Radical pair disproportionation

Quantum yields for adduct 63 and total product (63-65) formation from the reaction of - -t with several tertiary amines are summarized in Table 12. Quantum yields measured at 1.0 M amine concentration are lower than the values extrapolated to infinite amine concentration due to incomplete quenching of It. Extrapolated total quantum yields range from 0.07 to 0.33, providing a lower limit for the efficiency of the proton transfer step, kh> in Fig. 11. The other reaction products, 1,2-diphenylethane (64) and 1,2,3,4-tetraphenylbenzene (65), are formed mainly via in-cage radical pair disproportionation and out of cage combination, respectively. The relative importance of radical pair combination, disproportionation, and cage escape is dependent... 

It has been proposed that the decarbonylation of aldehydes by the Wilkinson catalyst [RhCl(PPh3)3] involves a radical pair disproportionation or recombination reaction. A radical pair intermediate in solution is equivalent to a cage reaction (Scheme 6). Table 15 shows the results obtained from the decarbonylation of a series of chiral cyclopropyl aldehydes ... 

Xu and Li (1989) investigated H — CIDNP spectra of fifteen substituted benzene-diazonium ions during reduction with NaBH4. The spectra are consistent with a mechanism in which the first step is the addition of a hydride ion to the diazonium ion. The diazene formed (Ar - N2 - H) is assumed to dimerize and disproportionate into a radical pair [Ar-N-NH2 N = N — Ar] which loses one equivalent of N2 yielding [Ar—N —NH2 Ar] and recombines to give the diarylhydrazine. A proportion of the aryl radicals escape and form the hydro-de-diazoniation product. 

Correlated or geminate radical pairs are produced in unimolecular decomposition processes (e.g. peroxide decomposition) or bimolecular reactions of reactive precursors (e.g., carbene abstraction reactions). Radical pairs formed by the random encounter of freely diffusing radicals are referred to as uncorrelated or encounter (P) pairs. Once formed, the radical pairs can either collapse, to give combination or disproportionation products, or diffuse apart into free radicals (doublet states). The free radicals escaping may then either form new radical pairs with other radicals or react with some diamagnetic scavenger... 

Another mechanism of nitroxyl radical regeneration was proposed and discussed in the literature [67-71]. The alkoxyamine AmOR is thermally unstable. At elevated temperatures it dissociates with cleavage of the R—O bond, which leads to the appearance of an [AmO + R ] radical pair in the cage of polymer. The disproportionation of this radical pair gives hydroxylamine and alkene. The peroxyl radical reacts rapidly with hydroxylamine thus... 

The thermal decomposition of azoalkanes bearing geminal a-cyano and a-trimethylsiloxy groups has been the subject of a report. The symmetrical compound (107) decomposes near room temperature to afford entirely C—C dimers, whereas the unsymmetrical azoalkane (108) requires heating to 75 °C. A NMR product study of photolysed (107) in the presence of TEMPO showed that the fate of caged t-butyl-l-trimethylsiloxy-l-cyanoethyl radical pairs is disproportionation (17%), cage recombination (20%), and cage escape (63%). 

At temperatures low enough to suppress thermal decomposition, a nltrosamlde XIV In polar or non-polar solvents Is photolytlcally decomposed to amldyl and nitric oxide radicals 4,16,17,18), This Is In sharp contrast to the photostahility of nitrosamines in neutral solvents (Including acetic acid) (ii), although the pattern of photodecomposition Is similar to that of nitrosamines In dilute acidic conditions. However, the overall photolysis pattern of nltrosamldes Is complicated by disproportionation of nitric oxide and existence of a radical pair XV (20,21,22). ... 

The spectra shown in Figure 4.4 were obtained from a reaction of fert-butyl-lithium with l-bromobutane. ° One deduces that both the fert-butyl and the butyl radical were produced and that they reacted in disproportionation reactions to give in part isobutylene and 1-butene, respectively. In the spectrum recorded at 30 s, vinyl proton signals from 1-butene are in emission (5 5.0 and 5 6.1) and enhanced absorption (5 5.6). The isobutylene vinyl proton signal at 5 4.6 is in emission on the left side and enhanced absorption is on the right side. This phenomenon is known as a multiplet effect, and it is due to differences in ISC rates for radical pairs containing rcrt-butyl radicals with different proton nuclear spins. Note that the tert-butyl-lithium sample contained an impurity of isobutylene before the reaction, and the amount of isobutylene was increased after the reaction. 

The participation of such ionic states would load one to expect disproportionation rates to increase in the series CH8, C2H5J iso-C8H7, and ferf-butyl, since the ionization potentials of these radicals form a descending series 10.0 e.v., 8.8 e.v., 7.9 e.v., and 6.9 e.v., respectively. This effect of decreasing ionization potential, which might be expected to push the ion-pair state down in energy, considerably below the radical pair state, is considerably off-set by the increasing value of rt (eqs. 5 and 6). ... 

It would appear from the foregoing that the reactions of recombination and disproportionation of radicals fall into a reasonable relation with each other and with abstraction reactions if in the approach of the radical pair, either head-to-head as in recombination or head-to-tail as in disproportionation, there occurs an appreciable contribution of ionic structures to the attractive potential of the radical pair. These then also fall into a consistent relation with the insertion reactions of carbenes and the snap-out reactions of stable molecules from which latter, the present experimental evidence requires a large charge separation in the transition state. 

Values of kd/kc of 0.055 and 0.28 (kd is the rate constant for the disproportionation process and kc is that for the coupling process) were measured for cumyl radicals (36) in benzene and on silica, respectively (Scheme 21) [161], Similarly, increases in the yields of disproportionation products have been observed for a-methylbenzyl radical pairs at low temperature on porous glass and several types of silica [101]. For example, kd/kc ratios of 0.054 and 1.33 were obtained in pentane and on porous glass at... 

The pathways followed by radical pairs, 42 (disproportionation or coupling with rearrangement) resulting from the type I cleavage of 2-phenylcycloalkanones (41) are influenced by cyclodextrin complexation [164], The product ratio depends both on the size of the cyclic ketone and on... 

It is generally much easier to study unimolecular than bimolecular processes. It should be noted in this connection that some reactions that are bimolecular in solution are unimolecular in crystals. For example, the coupling or disproportionation of two radicals generated in a crystal cage from a single precursor molecule is a first-order reaction of a radical pair, not a second-order reaction of independent radicals. 

Exercise 26-18 Acidification of a solution containing semiquinone radicals such as 1 tends to cause the radicals to disproportionate to the arenediol and arenedione. Why should acid cause changes in the relative stabilities of the semiquinones and the corresponding diol-dione pairs ... 

The observation of decreased exciplex fluorescence intensity and increased adduct formation with increasing solvent polarity (Fig. 10) led to the proposal that adduct formation proceeds via initial one-electron transfer to yield a radical ion pair, followed by proton transfer to yield a 1,2-diphenylethyl and a-di-alkylaminoalkyl radical pair, which subsequently combines to yield 63, disproportionates or diffuses apart (114). Subsequent investigation of this reaction led to the proposal that proton transfer occurs only from the initially formed exciplex or contact radical ion pair prior to solvation to yield a solvent separated radical ion pair. The detailed mechanism for reaction of It with tertiary amines in acetonitrile solution is summarized in Fig. 11 (116c). 

The reaction stoichiometry requires that the formation of the implicated closed shell cation occur by electron transfer within the restricted radical pair rather than via disproportionation (182). 

R. A. Sheldon and J. K. Kochi, J. Amer. Chem. Soc., 92, 4395 (1970). Note that in the t-butyl radical, with nine hydrogens, seven times as many radical pairs will disproportionate as combine even though the disproportionation rate per hydrogen is less than comb. 

In the course of irradiation of acetophenone in the presence of 1-phenylethanol, the actual quantum yields for pinacol formation do not exceed 50%, but rise to 71% when PhCH(OD)Me is used for photoreduction of acetophenone in acetonitrile683,684. A conclusion has been reached from this inverse DIE that half the reaction of triplet acetophenone with 1-phenylethanol involves abstraction of an OH hydrogen followed by disproportionation of the initial radical pair back to reactants. A transfer of an O-bonded hydrogen to a triplet ketone is taking place (equation 318) besides the abstraction of hydrogen from... 

It is known that for phenyl alkyl ketones both the Type I and Type II reactions occur only from triplet state. The radical pair formed from a-cleavage undergo disproportionation resulting in Type I products or recombine to regenerate the ground state ketone. The latter process reduces the efficiency of Type I product formation. 

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