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Photolysis of 3-halo-2,3-dihydrobenzopyran-4-ones 1a,b and 2a,b in acetonitrile and hexane led to dehalogenated dihydrobenzopyran-4-ones 3a,b and benzopyran-4-ones 4a,b. Their formation is accounted for in terms of primary cleavage of the carbon halogen bond to give α-carbonyl radicals (I) and/or cations (II). Intermediates II, precursors of 4, can be generated either directly by heterolysis or by initial homolysis followed by electron transfer. The ionic routes are more favored in acetonitrile than in hexane. The involvement of radical and ionic pathways in the photochemistry of 3-halo-2,3-dihydrobenzopyran-4-ones is supported by detection of Br2 -. in the laser flash photolysis experiments performed on 2a.

Chemical and transient spectroscopic evidence for radical and ionic pathways in the photolysis of 3-halo-2,3-dihydrobenzopyran-4-ones