Iron-catalyzed tandem oxidative coupling and annulation: an efficient approach to construct polysubstituted benzofurans.

The combination of FeCl(3) x 6 H(2)O and di-tert-butyl peroxide offers a novel and efficient method for the construction of polysubstituted benzofurans 3 from the reaction of simple phenols 1 and beta-keto esters 2, which are expected to give coumarins in the well-known Pechmann condensation. A variety of phenols reacted with beta-keto esters to provide a range of benzofuran products in moderate to excellent yields. The regio-specific annulation was proven by the X-ray molecular structure of the product 3k. Hydrate of FeCl(3) is essential for an achievement of the present transformation. The kinetic isotopic effect (KIE) experiments were carried out by competition experiments and displayed a k(H)/k(D) = 1.0 +/- 0.1. The kinetic isotopic effect indicated that aromatic C-H bond cleavage is not involved in the rate-determining steps of the present transformation. Moreover, the results clearly demonstrate that the dichotomous catalytic behavior of the iron catalyst, which is transition-metal catalyst in the oxidative coupling step and Lewis acid in the condensation step. The possible intermediate 5 was synthesized and converted into the desired benzofuran 3a under the reaction conditions. A tentative mechanism of the formation of benzofurans 3 was proposed.