Silacyclopropenes

Aikynes insert into the silacyclobutane 168 to form the silacyclohe.xene 169[93]. Also, the silacyclopropene 170 is expanded to the silacyclopentadiene 171 by the insertion of an alkyne[94]. The insertion product 173 was obtained by the Pd-catalyzed reaction of the neopentylidenesilirane 172 with acety-lene[95]. 

Silacyclopropane, 1,1 -difluorotetramethyl-decomposition, 1, 587 Silacyclopropanes, I, 575-581 ring expansion, I, 587 Silacyclopropenes, I, 583-587 minimum energy geometry, I, 587 ring expansion, 1, 587 7-Siladispiro[2.0.2.1 jheptane synthesis, I, 576... 

When alkynyldisilanes 13a and b were photolyzed in the presence of freshly generated dimesitylsilylene (Mes2Si ), the silylene added to the Si=C double bond of 1-silaallenes 14a and b to form disilacyclopropanes 15a and b (Scheme 5). Even without the independently generated silylene, photolysis of 13b produced 15b in 8% yield, but compound 13a gave only traces of 15a. In the case of 15b, the dimesitylsilylene most likely originated from silacyclopropene 16. 

The Kumada/Ishikawa group also investigated thermolytic reactions of alkynyl-polysilanes and silacyclopropenes in the presence of nickel catalysts and implicated a 1-silaallene-nickel complex as an intermediate in the reaction pathway... 

To see if silacyclobutenes (28) would react in the same manner as the alkynyl-polysilanes (1) and respond similarly to steric differences, compounds 28a and b were heated in the presence of NiCl2(PEt3)2 and PTMSA (Scheme 9). " Compound 28a gave silole 20a in 94% yield,but the only isolable products from the thermolysis of 28b were 26 (51%) and 27b (36%). The products from the thermolysis of silacyclopropene 28b were very similar to that for alkynylsilane lb, but for some reason there were many fewer products for the thermolysis of 28a than for alkynylsilane la. These results suggest that the more sterically hindered lb... 

Kumada and colleagues found that the 1,3-silyl rearrangements from silicon to carbon occurred during the photolysis of disilylalkynes, giving mixtures of silaallenes and silacyclopropenes,102 as illustrated by Eq. (12) ... 

Ishikawa71191 has described the thermal rearrangement at 280°C of the silacyclopropene 160 to the silaallene 161. It was suggested that a 1,2-trimethylsilyl rearrangement from silicon to carbon could lead to the sily-lene 162 on insertion into the C—H bond of one of the trimethylsilyl methyl groups 162 would give 163, which was isolated in 49% yield. 

Reaction of the 1-silacyclopropene (932) with [Ni(PEt3)4] gives nickelasilabutene (933).2311... 

Several di- or polysilyl systems have been found to be useful precursors for the photochemical generation of silenes. Vinyldisilanes cleanly yield silylmethylsilenes (133), while alkynyldisilanes yield mixtures of silylated silaallenes and silacyclopropenes [Eq. (8)] (119,136). Aryldisilanes when photolyzed form species presumed to be silenes, but showing unusual chemical behavior (see below) [Eq. (9)] (97-102). 

In the reaction of 1 with alkynes possessing electron-withdrawing substituents, the corresponding silacyclopropene derivatives 66 and 67 are formed, as described in Scheme 23.29 An unexpected pathway was observed in the reaction with the electron-poor hexafluorobutyne(2) the X-ray characterized heterocycle 68 was most likely obtained by nucleophilic attack of 1 at the triple bond. A subsequent shift of a fluorine atom from carbon to silicon creates an allene-type molecule which was stabilized by a [2 + 2] cycloaddition process involving a double bond from the pentamethylcyclopentadienyl unit, as described in Scheme 24.33... 

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. 

Several polysilabicyclosilirenes (79) have been obtained in the reactions of polysil-acyclooctynes (78) with dimesitylsilylene generated from the photolysis of 2,2-dimesityl-l,l,l,3,3,3-hexamethyltrisilane (Eq. 8). Thermolysis of bis(silacyclo-propane) (80) in the presence of bis(trimethylsilyl)acetylene at 60 °C affords bis-(silacyclopropene) (81) in 61% yield (Scheme 14.41). ... 

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). 

The photolysis of tris(trimethylsilyl)phenylsilane in the presence of a series of alkynes alforded the silacyclopropene through silylene addition to the triple bond. Those obtained from monosubstituted alkynes underwent photochemical isomerization to the disilanyl-alkyne through a 1,2-hydrogen shift (Scheme 48) (80JOM(190)117). Disubstituted alkynes form silirenes that can be isolated by preparative GLC. 

The first 1,2-disilacyclobutene (82) was prepared in 1973 by the gas phase reaction of dimethylsilylene and 2-butyne (73JOM(52)C21). It probably results through silylene insertion into the intermediate silacyclopropene (Section 1.20.3.4), but silylene dimerization followed by addition to the alkyne is also suggested (76JA7746), since (82) is formed in good yield if the disilene is generated directly (Scheme 127) (78JOM(162)C43). 

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. 

The classical open /l-silylethyl cation 9 and /f-silylvinyl cation 11 are no minima at higher level of theory3,4. They collapse to the bridged protonated silacyclopropane 21 and silacyclopropene 22, respectively. On the basis of their calculated structures (Figure 3) both cyclic molecules are best described as -complexes between a silylium ion and ethene or acetylene, respectively. 

In contrast to carbon chemistry, the three-membered cyclic C2FLtSi isomers are relatively stable. Thus, silacyclopropylidene 608 and 2-silacyclopropene 609 are more stable than 604 by 10.9 and 13.8 kcalmol-1, respectively28513, while cyclopropene 610 is by 22.4 kcalmol-1 less stable than allene 611 and more stable by 62.0 kcalmol-1 than singlet cyclopropylidene 612286. 

Silaallenes 613 have been proposed by Ishikawa, Kumada and coworkers as transient reactive intermediates in the photolytic or thermolytic degradation of alkynyldi-silanes 614 as minor byproducts, the main product being silacyclopropenes 615184,192,287 (equation 203). 

Following this reaction sequence established by Kumada and Ishikawa, Leigh and coworkers detected the transient 1-silaallene 597 in a laser flash photolysis of the ethynyl-disilane 619. 597 was identified by its characteristic UV absorptions at 275 nm and 325 nm278 and by the trapping reaction with methanol. It is formed, however, in a mixture with silacyclopropene 620, dimethylsilylene 621 and acetylene 622. Based on the quantitative analysis of the products of methanolysis (623-626) a chemical yield of 597 of 12-15% was deduced278 (equation 205). 

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