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

Triaryl Carbenium Ion Pair Mediated Electrocatalytic Benzylic C-H Oxygenation in Air

The selective oxidation of benzylic C-H bonds is a pivotal transformation in organic synthesis. Undoubtedly, achieving efficient and highly selective aerobic oxidation of methylarenes to benzaldehydes has been highly challenging due to the propensity of benzaldehyde to undergo overoxidation under typical aerobic conditions. Herein, we propose an innovative approach to address this issue by leveraging electrocatalytic processes, facilitated by ion-pair mediators [Ph3C]+[B(C6F5)4]-. By harnessing the power of electrochemistry, we successfully demonstrated the effectiveness of our strategy, which enables the selective oxidation of benzylic C-H bonds in benzylic molecules and toluene derivatives. Notably, our approach exhibited high efficiency, excellent selectivity, and compatibility with various functional groups, underscoring the broad applicability of our methodology.

A perspective on the role of lanthanides in biology: Discovery, open questions and possible applications

Because of their use in high technologies like computers, smartphones and renewable energy applications, lanthanides (belonging to the group of rare earth elements) are essential for our daily lives. A range of applications in medicine and biochemical research made use of their photo-physical properties. The discovery of a biological role for lanthanides has boosted research in this new field. Several methanotrophs and methylotrophs are strictly dependent on the presence of lanthanides in the growth medium while others show a regulatory response. After the first demonstration of a lanthanide in the active site of the XoxF-type pyrroloquinoline quinone methanol dehydrogenases, follow-up studies showed the same for other pyrroloquinoline quinone-containing enzymes. In addition, research focused on the effect of lanthanides on regulation of gene expression and uptake mechanism into bacterial cells. This review briefly describes the discovery of the role of lanthanides in biology and focuses on open questions in biological lanthanide research and possible application of lanthanide-containing bacteria and enzymes in recovery of these special elements.

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

A biomimetic alcohol dehydrogenase (ADH)-like oxidation protocol was developed using an ortho-naphthoquinone catalyst in the presence of a catalytic amount of base. The developed organocatalytic aerobic oxidation protocol proceeds through the intramolecular 1,5-hydrogen atom transfer of naphthalene alkoxide intermediates, a mechanistically distinctive feature from the previous alcohol dehydrogenase mimics that require metals in the active form of catalysts. The ADH-like aerobic oxidation protocol should provide green alternatives to the existing stoichiometric and metal-catalyzed alcohol oxidation reactions.

Ion Mobility Studies of Pyrroloquinoline Quinone Aza-Crown Ether-Lanthanide Complexes

Lanthanide-dependent enzymes and their biomimetic complexes have arisen as an interesting target of research in the past decade. These enzymes, specifically, pyrroloquinoline quinone (PQQ)-bearing methanol dehydrogenases, efficiently convert alcohols to the respective aldehydes. To rationally design bioinspired alcohol dehydrogenation catalysts, it is imperative to understand the species involved in catalysis. However, given the extremely flexible coordination sphere of lanthanides, it is often difficult to assess the number and nature of the active species. Here, we show how such questions can be addressed by using a combination of ion mobility spectrometry, mass spectrometry, and quantum-chemical calculations to study the test systems PQQ and lanthanide-PQQ-crown ether ligand complexes. Specifically, we determine the gas-phase structures of [PQQH₂]^-, [PQQH₂+H₂O]^-, [PQQH₂+MeOH]^-, [PQQ-15c5+H]⁺, and [PQQ-15c5+Ln+NO₃]²⁺ (Ln = La to Lu, except Pm). In the latter case, a trend to smaller collision cross sections across the lanthanide series is clearly observable, in line with the well-known lanthanide contraction. We hope that in the future such investigations will help to guide the design and understanding of lanthanide-based biomimetic complexes optimized for catalytic function.

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

Understanding the role of metal ions in biology can lead to the development of new catalysts for several industrially important transformations. Lanthanides are the most recent group of metal ions that have been shown to be important in biology, that is, in quinone‐dependent methanol dehydrogenases (MDH). Here we evaluate a literature‐known pyrroloquinoline quinone (PQQ) and 1‐aza‐15‐crown‐5 based ligand platform as scaffold for Ca²⁺, Ba²⁺, La³⁺ and Lu³⁺ biomimetics of MDH and we evaluate the importance of ligand design, charge, size, counterions and base for the alcohol oxidation reaction using NMR spectroscopy. In addition, we report a new straightforward synthetic route (3 steps instead of 11 and 33 % instead of 0.6 % yield) for biomimetic ligands based on PQQ. We show that when studying biomimetics for MDH, larger metal ions and those with lower charge in this case promote the dehydrogenation reaction more effectively and that this is likely an effect of the ligand design which must be considered when studying biomimetics. To gain more information on the structures and impact of counterions of the complexes, we performed collision induced dissociation (CID) experiments and observe that the nitrates are more tightly bound than the triflates. To resolve the structure of the complexes in the gas phase we combined DFT‐calculations and ion mobility measurements (IMS). Furthermore, we characterized the obtained complexes and reaction mixtures using Electron Paramagnetic Resonance (EPR) spectroscopy and show the presence of a small amount of quinone‐based radical.