Benzonitrile photochemical cycloadditions

Cantrell, T. S., Photochemical cycloadditions of benzonitrile to alkenes, factors controUing the site of the addition, /. Org. Chem., 42, 4238, 1977. 

Photochemical [2 + 2] cycloaddition of benzonitrile and of 1- and 2-naphthonitriles to electron-rich alkenes such as 2,3-dimethyIbut-2-ene gives the corresponding 2-aryl-l-azetines in poor yield 72JA5929,76CC729, 77JOC4238). This does not appear to be a versatile route to 1-azetines. 

The ortho cycloaddition is thermally forbidden in a suprafacial-suprafacial manner and the photochemical reaction is forbidden with S benzene and ground-state alkene. On the basis of these considerations, it could be understood that the ortho addition had only been observed with systems where the alkene is the lowest excited singlet species (as with maleimides [37,74,75] or where either the alkene or the arene has marked acceptor properties (the only examples known at that time were benzene-acrylonitrile [127] and benzonitrile + a mono-olefin [1,73], Benzene-acrylonitrile and benzonitrile-olefin systems do not display charge-transfer absorption, but charge transfer could well follow excitation. Bryce-Smith further stated that irradiation of benzene in the presence of simple mono-olefins normally provides B2u (Si) benzene as the lowest excited singlet species, which leads to meta rather than ortho addition, but the latter process might, in principle, be able to occur under conditions where a Biu (S2) state of benzene is populated. 

A review has highlighted the photocycloaddition reactions of alkenes with aromatic esters and nitriles. Cycloadditions occur by a (3+2)-mode and provides a path to medium size ring systems. When the cinnamic acid derivative (9) is irradiated at 359 nm in ethanol with added Ti02 the product (10) is formed in 30% yield. Analogous products are formed from other straight chain alcohols such as (11) from propan-l-ol. Benzonitrile can be photochemically hydrated in the presence of oxophosphorus porphyrins. " ... 

A-phenylmaleimide/ ° The complex nitrile oxide (736), prepared from dichloroglyoxime (HON=CCl—CC1=N0H) and dipropargylamine, forms the intramolecular cyclo-adduct 137) 3,6-Diphenylpyridazine 1,2-dioxide (738) rearranges photochemically to the isoxazolo-isoxazole (739), which is also produced by the oxidation of the dioxime HON=CPhCH=CHCPh=NOH. Regiospecific cycloaddition of benzonitrile oxide to the pyrazoline (740) affords the pyrazolo-oxadiazole (741)/ ... 

Particularly significant contributions have been made by Padwa and Schmid in the photochemical generation of benzonitrile methylides from 2//-azirines. For example (Scheme 21), photolysis of (67) in the presence of carbon dioxide as dipolarophile leads to 4-phenyl-3-oxazolin-5-ones (68), whereas the intramolecular dipolar cycloaddition reaction of (69) can afford a useful synthesis of the bicyclic product (70). ... 

Photochemical meta cycloaddition reactions of benzonitrile and the methylbenzonitriles with alkenes and dienes have been reported previously, and the results are summarized in a recent review. Examples include benzonitrile itself with seven alkenes and dienes (Table 2 in Reference 9) and each of the three isomeric methylbenzonitriles with -l,2-dichloroethene, fiiran, and cyclopentene (Table 3 in Reference 9). In all cases, cycloadditions were observed to give, along with other products, the dihydrosemibullvalenes expected from the trapping of a bicyclic intermediate analogous to that proposed for the phototranspositions and photoadditions to TFE. However, none of these products resulted from an intermediate that had the cyano group at C6. 

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