Regioselective palladium-catalyzed ring-opening reactions of C1-substituted oxabenzonorbornadienes

The effect of ethyl and methoxycarbonyl substitution on the C-1 position of oxabenzonorbornadienes undergoing palladium-catalyzed nucleophilic ring opening was investigated with a variety of aryl iodide nucleophiles. Electron-withdrawing groups in the C-1 position or on the aryl iodide afforded lower yields and led to aromatization of products. The presence of an electron-donating group in either position provided high yields in all cases. Despite variances in electronic nature, all trials produced a single regioisomeric product resulting from addition of the aryl group to the olefin carbon farthest from the C-1 substituent. Based on these findings, a mechanism has also been proposed.