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

Abstract

Flavonoids, which are ubiquitous plant secondary metabolites obtained from chalcones, mostly possess 6membered 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 Crings biosynthesized from chalcones by mainly performing Bring 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 CeO 2 using O 2 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 CeO 2 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.

Aurones: A Golden Resource for Active Compounds

“…The following reductive elimination of Pd from the palladacycle leads to the desired aurone 47 in its (Z)-predominant isoform, while the resulting reduced Pd(0) is reoxidized by oxygen from air, thus maintaining the catalytic efficiency of the system (Scheme 7) Scheme 7. Cyclization of chalcones according to Yatabe and co-workers [60].…”

“…A further example of chalcones cyclization has been provided by Yatabe and co-workers, who produced the expected aurones by treating the starting compounds with oxygen from air in the presence of an heterogeneous catalyst, made of Pd-on-Au nanoparticles supported on CeO 2 [ 60 ]. At first, the deprotonation of the hydroxy group of 44 occurs, which allows the coordination of chalcone with the heterogenous Pd catalyst as in 45 .…”

Supported Metal Nanoparticles Assisted Catalysis: A Broad Concept in Functionalization of Ubiquitous C−H Bonds

“…In 2018, Yatabe et al. published their report on Pd/Au NPs catalysed olefinic C−H functionalization of simple chalcones and formation of unusual 5‐membered ring resulting aurones [160] . Aurones are a class of flavonoids, secondary metabolites in plants.…”

A decennary update on applications of metal nanoparticles (MNPs) in the synthesis of nitrogen- and oxygen-containing heterocyclic scaffolds

“…Here, the Pd catalyst assists in the transformation of aurone from chalcone, Au improves the catalytic activity and longevity, CeO 2 inhibits 6-endo-trig cyclization, leading to the synthesis of avone, and Pd on Au together inhibit the Aucatalyzed synthesis of avone. 423 Zhang et al reported the CuFeO 2 NP-catalyzed green synthesis of imidazo[1,2-a]pyridines (151b) via the threecomponent reaction of 2-aminopyridine (150a), substituted benzaldehyde (21a) and substituted phenyl acetylene (3a) in a deep eutectic solvent such as citric acid-dimethyl urea (DMU) at 65 C (Scheme 239). 424 The NPs were prepared via a sol-gel process, followed by sequential annealing of a mixture of copper acetate Cu(CH 3 COO) 2 $H 2 O and ferric nitrate Fe(NO 3 ) 3 $9H 2 O in ethanol and triethanolamine.…”

“…Synthetic transformation from chalcone (2 0 -hydroxychalcone, 119b) into aurone(341), flavanone (323b), and flavones (323c) using Au and Pd metal catalysts 423. This journal is © TheRoyal Society of Chemistry 2020 RSC Adv., 2020, 10, 32740-32820 | 32799 Review RSC Advances Open Access Article.…”