Unusual Olefinic C–H Functionalization of Simple Chalcones toward Aurones Enabled by the Rational Design of a Function-Integrated Heterogeneous Catalyst

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Abstract

Flavonoids, which are ubiquitous plant secondary metabolites obtained from chalcones, mostly possess 6-membered C-rings derived from 6-endo-trig cyclization of chalcones. However, aurones, which are a class of flavonoids that rarely occur naturally, possess unusual 5-membered C-rings biosynthesized from chalcones by mainly performing B-ring oxidation. Therefore, the chemical catalytic transformation from simple chalcones into aurones is attractive, because it overcomes the drawback of known limited enzyme catalysis. The catalytic transformation, however, has not yet been reported because of the preferential 6-membered ring formation as with the biosynthesis and the need for rare intramolecular olefinic C–H functionalization. Here, we developed the catalytic olefinic C–H functionalization of simple chalcones toward various aurones enabled by the rational design of a function-integrated heterogeneous catalyst—a Pd-on-Au bimetallic nanoparticle catalyst supported on CeO2—using O2 in air as the sole oxidant without any additives. In this system, the four conditions that were required for the challenging transformation toward aurones were achieved by the respective components of the catalyst: (a) a supported Pd catalyst: a catalyst for the olefinic C–H functionalization of chalcones toward aurones, (b) an Au promoter: an improvement in the catalytic activity by stabilizing Pd(0), (c) a CeO2 support: the inhibition of the 6-endo-trig cyclization utilizing the adsorption of chalcones, and (d) a Pd-on-Au structure: the inhibition of Au-catalyzed flavone synthesis. This catalytic transformation will promote not only the pharmaceutical study of aurones but also the rational design of a heterogeneous catalyst for the development of organic reactions that are not yet realized by homogeneous catalysts or biocatalyst.

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