Disilacyclobutanes silenes from

The first conclusive evidence for the generation of silaethylenes (silenes) from silacyclobutanes came in 1967. If heated to 400 °C in the absence of trapping agents, ethylene and 1,3-disilacyclobutane resulted. Copyrolysis of a mixture of two silacyclobutanes with different substituents on silicon gave three 1,3-disilacyclobutanes (Schemes 81 and 82) (67CC864, 68JCS(B)419, 68JCS(B)1396). 

The pyrolysis of 1,1-disubstituted silacyclobutanes has been used as a method for the preparation of 1,1,3,3-tetrasubstituted 1,3-disilacyclobutanes183. Copyrolysis of two different 1,1-disubstituted silacyclobutanes yielded mixtures containing 1,3-disilacyclobutanes arising from all possible combinations of the two silenes present (equation 51). The preparative method failed in the case of cyclopentadienyl-substituted silacyclobutanes183, 184, presumably due to competing intramolecular reactions of the silene. The thermolysis of trimethylsilylcyclopentadiene may also proceed via a silene184. 

Generally, only simple silenes having small groups (H, Me, CH2=CH) are obtained as transient species from the thermolysis of silacyclobutanes. In part this is due to the high temperatures (usually above 450°C) required for the ring cleavage. Substitution on the carbon atom adjacent to silicon in the ring can lead to carbon-substituted silenes. 1,3-Disilacyclobutanes do not readily revert to silenes under thermal conditions, but examples... 

The laser flash photolysis of gaseous silacyclobutanes 2224 and 2325 and 1,3-disilacyclobutane 24 produced the transient silenes 25 (from 22 and 24) and 26 (from 23) as the major primary product. The silenes 25 and 26 were identified by their UV spectra with Xmax s=ss 260 nm. Rate constants for the decay processes of the transient silenes were also measured. 

The silene 124 is probably formed as its THF adduct and can be trapped by, e.g., 1,3-dimethyl 2,3-butadiene to give a [4+2] cycloadduct. The attempt to liberate the silene 124 from its donor adduct results in the formation of a disilacyclobutane 125. This is ascribed to the prolonged life-time of the intermediate 359 formed by the methyl migration in the silene (equation 96), which allows for a hydrogen migration to take place. 

Barton and Klein91 showed that dimethylsilene could be photochemically extruded from a bicyclo[2.2.2] bridged ring system using a mercury arc. When photolyzed neat at either 28 K or 77 K, both the 1,3-disilacyclobutane (the silene dimer) and the anticipated bis-(trifluoromethyl)benzene were obtained in good yield. However, when photolyzed in cyclohexane solution at room temperature, while bis(trifluoromethyl)benzene was isolated in good yield, only traces of the silene dimer were observed. It was not clear what was the fate of the silene (equation 58). 

The retro-ene reaction was used to generate 6,6-dimethyl-6-silafulvene from allylcyclo-pentadienyldimethylsilane (equation 61)215. From this reaction the dimer of the presumably first formed 6-silafulvene was isolated. Unlike other silene dimers, the isolable product does not contain a four-membered ring, although the initially formed dimer may be a 1,3-disilacyclobutane. In solution, the isolable dimer has a fluxional structure due to rapid 1,5-silyl migrations. 

Thermal silylcarbene-to-silene rearrangements have been known for a long time1. The pyrolytic product from trimethylsilyldiazomethane, 1,1,2-trimethylsilene, was trapped in an argon matrix230, and the pyrolysis of bis(trimethylsilyl)diazomethane126 was reported to produce fair amounts of 2,4-bis(trimethylsilyl)hexamethyl-l,3-disilacyclobutane, the expected dimerization product of 2-(trimethylsilyl)-1,1,2-trimethylsilene. A second product was the disilane expected from an ene addition of one... 

A silene-to-silylene isomerization by a 1,2-shift of a trimethylsilyl group from silicon to carbon was originally proposed in order to account for the formation of 1,1,3-trimethyl-1,3-disilacyclobutane during the pyrolysis of allylpentamethyldisilane (equation 93)214. Since silenes are planar and since the rc-component of the C=Si double bond is quite strong ( 40 kcal mol ) 19, m, it is not easy for the molecule to align the migrating SiH or SiSi bond with the carbon p-orbital that forms the new bond. 

While several highly substituted 1,2-disilacyclobutanes are known to revert under very mild conditions to silenes , it is generally believed that 1,3-disilacyclobutanes need more drastic conditions to undergo the cycloreversion yielding silenes . Kinetic data for the pyrolyses of several 1,3-disilacyclobutanes (24, 47, 48) have been reported by Davidson and CO workers and are summarized in Table 2 . Silene formation was inferred from detection of trapping products with TMSOMe and HCl. It was found that methyl substitution at silicon slows down the pyrolysis rate. The initial process for the decomposition of... 

The complex reaction sequence shown in equation 34 might provide some rationalization. The formation of the silylcarbene 141 is suggested, based on experimental results from related reactions , but there is no evidence for the formation of 141 nor for a silylene intermediate. Thus, the transformation 137 142 might proceed via a dyotropic rearrangement as well. The facile 1,3-methyl shift in 2-trimethylsilylsilenes which interconverts 142 139 is well known from Wiberg -type silenes . 139 (R = i-Bu) is stable in solution at room temperature over days and isomerizes only slowly to 140 (R = t-Bu) which rapidly dimerizes giving a 1,3-disilacyclobutane . 

Obviously, the interplay between subtle steric and electronic effects determines the final product of the dimerization of Apeloig-Ishikawa-Oehme silenes. Thus, the p-dimethylaminophenyl-substituted silene 381 gives the expected dimerization products, the 1,2-disilacyclobutane 382 and the linear dimer 383, in a relative ratio of 1 4 (equation 108) °. In sharp contrast the vray similar < -dimethylaminophenyl-substituted silene 384 gives in 63% yield a 1 2 mixture of the E/Z-l,3-disilacyclobutane 385 (equation 109) °. It was suggested that this peculiar regiospecific outcome of the dimerization of 384 results from a directive intermolecular donor-acceptor interaction between the ortfjo-dimethylamino group and the Si=C double bond . Clearly, more work has to be done in order to understand the small effects which determine the dimerization behaviour of Apeloig-Ishikawa-Oehme silenes . 

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