Palladium‐Catalyzed Annulation of Aryl‐1,2‐diols and Propargylic Carbonates. Theoretical Study of the Observed Regioselectivities

Methyl 1‐methylprop‐2‐ynyl carbonate reacts with 3‐ and 4‐substituted benzene‐1,2‐diols to give 2,3‐dihydro‐3‐methyl‐2‐methylidene‐1,4‐benzodioxines, as a mixture of regioisomers in the case of methoxy‐substituted diphenol, and a single regioisomer for the nitro‐ and the formyl‐substituted diphenols. The oxyanion which attacks first corresponds to the less acidic hydroxy function, and the attack of the second oxyanion on the η 3 ‐allylpalladium intermediate is directed toward the more substituted carbon atom. In order to explain the regioselectivity of the cyclization, a theoretical study based on density functional theory (DFT) has been performed. The first step implies an equilibrium between the two oxyanions. The regioselectivity of the second step is due to a subtle balance between the nature of the phosphane, the bulkiness, and the electronic properties of the substituents. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)