Photolysis of cyclohexane

The photochemistry of [Cr(CO)e] has been investigated in several studies. Flash photolysis of cyclohexane solutions of [CrfCO) ] affords two species one has a of 470 nm and a lifetime of 5 ms and the other, = 440 nm, has a lifetime > 1 s. The relationship between photolysed species of [CrfCO) ] and photochemical substitution reactions described in Scheme 4 has been suggested from i.r. and u.v. spectroscopic studies of matrix-isolated species. ... 

The photodecomposition of -alkanes at excitation energies slightly above the absorption onset involves both C-H and C-C bond decompositions [18]. The dominant process is the C-H scission, (H2) 0.8-0.9, and the contribution of C-C decomposition is small. In the photolysis of cyclohexane, cycloheptane, cyclooctane, and cyclodecane, however, only hydrogen evolution was observed [[Pg.375]

Fig. 1 shows the transient absorption spectra observed during photolysis of cyclohexane solutions of CMS. The absorption spectrum with the peak at 520 nm (spectrum a) is identical to that of the excimer of polystyrene (spectrum C) (22) and has a similar lifetime of 20 ns. The quenching rate of the absorption at 520 nm by O2 is comparable to what one would expect for the CMS excimer. 

Early experiments to search for the Cr(CO)s molecnle involved flash photolysis of cyclohexane solutions of Cr(CO)6 with UV-visible detection. Problems arose owing to impurities in the solvent interacting with unsaturated intermediates. Although some evidence was obtained for naked Cr(CO)s it was not possible to confirm the existence of this species nor to distinguish between sqnare pyramidal, C4 (17), and trigonal bipyramidal, D h (18), geometries see Isomer, Types of). 

In the 1470-A. photolysis of cyclohexane-nitrous oxide solutions, nitrous oxide reacts with excited cyclohexane molecules to form nitrogen and oxygen atoms. The reaction of N20 with photoexcited 2,2,4-trimethylpentane molecules is much less efficient than with cyclohexane. In the radiolysis of these solutions, G(N2) is the same for different alkanes at low 5 mM) N20 concentrations. At higher concentrations, G(N2) from the radiolysis of cyclohexane is greater than G(N2) from the radiolysis of 2,2,4-trimethylpentane solutions. The N2 yields from 2,2,4-trimethylpentane are in excellent agreement with the theoretical yields of electrons expected to be scavenged by N20. The yield of N2 in the radiolysis of cyclohexane which is in excess of that formed from electrons is attributed to energy transfer from excited cyclohexane molecules to nitrous oxide. 

For photolysis at 1470 A. a combination Xe arc and reaction cell (15) was used. The light flux through the sapphire exit window was about 8 X 10"9 Einsteins/sec. The actinometer used was the photolysis of cyclohexane for which < (H2) has been shown to be 1.0 (25). In die cell the solutions were in contact with the sapphire window. Solutions were photolyzed for — 20 min., and the conversion was approximately 0.1%. The solutions were stirred during photolysis and cooled to 13 db 2°C. to prevent evaporation. The concentration of nitrous oxide in the liquid phase was calculated from the Bunsen coefficients of 3.3 for cyclohexane and n-hexane and 3.8 for 2,2,4-trimethylpentane (26). 

Table I. Photolysis of Cyclohexane-Nitrous Oxide Solutions...

There are several possible explanations which need to be considered for the formation of N2 in the photolysis of cyclohexane-nitrous oxide solutions. These include direct absorption of vacuum ultraviolet light by nitrous oxide, photoionization of the solvent followed by electron attachment by nitrous oxide, and reaction of nitrous oxide with either excited cyclohexene or excited cyclohexane molecules. Of these possibilities only the last explanation—reaction of excited cyclohexane molecules with nitrous oxide—is important. 

The intermediates in the photolysis of cyclohexane which might react with nitrous oxide to form N2 are hydrogen atoms, excited C6Hi0, and excited C6Hi2. Hydrogen atoms can be eliminated on the basis that (H) is only 0.14 (25) and that the rate of reaction of H atoms with nitrous oxide is too slow (5). The cyclohexene which is formed in the photolysis, Reaction 3, could initially have as much as 7 e.v. excess energy and could conceivably sensitize the decomposition of nitrous oxide. Such a reaction would produce N2 but would not affect the yield of hydrogen. Since < (H2) is reduced by nitrous oxide, excited cyclohexene cannot be the main source of N2. 

Figure 2. Kinetic plot of Equation I for the photolysis of cyclohexane nitrous oxide solutions. Abscissa is 1 /(N2) Ordinate is 1 / [N20] in M"J...

Benzene, a known quencher of excited molecules, reduces G(N2) in cyclohexane solutions but not significantly in 2,2,4-trimethyl-pentane solutions. In the photolysis of cyclohexane, benzene reduces the extent of decomposition as a result of energy transfer, Reaction 6 (9, 25). Further, in the photolysis of cyclohexane-nitrous oxide solutions,... 

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. 

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). 

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... 

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). 

First Order Rate Constants for Decay of Transients Seen by Longer Wavelength (A rr > 390 nm) Flash Photolysis of Ru3(C0)]o in Cyclohexane Solutions with Various Added Ligands a... 

Photolysis of the methylidyne cluster HRu3(CO)] (/1, 71"COCH3) (A) (14) in cyclohexane solution leads to an unprecedented oxygen-to-carbon alkyl migration to form the bridging acyl complex HRu3(CO)10( i-> 2-C(O)CH3) (B) ... 

This transformation was demonstrated (14) by evaluating changes in the UV, IR and NMR spectra and comparing these to the spectra of authentic samples of each cluster (15., 16.). Quantum yields for the photoisomerization depicted in Equation 17 were found to be notably dependent both on the CO concentration and on the A rr. Although the resulting optical changes were the same for different A rr, the quantum yields in CO saturated cyclohexane ranged from < 10 at 405 run to 4.9 x 10 at 313 nm. Furthermore, 2 varied linearly from 1.2 x 10" at Pqq " 0.0 to 4.9 x 10 at P q - 1.0 atm for 313 nm photolysis in cyclohexane. 

Note added in typing In a very recent paper (81) Vaida and co-workers have used picosecond laser photolysis to show that, in cyclohexane solution, Cr(CO)5...cyclohexane (Amax 497 nm) is formed within 25 ps of the photolysis of Cr(C0)5 This suggests that, in solution, the primary photoproduct is Cr(C0)5 and that there is essentially no activation energy for the reaction of Cr(C0)5 with the solvent. Clearly, experiments with pulsed KrF lasers on carbonyls in solution and matrix may be very revealing. 

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