Photo-NOCAS process

The best-known reactions belonging to this class are based either on the photo-NOCAS process [32] or on the photochemistry of Barton esters [69]. In the first case, a three-component reaction involving a cyanoarene, an olefin, and a nucleophile (usually the solvent) occurs. The reaction is generally initiated by a PET process between the aromatic and the olefin. The examples presented below are chosen from among the most representative and most recent. A typical reaction is illustrated in Scheme 3.26, where an enolized P-dicarbonyl compound acts as an added nucleophile. 

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 process has also been reported with P-myrcene (57) as the reactant. The resultant radical cation, generated using dicyanobenzene as the sensitiser, affords the five products (58-62) shown and cyclization within the myrcene radical cation is an essential feature of this reaction sequence. SET photochemistry of aliphatic electron donors can provide a source of radicals. Thus irradiation of donors such as (63), (64), (65) and (66) results in bond fission and the formation of alkyl radicals which undergo addition to alkenes e.g. 67) or alkynes e.g. 68) to give the adducts (69) and (70), respectively. ... 

Other studies have sought to establish the scope and limitations of the photo-NOCAS process. Thus Arnold and co-workers have examined the reactions of alkenes with 1,4-dicyanobenzene (DCB). A typical result from this reaction is shown in Scheme 1. All of the products arise from the attack of the radical cation of the alkene on the DCB sensitizer with loss of the cyano function. A further study of photo-NOCAS reactivity has demonstrated that the radical cation of 2,3-dimethylbut-2-ene, formed by irradiation in the presence of DCB/biphenyl, can be trapped by fluoride ion. The resultant radical (39) reacts with the radical anion of DCB to yield the adduct (40). The radical cation of methylenecyclopro-pane (41) can be formed by irradiation in the presence of DCB as the sensitizer. The products are illustrated in Scheme 2 and, as shown, in all cases the cyclopropane ring remains intact. The diene (42) undergoes SET to dicyanoben-zene as the sensitizer with biphenyl as the co-sensitizer. In the absence of nucleophiles many products are formed such as (43) and (44) by reaction with the solvent acetonitrile or the sensitizer, respectively. In the presence of alcohols low yields of (45) and (46) are formed by reaction of the alcohol with the radical cation of the diene (42). 

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... 

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). 

Solvent addition to the alkene radical cation is often preferred to fragmentation in nucleophilic media such as alcohols (this gives rise to another useful photosubstitution process on aromatics, the photo-NOCAS reaction discussed elsewhere in this Handbook). 

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