Alkyl radicals, disproportionation rearrangements

The study of the addition of more complex alkyl radicals to olefins and acetylenes is fraught with experimental difficulties. The initial adducts are just about as reactive as the initial alkyl radicals. There are inherent problems of rearrangements, hydrogen abstraction, disproportionation. 

The first question is if no cyclization to the (Cy5) compounds is observed when the 5-hexenyl radical is chosen, is it possible to rule out the formation of an alkyl radical R on the reaction pathway The answer is no, not if fast competitive intermolecular reactions are expected. In this case, it is necessary to work at low concentrations in order to favor the intramolecular process but even under these conditions, very low yields of cyclized products are sometimes obtained. The use of the faster ring-opening processes (see Section XII. 1.D) will be then a useful complementary probe. But there is a case where even these faster reactions cannot afford a positive answer, when the radical intermediate reacts, by dimerization or disproportionation, with another radical in the solvent cage, since these processes are faster than the rearrangement processes. 

When one of the aromatic groups of the triarylmethyl free radical is replaced by an alkyl group, a decrease in stability due to a loss of resonance stabilization is to be expected. The paramagnetism and reactions associated with these less stable radicals will therefore appear only when the ethane is heated well above room temperature, the dissociation being endothermic. The rate of formation, but not the equilibrium constant, is experimentally accessible for these radicals since the radical once formed is subject to rearrangement, cleavage, and disproportionation reactions ... 

The initially formed radical R disproportionates (path a),dimerizes (path b), reacts with active cathodes (path c), rearranges (path d), adds to double bonds (path e) or is reduced to an anion (path f). Products of radical origin (a—e) occur mainly in the reduction of alkyl iodides, benzyl halides and in some cases of alkyl bromides. 

An attractive, although tentative, alternative would be an alkyl-substituted silylsilylene formed from the polymer chain. Two thermodynamically reasonable routes to such intermediates are possible. The first route (equation 4) involves 1,1-elimination to produce the silylsilylene directly. This route has a precedent in organosilane thermal processes (78, 79). The second route (equations 5a and 5b) involves rearrangement from a silene produced by the disproportionation (46, 80, 81) of two silyl radicals caused by bond homolysis. This type of rearrangement has also been described in the literature (82). The postulated silylsilylenes are also attractive intermediates to explain the rebonding of silicon to carbon atoms other than those in the original a positions (CH insertion), which is obvious from the mass spectral analysis of gaseous products from the laser ablation of isotopically labeled poly(di-n-hexylsilane). 

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