Mechanism of Photoisomerization of Xanthene to 6H-Dibenzopyran in Aqueous Solution

Article abstract of DOI:10.1021/jo00018a044

A new photoreaction, the photoisomerization of xanthene (1) to 6H-dibenzopyran (2) (ca. 70percent) (Φ = 0.0035 in 7:3 H2O-CH3CN) in aqueous CH3CN solution is reported.In addition to 2, 2-benzylphenol (3) (Φ ca. 0.001), 9,9'-bixanthyl (4) (Φ < 0.001), and 2-(2'-hydroxyphenyl)benzyl alcohol (5) (Φ < 0.001) are also observed as minor products. 9-Methylxanthene (9) also photoisomerizes to the corresponding pyran derivative 10, although in this case much lower yields are observed due to secondary photochemistry of 10.The photoisomerization of 1 is most efficient in aqueous solution: Φ increased with increasing water content in a H2O-CH3CN solvent mixture.A mechanism of photoisomerization is proposed that involves initial Ar-O bond homolysis from S₁, to generate a singlet biradical.Trivial recombination gives back unreacted 1.Hydrogen abstraction from solvent leads to the photoreduction product 3.However, recombination of the biradical at the ipso benzylic position (ortho to phenol) gives a cyclohexadienone intermediate 17.Subsequent transformation of this species to 2 occurs via an o-quinone methide intermediate 18, which itself can be trapped with MeOH or H2O, to give the corresponding methyl ether derivative 8 or alcohol 5.