Radical photo-induces

The radical cation of 1 (T ) is produced by a photo-induced electron transfer reaction with an excited electron acceptor, chloranil. The major product observed in the CIDNP spectrum is the regenerated electron donor, 1. The parameters for Kaptein s net effect rule in this case are that the RP is from a triplet precursor (p. is +), the recombination product is that which is under consideration (e is +) and Ag is negative. This leaves the sign of the hyperfine coupling constant as the only unknown in the expression for the polarization phase. Roth et aJ [10] used the phase and intensity of each signal to detemiine the relative signs and magnitudes of the... 

There are, indeed, many biological implications that have been triggered by the advent of fullerenes. They range from potential inhibition of HIV-1 protease, synthesis of dmgs for photodynamic therapy and free radical scavenging (antioxidants), to participation in photo-induced DNA scission processes [156, 157, 158, 159, 160, 161, 162 and 163]. These examples unequivocally demonstrate the particular importance of water-soluble fullerenes and are summarized in a few excellent reviews [141, 1751. 

A second degradation process is oxidation, often photo-induced especially by exposure to light not filtered for uv. The radicals resulting from this reaction promote depolymerization of the cellulose, as well as yellowing and fa ding of paper and media. Aging causes paper to become more crystalline and fragile, and this can be exacerbated particularly if the paper is subjected to poor conditions. 

Hydroperoxides are photo- and thermally sensitive and undergo initial oxygen—oxygen bond homolysis, and they are readily attacked by free radicals undergoing induced decompositions (eqs. 8—10). 

Photo-induced Diels Alder reaction occurs either by direct photo activation of a diene or dienophile or by irradiation of a photosensitizer (Rose Bengal, Methylene Blue, hematoporphyrin, tetraphenylporphyrin) that interacts with diene or dienophile. These processes produce an electronically excited reagent (energy transfer) or a radical cation (electron transfer) or a radical (hydrogen abstraction) that is subsequently trapped by the other reagent. 

Acetylchloride is a trapping agent that allows the reaction to go completion, transforming the product into a less oxidizable compound.The results of other reactions between indole (57) and substituted cyclohexa-1,3-dienes show that the photo-induced Diels-Alder reaction is almost completely regioselective. In the absence of 59 the cycloaddition did not occur the presence of [2+2] adducts was never detected. Experimental data support the mechanism illustrated in Scheme 4.14. The intermediate 57a, originated from bond formation between the indole cation radical and 58, undergoes a back-electron transfer to form the adduct 60 trapped by acetyl chloride. 

The stoichiometry of the induced reaction depends, as in the Fe(II)-S20 system, on the iron(II)/iron(III) ratio and on the pH. Therefore, it can be expected that under identical experimental conditions (actor, inductor, and hydrogen ion concentration) the induction factors for the two systems should be identical. The data obtained show that this expectation is fulfilled. For the photo-induced oxidation of arsenic(III) the value of ks /k -j was found to be 2, while in the present system k Jk -j = 4. (Comparing these values with the value of k jk = 21, it can be concluded that the SO4 radical, formed by reaction (43), is not removed by the reaction... 

Brezonik PL, J Fulkerson-Brekken (1998) Nitrate-induced photoysis in natural waters controls on concentrations of hydroxyl radical photo-intermediates by natural scavenging agents. Environ Sci Technol 32 3004-3010. 

Photo-induced electron-transfer decarboxylation reactions have been reviewed. A variety of methyl- and methoxy-substituted phenol radical cations have been generated by either photo-induced electron transfer or photo-ionization in dry solvents such as acetonitrile. In the presence of small amounts of water the radical cations are not detected and the phenoxyl radical is the only transient species observed. The 2-methoxyphenol radical cation was found to be more reactive than the 4-methoxy radical cation. 

The reactivity and jr-facial selectivity of CH3OH and H2O additions to the radical cations of 7-benzhydrylidenenorbornene derivatives (68), generated by photo-induced... 

A photo-induced electron transfer (from either the sensitizer in its excited state to the oxadiazole in its ground state or from the electron-donor reagent such as triethylamine to the excited oxadiazole) has been suggested as an explanation for the breaking of the O—N bond of 5-aryl-3-methoxy-(or 5-aryl-3-phenyl-)-l,2,4-oxadiazoles (71) upon irradiation. The resulting oxadiazole radical anion underwent either a heterocycliza-tion to give quinazolin-4-ones or reduction to give open-chain products. 

Photolysis of vinyl halides can induce both heterolysis of the C-X bond, thereby generating vinyl cations, and homolysis giving vinyl radicals. This competition between the two mechanisms was studied for 3-vinyl halides, 1,2,2-triphenylbromoethane (136) and 1-phenyl-2,2-bis(o-methoxyphenyl)-l-bromoethene and /3-styrene. Incursion of the photo-induced SrnI process, through the intermediate vinyl radical, is verified in the presence of reducing nucleophiles, such as the enolate ions of ketones and in part with (EtO)2PO . Incursion of the heterolytic pathway and the intermediacy of the radical cation, occurs in the presence of weak electron-donor anions, such as N02, Ns and Cl . The vinyl cation of /3-styrene gives phenylacetylene via an El-type elimination. 

The stereoselectivity of anti-Markovnikov adducts (161) and (162) produced through photo-induced electron-transfer reaction of (160) with MeOH in MeCN depends on the optimum structures and stabilities of the corresponding radical and carbanion intermediates (163) and (164). In PhH, steric hindrance in an exciplex, comprising an excited singlet sensitizer and (160), forced cis addition of MeOH to (160) to give trans-isomer (161) as the major addition product. 

From our observations, we feel it is unlikely that photo-induced redox reactions are occurring in the present studies. However, at wavelength < 300 nm photodissociation of oxyhemoglobin results in the formation of the 02" radical anion (34). 

OXYGEN, OXIDES 0X0 ANIONS Oxygen, photo-induced incorporation, PHOTO-OXYGENATION OXYGEN RADICALS Oxygen transport in erythrocytes,... 

Actually, the earliest derivative of a vinylcyclopropane radical cation was a serendipitous discovery. It was formed by an unusual hydrogen shift upon photo-induced electron transfer oxidation of tricyclo[4.1.0.0 ]heptane (26). This result has been questioned on the grounds that the same rearrangement was not observed in a Freon matrix. However, there is no basis for the assumption that radical cation reactions in frozen matrices at cryogenic temperatures should follow the same course as those at room temperature in fluid solution and in the presence of a radical anion, which is potentially a strong base. In several cases, matrix reactions have taken a decidedly different course from those in solution. For example, radiolysis of 8 in a Freon matrix generated the bicyclo[3.2.0]hepta-2,6-diene radical cation (27 ), or caused retro-Diels-Alder cleavage yet, the... 

---