Photochemistry electron transfer

Electron Transfer Photochemistry.—It has been suggested that electron transfer can bring about base hydrolysis of cobalt(m) complexes. Recent examples include the aquation of sulphite complexes, where detection of dithionate as one of the products is good evidence for an electron transfer mechanism, and of the dicobalt /M-superoxo complex [(H3N)6Co 02 Co(NH3)5] + in the presence of sulphite.  

Photochemical and thermal substitution reactions of [Co(CN)g] have been compared. The comparison permits conclusions to be drawn concerning the role of solvation and of ion-pair formation in determining the rates and course of these reactions.Charge-transfer also takes place during radiolysis of cobalt-cyanide solutions. The previously postulated species [Co (CN)g] has been detected in solutions prepared from [Co (CN)5] , while the attempt to prepare the elusive species [Co (CN)e] from [Co (CN)o] may have been successful.  

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J. L. Habib Jiwan and J. P. Soumillion, Electron transfer photochemistry initiated from a twisted intramolecular charge transfer state used as an electron donor and as an acceptor, J. Photochem. Photobiol. A Chem. 64, 145-158 (1992) J. P. Soumillion, Photoinduced electron transfer implying organic anions, in Topics in Current Chemistry, Vol., Photoinduced Electron Transfer (J. Mattay, ed.), Springer Verlag, Berlin, pp. 93-141 (1993). 

ELECTRON TRANSFER PHOTOCHEMISTRY OF CYCLOPROPANE SYSTEMS RADICAL CATION REACTIVITIES... 

Gould IR, Young RH, Moody RE, Farid S (1991) Contact and solvent-separated geminate radical ion-parrs in electron-transfer photochemistry. J Phys Chem 95 2068... 

U. C. Yoon u. P.S. Mariano, Acc. Chem. Res. 25, 233-240 (1992) . .Mechanistic and Synthetic Aspects of Amine-Enone Single Electron Transfer Photochemistry". 

In addition to nucleophilic capture by alcohols, nonprotic nucleophiles also react with these intermediates. For example, the distonic dimer radical cation 96 + can be trapped by acetonitrile a hydride shift, followed by electron return, gave rise to the pyridine derivative 131. Similar acetonitrile adducts are formed in the electron-transfer photochemistry of terpenes such as ot- and (3-pinene ° or sabinene. ... 

Oxygen Sensitive Systems. The more general case of oxygen-sensitive photoreducible dye sensitization is now thought to involve exciplex-mediated electron transfer photochemistry. In the time since Oster s original report many activators have been disclosed in both the patent and open literature. Table 1 lists the known broad classes of activators reported to date. Also listed are specific examples within the classes. 

Mesitylfhjorenyl anion (9MsF ) is unreactive towards Mel at temperatures below —78 °C.100 Above —60 °C the absorption spectrum of 9MsF in the presence of Mel is replaced by that of the corresponding 9-mesitylfluorenyl radical (9MsF), and 9-methyl-9-mesitylfluorene is formed in low yield, hi a study of the electron-transfer photochemistry of chrysanthemol, an intramolecular S 2 reaction of a vinylcycloprop-ane radical cation has been observed.101 hi a long series of studies of the reactivity of the acids of trivalent phosphorus and their derivatives, the behaviour of P—O nucleophiles towards arylmethyl bromide systems has been examined.102 Further evidence for an X-philic substitution/SET tandem mechanism has been obtained. 

The key point is the deprotonation rather than the desilylation of the MSMA s cation radical, an unprecedented feature. Aimed at giving some support to this mechanism, the amine-flavine electron transfer photochemistry has been studied.493 Later, a polar mechanistic pathway was established in place of the SET mechanism mentioned above.494... 

The electron transfer photochemistry of benzocyclobutenes in the presence of dienophiles exhibits another interesting feature of CIP reactions [93]. For example, charge transfer irradiation of the EDA complex composed of 1,2-diphenylbenzocy-clobutene (36) and TCNE (IS) leads to the CIP (36 15 ), of which 36 spontaneously ring-opens to the o-xylylene (37) radical cation, followed by the collapse of the new CIP (Eqs. (20, 21)), leading to the Diels-Alder adduct 39 in high yield and with high efficiency. 

Weng, H., Scarlata, C., and Roth, H.D. (1996) Electron transfer photochemistry of geraniol and (E, E)-farnesol. A novel tandem , 1,5-cyclization, intramolecular capture. Journal of the American Chemical Society, 118, 10947-10953. 

Yoon, U.C., Quillen, S.L., Mariano, P.S., Swanson, R., Stavinoha, J.L., and Bay, E. (1983) Exploratory and mechanistic aspects of the electron-transfer photochemistry of olefin-N-heteroaromatic cation systems. Journal of the American Chemical Society, 105, 1204—1218. 

The electron transfer photochemistry of geraniol (42) employing DCA and 1,4-dicyanobenzene (DCB) as sensitizers has been examined by Roth et al. [45]. The course of the reaction is controlled by the energy of the electron transfer step and thus by the sensitizer applied (Sch. 23). 

In 1,2,3-triphenylallyl carbanions, electron transfer photochemistry is found in the presence of acceptors like stilbene as demonstrated by the sensitized cis-trans isomerization of this molecule according to the following pathway [145],... 

Cyclic conjugated earbanions have also been studied and rarely found to give an electron transfer photochemistry in the absence of an electron acceptor. One example was however found in the case of cyclooctadienyl carbanion irradiated in THF and in the presence of chlorobenzene or rrans-stilbene. Dimeric radical products and phenylated cyclooctadiene were formed in the first case while trans-cis isomerization of stilbene was observed in the second one... 

The secondary electron-transfer processes, often used in mechanistic in estiga-tions and in preparative applications of electron-transfer photochemistry, enhance the quantum yields of product formation [167], In fact, as we have already pointed out in a previous section, the efficiency of separation of the geminate pair is strictly dependent on the redox potentials (see also indirect photooxygenation processes) [43, 50, 80-83, 135], Anyway, although in the present case the subsequent electron-transfer from epoxide to biphenyl radical cation BP is endothermic enough, in the absence of very fast competing reactions this primary radical cation may still undergo an endothermic electron-transfer process with epoxides. 

The discussion above grossly oversimplifies the complexity of silver halide photographic chemistry. A more detailed discussion is beyond the scope of this review. The interested reader is referred to monographs on the topic [7]. More germane to the present review is the evolving picture of the electron transfer photochemistry which occurs during the spectral sensitization of silver halide materials. 

Addition Reactions.—As the years go by, the importance of electron transfer processes is becoming increasingly apparent, and hardly a month passes without the reinterpretation of a reaction as involving such a process. This has stimulated the publication of review articles such as that by Mattes and Farid on the electron transfer reactions of alkenes, and the more specific reviews by Mariano on the application of electron transfer photochemistry to iminium salts. In this area Mariano and his co-workers have reported further on the electron-transfer-initiated photochemistry of iminium salts (1), and in detail on the spiro-cyclization methodology of iminium salt cyclization. ... 

Studies of the electron transfer photochemistry of l,2-diarylethanes and of aryl glycopyranosides have been reptorted. The wavelength-dependent photochemistry of 2,3-dimethylbut-2-ene and of oct-2-ene has been described by Inoue et al Direct irradiation (185 nm) of the bicycloalkenes (42) in pentane solution affords the corresponding cycloalkadiene (43) as well as the fragmentation products acetylene and (44), produced via the Intermediate carbene (45). ... 

Brus L (2008) Noble metal nanocrystals plasmon electron transfer photochemistry and single-molecule Raman spectroscopy. Acc Chem Res 41 1742... 

The vinylcyclopropane radical cation, is another radical cation of structure type B, which is stabilized by conjugation. Its proposed structure was based exclusively on ab initio calculations (B3LYP/6-31G ) because the electron-transfer photochemistry of this species failed to provide clear-cut CIDNP effects [128], In this context it is worth noting that product studies cannot, in principle, establish the cyclopropane radical cation structure type. Irrespective of the structure, nucleophilic capture is expected to result in the cleavage of the strained ring. 

The prominent electron transfer photochemistry of polypyridine complexes stems from a unique combination of their ground state redox reactivity and excited state properties ... 

Irradiation at 254 nm of the imidate (Scheme 2) in aqueous solutions results in the formation of 4-cyanophenol and A -isopropylbenzamide. The photochemical step involved in the formation of the products has been shown to be C-O bond fission with the production of the phenoxide and the benzonitrilium ion. Mariano and co-workers have reported a detailed study of the electron-transfer photochemistry of a-anilino carboxylates, p-anilinoalcohols and a-an-ilinosilanes. This study has measured the rates of decarboxylation of aniliniumcarboxylate radicals. The base induced retro-Aldol fragmentations of the radical-cations formed from the p-anilinoalcohols and the influence of substituents on the nitrogen on the desilylation of the a-anilinosilanes was also investigated. In addition, the synthetic potential of some of the electron-transfer photochemistry of the carboxylate salts (181) and (182) has been examined. In these examples irradiation, using DCA in methanol or acetoni-... 

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