Disilacyclopentenes

3-disilacyclopentenes are formed from the pyrolysis of SiMe, and are involved in further reaction processes of the gas phase pyrolysis. Hence, the chemical qualities pertaining to these compounds are of special interest. 

The reaction of with HBr between -80 °C and —30 °C depends on the HBr concentration. By employing a mole ratio of 1 1, the following reaction products are generated  

Reaction of 412 with bromine in a 1 1 molar ratio in CCl has been studied meticulously. First, bromine is added to the double bond  

The compound 3 is isolated in the form of colorless crystals. In the presence of a suitable catalyst, Ml rearranges producing BrMe2Si—CH2 

After addition of Br2 is complete, an intramolecular p-elimination occurs. This is also the case in the reaction with bromine in the absence of solvent, which proceeds likewise to BrMe2Si—CH2—SiMe2Br and trans-BrHC = CHBr as additional products [145]. 

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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... 

Thermal isomerization of alkenes to carbenes via a 1,2-silyl shift was examined both experimentally and theoretically (Scheme 3). 2,4-Dimethylene-l,3-disilacyclobutane undergoes thermal ring expansion to a 2-methylene-1,3-disilacyclopentene. The analogous all-carbon system failed to ring expand. Ab initio calculations revealed that this was... 

A stoichiometric reaction of the four-membered complex 34 with terminal and internal alkynes leads to spiro-disilacyclopentene derivatives 35 in 90-98% yields (Equation 3) <1996BCJ289>. 

C-13 satellites of the vinyl resonance offer a simple means of determining the structure of 180. As these signals are very weak, they are difficult to detect. However, sufficient amounts of compound 87 (1,3-disilacyclopentene) were provided by an organometallic synthesis35, s4), for comparison with the C-13 satellites in the vinyl group of both compounds ... 

In this section, ring formation of silicon-containing metallacycles is described and classified according to the heteroatom involved in the reaction. Therefore, various syntheses of disilacyclopentanes, disilacyclopentenes, silaboracyclopentanes, azasilacyclopentanes, oxasilacyclopentanes, and finally thiasilacyclopentanes are described. 

While disilacyclopentanes and disilacyclopentenes are often unstable, numerous routes for their formation have been described in the literature. Most of the authors proposed reaction mechanisms and it is considered relevant to integrate them in this report. 

Evidence for the formation of intermediates 113 and 114 has not been obtained yet. Once, disilacyclopentene 109 was isolated and its stmcture analyzed by spectroscopic and elemental analyses. It was treated with dimethylphe-nylsilane again to afford 117, in 1% yield. The formation of 117 was explained by a series of intermediates shown in Scheme 14. Insertion of a nickel species into one of the two trimethylsilyl-carbon bonds in 109 gave nickel complex 115. This, it was proposed, is followed by a trimethylsilyl shift from the sp -hybridized carbon to a nickel atom to give the reactive vinylidene carbene-nickel complex 116. The reaction of this nickel species with (dimethylphenyl)silane affords product 117. 

Direct photolysis of l,l,3,3-tetramethyl-l,3-disilacyclopentene (23) at 185 nm and 254 nm in deoxygenated pentane produces a ring contraction product l,l,3,3-tetramethyl-2-methylene-l,3-disilacyclobutane (24) and a cleavage product, 2,4,4-trimethyl-2,4-disilahex-5-yne (25) in absolute... 

Of the ring systems which come under the purview of this chapter the most widely studied are those containing silicon, especially the disilacyclopentanes and disilacyclopentenes. While such species are often unstable, numerous routes to their formation have been described. 

In another interesting variant of this process, disilacyclopentenes (e.g., (57)) were prepared by the reaction of diisobutylaluminum hydride with alkynes such as (56) (Scheme 8) <70ZAAC(377)48>. In this case, cu-addition of A1—H to the alkyne triple bond results in an increased reactivity of the —CH2Br group to the extent that spontaneous ring closure takes place in solvents of weak Lewis basicity (ether or dioxane) to afford the corresponding disilacyclopentene after hydrolytic loss of aluminum. 

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

The pathway chosen depends on the geometry of the disilametallacycle intermediate (%), the electronic properties (e.g., hardness) of the central metal and the steric effects of the substituents on the 1,3-diene. The disilacyclopentene products (e.g., (93)) generally are favored by the use of dienes with at least one end unsubstituted (to allow 1,1-addition), steric crowding in the coordination sphere around the metal in the metallacycle intermediate (preventing 1,4-addition) and use of a soft metal (such as Fe) to assist in the migration of the softer base, H (rather than the harder F ). 

Since the reaction of styrene with o-bis(deuteriodimethylsilyl)benzene produced the expected disilacyclopentene with no deuterium incorporation, the mechanism of this reaction is believed to involve the intermediacy of the cyclic bis(silyl)platinum complex (116) first reported by Eaborn et al. <73JOM(63)107> from the reaction of (113) and the platinum catalyst. However, complex (116) itself did not react with styrene but when 1.1 equivalents of (113) was added, an 83% yield of (114 Ri = H, R2 = Ph) was obtained. 

An excess of MeMgCl caused a preferred development of linear derivatives. The foAnation of 1,3-disilacyclopentene, 273, is obviously a deciding stage in the overall reaction path. It was shown that compounds with this molecular skeleton are formed in a 98% yield in the reaction of (Cl2Si—CCl2)3 with 4 mole equivalents MeMgCl. Table 56 presents the reaction products obtained from the reaction of l,3-disila-4-trimethylsilyl-cyclopentene 273 with MeMgCl. 

The 1,3-disilacyclopentene 273 reacts further with MeMgCl to yield its acetylene derivatives, which are obtained directly from reaction of (Cl2Si— 12)3 with excess MeMgCl, as shown in the mechanism... 

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