Silenes kinetics

The mechanism of substitution reactions at saturated silicon centers is well studied, regarding both kinetics and stereochemistry13,14. In contrast, addition reactions to unsaturated silicon centers, such as to disilenes and silenes, are relatively unexplored. The reason is clear suitable substrates for investigations of regio- and stereochemistry and reaction kinetics are not readily available due to inherent kinetic instability of disilenes and silenes. Kinetically stabilized disilenes and silenes are now available, but these are not always convenient for studying the precise mechanism of addition reactions. For example, stable disilenes are usually prepared by the dimerization of silylenes with bulky substituents. Therefore, it is extremely difficult to prepare unsymmetrically substituted disilenes necessary for regio- and/or stereochemical studies. 

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... 

A number of relatively stable silenes with bulky substituents are known at present, but stable silanones have not been isolated till now. Their instability, like that of silenes, is caused by a kinetic factor, according to various calculations. Thus, cyclooligomerization of silanones should proceed with zero activation barrier (Kudo and Nagase, 1985). 

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 However, a few reports are listed in Table I of the presumed formation of silenes having previously unpublished substitution patterns, prepared either thermally or photo-chemically from four-membered ring compounds containing silicon.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. 

Conlin148 also studied the pyrolysis of 1-methyl-1-silacyclobutane in the presence of excess butadiene at various temperatures where the decomposition followed first-order kinetics and where the silene isomerized to the isomeric silylene prior to reacting with the butadiene. The value for the preexponential factor A for the silene-to-silylene isomerization was found to be 9.6 0.2 s-1 and the Ewl for the isomerization was 30.4 kcal mol-1 with A// = 28.9 0.7 kcal mol-1 and AS = -18.5 0.9 cal mol-1 deg. More recently, the photochemical ring opening of l,l-dimethyl-2-phenylcyclobut-3-ene and its recyclization was studied. The Eact for cycli-zation was 9.4 kcal mol-1.113... 

Reactions favoring [2 + 2] cycloaddition tended to be those that had strongly electronegative groups on the sp2-hybridized silicon but only H and the neopentyl group on the sp2-hybridized carbon atom. Butadiene and cyclohexadiene generally favored [2 + 2] cycloaddition with these silenes. The [2 + 2] adducts with cyclohexadiene appear to be kinetic products, since they cleanly isomerized to the Diels-Alder adducts over time.182... 

Whether the [2+2] or [2+4] cycloaddition product is formed in the first reaction depends on the reaction conditions however, one product is convertible to the other via the silene intermediate. In the other reactions shown, the identity of the product formed, [2+2] or [2+4], seemed to be a function of the structures of both the silene and imine. In the last case, the [2+4] product appeared to be the kinetic isomer since conversion to the [2+2] isomer slowly occurred on standing in the dark, or faster if photolyzed, even at room temperature. 

Wiberg has studied the kinetics of several systems involving the silene Me2Si=C(SiMe3)2. The kinetics for the complex system of the silene with jV-trimethylsilylbenzophenonimine, namely [2+4] adduct <=> silene + imine <=> [2+2] adduct as shown in Eq. (62), were measured174,198 as were the data for the corresponding system with benzophenone, viz. [2+4] adduct <=> silene + benzophenone <=> [2+2] adduct.220... 

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 effect of ring substituents on the rate constants, deuterium kinetic isotope effects and Arrhenius parameters for ene-additions of acetone to 1,1-diphenylsilane have been explained in terms of a mechanism involving fast, reversible formation of a zwitterionic silene-ketone complex, followed by a rate-limiting proton transfer between the a-carbonyl and silenic carbon. A study of the thermal and Lewis acid-catalysed intramolecular ene reactions of allenylsilanes with a variety of... 

Silene (27) can undergo a 1,2-shift to give either methylsilylene (28) or, less favorably, to silylcarbene (29). The thermochemistry and the kinetics of these reactions have been points of major disparity between theory and experiments... 

Silenes of the family Me3SiR1Si=C(OSiMe3)Ad-l 137 undergo a complex silene-to-silene photoisomerization reaction90,94,96. When silenes 137 are generated by photolysis of acylsilanes 138, the isomeric silenes 139 and 140 are formed in a subsequent reaction. The reaction was followed by UV and NMR spectroscopy. The disappearance of 138 cleanly follows first-order kinetics and the overall kinetics were consistent with the transformation 138 -> 137 -> 139. 137 as well as 139 were characterized by NMR spectroscopy and, in addition, the structure of 137 was established by trapping with methanol. The identity of 139 and 140 was confirmed by the isolation of their head-to-tail dimers from which crystals, suitable for X-ray analyses, were isolated (equation 34)90. 

In contrast to ethene which is separated by a high potential barrier from the isomeric methylcarbene, silene 25 can undergo a 1,2 shift to either methylsilylene 276 or, less favourably, to silylmethylene 277 (equation 68). The thermochemistry and the kinetics of... 

In recent years the application of laser techniques has become an important tool in the study of the kinetics of relatively simple reactive organosilicon species (silylenes, silenes etc.). Gaspar and coworkers have reviewed the use of laser techniques to study the generation and reactions of silylenes14. 

An extensive theoretical investigation does not exist for the siloles, but PM3 calculations of formation enthalpies of 2 and its tautomers have indicated that the l//-silole is the most thermodynamically stable species200. The activation barrier for 11 — 2 isomerization was calculated to be 96 kJ moC1, comparable to that for cyclopentadiene2d 116. The (1H + 1H) dimer 1019 is isolated rather than the (2H + 1H) dimer as in the case of phosphole. This directly confirms the thermodynamic stability and the Diels-Alder kinetic instability of 2. The marked difference in the stability of the parent silole and phosphole was explained3 by the relative stabilities of the a bonds in silanes and phosphines (Si > P) and of the ji bonds in silenes and phosphenes (P > Si)117. 

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