Peter K. Freeman

The photochemistry of polyhaloarenes XIII. The photohydrodehalogenation of 3,4-dibromobiphenyl

Irradiation of 3,4-dibromobiphenyl (BpBr2) in acetonitrile resulted in the formation of 4-bromobiphenyl (BpBr) and 3-bromobiphenyl (3-BpBr) in a ratio of 7.6 ± 0.1 up to 22% conversion of starting material. The dependence of the reciprocal of the quantum yield (1Φ) upon 1BpBr2 is linear. Reduction of BpBr2 with lithium di-tert-butylbiphenylide generates a 3.6:1.0 ratio of BpBr to 3-BpBr, while photohyrodebromination of BpBr2 in the presence of triethylamine produces a similar ratio of 1.8:1.0. Product determining radical stability was assessed by radical debromination of BpBr2 using Ph3SnH in THF, which resulted in a BpBr:3-BpBr ratio of 1.5:1.0. AM1 calculations on the product determining radicals provide the basis for rationalization of products via excimer and radical anion intermediates.

Differential photohydrodehalogenation reactivity of bromobenzenes (1,2,4-tribromobenzene, 1,2,3,5-tetrabromobenzene) and pentachlorobenzene: Sunlight-based remediation

The photochemical dehalogenation of 1,2,4-tribromobenzene, 1,2,3,5-tetrabromobenzene and pentachlorobenzene in open-air solutions of acetonitrile using natural and artificial sunlight as the irradiation source has been investigated. The regiochemistry of mono-dehalogenation has been determined for 1,2,4-tribromobenzene and 1,2,3,5-tetrabromobenzene. Pentachlorobenzene did not react. 1,2,4-Tribromobenzene yielded three products, 1,4-dibromobenzene, 1,3-dibromobenzene and 1,2-dibromobenzene with isomer percentages of 52 +/- 1%, 39 +/- 2% and 9 +/- 1%, respectively. 1,2,3,5-Tetrabromobenzene yielded 1,3,5-tribromobenzene, 1,2,4-tribromobenzene and 1,2,3-tribromobenzene with relative product percentages of 60 +/- 2%, 29 +/- 2%, 11 +/- 1%.

The photochemistry of polyhalocompounds, dehalogenation by photoinduced electron transfer, new methods of toxic waste disposal

Recent work on the mechanisms of the photohydrodehalogenation of haloarenes, with an emphasis on polyhaloarenes, and the related mechanisms of phototransformations of aliphatic halocompounds are reviewed. Attention is focused on the nature of the excimer in the photochemical transformations of haloarenes without additional electron transfer reagent and on the nature of the exciplex formed in phototransformations in the presence of electron transfer reagent. Applications of surface catalyzed photochemical transformations and photohydrodehalogenation of polyhaloarenes to toxic waste disposal is discussed.

The photochemistry of methyl geranate, a model chromophore for insect juvenile hormone analogs

Abstract The irradiation of methyl geranate (3) in ether using 254 nm lamps produces methyl 5-methyl-2-isopropenyl-4-hexenoate (13), 1,6,6-trimethyl-endo-5-carbomethoxybicyclo-[2.1.1]hexane (11), methyl 2-isopropenyl-5-methylcyclopentanecarboxylate (12) and methyl (3Z)-3,7-dimethyl-3,6-octadlenoate (14). Photolysis run in water or in ether in the presence of base generates two additional dienes: methyl 3-methylene-7-methyl-6-octenoate (16) and methyl (3E)-3,7-dimethyl-3,6-octadienoate (17). The photolysis of methyl (2E, 6E)-3,7-dimethyl-2,6-nonadienoate (4a) in ether produces all Z E isomers, 4a – 4d.

The photochemistry of three tetrachloronaphthalenes

Abstract The regiochemistry of the photolysis of three tetrachloronaphthalenes is sharply dependent upon the presence of triethylamine. Analysis of the steady state kinetics in the presence and absence of quenchers and triethylamine provides a mechanistic rationale in terms of triplet, triplet excimer, and singlet exciplex.

The kinetics of photochemical deconjugation reactions of methyl geranate

Abstract The photochemical transformations of methyl geranate are analyzed in terms of the dependence of quantum yield upon base, 1,2-dimethylimidazole, concentration. The dependence of quantum yield of deconjugated esters methyl (3Z)-3,7-dimethy1-3,6-octadienoate (12), methyl 3-methy1ene-7-methyl-6-octenoate (13) and methyl (3E)-3,7-dimethyl-3,6-octadienoate (14) and the ratio of (13)/(12) and (13)/(14) upon base concentration, as well as the dependence of the ratios (methyl 2-isopropenyl-5-methy1cyc1opentanecarboxy1ate (15): deconjugated ester) (10)/(12), (10)/(13) and (10)/(14) upon the reciprocal of the base concentration, are consistent with relative rate constant ratios for ([1,5] sigmatropic shift)/(dienol deprotonation) for photodienols 15, (precursor of 13), 17 (precursor of 12) and 19 (precursor of 14) of 72, 1.0 and 85.

The rearrangement of 4,4-diphenylcyclohexa-2,5-dienylidene

Abstract Pyrolytic decomposition of the Li salt of the tosylhydrazone of 4,4-diphenyl-2,5-cyclohexadienone produces a mixture of biphenyl, o -terphenyl, p -terphenyl, methyl- o -terphenyl and the azine of 4,4-diphenyl-2,5-cyclohexadienone ( 13 ). Insight into the reaction pathway was provided by the pyrolytic decomposition of 2-deuterio tosylhydrazone 8a which generates o -terphenyl 10a and 10b in ratio of 69:31. These results are interpreted in terms of the carbene rearrangements of Schemes 2, 3 and 5.