MECHANISM OF PHOTOISOMERIZATION OF XANTHENE TO 6H-DIBENZO[B,D]PYRAN IN AQUEOUS-SOLUTION

A new photoreaction, the photoisomerization of xanthene (1) to 6H-dibenzo[b,d]pyran (2) (almost-equal-to 70%) (PHI = 0.0035 in 7:3 H2O-CH3CN) in aqueous CH3CN solution is reported. In addition to 2, 2-benzylphenol (3) (PHI almost-equal-to 0.001), 9,9'-bixanthyl (4) (PHI < 0.001), and 2-(2'-hydroxyphenyl)benzyl alcohol (5) (PHI < 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: PHI 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.