J. Am. Chem. Sac. 1994, 116. 368CL3683
3680
Photochemistry of the 9-Hydroxyxanthenyl Radical in the Laser-Jet: Evidence for Photochemically Induced, Regioselective Radical Coupling W a l d e m a r Adam' a n d Fumio Kita Conrriburion from the lnsrirule of Organic Chemisrry. Uniuersity of Wirrzburg. Am Hubland. 0-97074 Wirrzburg. Germany Receiued Nocember 30. 1993'
Abstract: 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-io-head cross-coupling between the resultant 9-hydroxyxanthenyl (2) and I-hydroxyethyl radicals. Under laser-jet conditions (high intensity), in addition to products 3 and 4. 3-(l-hydroxyethyl)-9H-xanthen-9-one (M) was obtained as the major head-to-rail cross-coupling product. The observed dependence of the product ratio of 4 and M on the irradiation intensity is rationalized in terms of the higher spin density in the aryl rings, particularly a t the 3-position (para to the hydroxysubstituted carbon atom). of the electronically excited 9-hydroxyxanthenyl radical ( Y ) . For the ground state radical 2, as expected, the majority of spin density resides a t the hydroxy-substituted carbon atom and, thus, constitutes the precursor to the head-io-headcross-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 S O M O shifts from the hydroxy-substituted radical center (ground state) to the aromatic a-system (first excited state) and is concentrated a t the l a - and 3-positions.
It was recently reported that the regioselectivity of the radical coupling between the hydroxydiphenylmethyl radical (1) and the I-hydroxyethyl radical wasstrongly dependent upon the light intensity used during the irradiation.' Thus, under laser-jet2 conditions (high intensity), irradiation ofbenzophenone in ethanol produced a significant amount of ring cross-coupling product (head-ro-rail coupling). derived from the oxidation-sensitive lighr-absorbingrransienr (LAT). The latter has been of intense
hR H
LAT
mechanistic interest in the photoreduction of aromatic ketones in Hdonatingsolventsformore than threedecades.' Theobserved dependence of the product distribution on the light intensity indicated that the head-to-rail product resulted mainly from the Abrtrael published in Adoonce ACS Abnracts. April 1, 1994. (I)(a) Rubin. M. E. TelrohpdronLetr. 1982.23.46154618, (b) Wilson. R. M.: Hanncmann. K.: Adam. W. P m - I U P A C S y m p . Phormhcm.. I I i h 1986. Abrract 2ST-3. IC) Adam. W.: Schullc Oertrich. R. Chrm. BPI. 1992. 125. 2463-2466. (2) (a) Wilson. R. M.: Schnam. K. A,; Hannemann. K.: Ho. D. M.: Mcmarian. H. R.; Azadnia. A,: Pinhas. A. R.;Figley. T. M. Sprcrrmhim. Acro. Par1 A 1990, 46A. 551-558. (b) Wilson. R. M.; Adam. W.: Schulle Ocrlrich. R. Smcrrum 1991. 4. 8-17. le) Wilson. R. M.: Schnaoo. K. A. 1 3 ) l a ) Pill$.J I \ , J r , Lctr.ngcr. R . L ,Td!Ior, R P , Pattrrwn. J \ I , Rccktcnuald.G..Ma~l~n.R.B.J Am Chem .%x 19S9.RI. Inoh-Iul7 D I Sch~nck.G.O..C2iala.U..Eaainpcr.K..U(Jtlhi.r.C, . P a r r . \ t TePrrohdnm I . Lem 1967.8. 193-198: (ciSchcnek. C..O.:Matthias: G.'Tbrrohedr&Le;r. 1967. 8. 699-702. (d) Mauser. H.: Nickel. E.: Sprocrrcr. U.: Bihl. V. 2. Norurjorsch. 1967.22B.903-908. (e) Filipcrcu. N.: Minn. F. L. J. Am. Chem. Sor. 1969.90. 1544-1547.lnChalland. E.D . C m J . Chem. 1969.47.687688. (8) Wciner. S. A. J..4 ;, Chem. Sd.1971. 93.425429. (h)'Lambl F. A.; Vittimkrga. E. M. J. Ow C h m 1973.38.352&3524. (i) Cohen. S. G.: Caird Ramsay. C.: Stein. N . M.: Weinstein. S. Y. J. Am. C h o n So
