

The unexpected formation of the first bis(triorganosilyl)silyl dianions has been reported by Sekiguchi et al. in 1999. Thus, the reaction of l,l-bis(triorganosilyl)-2,3-bis(trimethylsilyl)silacyclopropenes 128 and 129 with lithium provided the dilithiosilanes Li2[Si(R2R Si)2] (130 R = R1 =/-Pr 131, R = Me, R1 =/-Bu) with the only byproduct being bis(trimethylsilyl) acetylene Me3SiCCSiMe3 132 (Scheme 21 ).281>282 For reactions of silyl dianions at preparing unsaturated silanes, see Section 5.2. 

At the time, these suggestions could not be tested as silacyclopropenes were unknown, but an alternative mechanism was proposed involving the initial formation of the 1,2-disilacyclobutene, which ring opens to the 1,4-disilabuta-l,3-diene reversibly to add a second molecule of alkyne. The disilacyclobutene (27), formed from 2-butyne, adds 3-hexyne to give the disilin (28 Scheme 37) (76JA7746). 

Tamao et al,83 found that a higher coordinated silylene 119 can be formed from penta-coordinated silane 118 (Scheme 31). Warming a solution of 118 in toluene or dimethylformamide in the presence of diphenylacetylene or 2,3-dimethyl-l,3-butadiene resulted in the formation of silylene-trapping products 120 and 121. Interestingly, no 1 1 reaction product between the silylene and the acetylene was isolated. Thus, it must be concluded that the insertion of silylene 119 into a Si-C bond of initially formed silacyclopro-pene is faster than the addition to the triple bond of the acetylene so that the silacyclopropene cannot be isolated under the reaction conditions. 

A different decomposition channel is utilized by the silyl-substituted 1-silaallene 617289,29o. In the absence of trapping reagents the transient 617 formed in the thermolysis of 646 at 280 °C undergoes a 1,2-trimethylsilyl shift giving the silylene 648 and finally the 3,5-disilacyclopentene 649 in 25% yield. Alternatively 648 can also be formed from the silacyclopropene 616. The silaindene 650, the formal insertion product of the Si=C bond into the ortho C—H of the phenyl ring, is isolated in 18% yield289. The formation... 

Monitoring the reaction by 111-NMR spectroscopy clearly indicates the formation of the 1,2-disilacyclobutenes by a two-step addition-insertion process. Primarily, the silylene generated by thermal decomposition of the cyclotrisilane adds to the C—C triple bond yielding the silacyclopropene. Further insertion of a second silylene into the Si—C bond of the silacyclopropene finally affords the corresponding 1,2-disilacyclobutene. 

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... 

In contrast to aforementioned silacyclopropenes, copper-catalyzed ring expansion was achieved with silyloxy-orboryl-substituted silacyclopropenes (Scheme7.34). 101,104 Simply changing the ether substituent from an alkyl group to a silyl group enabled oxasilacyclopentene formation from 112b. These results reveal that the reactivity modes of the silacyclopropene are controlled by the identity of the substituent. 

To identify potential dipolarophiles, a series of competition experiments were performed (Scheme 7.45).117 While inclusion of terminal acetylenes resulted in preferential silacyclopropene 99c or dioxosilacyclopentane 161 formation, the desired oxasilacylopentene (163) was obtained when diethylacetylene dicarboxylate was added... 

Similar photolysis of 20 in the presence of 1-trimethylsilylpropyne produces a silacyclopropene in 33% yield. In this case, small amounts of two other compounds, l-trimethylsilyl-l-(r-phenyl-2, 2, 2 -trimethyldi-silanyl)propadiene (2% yield) arising from a 1,3-hydrogen shift of the initially formed silacyclopropene and l-bis(trimethylsilyl)phenylsilylpro-pyne (2% yield), are also obtained. Formation of the latter compound can be best explained in terms of another type of photochemical migration in-... 

Irradiation of 1-ethynyl-l-phenyltetramethyldisilane (55) leads to the formation of the silacyclopropene and silapropadiene which can be trapped by methanol to give 1-trimethylsilyl-l-methoxymethylphenylsil-ylethene (29% yield) and cis- and trans-1 -trimethylsily 1-2-methoxymethy 1-phenylsilylethene (19 and 15% yield). In contrast to 52, the photolysis of... 

The photolysis of phenylethynylpentamethyldisilane (56) takes place simultaneously by at least two different processes. The main route proceeds through a silacyclopropene and the other involves transient formation of a silapropadiene. Irradiation of a benzene solution of 56 in the presence of acetone gives four products, 2,2,5,5-tetrameth-yl-3-trimethylsilyl-4-phenyl-l-oxa-2-silacyclo-3-pentene (57), 2,2,5,5-tetra-methyl-3-phenyl-4-trimethylsilyl- l-oxa-2-silacyclo-3-pentene (58), phen-yltrimethylsilylacetylene, and 1-pheny 1-1 -trimethylsily 1-3-methyl-1,2-butadiene (59), in 51,2,10, and 5% yield, respectively, with 81% conversion, of the starting disilane. The formation of 57 and 58 can be explained by insertion of acetone into the silacyclopropene. Liberation of dimethylsilyl-ene species from either direct photolysis of 56 or decomposition of the silacyclopropene results in the formation of PhC=CSiMe3. Product 59... 

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>. 

Irradiation of l,4-bis(pentamethyldisilanyl)butadiyne 88 in methanol yields two 1 1 photoaddition products 90 and 91 as well as one 1 2 photoadduct 93. Adduct 90 is the primary photoproduct, while 91 and 93 are the secondary products. Structure 93 is formed via two silacyclopropene intermediates, 89 and 92 (Scheme 27). Irradiation of 88 with acetaldehyde and acetone in deareated methylene chloride yields only the 1 1 photoadduct with a l-oxa-2-silacyclopent-3-ene ring due to steric effects, preventing formation of a second silacyclopropene intermediate. Trapping experiments for the silacyclopropene intermediates with methanol were performed <19960M2182>. 

Another type of reaction allowed us to study the philicity of silylene 4. As we have shown earlier, 4 adds smoothly to a variety of alkynes giving way to the silacyclopropene framework (Eq. 4) [10], Varying the / ara-substituents of diphenylacetylene offers us the possibility to tune the electron density of the triple bond, and we now studied the rates of the reaction of 3 with diphenylacetylenes lOa-c. The absolute reaction rates of these three first-order reactions are identical (Ai = 6.3 0.2 10 4s-l) this result is in accordance with a mechanism, in which the formation of silylene 4 from cyclotrisilane 3 is the rate determining step [6], However, the relative reaction rates of the addition of 4 to the triple bond of lOa-c, which were determined by competition experiments, turned out to differ appreciably from each other. Electron withdrawing substituents favor the addition of 4 to the alkyne, whereas electron donating substituents, such as a methyl group, slow down the reaction rate. As shown in Fig. 5, the relative reactions rates correlate well with the [Pg.62]

Decamethylsilicocene (1) reacts with alkynes having electron-withdrawing substituents (Me02CC=CC02Me, Me3SiC=CS02Ph) under formation of silacyclopropene derivatives [12] A surprising reaction is observed between 1 and hexafluorobutyne. 

A vinyl silylene intermediate is implicated in the formation of disilacyclopentenes by thermolysis of silacyclopropenes <920M597, 950M1204>. 

The l,2-azasilolo[5,l-e]-l,2-azasilole (201) was obtained along with (200) by photochemical addition of the silacyclopropene (199) to acrylonitrile <78CC80>. The mechanism for the formation of (201) is believed to involve two successive (2 + 2) cycloaddition processes. 

The formation of metallasilacyclobutene has been proposed in the transition metal catalyzed reaction of silacyclopropenes with an acetylene [86,88-93]. Ishikawa et al. [97] suggested the presence of nickelasilacyclobutenes from C-, Si-, and P-nmr data. Irradiation of a solution of 2-mesityl-2(phenyl-ethynyl)-l,l,l,3,3,3-haxamethyltrisilane in hexane at room temperature gave silacyclopropene and then reacted with tetrakis(triethylphosphine)nickel(0) to... 

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