Isomerization, photolytic

Photolytic reactions of dienes frequently give complex mixtures of rearranged products. Described here, however, is a photolytic isomerization of 1,5-cyclooctadiene (present in solution, in part, as a complex with cuprous chloride) that affords a good yield of one product. 

In all cases when photolytic isomerization into aliphatic alkanes was observed it was found that the quantum yield of primary decomposition calculated from the measured end products is slightly smaller than unity. 

Pentakis(pentafluoroethyl)-l-azaprismane 52 can be isolated in 91% yield by irradiation of the corresponding pyridine. The photolytic isomerization of alkylpyridines (e.g., 2-picoline to 3- and 4-picolines) is also believed to involve azaprismane intermediates. 

Related carbenes, i.e. 3//-benzocycloheptatrien-3-ylidene and 5/7-dibenzo[a,c]cycloheptatrien-5-ylidene generated from the corresponding tosylhydrazones, thermally or photolytically, isomerize to benzo-12 or dibenzobicyclo[4.1.0]hepta-2,4,6-trienes, respectively. In contrast to cycloheptatrienylidene, 5/7-dibenzo[a,c]cycloheptatrien-5-ylidene underwent ring contraction to form a cyclopropene derivative 13 before reaction with alkenes. 

Finally, photolytic isomerization of bromomethyl derivative (12) gives exomethylene derivative (157) (Equation (13)) <8lJOCl666>. 

The photochemical behavior of the isomeric 3-methyl-2-phenyl-2-allyl-l-azirine (66) system was also studied. Irradiation of (66) in cyclohexane gave a quantitative yield of azabicyclohexenes (67) and (68). Control experiments showed that (65) and (66) were not interconverted by a Cope reaction under the photolytic conditions. Photocycloaddition of (66) with an added dipolarophile afforded a different 1,3-dipolar cycloadduct from that obtained from (65). The thermodynamically less favored endo isomer (68b) was also formed as the exclusive product from the irradiation of azirine (66b). 

The photo-induced exo selectivity was observed in other classic Diels-Alder reactions. Data relating to some exo adducts obtained by reacting cyclopentadiene or cyclohexadiene with 2-methyl-1,4-benzoquinone, 5-hydroxynaphtho-quinone, 4-cyclopentene-l,3-dione and maleic anhydride are given in Scheme 4.13. The presence and amount of EtsN plays a decisive role in reversing the endo selectivity. The possibility that the prevalence of exo adduct is due to isomerization of endo adduct under photolytic conditions was rejected by control experiments, at least for less reactive dienophiles. 

The a- and [3-isomers of endosulfan undergo photolysis in laboratory tests after irradiation in polar solvents and upon exposure to sunlight on plant leaves. The a-isomer also undergoes isomerization to the P-isomer, which is relatively more stable (Dureja and Mukerjee 1982). A photolytic half-life of about 7 days was reported for endosulfan by EPA (1982c). The primary photolysis product is endosulfan diol, which is subsequently photodegraded to endosulfan a-hydroxyether. Endosulfan sulfate is stable to direct photolysis at light wavelengths of >300 nm however, the compound reacts with hydroxy radicals, with an estimated atmospheric half-life of 1.23 hours (HSDB 1999). 

Stilbenes that are used as fluorescent whitening agents are photolytically degraded by reactions involving cis-trans isomerization followed by hydration of the double bond, or oxidative fission of the double bond to yield aldehydes (Kramer et al. 1996). 

In order to isolate a single isomer of diphosphathienoquinone, two bromo groups were attached to 3,4-positions to avoid formation of the ( )-form by steric repulsion between Br and Mes groups. Analogous dehydrochlorination of the precursor afforded (Z,Z)-diphosphathienoquinone 4b as a single isomer (Scheme 11) [11], Diphosphathienoquinone 4b did not undergo iiZ-isomerization either under thermal or under photolytical condition. 

The Nazarov cyclization of vinyl aryl ketones involves a disruption of the aromaticity, and therefore, the activation barrier is significantly higher than that of the divinyl ketones. Not surprisingly, the Lewis acid-catalyzed protocols [30] resulted only in decomposition to the enone derived from 46,47, and CO. Pleasingly, however, photolysis [31] readily delivered the desired annulation product 48 in 60 % yield. The photo-Nazarov cyclization reaction of aryl vinyl ketones was first reported by Smith and Agosta. Subsequent mechanistic studies by Leitich and Schaffner revealed the reaction mechanism to be a thermal electrocyclization induced by photolytic enone isomerization. The mildness of these reaction conditions and the selective activation of the enone functional group were key to the success of this reaction. 

The metabolism of synthetic pyrethroids in plants has been extensively studied and many reviews are available [74, 117, 131]. After application as a formulation to plants, pyrethroid molecules are considered to be dissolved in epicuticular waxes followed by penetration to interior tissues where various chemical and enzymatic reactions proceed. The existing metabolism studies using 14C-labeled pyrethroids clearly show insignificant translocation from treated sites to other parts of plants due to their hydrophobic nature. The reactions in plants can be generally classified into three types photolytic and chemical reactions on plant surface and so-called phase I and II reactions successively proceeding in tissues [60]. Not only the photo-induced cis-trans isomerization for cypermethrin (5) and deltamethrin (6) but also... 

Table 5.8 summarizes the glycals that had been converted into 2-aminoglycosides so far. In general, photolytic ds/trans isomerization of trans-azodicarboxylates is necessary to accomplish good yields of the initial cycloadducts [335-337]. However, bis-trichloroethyl azodicarboxylate also reacts under thermal conditions and also gives a more reactive cycloaddition product [338,339]. 

The crystalline state of 193 was irradiated with sunlight at 5 °C (equation 93) to afford the cyclobutanes 194 and 195 in a 3 1 ratio117. Compound 195 obviously arose from the dimerization of the c/s-isomer of 193. The disordered crystal structure of 193 permits isomerization of 193 to the (. A-isomer which photolytically reacted with 193 to give 195. Interestingly, the crystalline state of compound 196 and 198 was photolysed to 197 and 198, respectively (equations 94 and 95), but /i-nitro-p-methylstyrene was photostable. 

Photolytic. Photolysis of thin films of solid endrin using UV light X = 254 nm) produced 5-ketoendrin and endrin aldehyde and other compounds (Rosen et al., 1966). When exposed to sunlight for 17 d, endrin completely isomerized to 5-ketoendrin (1,8 eYO-9,10,11,11-hexachloro-yclo[6.2.1.1 .0 0 °]dodecan-5-one) and minor amounts of endrin aldehyde (Burton and Pollard, 1974). Irradiation of endrin by UV light X = 253.7 and 300 nm) or by natural sunlight in cyclohexane and hexane solution resulted in an 80% yield of l,8-eA o-9,ll,ll-pentachloro-cyclo[6.2.1.1 0 0 °]dodecan-5-one (Zabik et al, 1971). This compound also formed from the sunlight photolysis of endrin in hexane solution (Fujita et al., 1969). 

Photolytic. A carbon dioxide yield of 53.7% was achieved when phenylenediamine (presumably an isomeric mixture) adsorbed on silica gel was irradiated with light (A, >290 nm) for 17 h (Freitag et al., 1985). 

Photolytic. Photolysis of permethrin in aqueous solutions containing various solvents (acetone, hexane, and methanol) under UV light (1 >290 nm) or on soil in natural sunlight initially resulted in the isomerization of the cyclopropane moiety and ester cleavage. Photolysis products identified were 3-phenoxybenzyldimethyl acrylate, 3-phenoxybenzaldehyde, 3-phenoxybenzoic acid, mono-chlorovinyl acids, cis- and fra/is-dichlorovinyl acids, benzoic acid, 3-hydroxybenzoic acid, 3-hydroxybenzyl alcohol, benzyl alcohol, benzaldehyde, 3-hydroxybenzaldehyde, and 3-hydroxybenzoic acid (Holmstead et ah, 1978). 

Sarel and co-workers have examined some reactions of alkynylcyclopropanes with iron carbonyl compounds [1]. Treatment of cyclopropylacetylene (5) with iron pentacarbonyl under photolytic conditions gives, after cerium(IV) oxidation, isomeric quinones 6 and 7, derived from two molecules of 5 and two carbonyls with both cyclopropane rings intact [6]. Furthermore, the photoreaction of dicyclopropylacetylene (8) with iron carbonyl gives some ten different products depending on the reagents and the reaction conditions, and some of them have the cyclopentenone skeleton formed by the opening of cyclopropane ring coupled with carbonyl insertion [7] (Scheme 2). 

ElZ isomerizations are usually not expected in the solid state. They have been widely studied in solutions or in liquids. This includes thermal, catalytic, and photolytic processes and ElZ isomerization was also observed in competition with biphotonic excimer laser photodecompositions [47]. Most of the ElZ isomerizations in the solid state have been photo chemically observed [48], but mostly not as uniform quantitative reactions. If these isomerizations cannot be performed under selective conditions of irradiation (an exception is 83/84) [49], the only chance to have these reactions uniform with 100% yield is a very efficient isomerization (according to the phase rebuilding mechanism) that leads to an isomeric product with heavily interlocked crystal lattice. Under such circumstances side reactions of the substrate and photoconversions of the product are prohibited (including the back reaction, of course). Four favorable cases... 

The chemistry of unsaturated azepines is dominated by their polyene character. The absence of ir-delocalization confers instability on the ring system as witnessed by the many and various ring transformations undergone in acid and base solution, and under thermal and photolytic conditions. Most of the major reactions of azepines involve the neutral molecule e.g. cycloadditions (Section 5.16.3.8.1), metal carbonyl complexation (Section 5.16.3.8.2), dimerizations (Section 5.16.3.2.3) and photo- and thermo-induced valence isomerizations (Section 5.16.3.2.1). 

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