Toluene photochemical decomposition

Photochemical decomposition of diazomethane yields methylene, which reacts with benzene to form toluene and cycloheptatriene (via norcaradiene) (Scheme 5).65 66... 

X 109 and 8. 9 x 109 s 1 for Ndm and Ybm, respectively. Both the above-described in-termolecular mechanism, as well as an intramolecular pathway in the ternary complex with aad which forms in solution, are responsible for the observation of NIR luminescence in these systems. Addition of water to the toluene solutions quenches the NIR luminescence, while it enhances the visible CL emission of the corresponding solution of Eum and Tbm (Voloshin et al., 2000c). Neodymium and ytterbium tris(benzoyltrifluoroacetonates) display the same CL as tta complexes, although for Ybm its intensity is about 2.5 times lower than for the tta chelate. On the other hand, almost no CL is detected for acetylacetonate complexes (Voloshin et al., 2000a). Thermal or photochemical decomposition of aad also triggers CL from [Pr(dpm)3] and Pr(fod)3], both in the visible (from the 3Pi, 3Po, and 1D2 levels) and in the NIR at 850 nm ( Do -> 3F2 transition) and 1100 nm ( D2 3F4 transition). The excited... 

Singer and Chen demonstrated the inability of an a-chlorine substituent to stabilize the configuration of a cyclopropyl radical. They showed that the photochemical decomposition of both exo- (20) and endo-t-buty 6-chlorobicyclo[3.1.0]hexane-6-percarboxylate (21) in diisopropylbenzene resulted in an identical mixture of exo- (22) and endo-6-chlorobicyclo[3.1.0]hexane (23). A similar result" was obtained in the thermal decomposition of both exo- (24) and endo-t-buty 7-chlorobicyclo[4.1.0]heptane-7-percarboxylate (25). In solvents such as toluene, cumene or bromotrichloromethane the same ratio (20 80) of exo-26 and endo-21 products was formed within experimental error. 

Photochemical decomposition of malonic acid by irradiation in solution has been reported. Some of the radical species produced by this treatment are identical to those formed by the Ce decomposition of malonic acid in the Belousov-Zhabotinsky reaction. The (2 + 2)-cycloadducts (172) can be readily prepared by irradiation of mixtures of the corresponding enone and alkene, and these adducts can conveniently be converted into the hydroperoxide (173) by irradiation at 366 nm in the presence of air and acridine in toluene.The decarboxylation occurs by a free radical pathway and treatment of the hydroperoxide with dimethyl sulfide brings about formation of the ring-expanded ketones or lactones (174),... 

Trost and Whitman have established a new route to diphenylcyclopropenone, using the photochemical decomposition of the aa -bisdiazoketone (161). The product (162) was stable to the reaction conditions when irradiation (436 nm) was carried out in methanol, but in toluene decarbonylation afforded a quantitative yield of diphenyl-acetylene. Application of the reaction to oea -bisdiazocyclohexanone gave only cyclopentane derivatives (Scheme 13), indicative of the transient intermediacy of a cyclopropenone. 

The Arrhenius activation energy for this rate-determining dissociation is 13 kcal/mole (in toluene solution). This value agrees well with the activation energy of the thermal and photochemical decomposition of nickel carbonyl in which the above dissociation (Eq. 16) is also rate determining (33,34,35). 

Stirrer-heater device. When the reaction mixture has reached 85 °C, 2-isocyano-1,3-dimethylbenzene (0.786 g, 6.0 mmol) is added to give an immediate green coloration. Vigorous CO evolution occurs for a period of 10 min. The completion of the reaction is indicated by the cessation of CO evolution, but is best confirmed by IR spectroscopy (Table I). The reaction mixture is cooled to 0 °C, filtered to remove the catalyst, and then passed down a short silica gel column ( 20g, wrapped in foil to minimize photochemical decomposition) in order to remove any remaining traces of catalyst. Portions of toluene may be used as eluent. The solvent is removed under reduced pressure to give the product as a yellow, crystalline solid. Yield 2.12 g (88%). This material is sufficiently pure for most purposes, but it may be recrystallized from dichloromethane-pentane to give the analytically pure product. 

CgoO (1) can also be prepared by allowing toluene solutions of CgQ to react with dimethyldioxirane (Scheme 8.3) [28], The so-obtained product is identical to that prepared by photochemical epoxidation [15], Upon treatment of CgQ with dimethyldioxirane, a second product is formed simultaneously (Scheme 8.3), which was identified to be the 1,3-dioxolane 6. Upon heating 6 in toluene for 24 h at 110 °C, no decomposition could be observed by HPLC, implying that 1 and 6 are formed by different pathways. Replacement of dimethyldioxirane with the more reactive methyl(trifluoromethyl)dioxirane allows much milder reaction conditions [29]. At 0 °C and a reaction time of only some minutes this reaction renders a CgQ conversion rate of more than 90% and higher yields for CgoO as well as for the higher oxides. 

The products formed in these reactions are very sensitive to the functionality on the carbenoid. A study of Schechter and coworkers132 using 2-diazo-1,3-indandione (152) nicely illustrates this point. The resulting carbenoid would be expected to be more electrophilic than the one generated from alkyl diazoacetate and consequently ihodium(II) acetate could be used as catalyst. The alkylation products (153) were formed in high yields without any evidence of cycloheptatrienes (Scheme 33). As can be seen in the case for anisole, the reaction was much more selective than the rhodium(II)-catalyzed decomposition of ethyl diazoacetate (Scheme 31), resulting in the exclusive formation of the para product. Application of this alkylation process to the synthesis of a novel p-quinodimethane has been reported.133 Similar alkylation products were formed when dimethyl diazomalonate was decomposed in the presence of aromatic systems, but as these earlier studies134 were carried out either photochemically or by copper catalysis, side reactions also occurred, as can be seen in the reaction with toluene (equation 36). 

A diazosilene is probably also involved in the photochemical or copper-catalyzed decomposition of bis(diazoacetate) 156 in benzene (equation 36). In both cases, dia-zoketene 157 was the only identified product72. Its formation was explained by the silylcarbene-to-acylsilene-to-silylketene sequence outlined in Scheme 5. Efforts to achieve the N2 extrusion from the remaining diazo function by thermolysis in boiling toluene or by prolonged photolysis resulted only in unspecific decomposition. 

The pyrolysis of methyl mercaptan was investigated by the toluene flow carrier technique in the range 732-829 °C . The reaction is first order and leads to the formation of CH4, HjS, Hj and bibenzyl in contrast to the photochemical case, the thermal decomposition is undoubtedly initiated by... 

Isomerization of the polymer from a predominantly cis configuration to a predominantly trans configuration could be accomplished either thermally or photochemically. Thermal isomerization was accomplished by heating the sample in benzene or THF at 60-80°C in a tube sealed with a Teflon Kontes screw-top until the visible absorption spectrum showed no change. Photochemical isomerization was accomplished at 0°C (monitored by thermocouple and external meter in the bath) by exposure of the sample dissolved in THF or benzene to light from a Pyrex-filtered, 350 W, medium-pressure mercury Hanovia lamp (approximately 6-12 h for a sample concentration of 1 mg mL ). Overexposure resulted in a decrease in color indicating decomposition of the material in solution. THF, toluene and benzene were suitable solvents for this experiment. Chlorinated solvents sometimes lead to photobleaching. 

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