A Bio-Inspired Lanthanum-Ortho Quinone Catalyst for Aerobic Oxidation of Alcohol

Abstract

<p>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 metal serves only as a Lewis acid promoter and the redox process occurs exclusively on the semiquinone ligand. The catalysis is initiated by 1e<sup>-</sup>-reduction of lanthanum-activated <i>ortho</i>-quinone to a semiquinone-lanthanum complex La(<b>SQ<sup>-.</sup></b>)<sub>2</sub>, which undergoes a coupled O-H/C-H dehydrogenation for aerobic oxidation of alcohols with up to 330 h<sup>-1</sup> TOF. This study suggests a possible functional mode of semiquinone radicals, widely observed with quinoproteins in Nature. Moreover, this unique reductive activation strategy as well as the resulted radical anion as redox ligand creates a new turning point in the development of efficient aerobic oxidation catalysis.</p>

Pyrroloquinoline Quinone Aza‐Crown Ether Complexes as Biomimetics for Lanthanide and Calcium Dependent Alcohol Dehydrogenases**

“…Recently, Luo et al have reported in their preprint that a semiquinone anionic radical species forms upon coordination of LaI 3 with the ortho-quinone o-Q in acetonitrile. [10] An EPR spectrum of the system showed the presence of an organic radical species with g = 2.003, which could not be observed in absence of the metal salt. Furthermore the emergence of traces of radicals when PQQ and lanthanide salts are combined has been observed previously by Daumann and co-workers (see note in the Supporting Information).…”

“…Recently Luo et al showed that the semiquinone-lanthanum complex [La(o-Q * À ) 2 ] can also serve as an efficient catalyst for the oxidation of alcohols under aerobic conditions. [10] The catalysis was initialized by reduction through single electron transfer of the ortho-quinone ligand o-Q after coordinating with LaI 3 . A screening of different conditions showed that the addition of a base did not have an obvious effect on the reaction outcome.…”

“…Numbering scheme of the PQQ-moiety in 1 according to Unkefer et al [6] performed with calcium triflate showed no substrate conversion at all. Given the fact that the pyridine (and pyrrole) moiety is lacking in ligand o-Q and the coordination of Ln is taking place at both quinone oxygens, the system cannot be considered a Proposed coupled proton-hydride transfer mechanism by Luo et al [10] (C) Proposed addition-base catalyzed elimination mechanism for the oxidation of ethanol by Itoh and Fukuzumi. [7] (D) Proposed hydride-transfer mechanism for the oxidation of 4-methyl benzyl alcohol by Schelter et al [9] The tetracyclohexyl-3-(2-aminophenyl)pentanediamide moiety is omitted for clarity.…”

PQQ-Aza-Crown Ether Complexes as Biomimetics for Lanthanide and Calcium Dependent Alcohol Dehydrogenases

“…Recently, Luo et al have reported in their preprint that a semi-quinone anionic radical species forms upon coordination of LaI3 with the ortho-quinone o-Q in acetonitrile. [12] An EPR spectrum of the system showed the presence of an organic radical species with g = 2.003, which could not be observed in absence of the metal salt. Furthermore the emergence of traces of radicals when PQQ and lanthanide salts are combined has been observed previously by Daumann and coworkers (see note in SI there).…”

“…Recently Luo et al showed that the semiquinone-lanthanum complex [La(o-Q -• )2] can also serve as an efficient catalyst for the oxidation of alcohols under aerobic conditions. [12] The catalysis was initialized by reduction via single electron transfer of the ortho-quinone ligand o-Q after coordinating with LaI3. A screening of different conditions showed that the addition of a base did not have an obvious effect on the reaction outcome.…”