Silenate formation from

The first evidence for the formation of silenes came from the thermolysis of silacyclobutanes, which resulted in a retro-[2+2] process leading to the silene Me2Si=CH2 and ethylene1 ... 

Conlin and co-workers have also studied the fragmentation of a siletane (silacyclobutane). In this case, both the E- and Z-isomers of 1,1,2,3-tetra-methylsilane 45 were prepared and thermolyzed (Scheme 8).144 Both E-and Z-isomers of 45 led to the same products in slightly different ratios the major products were propene with silene 46, and E- and Z-2-butenes with silene 47. Silene formation was inferred from detection of the disila-cyclobutane products. During these processes, the stereochemical integrity of the compounds was largely preserved. 

Two reactions have come to be extensively used with silenes, arising from the need to trap the short-lived species cleanly and in high yield, as evidence either of their formation or of the extent of their formation. These are the addition of alcohols, usually methanol, across the double bond to yield an alkoxysilane, and the Diels-Alder reaction with a diene, often 2,3-dimethylbutadiene. Each is an example of the two different types of addition to the Si=C double bond. 

Analysis of the data in Table XVIII suggests that silene formation is kinetically the most favorable process. However, according to experiment, metallated silenes are formed. This is related to the fact that in polar solvents proton transfer from the carbon atom to silicon is intermolecular, which leads to a considerable decrease in the reaction barrier. We believe that when the migration of substituents from the carbon atom to silicon is suppressed, for example, by the introduction of two alkyl radicals, the elimination of phosphines resulting in silene formation becomes the most probable process. 

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. 

Formation from silenes and silicon-heteroatom double-bond... 

A mechanism involving the formation of a divalent silicon (silene) intermediate from an a-elimination of the alkoxy-substituted di- or trisilanes, followed by a series of insertion reactions has been proposed (4). For instance, the thermolysis of 1,2-dimethoxytetramethyldisilane may proceed as follows ... 

Again, the same general trends as were found for MeOH, AcOH, n-BuNII2 and MeOTMS are observed in the temperature dependences for reaction of the two silenes, 19a and 19e, with acetone. In both solvents, the differences are consistent with the efficiency of product formation from the intermediate complex, being greater for 19e than for 19a. Interestingly, the curvature observed in the temperature dependence for silene 19e in hexane solution indicates a turnover temperature of ca —15 °C, and the calculated complexation rate constant at this temperature, kc 1.5 x 1010 M-1 s 1, is identical to the diffusional rate constant within experimental error (fcdiff = 1.3 x 1010 M s-1). 

Very recently, Berry and coworkers reported the formation of a ruthenium-silene complex from (PMe3)4RuHSiMe3 and BPh4, and they observed the interconversion between silene and a 16-electron ruthenium silyl complex (Eq. 12) [13a]. 

The product formation from reactions of 9 and 10 with MeLi and LiAlH4 yields the stereo-and regioisomeric substituted derivatives (MeLi 11, 12 LiAlH4 14, 15), whereas PhMgBr reacts selectively with 10 to give the silacyclobutane 13. The reactions of silene 3 with other pentafulvenes (e.g., 16,17 and 18) lead to similar results. 

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... 

While the decomposition of silacyclobutanes as a source of silenes has continued to be studied in the last two decades, the interest has largely focused on mechanisms and kinetic parameters. However, a few reports are listed in Table I of the presumed formation of silenes having previously unpublished substitution patterns, prepared either thermally or photo-chemically from four-membered ring compounds containing silicon. Two cases of particular interest involve the apparent formation of bis-silenes. Very low-pressure pyrolysis of l,4-bis(l-methyl-l-silacyclobutyl)ben-zene94 apparently formed the bis-silene 1, as shown in Eq. (2), which formed a high-molecular-weight polymer under conditions of chemical vapor deposition. 

Another variant of the above-mentioned routes to silenes involved treatment of the carbinols (Me3Si)3SiC(OH)RR, formed from the addition of organometallic reagents R Li to polysilylacylsilanes, with bases such as NaH64 or MeLi,57,64 leading to the formation of alkoxides. These alkoxides spontaneously lost trimethylsilanolate ion, yielding silenes references for these reactions are listed in Table I. 

The second form of head-to-head dimerization involved the formation of a linear (as distinct from a cyclic) species in which two molecules of silene form a silicon-silicon bond. If this follows the pathway suggested above in Eq. (25), the resulting 1,4-diradical must then disproportionate by hydrogen abstraction, forming a molecule saturated at one end and unsaturated at the other. Recent examples are given in Eq. (27).86... 

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