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.

Photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Part 6: methanol, nonconjugated dienes, and 1,4-dicyanobenzene

1994 ◽  
Vol 72 (2) ◽  
pp. 415-429 ◽  
Author(s):  
Donald R. Arnold ◽  
Kimberly A. McManus ◽  
Xinyao Du
Keyword(s):  
Electron Transfer ◽  
Double Bond ◽  
Radical Cation ◽  
Oxidation Potential ◽  
Aromatic Substitution ◽  
Double Bonds ◽  
Excited Singlet ◽  
Reaction Conditions ◽  
Methanol Solutions ◽  
Cyclic Adducts

Irradiation, through Pyrex, of an acetonitrile–methanol (3:1) solution of 1,4-dicyanobenzene (1) and 1,5-hexadiene (9) leads to formation of ortho and meta cyclic adducts (13–16) arising from the intermediate exciplex. There was no evidence for interaction between the two double bonds of this nonconjugated diene. The oxidation potential of 9 is high enough (> 3 V vs. sce) to preclude single electron transfer (SET); no photo-NOCAS products are formed. Similar irradiation of acetonitrile–methanol solutions of 1 and 2-methyl-1,5-hexadiene (10) does yield a photo-NOCAS product (17); reaction occurs only on the more heavily substituted double bond. The additional substitution on the double bond lowers the oxidation potential (2.70 V vs. sce) of this diene to the point where SET from 10 to the excited singlet state of 1 can occur. In this case, no cycloaddition products are formed; the exciplex is quenched by electron transfer. There was no evidence for interaction between the two double bonds of the initially formed radical cation 10+•, or between the terminal double bond and the β-alkoxyalkyl radical of the intermediate leading to the photo-NOCAS product. The photo-NOCAS product (19) was also formed when 2,5-dimethyl-1,5-hexadiene (11) was subjected to these reaction conditions. In this case, when biphenyl (4) was added as a codonor, in addition to the photo-NOCAS product, products (21cis and trans) resulting from cyclization of the initially formed acyclic radical cation 11+• to give the 1,4-dimethylcyclohexane-1,4-diyl radical cation were also observed. This 1,6-endo, endo cyclization of 11+• must be rapid enough to compete with reaction with methanol. There was no evidence for cyclization (neither 1,4-exo nor 1,5-endo) of the intermediate β-alkoxyalkyl radical. When the radical cation of 2,5-dimethyl-1,4-hexadiene (12+•) is generated under these reaction conditions, photo-NOCAS products 22 and 23 are formed at the more heavily substituted double bond, along with the conjugated tautomer 2,5-dimethyl-2,4-hexadiene (24). The mechanisms for these transformations are discussed.

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.

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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