Phenols 3 + 2 photocycloaddition

Dihydropyran-4-ones are a source of phenols via an intramolecular [2+2] photocycloaddition reaction and a Lewis-acid catalysed cleavage of the cyclobutane moiety <96TL1663>. 

Intermolecular photocycloaddition also occurs between alkenes and simple phenols. The swing from meta addition illustrated above in the [3 + 2]-mode to ortho-addition is a result of charge-transfer interactions between the alkene and the phenol and a greater charge transfer favours the ortto-addition mode. These aspects have been the subjects of reviews . This reaction mode is exemplified by the addition of acrylonitrile... 

The meta photocycloaddition of ( )-l,2-dichloroethene with benzonitrile, toluonitriles, benzene, phenol, cresols, fluorobenzene, chlorobenzene, trifluoromethylbenzene and 3-trifluoro-methylbenzonitrile shows not only the influence of the aromatic substituents on the direction of the alkene attack, but also a control of the cyclopropane closure by the vinyl chlorine to give exclusively the exo-(>, ent/o-7-dichloro adduct isomer 5 which was, in the most favorable cases, isolated in gram quantities. ... 

Azocine formation by photocycloaddition of acrylonitrile to arenas is also observed when cyanobenzenes and cyanonaphthalenes are irradiated with ultra-violet light in the presence of phenols. The products have the structure (70) and are presumed to be formed by thermal ring opening of the primary ortho adducts (71). 

Gilbert, A. and Jones, D. T., Formation of bicyclo[3.2.1]oct-2-en-8-ones and 1-hydroxydihy-drosemibullvalenes from the mefci-photocycloaddition of cyclopentene to phenols, /. Chem. Soc., Perkin Trans. 2, 1385,1996. 

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