Photo-oxidation and -reduction

Investigation of the temperature dependence of the photochemistry of o-methylacetophenone has revealed that the rate of decay of the biradical generated in the photoenolization is associated with a low A factor. An explanation for the effect lies in the requirement for spin inversion in the process. The key step in a newly developed synthesis of ( )-oestrone is the photoinduced cyclization (1) (2). Excitation of (1) induces photoenolization to the kinetically 

In a flash photolysis study of the photoreduction of chloranil to tetra-chlorohydroquinone, solvent and isotope effects have been interpreted in terms of 

Photoreduction of 1,2-naphthoquinone by acetaldehyde proceeds by a mechanism involving the initial radical pair, as shown by measurements of H CIDNP and of the change of product distribution with temperature. However, at 20 °C at least 6.7% occurs by addition of a free acyl radical to the quinone in its ground state. In some related work, strong evidence has been obtained for secondary polarization of 1,4-naphthosemiquinone radicals during photoreduction. The temperature dependence of the chemical decay rate constant showed that the termination process is diffusion controlled. 

The photoinduced reduction of some quinones by zinc porphyrin and also by its tetraphenyl derivative has been studied in micellar systems. The mean time for intramicellar electron transfer has been established as 0.2 ps, and for duroquinone the rates of entry and exit from the micelle have been found to be 5 x 10 m s and6 x 10 m s respectively. Quinones possessing long chains are less mobile and partial charge separation could be achieved. Irradiation of anthraquinone in aqueous sodium dodecyl sulphate leads to anthraquinol and the surfactant-anthrahydroquinone ether as major products via the triplet state of the anthraquinone.  

On irradiation in MeOH, (3 R = Me or Et) undergoes y-hydrogen abstraction followed by intramolecular radical reaction to give a jS-lactam. However, excitation in the nn region of the thiocarbonyl group does not induce this transformation, suggesting that photocyclization of the thioxoacetamides proceeds from upper excited states as in the case of thiones.  

Reviews have appeared of advances in alkane functionalisation, non-sensltized photooxygenation of organic conpounds, sensitized 

Involving copper complexes,and photoreduction of aromatic species.  

The photoreduction efficiency of carbonyl compounds such as 9,10-anthraquinone, 1,4-anthraquinone, 6,13-pentacenequinone, and 2-phenyl-9.10-anthraquinone, in the presence of anthracene, pyrene, naphthalene, biphenyl, 1,4-benzoquinone, and 1,4-naphthoqulnone has been shown to depend on the relative positions and nature of the electronic levels of the substrate and acceptor. Photoreduction of aromatic ketones to alcohols or pinacols can be catalysed by CdS powders using triethylamine as sacrificial donor in MeCN, and morphology is reported by the same authors to affect the two-electron transfer photoreductions of aromatic ketones on CdS induced by visible llghti under analogous conditions olefins behave similarly. Aldehydes of the type RCHO (R -jB -tolyl, g-anlsyl, hexyl) have been photoreduced to the corresp onding 

The effect of lethylene chain length on the photophysical and photocheilcal processes of n-alkyl anthraqulnone-2-carboxylates has been studied by a variety of techniques.Hydrogen atoi abstraction froi the ethylene side chain by the triplet state of the anthraqulnone occurs with 

An investigation of the excited state tautoaerisation of 7-hydroxyquinoline using ps tiae-resolved fluorescence has revealed the presence of two different excited state tautoaerisation processes.The faster of the two occurs within electronically excited li2 solute-alcohol coaplexes and the slower is associated with tautoaerisation in excited solute-alcohol coaplexes containing aore than two solvent aolecules. 

Reviews have appeared of the solid-state dimerisation of thiones, the photooxidation of sulphur compounds, thiocarbonyl photochemistry, the DCA sensitized photooxygenation of olefins, and of the role of intersystero crossing steps in singlet oxygen chemistry.  

MeCOCHMeCHjOH, MeCOCHjCMejOH, and MeCOCCH ) OH, have been irradiated in heptauie or PrCN and the a-hydroxyketones found to decompose by a Type I mechamism. The 0-hydroxyketones decompose by H-0 intraunolecular H-adsstraction auid reactions of the 

Details of the photoenolisation reactions of some acyclic a,0-unsatuzated ketones have appeared aind competing reketonisation mechanisms used to rationalise the wide variation in the photochemical deconjugation reactivity of enones. A study of the photoenolisation of 

Photochemical deoxygenation of triphenodioxazine N-oxides has also been described.A singlet state-driven photogalvanic cell based on the photoreduction of 3,7-diaminophenoxazinylium chloride (oxonine) by Pe(II) has been described and 

N-arylldenamines have been reduced to the corresponding unines by irradiation in benzene solution under nitrogen. The reaction also occurs in the solid state, and a t%ro-phase system has been successfully tested fox recycling an MADH model for photoreduction of IJ-arylideneanilines using sunlight.Aromatic amines have been used as sensitizers for the photoreduction of 

Topics which have formed the subjects of reviews this year include organic photo-oxidation reactions, synthetic aspects of organic photochemistry in the presence of oxygen, electron transfer reactions involving oxygen, and the photo-oxidation of mercaptopteridines/ 

Heckel and W. Pfleiderer, Chem. Biol. Pteridines, Proc. Int. Symp. Pteridines Folic Acid Deriv. Chem., Biol. Qin. Aspects, 7th, 1982, 487, ed. J. A. Blair. 

Ronfard-Haret, R. V. Bensasson, and J. C. Gramain, Chem. Pkys. Lett., 1983, 96, 31. 

Zinc tetraphenylporphine has been found to sensitize the photoredox reaction between N-phenylglycine and p-benzoquinone in polar solvents. In MeCN and at wavelengths above 500 nm, the ketone is reduced to hydro-quinone, and following decarboxylation of the glycine to PhN=CH2 and hydrolysis, aniline is formed. In ethanol solution, photosensitized reduction 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes, supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

The role of excitation lifetime in electron transfer reactions,and photoinduced electron transfer in isolated jet-cooled molecular systems, have also been discussed. 

Some general rules for photochemical hydrogen abstractions by n,7i -excited states have appeared.  

Computer simulation of intramolecular hydrogen atom transfer to carbonyl oxygen has been achieved by a Monte Carlo method. In particular, the model has been found to give good agreement when applied to intramolecular P-, y-, 6-, -, and ( -hydrogen atom abstraction in ketones as well as q- and 0-proton transfer in oxoesters, and it has also been used to predict the reactivities of 

N-dibenzylamino)ethyl 2-, 3-, and 4-benzoylbenzoate. The first fluorescence and fluorescence quantum yield measurements from the excited state of the radical anion of benzo-l,4-quinone have appeared, and since these species are powerful reducing agents generally, such parameters are of considerable impor- 

Topics that have formed the subjects of reviews this year include contemporary issues in electron transport research, dynamics of bimolecular photoelectron transfer reactions, photophysical properties of functionalised fullerene derivatives, carbon-carbon bond formation via radical ions, photoinduced electron transfer processes in ketone, aldehyde, and ester synthesis, photochemical reactions between arenenitriles and benzylic donors, photo-oxidation of conjugated dienes, photoredox reactions of aromatic nitro compounds, electron transfer-mediated photochemistry of some unsaturated nitrogen-containing compounds, reactions of 02( Ag), carbon dioxide activation by aza-macrocyclic complexes, and photochromism of chalcone derivatives.  

A general kinetics model for all photochemical reaction orders, and a photosystem designed to harvest visible light as a source of electrons for use in redox reactions applicable to organic synthesis have also been described.  

4-Hexyl-bridged anthraquinone undergoes intramolecular photoinduced hydrogen atom transfer on the picosecond time-scale to form the biradical methide 

The role of solid state reactions controlled by crystal lattices has also been discussed. 

The ground state structure of the photochemical reaction site of benzo-phenones has been examined by the MO method and IR absorption spectrometry, and comparisons made with fluorenone.  

An assessment of experimental observations concerning the influence of butyl methacrylate on the primary photochemical processes occurring in the reduction of benzophenone by triethylamine suggests that they may point to the participation of ternary exciplexes. Such species may be of the form [donor + acceptor + alkene] and may play an important role in the electron transfer processes. Studies have been reported which characterise the triplet states of m-, and p-hydroxybenzophenones. In non-hydrogen bond 

Cyclobutanones in acetic acid undergo a regiospecific photoconversion into 2-acetoxy-5-alkoxytetrahydrofurans with retention of configuration at the migrating a-position, and an investigation of ultrafast relaxation processes in A, A -dimethylaminobenzylidene indan-l,3-dione as a molecular film has shown that formation and vibronic relaxation of the exciton states occurs in less than 100 fs. Equilibration of the two trapped exciton states seems to occur within 20 ps. 

The two rigidly linked porphyrin-naphthoquinone dyads (1) and (2), whose quinone carbonyl groups lie at different distances from the porphyrin macrocycle, have been prepared. The rate constants for the photoinduced electron transfer in (1) appear to be independent of solvent dielectric constant, whereas (2), the quinone carbonyl groups of which are further from the porphyrin. 

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