PHOTOCHEMISTRY OF THE 9-HYDROXYXANTHENYL RADICAL IN THE LASER-JET - EVIDENCE FOR PHOTOCHEMICALLY INDUCED, REGIOSELECTIVE RADICAL COUPLING

The conventional irradiation (low intensity) of xanthone in ethanol was observed to produce pinacol 3 by photoreductive coupling and 9-hydroxy-9-(1-hydroxyethyl)-9H-xanthene (4) by head-to-head cross-coupling between the resultant 9-hydroxyxanthenyl (2) and 1-hydroxyethyl radicals. Under laser-jet conditions (high intensity), in addition to products 3 and 4, 3-(1-hydroxyethyl)-9H-xanthen-9-one (5d) was obtained as the major head-to-tail cross-coupling product. The observed dependence of the product ratio of 4 and 5d on the irradiation intensity is rationalized in terms of the higher spin density in the aryl rings, particularly at the 3-position (para to the hydroxy-substituted carbon atom), of the electronically excited 9-hydroxyxanthenyl radical (2'). For the ground state radical 2, as expected, the majority of spin density resides at the hydroxy-substituted carbon atom and, thus, constitutes the precursor to the head-to-head cross-coupling product 4. The theoretical spin densities for the ground and excited state 9-hydroxyxanthenyl radicals 2, calculated by semiempirical MO methods (PM3), indicate that upon photoexcitation the electron density in the SOMO shifts from the hydroxy-substituted radical center (ground state) to the aromatic pi-system (first excited state) and is concentrated at the 1a- and 3-positions.