Energy transfer quenching of a fluorescent excited radical cation by counter radical anion: dissipation of radical ions generated by photoinduced electron transfer

2000 ◽  
Vol 326 (3-4) ◽  
pp. 293-298 ◽  
Author(s):  
Nobuyuki Ichinose ◽  
Tomoko Tanaka ◽  
Shun-ichi Kawanishi ◽  
Tetsuro Majima
Keyword(s):  
Electron Transfer ◽  
Energy Transfer ◽  
Radical Cation ◽  
Photoinduced Electron Transfer ◽  
Radical Anion ◽  
Radical Ions ◽  
Excited Radical

Radical ions in photochemistry. 18. The photosensitized (electron transfer) tautomerization of alkenes; the 1,1-diphenyl alkene system

1987 ◽  
Vol 65 (9) ◽  
pp. 2312-2314 ◽  
Author(s):  
Donald R. Arnold ◽  
Shelley A. Mines
Keyword(s):  
Hydrogen Bond ◽  
Electron Transfer ◽  
Radical Cation ◽  
Radical Anion ◽  
Relative Rate ◽  
Radical Cations ◽  
Radical Ions ◽  
Back Electron Transfer ◽  
Ambident Anion ◽  
Determining Factor

The photosensitized (electron transfer) irradiation of several conjugated 1,1-diphenyl alkenes, in acetonitrile with 1,4-dicyanobenzene or 1-cyanonapthalene as electron accepting sensitizer and 2,6-lutidine as base, leads essentially quantitatively to tautomerization to the less stable unconjugated isomer(s). The proposed mechanism for this reaction involves formation of the alkene radical cation and sensitizer radical anion followed by deprotonation of the radical cation, reduction of the resulting radical to the ambident anion by back electron transfer from the radical anion, and reprotonation. There are several steps in this mechanism that could control the ratio of isomers. Evidence is provided that, at least in some cases, it is the relative rate of deprotonation from the isomeric radical cations that is the determining factor. This rate is influenced by the conformation of the radical cation; the carbon–hydrogen bond involved in the deprotonation step must overlap with the singly occupied molecular orbital.

Photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Part 9: methanol-2,6-dimethyl-1,6-heptadiene, and 1,4-dicyanobenzene

1995 ◽  
Vol 73 (6) ◽  
pp. 762-771 ◽  
Author(s):  
Dennis A. Connor ◽  
Donald R. Arnold ◽  
Pradip K. Bakshi ◽  
T. Stanley Cameron
Keyword(s):  
Electron Transfer ◽  
Radical Cation ◽  
Photoinduced Electron Transfer ◽  
Aromatic Substitution ◽  
Radical Ions ◽  
Double Bonds ◽  
Alkoxy Radical ◽  
Product Ratio

The photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction of methanol, 2,6-dimethyl-1,6-heptadiene, and 1,4-dicyanobenzene yields three distinct types of 1:1:1 adducts: an acyclic product, 4-(1-methoxymethyl-1,5-dimethyl-5-hexenyl)benzonitrile (8, 5%); a cis–trans pair of cyclohexanes, 4-(3-methoxymethyl-1,3-dimethylcyclohexyl)benzonitrile (9cis (12%) and 9trans (11%)); and a cis–trans pair of cycloheptanes, 4-(4-methoxy-1,4-dimethylcycloheptyl)benzonitrile (10cis (12%) and 10trans (10%)). Variation in the concentration of the nucleophile, methanol, and codonor, biphenyl, affects the product ratio and it has been possible to establish the mechanisms for the formation of these products. The acyclic product is formed by a typical photo-NOCAS reaction, that is, addition (anti-Markovnikov) across one of the heptadiene double bonds. The cyclohexane products are formed following 1,6-endo cyclization of the intermediate β-alkoxy radical. And the cycloheptane products result from 1,7-endo,endo cyclization of the initially formed 2,6-dimethyl-1,6-heptadiene radical cation. Comparison of the relative rates of these cyclization processes can be made with those of the next smaller homolog, 2,5-dimethyl-1,5-hexadiene. Keywords: photochemistry, photoinduced electron transfer, radical ions, radicals, cyclization.

Radical ions in photochemistry. 21. The photosensitized (electron transfer) tautomerization of alkenes; the phenyl alkene system

1989 ◽  
Vol 67 (4) ◽  
pp. 689-698 ◽  
Author(s):  
Donald R. Arnold ◽  
Shelley A. Mines
Keyword(s):  
Hydrogen Bond ◽  
Electron Transfer ◽  
Radical Cation ◽  
Radical Anion ◽  
Phenyl Group ◽  
Oxidation Potential ◽  
Original Position ◽  
Acetonitrile Solution ◽  
Steric Interaction ◽  
Ambident Anion

Alkenes, conjugated with a phenyl group, can be converted to nonconjugated tautomers by sensitized (electron transfer) irradiation. For example, irradiation of an acetonitrile solution of the conjugated alkene 1-phenylpropene, the electron accepting photosensitizer 1,4-dicyanobenzene, the cosensitizer biphenyl, and the base 2,4,6-trimethylpyridine gave the nonconjugated tautomer 3-phenylpropene in good yield. Similarly, 2-methyl-1-phenylpropene gave 2-methyl-3-phenylpropene, and 1-phenyl-1-butene gaveE- and Z-1-phenyl-2-butene. The reaction also works well with cyclic alkenes. For example, 1-phenylcyclohexene gave 3-phenylcyclohexene, and 1-(phenylmethylene)cyclohexane gave 1-(phenylmethyl)cyclohexene. The proposed mechanism involves the initial formation of the alkene radical cation and the sensitizer radical anion, induced by irradiation of the sensitizer and mediated by the cosensitizer. Deprotonation of the radical cation assisted by the base gives the ambident radical, which is then reduced to the anion by the sensitizer radical anion. Protonation of the ambident anion at the benzylic position completes the sequence. Reprotonation at the original position is an energy wasting step. Tautomerization is driven toward the isomer with the higher oxidation potential, which is, in the cases studied, the less thermodynamically stable isomer. The regioselectivity of the deprotonation step is dependent upon the conformation of the allylic carbon–hydrogen bond. The tautomerization of 2-methyl- 1-phenylbutene gave both 2-phenylmethyl-1-butène and 2-methyl-1-phenyl-2-butene (E and Z isomers), while 2,3-dimethyl- 1-phenylbutene gave only 3-methyl-2-phenylmethyl-1 -butene. In the latter case, steric interaction of the methyls on the isopropyl group prevents effective overlap of the tertiary carbon–hydrogen bond with the singly occupied molecular orbital, thus inhibiting deprotonation from this site. Keywords: photosensitized, electron transfer, alkene, tautomerization, radical cation.

ChemInform Abstract: Electron Transfer Photochemistry of Aromatic Imides and Phenylcyclopropane. Radical Anion-Radical Cation Cycloaddition.

ChemInform ◽  
1987 ◽  
Vol 18 (11) ◽  
Author(s):  
P. H. MAZZOCCHI ◽  
C. SOMICH ◽  
M. EDWARDS ◽  
T. MORGAN ◽  
H. L. AMMON
Keyword(s):  
Electron Transfer ◽  
Radical Cation ◽  
Radical Anion ◽  
Anion Radical

Article

1999 ◽  
Vol 77 (10) ◽  
pp. 1655-1670 ◽  
Author(s):  
Dino Mangion ◽  
Donald R Arnold
Keyword(s):  
Electron Transfer ◽  
Photoinduced Electron Transfer ◽  
Photochemical Reactions ◽  
Transfer Mechanism ◽  
Radical Ions ◽  
Reaction Conditions ◽  
Electron Transfer Mechanism ◽  
Photochemical Reactivity ◽  
Bond Homolysis

The photochemical reactivity of a series of 4-halobenzonitriles and 4-haloanisoles with 1,1-diphenylethene in a nucleophilic solvent (methanol) has been investigated. Analysis of the photochemical reactions involving the 4-halobenzonitriles revealed formation of alkene-methanol adducts, such as 1-methoxy-2,2-diphenylethane, 1-methoxy-2,2-diphenylethene, and 1,1-dimethoxy-2,2-diphenylethane, indicative of a photochemical electron-transfer mechanism. These products were not significant in the photochemical reactions involving the 4-haloanisoles. Both the 4-halobenzonitriles and the 4-haloanisoles produced an arene-alkene-methanol Markovnikov adduct, 1-aryl-2-methoxy-2,2-diphenylethane (aryl = 4-cyanophenyl or 4-methoxyphenyl). This compound was shown to undergo an acid-catalysed elimination to 1-aryl-2,2-diphenylethene under the reaction conditions, which subsequently underwent a 6pi-electrocyclization to the 3-substituted(cyano or methoxy)-9-phenylphenanthrene. Possible mechanisms for the observed reactivity are discussed and evaluated.Key words: photochemistry, photoinduced electron transfer, bond homolysis, radical ions, radicals, exciplexes.

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