Photo-NOCAS system,

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... 

As can be seen from the few examples cited above SET processes are now fairly common in organic photochemistry. One of the areas where considerable study has taken place is the process referred to as a photo-NOCAS. Within this framework Albini and coworkers have shown that the products formed from the reaction of 2,3-dimethylbut-2-ene with 1,4-dicyanobenzene are compounds (22)- 25). The reaction was brought about using phenanthrene as the initial light absorber. This technique leads to cleaner reactions than those where the 1,4-dicyanobenzene is irradiated directly. The solvent system used is methanol/ acetonitrile and products (24) and (25) are the result of solvent incorporation. A further example of photo-NOCAS chemistry has been reported by Arnold and coworkers.Typical of the examples studied is the reaction illustrated in Scheme 2. The cyclization of the dienes (26) was also examined. This specific example deals with the generation of radical cations from (/ )-(+)-a-terpineol and (/ )-(+)-limonene with 1,4-dicyanobenzene as the electron accepting sensitizer. In another detailed study on reactions of this type the factors that control the regiochemistry in photo-NOCAS processes have been assessed. ... 

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. 

The photo-NOCAS reaction is restricted to mononuclear, electron-deficient aromatics as the electron acceptors. Using polynuclear systems such as 1,4-dicyanonaphthalene, for example, leads to the ole-fin-nucleophile adduct radical adding, rather than substituting, at the ipso-position of the arene (Scheme 18). This is most Hkely due to the reduced rearomatization energy gained in binuclear, as opposed to mononuclear, systems. The photo-NOCAS reaction is Hmited to aliphatic olefins as electron... 

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