Photolysis intermediate

Interestingly, Sarkisian and Binkley photolyzed the benzocyclobutaqui-noxaline 56 in methanol and obtained isoindolobenzimidazole 96. They postulated the diazocine diradical 103 and monoradical 104 as photolysis intermediates (70JOC1228). [Peracid oxidation of 56 did afford the diazo-cinedione 57 (see Section II,B,4) (78T495).] During this period, Reid and... 

Jager S, Lewis JW, Zvyaga TA, Szundi I, Sakmar TP, Kliger DS. Time-resolved spectroscopy of the early photolysis intermediates of rhodopsin Schiff base counterion mutants. Biochemistry 1997 36 1999-2009. 

Quantum yields for H2 formation from HFe(CO)4 in 9/1 THF/H2O were determined to be 0.08 at X-irr 366 nm and 0.14 at 313 nm under either a Ar or CO atmosphere. If one assumes the validity of the scheme, the insensitivity of photochemical H2 production toward the presence of CO must lead to the conclusion that the reactivity of HFe(CO)4 with the photolysis intermediate HFe(CO)3 is a least comparable to that of CO (The concentration ratio [Fe(CO)4 ]/[CO] is about 10/1 under the conditions of the experiment.) The scheme also requires that conversion of Fe(CO)5 to the hydride anion HFe(CO)4 be rapid relative to photochemical H2 production. Consistent with this is the very large second order rate constant for the thermal reaction of Fe(CO)5 with hydroxide in solvent mixtures and conditions comparable to those used for the photoreaction [104]. Interestingly, the proposed bimolecular pathway for H2 formation described in the photochemical scheme has not received serious consideration in discussions of thermal WGSR catalysis by Fe(CO)5. 

Lewis, J.W., Winterle, J.S., Powers, M.A., Kliger, D.S., and Dratz, E.A., Kinetics of rhodopsin photolysis intermediates in retinal rod disk membranes. I. Temperature dependence of lumirhodopsin and metarhodopsin I kinetics, Photochem. Photobiol, 34, 375, 1981. 

Lewis, J.W., Liang, J., Ebrey, T.G., Sheves, M., and Khger, D.S., Chloride effect on the early photolysis intermediates of a gecko cone-type visual pigment. Biochemistry, 34, 5817,1995. 

The photosensitized dimerization of isoprene in the presence of henzil has been investigated. Mixtures of substituted cyclobutanes, cyclohexenes, and cyclooctadienes were formed and identified (53). The reaction is beheved to proceed by formation of a reactive triplet intermediate. The energy for this triplet state presumably is obtained by interaction with the photoexcited henzil species. Under other conditions, photolysis results in the formation of a methylcydobutene (54,55). 

Pyridazin-3(2H)-ones rearrange to l-amino-3-pyrrolin-2-ones (29) and (30) upon irradiation in neutral methanol (Scheme 10), while photolysis of 5-amino-4-chloro-2-phenylpyridazin-3(2H)-one gives the intermediate (31) which cyclizes readily to the bis-pyridazinopyrazine derivative (32 Scheme 11). 

The photolysis of azirines has been shown to result in dimerization to pyrazines (72JA1395) and although this formally corresponds to a type B synthesis it involves an isolable intermediate (105) and does not proceed by simple dimerization (Scheme 70). 

Apart from the nuclear bromination observed (Section 2.15.13.1) in the attempted radical bromination of a side-chain methyl group leading to (396), which may or may not have involved radical intermediates, the only other reaction of interest in this section is a light-induced reduction of certain hydroxypyrido[3,4-f)]pyrazines or their 0x0 tautomers analogous to that well-known in the pteridine field (63JCS5156). Related one-electron reduction products of laser photolysis experiments with 1 -deazaflavins have been described (79MI21502). 

The light-induced rearrangement of 2-phenyl- to 3-phenyl-thiophene may occur by a similar mechanism an equilibrium between the bicyclic intermediate (26) and the cyclopro-penylthioaldehyde (27) has been suggested (Scheme 2). The formation of IV-substituted pyrroles on irradiation of either furans or thiophenes in the presence of a primary amine supports this suggestion (Scheme 3). Irradiation of 2-phenylselenophene yields, in addition to 3-phenylselenophene, the enyne PhC=C—CH=CH2 and selenium. Photolysis of 2-phenyltellurophene furnishes solely the enyne and tellurium (76JOM(108)183). 

Phenylthiazirine (40) can be isolated as an intermediate in the photolysis of 5-phenyl-1,2,3,4-thiatriazole and also from other five-membered ring heterocycles capable of losing stable fragments see Scheme 2 (81AHC(28)231). Photolysis of 5-phenylthiatriazole in the presence of cyclohexene yields cyclohexene episulfide (60CB2353) by trapping the sulfur atom. 

However, in some cases carboxylic acid-derived groups can participate in ring fission-reclosure reactions. Thus photolysis of 1,5-disubstituted tetrazole (399) gives nitrogen and appears to involve the amino-nitrene intermediate (400), which reacts further to give (401) (77AHC(21)323). 

A similar intramolecular trapping of the intermediate (511) from the photolysis of the corresponding methyl tetrazole-l,5-dicarboxylate (510) gave methyl 5-methoxy-l,2,4-oxadiazole-3-carboxylate (512). 

Photochemical elimination of carbon dioxide from suitable precursors has given a variety of reactive intermediates at low temperatures where they are often stable and can be studied further. This approach has been utilized in attempts to generate new 1,3-dipolar species, and photolysis of (515) gave an azomethine nitrene intermediate (516) (see Section 4.03.6)... 

Salicylonitrile is believed to arise by direct cleavage with subsequent hydrogen transfer, while the benzoxazoles were produced by an isocyanide intermediate (73JA919, 74HCA376). Photolysis in D2O tends to confirm this possibility and rule out an azirine intermediate (39), due to deuterium corporation into the molecule (Scheme 10) (74HCA376). 

Photochemical studies on the ring degradation of 3-hydroxy-l,2-benzisoxazole also yielded benzoxazolone, and (40), (41) and (42) (Scheme 14) were believed to be potential intermediates. Low temperature IR measurements indicated the presence of (42) during the photochemical reaction (73JA919, 71DIS(B)4483, 71JOC1088). Sensitization studies indicate that the rearrangement is predominantly a triplet reaction, and the keto tautomer is believed to be the key orientation for the photolysis. 

The mechanism of thermolysis and photolysis of ethers of 3-hydroxy-1,2-benzisoxazole has also been studied. Heating of the allyl ether (43) gave minor amounts of (44) and two benzoxazoles. Photolysis of (45) in methanol gave a benzisoxazole and an iminoester, via intermediate (46). Thermolysis at 600 °C gave a benzoxazole, a benzoxazolone and cyano-phenol (Scheme 16) (71DIS(D)4483). 

Photolysis of 3-phenyl-2,l-benzisoxazole in 48% HBr produced reduction and substitution products via a proposed triplet state nitrenium ion intermediate (71HCA2111). Photolytic decomposition of 5-bromo-3-phenyl-2,l-benzisoxazole in 48% HBr gave 2-amino-5-bromoacetophenone and 2-amino-3,5-dibromoacetophenone (Scheme 18). A nitrenium ion intermediate was also proposed for the photolytic decomposition of 3-phenyl-2,l-benzisoxazole in concentrated HCl (Scheme 19) (7IHCA2111). 

Photolysis in concentrated HCl of 3,5-diphenyl-2,l-benzisoxazole gave 2-amino-3-chloro-5-phenylbenzophenone and 2-amino-5-(p-chlorophenyl)benzophenone via similar intermediates (Scheme 20) (71HCA2111). 

Photolysis in general produced oxazoles and a variety of other products including aminochalcones, nitriles, aldehydes and chalcone oximes. A number of photolytic intermediates have been postulated, represented by (151), (152), (153) and (154) (77CL1195, 75T1373, 73HCA2588, 73TL2283). 

The photolysis of 3-( p-cyanophenyl)-2-isoxazoline in benzene produced a tricyclic product along with six other materials (Scheme 46) (B-79MI41616). Irradiation of the bicyclic 2-isoxazoline (155) produced benzonitrile, /3-cyanonaphthalene and polymer via a proposed biradical intermediate (156) (Scheme 47) (B-79MI41615). 

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