Benzene from photolysis

The photochemistry of borazine delineated in detail in these pages stands in sharp contrast to that of benzene. The present data on borazine photochemistry shows that similarities between the two compounds are minimal. This is due in large part to the polar nature of the BN bond in borazine relative to the non-polar CC bond in benzene. Irradiation of benzene in the gas phase produces valence isomerization to fulvene and l,3-hexadien-5-ynes Fluorescence and phosphorescence have been observed from benzene In contrast, fluorescence or phosphorescence has not been found from borazine, despite numerous attempts to observe it. Product formation results from a borazine intermediate (produced photochemically) which reacts with another borazine molecule to form borazanaphthalene and a polymer. While benzene shows polymer formation, the benzyne intermediate is not known to be formed from photolysis of benzene, but rather from photolysis of substituted derivatives such as l,2-diiodobenzene ... 

Dewar benzene has actually been isolated, and found to revert only slowly to benzene (its half life is approximately 2 days at 25°C). This is remarkable given how similar its geometry is to that of benzene, and what is expected to be a huge thermodynamic driving force for the isomerization. The substituted Dewar benzene, 7, formed from photolysis of 1,2,4-tri-tert-butylbenzene, 6, is apparently even... 

More interesting from the preparative point of view are related rearrangements of substituted car-2-enes. On benzene sensitized photolysis (450-W mercury lamp, 6 h) of 4a-hydroxymethyl-car-2-ene in acetic acid, a mixture of 15% unchanged starting material, ( + )-endo-( 16a, 13%) and ( — )-e.vo-7-hydroxymethyl-l,4,4-trimethylbicyclo[3.2.0]hept-2-ene (17a, 72%) was obtained.75 Several more examples are listed. Several of these compounds have been used in natural product syntheses.74,76... 

Although we could not isolate and characterize a pure product from photolysis of [RuH2(CO)(PPh3)3] in benzene, we could trap the proposed intermediate by irradiation under carbon monoxide. When irradiation was conducted under CO, the ir and 31P NMR spectral changes showed that [Ru(CO)3(PPh3)2] was produced quantitatively (see Reaction 9) (35). The product could be isolated pure by solvent concentration. 

Compound 1 (500 mg, 1.39 mmol from photolysis of stilbene and 2-methylmer-capto-benzothiazole) was sealed under vacuum in a thick-walled Pyrex tube (the volume was about 100 mL) and heated to 200 °C for 20 h in the dark. Methan-ethiol (from the [2,l,3]-elimination) and dimethyldisulfide were condensed to a cold trap (-196 °C) at a vacuum line (65 mg, 97%). The residue was separated by preparative TLC on 200 g SiC>2 with benzene to give 340 mg (78%) of 2 (mp 108 °C, methanol) and 96 mg (22%) of 3. 

The product mixture obtained on photolysis of diaryliodonium salts is, in fact, much more complex than these data suggest. DeVoe et al [22] have identified, in addition to iodobenzene, acetanilide, biphenyl, two iodo-biphenyl isomers in the product mixture from photolysis of Ph2I+PFg in AN or HzO at 254 nm. With the chloride counterion, this list expanded to include chlorobenzene and hydroxybiphenyl (presumably the 2-isomer) Ph2I+I, on the other hand, photolyzed cleanly at 313 nm to iodobenzene. A similar mix of products, including benzene and a third iodobiphenyl isomer was observed by Dektar and Hacker [70] on photolysis of the triflate salt under a variety of conditions (see Table 4, below). 

Benzene formed from photolysis of the 1 1 complex is a cage-escape product from 3(Jul-CHO+ /Ph ). Benzene formed from the photolysis of the 2 1 complex is an in-cage product from 3((Jul-CHO)2 /Ph ). The formation of 2 1 complexes of amino-substituted ketones and iodonium salts has been suggested to account for the high photosensitivity of polymeric Mannich bases with iodonium salts [102]. Formation of 2 1 donor iodonium cation complexes has been rationalized by consideration of the crystal structures of diphenyliodonium halides, which crystallize as dimers with square planar iodine atoms with two bridging halide counterions [102,108]. 

NaOCl (Clorox) and 30% H2 02 reacts with (1) at -5 to -15° to form the endo-peroxide (2) in 37% yield. This substance readily rearranges at 45° to trans-benzene trioxide (3). Singlet oxygen generated from triphenyl phosphite ozonide (3, 323-324) reacts with (1) to form (2), but purification by subUma-tion results in conversion to (3). This trioxide also results from photolysis of (2) (27% yield). 

Triplet sensitization of sulfonium salts proceeds exclusively by the homolytic pathway, and that the only arene escape product is benzene, not biphenyl or acetanilide. However, it is difficult to differentiate between the homolytic or heterolytic pathways for the cage reaction, formation of the isomeric halobiaryls. Our recent studies on photoinduced electron transfer reactions between naphthalene and sulfonium salts, have shown that no meta- rearrangement product product is obtained from the reaction of phenyl radical with diphenylsulfinyl radical cation. Similarly, it is expected that the 2- and 4-halobiaryl should be the preferred products from the homolytic fragments, the arene radical-haloarene radical cation pair. The heterolytic pathway generates the arene cation-haloarene pair, which should react less selectively and form the 3-halobiaryl, in addition to the other two isomers. The increased selectivity of 2-halobiaryl over 3-halobiaryl formation from photolysis of the diaryliodonium salts versus the bromonium or chloronium salts, suggests that homolytic cleavage is more favored for iodonium salts than bromonium or chloronium salts. This is also consistent with the observation that more of the escape aryl fragment is radical derived for diaryliodonium salts than for the other diarylhalonium salts. 

Dewar benzene was prepared in 1963 by lead tetraacetate decarboxylation of the diacid from the photolysis of 1,2-dihydrophthalate esters. Benzvalene was isolated from photolysis of benzene in 1967 " and prepared in quantity from lithium cyclopentadienide, methylithium, and methylene chloride. Prismane was prepared from benzvalene in 1972. Bicyclopropenyl itself was prepared in 1989 although alkyl derivatives were characterized earlier and shown to undergo 3,3-sigmatropic shifts, vide infra. Substituted derivations of all the (CH)6 hydrocarbons, particularly the permethyl materials. 

UV absorption of a polymer influences the response to photolytic degradation. Polyolefins, which are relatively transparent in the UV, receive essentially constant radiation throughout the sample and thus yield photolysis products as a function of total sample weight. Highly UV absorbing polymers, such as PS, strongly attenuate the incident radiation. Measurements of the photolysis yield of benzene from PS films showed that photolysis is surface area controlled and products are formed within a thin surface layer. 

Carbenes are considered to be intermediates in the formation of the two indenes (26) and (27) from photolysis of the cyclopropene (28) in benzene. The 4 1 ratio of (26) (27) indicates preferential cleavage of bond (a) over... 

Photolysis of Cp2TiAr2 in benzene solution yields titanocene and a variety of aryl products derived both intra- and intermolecularly (293—297). Dimethyl titan ocene photolyzed in hydrocarbons yields methane, but the hydrogen is derived from the other methyl group and from the cyclopentadienyl rings, as demonstrated by deuteration. Photolysis in the presence of diphenylacetylene yields the dimeric titanocycle (28) and a titanomethylation product [65090-11-1]. 

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. 

Mixed aryl selenides have also proven to be excellent ree ents for group transfer reactions.Photolysis of selenides in an inert solvent such as benzene can initiate chain reactions. Substituted radicals can be generated in this manner, from a-selenoe-... 

Because of the low photostationary concentration of benzvalene, photolysis is not an efficient way of accumulating this compound. The highly reactive molecule can be trapped, however, if it is generated in the presence of other molecules with which it reacts. Irradiation of benzene in acidic hydroxylic solvents gives products formally resulting from 1,3-bonding in the benzene ring and addition of a molecule of solvent ... 

The methyl ester (100, R = CH3), derived from this A-nor acid by treatment with diazomethane, is different from the ester (102) obtained either by Favorskii rearrangement of 2a-bromo-5a-cholestan-3-one (101) or by the action of cyanogen azide on 3-methoxy-5a-cholest-2-ene (103) followed by hydrolysis on alumina. The ketene intermediate involved in photolysis of (99) is expected to be hydrated from the less hindered a-side of the molecule to give the 2j -carboxylic acid. The reactions which afford (102) would be expected to afford the 2a-epimer. These configurational assignments are confirmed by deuteriochloroform-benzene solvent shifts in the NMR spectra of esters (100) and (102). ... 

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