Photochemical disproportionation

As we have shown, the fixation of solar energy by photosynthesis causes a redox disproportionation. Abiotic photochemical processes can similarly induce redox disproportionation. Photochemical reactions produce highly reactive radicals and unstable redox species, which are important in the euphotic zone of natural waters. This will be discussed more in Chapter 12. 

Most chlorofluorocarbons are hydrolytically stable, CCI2F2 being considerably more stable than either CCl F or CHCI2F. Chlorofluoromethanes and ethanes disproportionate in the presence of aluminum chloride. For example, CCl F and CCI2F2 give CCIF and CCl CHCIF2 disproportionates to CHF and CHCl. The carbon—chlorine bond in most chlorofluorocarbons can be homolyticaHy cleaved under photolytic conditions (185—225 nm) to give chlorine radicals. This photochemical decomposition is the basis of the prediction that chlorofluorocarbons that reach the upper atmosphere deplete the earth s ozone shield. 

Photochemical oxidation of a mixture of 1,3,4,7-tctramethyliso-indole (51) and its tautomer (50) has been found to give the isoindolenine hydroperoxide (105). Conceivably, this hydroperoxide could disproportionate to give a hydroxyisoindolenine, since 106 is a major by-product when the condensation between 2,5-hexanedione and... 

Carbonyl compounds can undergo various photochemical reactions among the most important are two types of reactions that are named after Norrish. The term Norrish type I fragmentation refers to a photochemical reaction of a carbonyl compound 1 where a bond between carbonyl group and an a-carbon is cleaved homolytically. The resulting radical species 2 and 3 can further react by decarbonylation, disproportionation or recombination, to yield a variety of products. 

Photochemical disproportionation of metal-metal bonded carbonyl dimers. A. E. Stiegman and D. R. Tyler, Coord. Chem. Rev., 1985, 63, 217 (59). 

Mullins71 have shown that the seven-membered cyclic sulfone (38) can be converted photochemically at 254 nm, as well as by thermolysis, into a mixture of 39 and 40. Interestingly, in this series, compound 40, the result of hydrogen transfer (disproportionation) in the presumed intermediate biradical, remained the major product when different solvents and different wavelengths were employed for the irradiation. 

The photolysis of dimethyl sulphoxide (at 253.7 nm) in a wide range of solvents has been studied in detail176. Three primary reactions occur, namely (i) fragmentation into methyl radicals and methanesulphinyl radicals, equation (60), (ii) disproportionation into dimethyl sulphone and dimethyl sulphide, equation (61) and (iii) deactivation of the excited state to ground state dimethyl sulphoxide. All chemical processes occur through the singlet state. Further chemical reactions of the initial photochemical products produce species that have been oxidized relative to dimethyl sulphoxide. 

Aqueous plutonium photochemistry is briefly reviewed. Photochemical reactions of plutonium in several acid media have been indicated, and detailed information for such reactions has been reported for perchlorate systems. Photochemical reductions of Pu(VI) to Pu(V) and Pu(IV) to Pu(III) are discussed and are compared to the U(VI)/(V) and Ce(IV)/(III) systems respectively. The reversible photoshift in the Pu(IV) disproportionation reaction is highlighted, and the unique features of this reaction are stressed. The results for photoenhancement of Pu(IV) polymer degradation are presented and an explanation of the post-irradiation effect is offered. 

Later experiments (4 ) were designed to determine a cell e.m.f. for the plutonium disproportionation system with a particular light source. Concentration quotients for the light and dark conditions, Qg and Qj, were determined, and an energy difference of 1.65 kcal (32 mV) was calculated by the relation -RTln C /Qd This reversible photochemical shift may be the only single-element system known at this time and certainly is the simplest such system. Even though the radioactive properties could prevent development and utilization of a plutonium photoconversion system, these studies certainly suggest that similar nonradioactive and more acceptable systems could be discovered and developed. 

For example, photolysis of a suspension of an arylthallium ditrifluoro-acetate in benzene results in the formation of unsymmetrical biphenyls in high yield (80-90%) and in a high state of purity 152). The results are in full agreement with a free radical pathway which, as suggested above, is initiated by a photochemically induced homolysis of the aryl carbon-thallium bond. Capture of the resulting aryl radical by benzene would lead to the observed unsymmetrical biphenyl, while spontaneous disproportionation of the initially formed Tl(II) species to thallium(I) trifluoroacetate and trifluoroacetoxy radicals, followed by reaction of the latter with aryl radicals, accounts for the very small amounts of aryl trifluoroacetates formed as by-products. This route to unsymmetrical biphenyls thus complements the well-known Wolf and Kharasch procedure involving photolysis of aromatic iodides 171). Since the most versatile route to the latter compounds involves again the intermediacy of arylthallium ditrifluoroacetates (treatment with aqueous potassium iodide) 91), these latter compounds now occupy a central role in controlled biphenyl synthesis. 

The donor-induced disproportionation in equation (91) leads to the EDA complex, i.e., [D, NO+]NO as the (first) directly observable intermediate. The critical role of the nitrosonium EDA complex in the electron-transfer activation in equation (92) is confirmed by the spectroscopic observation of the cation-radical intermediates (i.e., D+ ) as well as by an alternative (low-temperature) photochemical activation with deliberate irradiation of the charge-transfer band252 (equation 95). 

Dithiazolyl radical 228 photochemically and thermally disproportionates to afford the 1,2,5-thiadiazole 229 and the unstable 1,2,3-trithiole 230 (Equation 54) <2000JCD3365>. Thermolysis of perfluoro-l,3A4(i2,2,4-benzodithiadia-zine 231 affords complex mixtures of heterocycles including perfluoro-2,l,3-benzothiadiazole 232 and 7,8-difluoro-benzo[l,2- 3,4-f ]bis[l,2,5]thiadiazole 233 (Equation 55) <2005EJI4099>. 

As for any chain reaction, radical-addition polymerization consists of three main types of steps initiation, propagation, and termination. Initiation may be achieved by various methods from the monomer thermally or photochemically, or by use of a free-radical initiator, a relatively unstable compound, such as a peroxide, that decomposes thermally to give free radicals (Example 7-4 below). The rate of initiation (rinit) can be determined experimentally by labeling the initiator radioactively or by use of a scavenger to react with the radicals produced by the initiator the rate is then the rate of consumption of the initiator. Propagation differs from previous consideration of linear chains in that there is no recycling of a chain carrier polymers may grow by addition of monomer units in successive steps. Like initiation, termination may occur in various ways combination of polymer radicals, disproportionation of polymer radicals, or radical transfer from polymer to monomer. 

Azobisisobutyronitrile, 182, reacts thermally or photochemically to give the intermediate 183, which leads, in inert solvents, to combination products 184 and 185, and disproportionation products 186 and 187. The parent compound is dimorphic, and both crystal forms behave similarly on photolysis, yielding 95% disproportionation and 5% 184. In contrast, in both fluid and rigid solution the disproportionation products form only 5% of the total. The cage effect in the solid is almost quantitative. 

Most mechanistic studies " have utilized lV,Al-diphenylmethyl-amine. With this substrate in the absence of oxygen, photochemical cyclization to the observable 312 (R = H, R = Me) is followed by disproportionation to the carbazole and to a tetrahydrocarbazole believed to be... 

Studies on the photochemical reactions of dihydropyridines have proven to be interesting. There are a number of 1,4-dihydropyridines that are known to disproportionate when irradiated (equation 19) (B-76PH240). Analogous intramolecular reductions have also been observed by other workers (55JA447). In contrast to these results, the 1,4-dihydropyridine (59) rearranged to its 1,2-dihydro isomer (60). Further irradiation resulted in dimerization. Interestingly, the photodimer (61) cyclized to the cage compound (62). 

Gold] III) porphyrins have been used as acceptors in porphyrin diads and triads due to their ability to be easily reduced, either chemically or photochemically. A new method for incorporating gold(III) into porphyrins (Figure 1.67a) has been described and consists of the disproportionation of [Au(tht)2]BF4 in its reaction with the porphyrin in mild conditions [321]. The metallation of [16]-hexaphyrin with NaAuCl4 yielded the aromatic gold(III) complexes (Figure 1.67b) and the two-electron reduction of the aromatic complexes provided the antiaromatic species [322]. 

The kinetic expression does not provide a definitive answer for the reaction mechanism. An alternative interpretation is also possible if one substitutes (37) with photochemical formation of Au11, (38) with disproportionation of Au11 to Au111 and Au1 and the chlorine atom in (39) with Au11. 

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