Alkyl radicals disproportionation pathways

During the oxidative formation of radicals of type 127 from the appropriate tetrahydrolumazine, various reactivities were found.399 Sequential oxidation in formic acid yielded 127 followed by 128, even when insufficient oxidant was used, implying a disproportionation mechanism. Oxidation of JV(J)-alkyl side chains to give N(J)-acyl substituents was also suggested to account for a further oxidation mode observed.399 Pfleiderer and co-workers have elucidated radical-mediated pathways in polarographic studies of pterins.400... 

Selective trapping of alkyl radicals from the alkyl halide component during the course of the catalytic disproportionation is the same as the previous observation with silver, and it indicates that the prime source of radicals in the Kharasch reaction lies in the oxidative addition of alkyl halide to reduced iron in Equation 47. Separate pathways for reaction of i-propyl groups derived from the organic halide and the Grignard reagent are also supported by deuterium labelling studies which show that they are not completely equilibrated.(49) Furthermore, the observation of CIDNP (AE multiplet effect) In the labelled propane and propene... 

The reaction of a nucleophilic alkyl radical R with benzene affords the a-complex 1, a fairly stable cyclohexadienyl radical, which under oxidizing conditions leads to cation 2 (Scheme 1). Depending on the stability of the attacking radical, the formation of 1 is a reversible process. Deprotonation eventually affords the homolytic aromatic substitution product 3. If the reaction is performed under non-oxidizing conditions, cyclohexadienyl radical 1 can dimerize (—> 4), disproportionate to form cyclohexadiene 5 and the arene 3, or further react by other pathways [3]. 

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. 

With primary halides, dimers (R—R) are formed predominantly, while with tertiary halides, the disproportionation products (RH, R(—H)) prevail. Both alkyl nickel(III) complexes, formed by electrochemical reduction of the nickel(II) complex in presence of alkyl halides, are able to undergo insertion reactions with added activated olefins. Thus, Michael adducts are the final products. The Ni(salen)-complex yields the Michael products via the radical pathway regenerating the original Ni(II)-complex and hence the reaction is catalytic. In contrast to that, the Ni(III)-complex formed after insertion of the activated olefin into the alkyl-nickel bond of the [RNi" X(teta)] -complex is relatively stable. Thus, further reduction leads to the Michael products and an electroinactive Ni"(teta)-species. 

The products arising from the reaction of 431 with the alkyl-substituted silenes 149 and 150 suggest that the reaction occurs by a radical pathway, initiated by a homolytic Si—C bond cleavage of 431 and subsequent Si—Si bond formation giving the biradical 434. Intramolecular disproportionation of 434 gives 435, while 436 and 437 are the results of ring closure reactions without or with expulsion of tetramethylethene, respectively (equation 139)181. 

The reactivity of 02 - with alkyl halides in aprotic solvents occurs via nucleophilic substitution. Kinetic studies confirm that the reaction order is primary > secondary > tertiary and I > Br > Cl > F for alkyl hahdes, and that the attack by 02 - results in inversion of configuration (Sn2). Superoxide ion also reacts with CCI4, Br(CH2)2Br, CeCle, and esters in aprotic media. The reactions are via nucleophilic attack by 02 on carbon, or on chlorine with a concerted reductive displacement of chloride ion or alkoxide ion. As with all oxyanions, water suppresses the nucleophilicity of 02 (hydration energy, lOOkcalmoL ) and promotes its rapid hydrolysis and disproportionation. The reaction pathways for these compounds produce peroxy radical and peroxide ion intermediates (ROO and ROO ). 

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