Palladium(II)-catalyzed ring enlargement of 2-(arylmethylene)cyclopropylcarbinols: strong effect of substituent electronic nature on the reaction pathway

摘要

In the presence of Pd(II) catalyst and copper(II) bromide, 2-(arylmethylene)cycIopropylcarbinols undergo ring enlargement to deliver (arylcyclobutenyl)carbinols or hydrogenated furans in good yields under mild conditions; mechanisms accounting for the distinct products have been proposed on the basis of control and deuterium labeling experiments. Methylenecyclopropanes (MCPs) are generally used as building blocks in organic synthesis for their ready accessibility as well as diverse reactivity driven by the relief of ring strain. Over the past decades, a series of review articles have been published on the transformations of these exo-methylene three-membered carbocycles. Transition metal catalysts, now one of the most powerful tools for synthetic chemists, have played an increasingly important role in these transformations. Recently, we showed that, catalyzed by palladium acetate and metal bromide, MCPs could undergo ring enlargement to provide the cyclobutene motif efficiently.