Electron transfer photo-NOCAS reaction

The presence of hetero-atoms within the system, remote from the alkene double bonds, does not have an adverse influence on the SET processes that occur. Thus irradiation of the diene 33 in benzene solution with 1,4-dicyanonaphthalene as the electron-transfer sensitizer affords the cyclobutane 34 in 78% yield. Various examples of the reaction were described giving cyclobutane derivatives in 54-69% yield. Benzene, or an arene solvent, is vital for the success of the reaction. When acetonitrile is used, allylation of the sensitizer (akin to the photo-NOCAS reaction) results in the formation of the three products 35-3718. (2 + 2)-Cyclization of this type described for 33 is also seen with the dialkenyl ether 38. When 38 is irradiated using X > 350 nm or X > 450 nm in acetonitrile... 

Various substituted cyclopropanes have been shown to undergo nucleophilic addition of alcoholic solvents. For example, the electron transfer reaction of phenylcyclopropane (43, R = H) with p-dicyanobenzene resulted in a ring-opened ether 44. This reaction also produced an aromatic substitution product (45, R = H) formed by coupling with the sensitizer anion. This reaction is the cyclopropane analog of the photo-NOCAS reaction, but preceded it by almost a decade. 

Photo-NOCAS reactions of p-dicyanobenzene with 2-methylpropene in acetonitrile afforded novel 3,4-dihydroisoquinoline derivatives, as shown in Scheme 132 [482], This photoreaction is initiated by a single electron transfer from olefin to p-dicyanobenzene. Acetonitrile as a nucleophile combined with the alkene radical cation and the resulting radical cation adds to the radical anion of 1,4-di-cyanobenzene. Cyclization to the ortho position of phenyl group followed by loss... 

In the presence of methanol as solvent and 1,4-dicyanobenzene as acceptor, photoinduced electron transfer from 1,4-bis(methylene)cyclohexane gives 4-(methoxymethyl)-1 -methylenecyclohexane and 4-(4-cyanophenyl)-4-(methoxy-methyl)-l-methylenecyclohexane which arise by nucleophilic attack of the solvent on the radical cations, followed either by reduction and protonation, or by combination with the radical anion of the electron acceptor.These observations are in accordance with the proposed mechanism of the nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction. The same group has also investigated the use of cyanide ion as nucleophile and report that irradiation of a mixture of 1,4-dicyanobenzene in the presence of biphenyl as donor, KCN, and 18-crown-6 gives a mixture of (79) and (80). These workers have also extended the scope of NOCAS to fluoride ion. In particular, use of 2,3-dimethylbut-2-ene and 2-methylbut-2-ene gives 4-cyanophenyl substituted... 

The photo-NOCAS reaction was first described by McMahon and Arnold and is a photonucleophilic Sfj2Ar aromatic substitution between dicyanobenzene and an olefin in the presence of electron donor photosensitizers (phenanthrene or biphenyl) in acetonitrile-methanol solutions. This reaction system has been researched extensively in recent times. As shown in Scheme 6, the single electron transfer from olefin to photo-excited electron-deficient dicyanobenzene forms the cation radical of the olefin, which initiates a quenching reaction with nucleophile solvent methanol molecules and forms the methoxyalkyl radical. Addition of an electron transfer photosensitizer (phenanthrene or biphenyl) to the reaction mixture increases the efficiency of the reaction simply by absorbing more Hght. The excited state of the photosensitizer donates an electron to dicyanobenzene to give the photosensitizer radical cation and dicyanobenzene radical anion. The photosensitizer radical cation then oxidizes the olefin. 

One of the photochemical electron transfer reactions developed quite extensively over the past decades, particularly by our research group, is the photochemical nucleophUe-olefin combination, aromatic substitution (photo-NOCAS) reaction. We have used this reaction as a framework in which to investigate the behavior of photogenerated radical ions as well as a well-understood reference against which to compare competing PET reactions. 

Electron Transfer Processes - A single electron-transfer mechanism is involved in the cycloaddition of alkenes, such as 2-methylpropene, to 1,2-dicyanonaphthalene. Reaction of the alkene radical cation with the radical anion of the sensitiser results in the products shown in the Scheme 1. Incorporation of solvent to give (55) occurs as one of the main products in addition to what are essentially photo-NOCAS products (56). 

Mangion, D., Arnold, D.R., Cameron, T.S., and Robertson, K.N., The electron transfer photochemistry of allenes with cyanoarenes. Photochemical nucleophQe-olefm combination, aromatic substitution (photo-NOCAS) and related reactions, /. Chem. Soc., Perkin Trans. 2, 48, 2001 and references cited therein. 

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