Bio-inspired lanthanum-ortho-quinone catalysis for aerobic alcohol oxidation: semi-quinone anionic radical as redox ligand

Abstract

Oxidation reactions are fundamental transformations in organic synthesis and chemical industry. With oxygen or air as terminal oxidant, aerobic oxidation catalysis provides the most sustainable and economic oxidation processes. Most aerobic oxidation catalysis employs redox metal as its active center. While nature provides non-redox metal strategy as in pyrroloquinoline quinone (PQQ)-dependent methanol dehydrogenases (MDH), such an effective chemical version is unknown. Inspired by the recently discovered rare earth metal-dependent enzyme Ln-MDH, here we show that an open-shell semi-quinone anionic radical species in complexing with lanthanum could serve as a very efficient aerobic oxidation catalyst under ambient conditions. In this catalyst, the lanthanum(III) ion serves only as a Lewis acid promoter and the redox process occurs exclusively on the semiquinone ligand. The catalysis is initiated by 1e--reduction of lanthanum-activated ortho-quinone to a semiquinone-lanthanum complex La(SQ-.)2, which undergoes a coupled O-H/C-H (PCHT: proton coupled hydride transfer) dehydrogenation for aerobic oxidation of alcohols with up to 330 h−1 TOF.

Bioinspired o-Naphthoquinone-Catalyzed Aerobic Oxidation of Alcohols to Aldehydes and Ketones

“…The kinetic isotope effect of 1a - D turned out to be 1.9, and the parallel experiments between 1a and 1a - D 2 also revealed the k H/D value of 2.7, although the oxidation of 2a - D stopped at about 35% conversion. The observed k H/D value of 2.0–2.7 strongly implies that the C–H bond cleavage event does directly affect the rate of the o -NQ-catalyzed aerobic oxidation process, as previously observed in the coupled O–H/C–H hydrogen exchange , as well as the cyclic hydrogen atom transfer process . Based on our experimental observation and control experimental data, a plausible reaction mechanism of the o -NQ-catalyzed aerobic oxidation of alcohols is proposed.…”

“…Schelter’s group in 2018 built a functional synthetic model for the quinoid alcohol dehydrogenase using a rare earth metal, La, and ferrocenium hexafluorophosphate, [Fc]­[PF 6 ], as a terminal oxidant, where the catalytic oxidation of 4-methylbenzyl alcohol was achieved in 96% yield (Scheme c) . In 2022, Luo’s group reported a La/ ortho -quinone-catalyzed aerobic oxidation of alcohols via a proton-coupled hydride transfer process through the 1e – -reduction of La-activated ortho -quinone to a La–semiquinone complex . While the ortho -quinone-containing photocatalyst, pyrenedione, has been utilized for the olefination of nitriles, presumably via oxidation of alcohols, to the best of our knowledge there is no literature report based on the metal-free biomimetic ortho -quinone-catalyzed aerobic oxidation of alcohols.…”

“…The subjection of hydrobenzoin to the current aerobic oxidation condition provided the desired benzil 2ze in 64% yield via double oxidation. The current o -NQ-catalyzed aerobic oxidation protocol readily oxidized the amine-containing alcohols 1q – 1r , which could not be tolerated under Luo’s La/ o -Q catalyst system . Also, the selective oxidation of benzylic alcohol positions was possible to give the products 2zf – 2zg in 43–76% yields.…”

“…12a In 2022, Luo's group reported a La/ortho-quinone-catalyzed aerobic oxidation of alcohols via a proton-coupled hydride transfer process through the 1e − -reduction of La-activated ortho-quinone to a La− semiquinone complex. 13 While the ortho-quinone-containing photocatalyst, pyrenedione, has been utilized for the olefination of nitriles, presumably via oxidation of alcohols, 14 to the best of our knowledge there is no literature report based on the metalfree biomimetic ortho-quinone-catalyzed aerobic oxidation of alcohols.…”

“…The current o-NQcatalyzed aerobic oxidation protocol readily oxidized the aminecontaining alcohols 1q−1r, which could not be tolerated under Luo's La/o-Q catalyst system. 13 Also, the selective oxidation of benzylic alcohol positions was possible to give the products 2zf− 2zg in 43−76% yields.…”