Pathways for Disproportionation

For simple alkyl radicals, the product distribution appears to be predictable using statistical arguments. 

For more highly substituted examples, it is clear that other factors are also important. Substitution at the radical center has a profound effect. For example, in disproportionation, radicals 27 and 28 show a marked preference for loss of a hydrogen from the (x-methyl substituent. 

With the radical 29, even though loss of an equatorial hydrogen should be sterically less hindered and is favored thermodynamically (by relief of 1,3 interactions of the axial methyl), there is an 8-fold preference for loss of the axial hydrogen (at 100 C), The selectivity observed in the disproportionation of this and other substituted cyclohexyl radicals led Beckwith to propose that disproportionation is subject to stereoelectronic control which results in preferential breaking of the C-H bond which has best overlap with the orbital bearing the unpaired spin. 

Reactions betw een carbon-centered radicals generally give a mixture of disproportionation and combination. Much effort has been put into establishing the relative importance of these processes. The ratio of disproportionation to combination (/ ,d/Atc) is dependent on the structural features of the radicals involved and generally shows only minor variation with solvent, pressure, temperature, etc. 

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Even when only the terminal monomer unit is considered, radical-radical termination in binary copolymerization involves at least seven separate reactions (Scheme 7.12). There are two homoterminalion processes and one cross termination process to consider. In the case of cross termination, there arc two pathways for disproportionation. There are then at least three pieces of information to be gained ... 

The rhodium(II) acetate dimer shows good Marcus behavior in reactions with metal ion oxidants and reductants. This allows evaluation of a self-exchange rate around 16 M s at 25°C and 1.0 M HCIO4 for [Rh2(OAc)4] . Dimeric species have also been noted in a study of [Rh(I)/(bipy)2] in aqueous solutions. Pulse radiolysis of [Rh(bipy)3] has been used to generate [Rh(bipy)3], which releases bipy to form [Rh(bipy)2(OH2)2f. Dimerization of [Rh(bipy)2(OH2)2f is rapid and thermodynamically favored and the dimer [Rh(bipy)2]2 slowly dispropor-tionates with a rate constant of 9 x 10 s to give Rh(I) and Rh(III). At higher pH, hydrolysis of [Rh(bipy)2(OH2)2] occurs and the major pathway for disproportionation involves reaction of [Rh(bipy)3] with... 

In contrast with the well-known Embden-Meyerhof-Pamass glycolysis pathway for the conversion of hexose sugars to alcohol, the steps in conversion of ethanol to acetic acid remain in some doubt. Likely, ethanol is first oxidized to acetaldehyde and water (39). For further oxidation, two alternative routes are proposed more likely, hydration of the acetaldehyde gives CH2CH(OH)2, which is oxidized to acetic acid. An alternative is the Cannizzaro-type disproportionation of two molecules of acetaldehyde to one molecule of ethanol and one molecule of acetic acid. Jicetobacter... 

The disproportionation reaction destroys the layered structure and the two-dimensional pathways for lithium-ion transport. For >0.3, delithiated Li, AV02 has a defect rock salt structure without any well-defined pathways for lithium-ion diffusion. It is, therefore, not surprising that the kinetics of lithium-ion transport and overall electrochemical performance of Li, tV02 electrodes are significantly reduced by the transformation from a layered to a defect rock salt structure [76], This transformation is clearly evident from the... 

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

Assuming that the latter reaction constitutes the major pathway for U(IV)-U(VI) exchange, determine the form of the rate law for U(V) disproportionation and its rate constant. 

The reverse reaction of the disproportionation equilibrium has been investigated by Masters and Schwartz. It provides a pathway for the exchange of U(VI) and U(IV) as has been previously mentioned. 

FIGURE 6.11 Confirmed pathway for the observed disproportionation of tocopheroxyl radical 2 into a-tocopherol (1) and o-QM 3, the latter immediately dimerizing into a-tocopherol spiro dimer (9). 5a-C-centered radicals 10 are not involved in this process. 

SCHEME 10.5 Proposed pathway for the nonenzymatic conversion of BHTOOH to BHT-QM in keratinocytes. BHTOOH is oxidized to a peroxy radical that spontaneously loses oxygen. Two BHT phenoxy radicals then undergo disproportionation. 

Specific reviews of the electrochemistry of mononuclear carbonyls have not appeared. The primary oxidation of the mononuclear carbonyls leads to the formation of 17-electron radical cations with half-lives in the order of seconds or less in MeCN electrolytes [14, 15]. Decay may take place by disproportionation, CO loss, and/or nucleophilic attack. Electrogeneration in solvents of low nucleophilicity such as trifluoroacetic acid can enhance the stability of the cations and indicates that nucleophilic attack is a major pathway for decay. This is concordant with the stability order [Cr(CO)g]+ > [Fe(CO)5]+ [Ni(CO)4]+, where the lower coordination numbers favor nucleophilic attack and... 

On the basis of these results a general pathway for the formation of HDF from sugars was assumed as exemplified for fructose-1,6-biphosphate in Figure 11 [88]. Elimination of the phosphate group at C-6 of the l-deoxyosone-6-phosphate, results in acetylformoine which was established as the key intermediate in HDF formation [88]. Reduction of acetylformoine either by a disproportionation reaction with a second molecule of acetylformoine or by further reductive agents present in foods, like vitamin C, then, after elimination of water, generates HDF. 

V vs. NHE). A reasonable pathway for the facile disproportionation is the formation of a dimer with subsequent dissociation to dioxygen and peroxide 16... 

Collins et al. have studied a number of reactions in excess tetralin at 400°C (15). They reported 99% conversion of indene to indane after 1 hour and conversion of cyclohexene and 1-cyclohexenylbenzene to cyclohexane and cyclohexylbenzene after 18 hours. At 400°C values in Table V predict nearly complete (>90%) hydrogenation of both indene and 1-cyclohexenylbenzene after 1 hour and a conversion of cyclohexene to cyclohexane at a rate of 40% per hour. Molecular disproportionation is a feasible pathway for these reactions. 

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 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 ). 

The HOO- species rapidly disproportionates to HOOH, which is not electroactive in DMF at potentials less negative than that for the O2/O2 - couple (-0.64 V versus NHE). A reasonable pathway for the facile disproportionation is the formation of a dimer with subsequent dissociation to dioxygen and peroxide (see Chapter 5). ... 

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