Self-disproportionation

For the deposition reactions carried out under an Ar atmosphere, up to 41% of the dark red tris-/3-diketonate complex Ru(hfac)3 is observed as a volatile byproduct. This finding suggests that the operation of a parallel pathway, which involves self-disproportionation of the source reagent, is necessary to account for the observed film growth ... 

The necessity for another radical to complete the substitution sequence opens the way for complications. The cyclohexadienyl radical 40 may well react in some other way. It may dimerize, undergo self-disproportionation, or couple. Furthermore, since the cyclohexadienyl radical has three positions at which it can react, isomeric products are possible in each case. 

Pd-Pt. The heterometallic nms-PdCl(PR3)2 C=C rans-PtCl(PR3)2 (73, R = Me, Et, Bu) were best prepared from rreaction between nms-PdCl(C=CH)(PEt3)2 and nms-PtCl2(PEt3)2 gave the Pd/Pt-C2 complex together with some of the homonuclear products as a result of ready self-disproportionation.226... 

The ions and radicals that formed may recombine to restore the original dye, or they may undergo further reactions with oxygen, water or undergo self-disproportionation to form colourless end products. 

Interestingly, it is reported that the scalemic samples melted with sublimation. These compounds could be good candidates for the ongoing interest on self-disproportionation during sublimation. 

On self-disproportionation of less numerous chain-end alkyl radicak, the chain-end double bonds and methyl groups are formed in polyethylene. 

Self-Disproportionation of Enantiomers During the Purification by Achiral-Phase HPLC... 

FIG. 4 Self-disproportionation of Michael adduct enantiomers during the purification on achiral HPLC column. 

The phenoxy radicals are relatively long-lived and can undergo self-disproportionation, recombination vith alkylperoxy radicals, or isomeric rearrangement followed by recombination. The resulting compounds may have some stabilization activity. Propionate-type hindered phenols constitute a special class in this category during reaction, the phenol is transformed into phenolic cinnamates, which are known to be efficient chain-breaking antioxidants [Eq. (85)]. 

Kurylo, M.J., Ouellette, P.A., Laufer, A.H. Measurements of the pressure dependence of the hydroperoxy (HO2) radical self-disproportionation reaction at 298 K. J. Phys. Chem. 90, 437-440 (1986)... 

A few free radicals are indefinitely stable. Entries 1, 4, and 6 in Scheme 12.1 are examples. These molecules are just as stable under ordinary conditions of temperature and atmosphere as typical closed-shell molecules. Entry 2 is somewhat less stable to oxygen, although it can exist indefinitely in the absence of oxygen. The structures shown in entries 1, 2, and 4 all permit extensive delocalization of the unpaired electron into aromatic rings. These highly delocalized radicals show no tendency toward dimerization or disproportionation. Radicals that have long lifetimes and are resistant to dimerization or other routes for bimolecular self-annihilation are called stable free radicals. The term inert free radical has been suggested for species such as entry 4, which is unreactive under ordinary conditions and is thermally stable even at 300°C. ... 

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). 

The self reaction of substituted phenylethyl radicals (1) has been widely investigated.92 96 The findings of these studies are summarized in Table 5.2. Unless R2 is very bulky (eg. r-butyl, see below ), combination is by far the dominant process with the value k /k typically in the range 0.05-0.16. Thus, a small amount of disproportionation is always observed. 

The self-reactions of 2-carboalkoxy-2-propyl radicals (8-10) have been examined.89 Ia> 104 The results of these studies are reported in Table 5.3. Combination is slightly favored over disproportionation. The value of kjkw for 8 was found to be essentially independent of temperature. 

Bizilj et at reported that disproportionation is more important for oligomeric radicals. While combination products were unequivocally identified, analytical difficulties prevented a precise determination of the disproportionation products. Accordingly, they were only able to state a maximum value of ktself reaction of 11 and <1.50 for reaction between 8 and 11. 

The reaction between the PMMA and PS model radicals (4 and 5, generated from the unsymmetrical azo-compound 3) has been studied as a model for crosstermination in MMA-S copolymerization (Scheme 7.13).178,179 The value for tcross reaction was 0.56. In disproportionation, transfer of hydrogen from the PS model 5 to the PMMA radical 4 was ca 5.1 times more prevalent than transfer in the reverse direction (from 4 to 5). The value of kJklc(90°C) is between those of Atd/ tc(90oC) for the self-reaction of these radicals... 

Analysis of the products from the thermal decomposition of the mixed azo compound 6 showed that in the cross-reaction of radicals 5 and 7 ld/A tt(90oC) is 0.61.179 This study also found that in disproportionation, hydrogen transfer from 5 to 7 is ca 2.2 times more frequent than transfer from 7 to 5. Both self-reactions involve predominantly combination (Scheme 7.14). The values of Ar1j/Aru.(80°C) are 0.16 and 0.05 for radicals 5 (Section 5.2.2.1.1) and 7 (Section 5.2.2.1.3) respectively. It is clear that values of kJkK for homotermination cannot be used as a guide to the value for kjkyt in cross-termination. 

Rh(OEP)H reacts with CNR (R = Me, n-Bu,) to give the adduct Rh(OEP)-(H)CNR (which has no parallel in CO chemistry) which then slowly transforms to the formimidoyl insertion product, Rh(OEP)C(H)=NR. The dimer Rh(OEP))2 reacts with CNAr (Ar = 2.6-Cf,H3Mc2) in aqueous benzene to give the carbamoyl product. Rh(OEP)C(0)NHAr (characterized by an X-ray crystal structure) together with the hydride, which it.self reacts further with the isocyanide. This is suggc.sted to form via a cationic carbene intermediate, formed by attack of HiO on coordinated CNAr in concert with disproportionation to Rh(III) and Rh(l). 

Parker, S.L., Fu, Y., McAllen, K. et al. Up-regulation of brain nicotinic acetylcholine receptors in the rat during long-term self-administration of nicotine disproportionate increase of the alpha6 subunit. Mol. Pharmacol. 65 611, 2004. 

The extent to which chain oxidation is inhibited depends on the activity and concentration of the antioxidant. A specific activity of an antioxidant as a retarding agent should be expressed per unit concentration of the inhibitor. If the antioxidant terminates chains, chain self-termination by the reaction of peroxyl radical disproportionation should be taken into account. As a result, one obtains the following expression for estimation of the activity F of the introduced amount of the antioxidant [18] ... 

The development of mild forms of anxiety and neuroveg-etative and/or cognitive responses to stress may represent an adaptive evolutionary step against environmentally (external) or self-triggered (internal) threats, but maladaptive reactions have also emerged in human evolution. Thus, anxiety disorders are maladaptive conditions in which disproportionate responses to stress, or even self-evoked responses, are displayed. Anxiety disorders are one of the most frequent psychiatric illnesses, and have a lifetime prevalence of 15- 20% [1, 89]. The most common presentations are generalized anxiety disorder, with a lifetime prevalence rate of close to 5% [1, 89] social anxiety disorder, with very variable lifetime prevalence rates ranging from 2 to 14% [90] panic disorder, with rates from 2 to 4% [1,89] and post-traumatic stress disorder (PTSD), with a prevalence rate close to 8%. Specific phobias, acute stress and obsessive-compulsive behavior are other clinical presentations of anxiety disorders. 

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