A Copper-Based Metal-Organic Framework Acts as a Bifunctional Catalyst for the Homocoupling of Arylboronic Acids and Epoxidation of Olefins

Abstract

A copper(I)-based metal-organic framework ({[Cu2 Br2 (pypz)]n ⋅nH2 O} (Cu-Br-MOF) [pypz=bis[3,5-dimethyl-4-(4'-pyridyl)pyrazol-1-yl] methane] has been synthesized by using an elongated and flexible bridging ligand. The structure analysis reveals that each pypz ligand acts as a tritopic ligand connected to two Cu2 Br2 dimeric units, forming a one-dimensional zig-zag chain, and these chains further connected by a Cu2 Br2 unit, give a two-dimensional framework on the bc-plane. In the Cu2 Br2 dimeric unit, the copper ions are four coordinated, thereby possessing a tetrahedral geometry; this proves to be an excellent heterogeneous catalyst for the aerobic homocoupling of arylboronic acids under mild reaction conditions. This method requires only 3 mol % of catalyst and it does not require any base or oxidant-compared to other conventional (Cu, Pd, Fe, and Au) catalysts-for the transformation of arylboronic acids in very good yields (98 %). The shape and size selectivity of the catalyst in the homocoupling was investigated. The use of the catalyst was further extended to the epoxidation of olefins. Moreover, the catalyst can be easily separated by simple filtration and reused efficiently up to 5 cycles without major loss of reactivity.

Keywords: arylboronic acids; copper; epoxidation; metal-organic frameworks; olefins.