Kinetics, silene reactions formation

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... 

The kinetics of acetone addition have also been studied for several aryldisilane-derived (l-sila)hexatrienes, including 21a-c47,51, 4651 and 62 (from photolysis of 61 equation 47), where the reaction follows a different course than that of simpler silenes such as 1952. In these cases, the reaction proceeds via two competing pathways, formal [2 2]-cycloaddition and ene-addition. Unlike the case with the simpler silenes, however, the ketone rather than the silene acts as the enophile in the reaction, presumably because this alternative has the formation of an aromatic ring as an added driving force. 

Encounters between silyl radicals in solution or in the gas phase usually result in recombination and disproportionation (45, 46). Disproportionation results in the production of silanes and highly reactive silenes. The disproportionation reaction is thermodynamically favorable because of the formation of a silicon-carbon double bond, which, although subsequently chemically reactive, is worth —39 kcal/mol (44). For pentamethyldisilanyl radicals, disproportionation is kinetically competitive with radical dimerization (46). In an earlier study, Boudjouk and co-workers (47) demonstrated conclusively by isotopic substitution and trapping that the silyl radicals generated by photolysis undergo disproportionation, as well as, presumably, dimerization (Scheme I). In deuterated methanol, the silanes produced were predominantly undeuterated, whereas methoxymethyldiphenylsilane was extensively deuterated in the a position. The results of these experiments strongly implicated the substituted silene produced by disproportionation. 

Disproportionation of Silyl Radicals. Faced with the task of generating a series of sterically hindered polysilyl substituted radicals upon irradiation, the authors have proposed a tentative reaction scheme involving silyl radical formation, disproportionation to silanes and silenes, and readdition of silyl radicals to the silenes. The disproportionation of silyl radicals is a well-established process that is kinetically competitive with recombination (46). Repetition of this process would lead eventually to highly sterically encumbered and undoubtedly persistent silicon-based radicals carrying only silicons in the a positions. Although such a scheme would explain much of the data in this obviously very complex process, it is very tentative, and other possible routes to and structures for the persistent silyl radicals have not been ruled out (58). 

Thus, a whole series of cases is now known. As the bulkiness of the substituents increases, the dimerization slows down, but the dimer is still more stable than the monomer (kinetic stabilization). As it increases further, the two forms are of comparable free energy since the dimer can form only long and weak new bonds, and finally, the monomer becomes the stable form relative to the dimer as formation of new bonds by dimerization becomes totally unprofitable (thermodynamic stabilization) the silene may of course still be thermodynamically unstable with respect to other types of reactions. 

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