The functionalization of aromatic compounds is a key step in the production of many organic chemicals and pharmaceuticals. Currently, inert C-H bonds need to be pre-functionalized before they can be used in transition metal mediated coupling reactions. The associated cost of the pre-functionalization renders this procedure inaccessible for most bulk chemicals, while a direct route circumventing any pre-functionalization would be more superior in terms of atom and step economy. C-H/C-H arene-arene couplings can in principle be used for the direct synthesis of biaryls from simple arenes, with e.g. Pd carboxylates as the catalysts. If O2 is used as the oxidant, water is the only by-product. Unfortunately, the multitude of C-H bonds present in organic reactants, and the poor differentiation between them often result in poor regioselectivity. Consequently, reactions with ‘simple’ arenes (e.g. toluene) lead to useless mixtures of many isomers. In this work, we implemented the molecular mechanisms of transition-metal catalyzed C-H activation in the shape-selective confinement of a porous material like a zeolite. Although extensively studied for acid-catalyzed reactions, the well-known concept of shape-selectivity in zeolite catalysis has not been used for confined molecular catalysts that activate C-H bonds of e.g. arenes to form more complex organics. Here we demonstrate that Pd(II) catalysts, site isolated in the framework of zeolite Beta or some other 12-MR zeolites, uniquely allow to oxidatively couple toluene and other monosubstituted aromatics to bitolyl, with exceptional selectivity for the para,para-dimer. Next, two different aromatic compounds were selectively coupled, in which the shape-selectivity enabled the selective formation of a single product out of 12 possible products.

Oxidative homo- and cross-coupling of aromatics to biarylic compounds by shape-selective C-H activation using molecular palladium in zeolites

ACS Spring 2022

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