Cyclohexane photolysis

The mechanism of H2 formation involves both unimolecular H2 elimination, when both of the H atoms of a hydrogen molecule originate from the same alkane molecule, and H atom elimination with subsequent H atom abstraction reaction. We show the mechanism on the example of cyclohexane photolysis hv = 7.6 eV) [91] ... 

Similarly to the fluorescence quantum yields, the yields of individual primary decomposition steps generally show considerable excitation energy dependence the yields of the unimolecular H2 and alkane eliminations and also those of the radical-type decompositions show a continuous variation with photon energy [27,39,42,107,115]. In cyclohexane photolysis the sum of the quantum yields of the two primary decompositions described by Reactions (5) and (6) is practically unity between photon energies 7.6 and 11.6 eV yield decreases with the energy, [Pg.382]

The [IrH2(Me2CO)2(PPh3)2]PF6/Bu CH—CH2 system, mentioned above, has also been used fw the atomatization of cyclohexane." Photolysis of pyridine AT-oxide in an alkane leads to dehydrogenation of the alkane." The formation of by-products in the reaction, as a result of (riiototeatrangemem of the A(-oxide, can be partially suppressed by the addition of BF3." The classic reaction involving heating with... 

Figure I. Quantum yield of nitrogen in the 1470-A. photolysis of alkane-nitrous oxide solutions at 13°C. Yields are relative to 4>(H2) = 1 for liquid cyclohexane photolysis at 1470 A. Ordinate is concentration of N20 in moles/liter...

CycJohexyl free radicals, generated by photolysis of t-butyl peroxide in excess cyclohexane, also possess nucleophilic character (410). Their attack on thiazole in neutral medium leads to an increase of the 2-isomer and a decrease of 5-isomer relative to the phenylation reaction, in agreement with the positive charge of the 2-position and the negative charge of the 5-position (6). 

Photolysis of pyridazine IV-oxide and alkylated pyridazine IV-oxides results in deoxygenation. When this is carried out in the presence of aromatic or methylated aromatic solvents or cyclohexane, the corresponding phenols, hydroxymethyl derivatives or cyclohexanol are formed in addition to pyridazines. In the presence of cyclohexene, cyclohexene oxide and cyclohexanone are generated. 

Photodecomposition of A -l,2,3-triazolines gives aziridines. In cyclohexane the cis derivative (304) gives the cis product (305), whereas photolysis in benzene in the presence of benzophenone as sensitizer gives the same ratio of cis- and trans-aziridines from both triazolines and is accounted for in terms of a triplet excited state (70AHC(ll)i). A -Tetrazo-lines are photolyzed to diaziridines. 

Dipolar cycloadditkm reactions of nitrones to olefins, 46, 1,3-Dipolar cycloadditions with 3-phenylsydnone, 45, 98 Dispiro[5 1 5 l]tetradecane-7,14-dione, photolysis to cyclohexylidene-cyclohexane, 47, 34 preparation from cyclohexanecarbonyl chlonde and triethylamine, 47,34 Displacement of bromine from 1-bromo-2-fluoroheptane to give 2-fluoro-heptyl acetate, 46, 37... 

Similarly, photolysis of l-(2-azidophenyl)-l/f-pyrazole in acetonitrile in the presence of dipropylamine affords AfN-dipropyl-7-(lF/-pyrazol-l-yl)-3//-azepin-2-amine in low yield (4%).192 Surprisingly, however, photolysis of the corresponding 1-(2-azidophenyl)-3,5-dimethyl-l//-pyrazole (84) in cyclohexane in the presence of the base yields 85 which, on the basis of H NMR spectroscopic evidence, has been formulated as a rare example of a stable 2 H-azepine. 

Photolysis of several 2-azidophenazines has been shown to afford quinoxahnes. Thus irradiation of 2-azidophenazine (576, R = H) in cyclohexane or acetonitrile gave, among other products, 3-(2-cyanovinyl)-2-quinoxalmecarbaldehyde (577) in <17% yield and irradiation of 2-azido-l-methoxyphenazine in degassed benzene or acetonitrile gave, among other products, a separable mixture of cis- and frawi-isomers of methyl 3-(2-cyanovinyl)-2-quinoxalinecarboxylate (578), each in low yield. 3 ... 

F ure 4.4. TRIR difference spectra averaged over the timescales indicated following 266 nm laser photolysis of diphenyl diazomethane (6.3mM) in C02-saturated cyclohexane and cyclohexane-4i2. Since the detection of transient species is more problematic in regions with strong solvent bands due to the low transmission of IR light, cyclohexane-4i2 was required for the spectral region below 1600 cm. Reprinted with permission from B. M. Showalter and J. P. Toscano, J. Phys. Org. Chem. 2004,14, 743. Copyright 2004, John Wiley Sons Limited. 

The photolysis of hexaarylcyclotrigermanes was used to synthesize the first stable digermenes. The photolyses are generally carried out at 254 nm in hydrocarbon solvents (e.g., cyclohexane or 3-methylpentane). Presumably, two equivalents of the cyclotrigermane form three equivalents of the digermene, the third equivalent being formed by dimerization of the diarylgermylene. 

In cyclohexane the same two ketones (12) and (13) are obtained from the photolysis of (11) but in aqueous dioxane two phenols are isolated as well as the bicyclic ketone (12). Swenton(10) suggested that the gas-phase reaction involves diradical species, whereas in polar solvents zwitterionic intermediates are favored ... 

In the presence of benzophenone, (8) was again the major product (>95°/0) and only trace amounts of the cyclohexane products were produced. These results suggest the intermediacy of a singlet 1,6-hexylene biradical in the direct photolysis and a longer lived triplet 1,6-diradical in the sensitized photolysis. In the triplet biradical more time is available for 1,6-hydrogen transfer to occur prior to spin inversion and hence more olefin (8) is produced. Similar results were reported for the direct and photosensitized photolysis of the 3,8-dimethyl derivative of (7). 

The photolysis of methylisopulegon in cyclohexane by Cookson et ah resulted in the interesting methylene cyclobutanol shown below in a 70% yield(99) ... 

In accord with these mechanistic ideas, photolysis of phosphoryl azides of type (RO)2PO—Nj in cyclohexane gives predominantly insertion products. Expectedly, no evidence is obtained for the intermediacy of a corresponding metaphosphate since 1,2-OR shifts do not generally occur. 

Photolysis of bis(dimethylamino)phosphoryl azide 2071401 represents an entirely different entry to a metaphosphorimidate. If the reaction is performed in cyclohexane, it gives only 7 % of the amide 209 which can be rationalized as the insertion product of the intermediate nitrene 208 into a CH bond of cyclohexane. The major product component is a polymer. The assumption that it is polymeric aminometa-phosphorimidate 212 is substantiated indirectly by the nature of the principal product of photolysis of 207 in methanol. A 1,2-shift of a NMe2 moiety which... 

In contrast to the problems encountered on photolysis of alkyl- and aryl-sulphonyl azides, we have found that ferrocenylsulphonyl azide 74 is smoothly decomposed by 3500 A light in cyclohexane or in benzene to give ferrocene 15, ferrocenylsulphonamide 16 and the novel bridged [2]ferrocenophanethiazine 1,1-dioxide 17 24>. The yield of 17 varied with the nature of the solvent, being 13.3% in cyclohexane, 67% in benzene, and zero in dimethyl sulphoxide or DMSO/benzene 25>. 

On the other hand, thermolysis of ferrocenylsulpkonyl azide (14) in aliphatic solvents may lead to the predominant formation of the amide (16) 17>. A 48.4% yield of (16) was obtained from the thermolysis in cyclohexane while an 85.45% yield of 16 was formed in cyclohexene. Photolysis of 14 in these solvents led to lower yields of sulphonamide 32.2% in cyclohexane, 28.2% in cyclohexene. This suggests again that a metal-nitrene complex is an intermediate in the thermolysis of 14 since hydrogen-abstraction appears to be an important made of reaction for such sulphonyl nitrene-metal complexes. Thus, benzenesulphonamide was the main product (37%) in the copper-catalyzed decomposition of the azide in cyclohexane, and the yield was not decreased (in fact, it increased to 49%) in the presence of hydroquinone 34>. On the other hand, no toluene-sulphonamide was reported from the reaction of dichloramine-T and zinc in cyclohexane. 

No insertion product was observed on photolysis of ferrocenylsul-phonyl azide in cyclohexane or in cyclohexene 25>, suggesting that the reactive intermediate formed is the triplet sulphonyl nitrene. The fact that addition to the olefinic bond of cyclohexene takes place under these conditions 25> does not necessarily argue against this conclusion (vide infra). 

Solution Photolysis. Solutions were prepared by dissolving the ap-propriate carbamate (la-4a) or amine in cyclohexane (spectrograde -... 

In order to understand these results it is necessary to consider the nature of the intermediates formed upon photolysis of arylamines. The absorption spectra of transients produced upon photolysis of aniline and various alkyl ring-substituted arylamines was obtained by Land and Porter (18) in different solvents using a flash photolysis apparatus. On this basis they identified both an anilinyl radical (PhNH-) and an anilinyl radical cation (PhNHj). The radical cation is present in polar media (H2O) but absent in cyclohexane. From these results, a homolytic cleavage... 

As discussed above, the solution environment provides for a set of time scales different from the gas phase environment. In solution, there are typically 1013 collisions second"1 of a solute molecule with solvent molecules. Thus, if a photolytically generated species is expected to have a large cross section for reaction with solvent and it is desired to monitor that reaction, both generation and monitoring must be done on a picosecond (psecond) or even sub-psecond timescale. That monitoring this rapid is necessary has been confirmed in an experiment on Cr(CO)6 in cyclohexane solution where psecond photolysis and monitoring was not rapid enough to detect the naked Cr(CO)5 that existed before coordination with cyclohexane (55). 

It had already been established by uv-vis flash photolysis (35) that Cr(CO)5 (solvent) was the first observable intermediate in the photolysis of Cr(CO)6. Figure 9 shows the IR spectrum (96) of the photoproduct Cr(CO)5(C6Hi2) in cyclohexane solution. The spectra were obtained using Cr(CO)5(13CO) (96). The extra spectroscopic information provided by the 13CO group was sufficient to show that the spectrum was consistent... 

Fig. 12. Transient IR difference spectra showing changes in absorbance (a) 5 / seconds, (b) 25 seconds, and (c) 1.25 mseconds after the UV flash photolysis of [CpFe(CO)2]2 in cyclohexane solution under 1 atm pressure of CO. Bands pointing upward represent an increase in absorbance (i.e., formation of a compound) and those pointing downward a decrease [i.e., depletion of starting material, (A)]. The bands are assigned as follows A, [CpFe(CO)2]2 B, CpFe(CO)2 and C, CpFe(p.-CO)3Fe(Cp). Points marked were recorded with a 12CO laser and those marked + with a 13CO laser. [Reproduced with permission from Moore et al. (61).]...

Time-resolved IR measurements by Moore, Simpson, and co-workers (61) showed that both CpFe(CO)2 and CpFe(/x-CO)3FeCp were formed within 5 / seconds of photolysis of [CpFe(CO)2]2 in cyclohexane solution. The spectra are shown in Fig. 12. CpFe(/i-CO)3FeCp has similar IR absorption frequencies in the matrix (6,7) and in solution (67). Interestingly, CpFe(CO)2 was the first unsaturated species to be identified by time-resolved IR without previous matrix isolation data being available. CpFe(/u.-CO)3FeCp reacts with CO [Eq. (16)] much more slowly (k —4.5 x 104 dm3 mol-1 second-1) than Mn2(CO)9 reacts (77)... 

Time-resolved IR measurements appear to be a general method for studying the photochemistry of [CpM(CO) ]2 compounds in solution. Thus, photolysis of [CpMo(CO)3]2 in cyclohexane solution produced two products (110) of which one is CpMo(CO)3, with IR absorptions close to those reported for matrix-isolated CpMo(CO)3 (112). 

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