Abstract
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.
Highlights
As an extension of our previous studies on the photochemistry of benzopyran-derived compounds,[1,2,3,4] we have been interested in the light induced fragmentation of 3-halo-2,3dihydrobenzopyran-4-ones (3-halochromanones), which leads to the corresponding α-keto radicals and/or cations; these species are key intermediates in the electron transfer processes followed by 4-acetoxychromenes.[4]
1a and 2a the only substituent of the dihydrobenzopyran ring is chlorine or bromine respectively, while compounds 1b and 2b possess a trans- phenyl group attached to the 2- position (Chart 1)
The presence of a phenyl substituent at the 2-position of the dihydrobenzopyran ring of 1a or 2a did not change the trend; again the benzopyran derivative was the major photoproduct flavanone 3b was formed; the latter was found in higher amounts when hexane was used as solvent (Table 1, entries 9-16)
Summary
As an extension of our previous studies on the photochemistry of benzopyran-derived compounds,[1,2,3,4] we have been interested in the light induced fragmentation of 3-halo-2,3dihydrobenzopyran-4-ones (3-halochromanones), which leads to the corresponding α-keto radicals and/or cations; these species are key intermediates in the electron transfer processes followed by 4-acetoxychromenes.[4]. When hexane was used as solvent (Table 1, entries 3 and 4), appreciable amounts of the corresponding dihydro derivative (4-chromanone, 3a) were found in the photolysate.
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