Recent advances in the transition-metal-catalyzed regioselective approaches to polysubstituted benzene derivatives.

method for the synthesis of polysubstituted benzenes. However, high regioselectivity (and yield) can only be achieved by the careful choice of the reagents and synthetic route. Furthermore, it is usually necessary to convert and/or protect-deprotect the substituents properly. Directed ortho metalation is another synthetic method which has been studied extensively, and this approach is a useful method for the introduction of a substituent at the ortho position.2 However, the scope of this reaction is clearly restricted by its nature. Compared to the following transition metal-catalyzed methods, these approaches have some drawbacks from the viewpoint of atom economy3 or environmental concern. In 1948, Reppe et al. discovered the transitionmetal-catalyzed [2+2+2] cyclotrimerization of alkynes,4 and this new synthetic method was applied to the synthesis of substituted benzenes (eq 2). In this