Photochemistry photo sensitization

Recently, Inoue et al. have established the concept of multidimensional control of asymmetric photochemistry [58]. Applying this strategy, the product stereoselectivity can be inverted by environmental factors such as the temperature, pressure or solvent, and this control has been interpreted in terms of the contributions of both enthalpy (AH+) and entropy (AS+). Although originally designed for enantiodifferentiating photo sensitization of cycloalkenes [58], Miranda et al. were able to apply it to PET cyclizations of o-allylaniline derivatives [59]. In particular, irradiations at different temperatures revealed a significant entropy-controlled diastereoselectivity for compound 72 and the equipodal temperature was found at 292 K (Sch. 34). 

Photochemistry. Photo-oxidation reactions, sensitized by dyes, of various tertiary amines and alkaloids have been investigated. The formation of... 

Rancan F, Helmreich M, Molich A, Jux N, Hirsch A, Roder B, Witt C, Bohm F (2005) Fullerene-pyropheophorbide a complexes as sensitizer for photodynamic therapy uptake and photo-induced cytotoxicity on Jurkat cells. Journal of Photochemistry and Photobiology B Biology 80 1-7. 

The photophysics and photochemistry of gaseous PuFe have been ex-amined. Studies involving zinc porphyrins have been reported and include photo-oxidations in aqueous solution, photoreductions of Zn-TPP with hydrazines, and the role of Zn-TPPSa/ethyl viologen in photoredox processes. The mechanism of the photo-oxidation of water to oxygen with silver chloride has been discussed, and the synthesis of bis(chlorosilyl)-mercury compounds described. Colloidal CdSe has been shown to sensitize the photoreduction of O2 and of methyl viologen by cysteine. ... 

The location of the oxime group in the product in Scheme 4 depends upon where the methyl group is located, but the yields can be high (79%). Irradiation of 2-methoxyfuran also induces side-chain dissociation and the formation of lactones (i.e., methylbutenolides).21 Here the abundant formation of ethane testifies to the participation by methyl radicals. Such reactions are too complex for synthetic purposes. This is true also of 2-acetoxyfuran photochemistry in which the photo-Fries reaction is not observed.21 Curiously, furan itself sensitizes the photo-Fries reaction in benzenoid esters.22... 

As can be seen from the few examples cited above SET processes are now fairly common in organic photochemistry. One of the areas where considerable study has taken place is the process referred to as a photo-NOCAS. Within this framework Albini and coworkers have shown that the products formed from the reaction of 2,3-dimethylbut-2-ene with 1,4-dicyanobenzene are compounds (22)- 25). The reaction was brought about using phenanthrene as the initial light absorber. This technique leads to cleaner reactions than those where the 1,4-dicyanobenzene is irradiated directly. The solvent system used is methanol/ acetonitrile and products (24) and (25) are the result of solvent incorporation. A further example of photo-NOCAS chemistry has been reported by Arnold and coworkers.Typical of the examples studied is the reaction illustrated in Scheme 2. The cyclization of the dienes (26) was also examined. This specific example deals with the generation of radical cations from (/ )-(+)-a-terpineol and (/ )-(+)-limonene with 1,4-dicyanobenzene as the electron accepting sensitizer. In another detailed study on reactions of this type the factors that control the regiochemistry in photo-NOCAS processes have been assessed. ... 

Benzophenone sensitized laser/liquid jet photochemistry generated electronically excited cyclopentadiyl species by two-photon processes. The first photon is required to generate the ground state cyclopentadiyl triplet diradical by benzophenone sensitized denitrogenation of the diazoalkane, the second one to produce the excited cyclopentadiyl and subsequent photo-... 

McEvoy A. J. and Gratzel M. (1994), Sensitization in photochemistry and photo-voltaics . Solar Energy Mat. Solar Cells 32, 221-227. 

Research into light-initiated chemical reactions and processes on solid surfaces is a growing new field which promises to yield a number of useful applications molecular photo-devices for super memory, photo-chemical vapor deposition to produce thin-layered electronic semiconducting materials, sensitive optical media, and the control of photochemical reaction paths, etc. In fact, photochemistry on solid surfaces is now a major field in a national research project on "Frontiers of Highly Efficient Photochemical Processes" sponsored by the Ministry of Education, Science and Culture of Japan. 

Reusability is a characteristic of the sensitizers prepared by stirring Ceo-fullerene with aminomethylated poly(styrene/vinylbenzene). They have been used to promote the standard O2 oxidation processes such as ene and Diels-Alder reactions (Scheme 7), and catalysts suitable for photoxidations in water have been prepared from them by reaction with poly(allylamine). The same reactions have been carried out using a novel solvent-free procedure which involves loading a porphyrin into solvent-swollen polystyrene beads and carrying out the photo-oxidation in neat liquid substrate. The formation of the allylic hydroperoxide (89) from p-pinene, with complete conversion and in 84% yield, is particularly noteworthy, as the standard liquid-phase reaction can be problematic. It is suggested that the possibility of using this approach under solar conditions is further evidence of the sustainable, green chemistry potential of synthetic photochemistry. 

Under conditions of nanosecond laser-flash photolysis, a long-lived transient absorption assigned to VI, A 465 nm, analogous to V, the intermediate proposed in formation of iodobiphenyls from iodonium salts, is observed [83], The diphenylsulfinyl radical cation, /Lmai340, 750 nm, is similarly observed in acetone sensitized laser-flash photolysis. Photo-CIDNP observations, namely emissive polarization for benzene, suggest that the homolytic cleavage pathway also operates under conditions of direct photolysis, accounting, in part, for the diphenylsulfide product [83], as well as the photochemistry of polymeric triarylsulfonium salts [81] (see above). 

In the field of techniques, one of the more unusual reports during the year is a new photochemical method for the analytical determination of carbon in organic compounds involving the formation of carbon dioxide by Ce(S04)2-sensitized photo-oxidation (Ivanov and Atanov). Brauchle et al. have described a new technique termed holographic photochemistry , and have used it to study H-abstraction by benzophenone in a polymer host. 

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. 

Naman, S.A., Khammas, Z.A.A. and Hussein, F.M. (2002) Photo-oxidative degradation of insecticide dichlorovos by a combined semiconductors and organic sensitizers in aqueous media. Journal of Photochemistry and Photobiology A Chemistry 153, 229-236. 

On irradiation of DOM, the one primary photochemical event is photo-ionization. This process is less sensitive to environmental factors than triplet formation. All of the remaining photochemistry depends on the subsequent fate of the three primary products of photoionization triplets, solvated electrons, and cation radicals [13]. Laser flash photolysis studies supplied early evidence for hydrated electron (e q) production on irradiation of DOM, with significant primary quantum yields [9,12,84,85]. The hydrated electron was identified by its characteristic broad absorption spectrum from 700 nm to 750 nm [86, 87]. The formation of e q is thought to result from the photo-ejection of an electron from excited-state humic substances ... 

A plano-convex lens was used to collimate and focus, an IR mirror was employed to eliminate non-productive IR irradiation and minimize associated heat, and a variety of UV filters could be employed to control wavelength. After evaluation of 728 reactions, they identified five which were further explored. Examples included a wavelength dependent [2-I-2]-cycloaddition of indole 27 to afford a mixture of indole 28 cyclobutane 29 (Scheme 5A). They observed that the photo-Fries rearrangement was predominant at shorter wavelengths while the [2 -I- 2]-cycloaddition was favoured at longer wavelengths. A second reaction they identified was an oxa-di-7i-methane reaction with bicycle 39 to afford the caged cyclopropane 35 in 70% yield after 2 min (Scheme 5B). In the screen they found both direct excitation and triplet sensitization conditions for the reaction. They also identified a 1,3-acyl shift with ketol 32 to afford cyclohutanone 33 in modest yield (Scheme 5C). This platform demonstrated that flow photochemistry can enable automated reaction discovery. 

Sensitization was another word requiring definition and correct application. Thus, a stable molecule could be made photosensitive by the additirai of a small amount of another molecule, but this might involve different phenomena, as illustrated by Berthoud [62]. These ranged from the photochemical formation of a reactive compound to that of a catalyst. Atoms could be generated and transfer their energy to other species, as demonstrated for atoms of mercury or of oxygen [63]. Noyes pointed out to the reaction of molecules, e.g., NO2, at an irradiated mercury surface, and on the differences and similarity between photochemistry and photo-ionization [64]. 

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