Can Donor Ligands Make Pd(OAc)2 a Stronger Oxidant? Access to Elusive Palladium(II) Reduction Potentials and Effects of Ancillary Ligands via Palladium(II)/Hydroquinone Redox Equilibria

Photocatalyzed Aminomethylation of Alkyl Halides Enabled by Sterically Hindered N-Substituents

The catalytic C(sp3 )-C(sp3 ) coupling of alkyl halides and tertiary amines offers a promising tool for the rapid decoration of amine skeletons. However, this approach has not been well established, partially due to the challenges in precisely distinguishing and controlling the reactivity of amine-coupling partners and their product homologues. Herein, we developed a metal-free photocatalytic system for the aminomethylation of alkyl halides through radical-involved C(sp3 )-C(sp3 ) bond formation, allowing for the synthesis of sterically congested tertiary amines that are of interest in organic synthesis but not easily prepared by other methods. Mechanistic studies disclosed that sterically hindered N-substituents are key to activate the amine coupling partners by tuning their redox potentials to drive the reaction forward.

Optimizing the Synthetic Potential of O2: Implications of Overpotential in Homogeneous Aerobic Oxidation Catalysis

Molecular oxygen is the quintessential oxidant for organic chemical synthesis, but many challenges continue to limit its utility and breadth of applications. Extensive historical research has focused on overcoming kinetic challenges presented by the ground-state triplet electronic structure of O2 and the various reactivity and selectivity challenges associated with reactive oxygen species derived from O2 reduction. This Perspective will analyze thermodynamic principles underlying catalytic aerobic oxidation reactions, borrowing concepts from the study of the oxygen reduction reaction (ORR) in fuel cells. This analysis is especially important for "oxidase"-type liquid-phase catalytic aerobic oxidation reactions, which proceed by a mechanism that couples two sequential redox half-reactions: (1) substrate oxidation and (2) oxygen reduction, typically affording H2O2 or H2O. The catalysts for these reactions feature redox potentials that lie between the potentials associated with the substrate oxidation and oxygen reduction reactions, and changes in the catalyst potential lead to variations in effective overpotentials for the two half reactions. Catalysts that operate at low ORR overpotential retain a more thermodynamic driving force for the substrate oxidation step, enabling O2 to be used in more challenging oxidations. While catalysts that operate at high ORR overpotential have less driving force available for substrate oxidation, they often exhibit different or improved chemoselectivity relative to the high-potential catalysts. The concepts are elaborated in a series of case studies to highlight their implications for chemical synthesis. Examples include comparisons of (a) NOx/oxoammonium and Cu/nitroxyl catalysts, (b) high-potential quinones and amine oxidase biomimetic quinones, and (c) Pd aerobic oxidation catalysts with or without NOx cocatalysts. In addition, we show how the reductive activation of O2 provides a means to access potentials not accessible with conventional oxidase-type mechanisms. Overall, this analysis highlights the central role of catalyst overpotential in guiding the development of aerobic oxidation reactions.

Mechanism-Guided Design of Robust Palladium Catalysts for Selective Aerobic Oxidation of Polyols

The palladium complex [(L₁)Pd(μ-OAc)]₂[OTf]₂ (L₁ = neocuproine) is a selective catalyst for the aerobic oxidation of vicinal polyols to α-hydroxyketones, but competitive oxidation of the ligand methyl groups limits the turnover number and necessitates high Pd loadings. Replacement of the neocuproine ligand with 2,2'-biquinoline ligands was investigated as a strategy to improve catalyst performance and explore the relationship between ligand structure and reactivity. Evaluation of [(L₂)Pd(μ-OAc)]₂[OTf]₂ (L₂ = 2,2'-biquinoline) as a catalyst for aerobic alcohol oxidation revealed a threefold enhancement in turnover number relative to the neocuproine congener, but a much slower rate. Mechanistic studies indicated that the slow rates observed with L₂ were a consequence of precipitation of an insoluble trinuclear palladium species─(L₂Pd)₃(μ-O)₂²⁺─formed during catalysis and characterized by high-resolution electrospray ionization mass spectrometry. Density functional theory was used to predict that a sterically modified biquinoline ligand, L₃ = 7,7'-di-tert-butyl-2,2'-biquinoline, would disfavor the formation of the trinuclear (LPd)₃(μ-O)₂²⁺ species. This design strategy was validated as catalytic aerobic oxidation with [(L₃)Pd(μ-OAc)]₂[OTf]₂ is both robust and rapid, marrying the kinetics of the parent L₁-supported system with the high aerobic turnover numbers of the L₂-supported system. Changes in ligand structure were also found to modulate regioselectivity in the oxidation of complex glycoside substrates, providing new insights into structure-selectivity relationships with this class of catalysts.

Thermodynamic-Kinetic Comparison of Palladium(II)-Mediated Alcohol and Hydroquinone Oxidation

Palladium(II) catalysts promote oxidative dehydrogenation and dehydrogenative coupling of many organic molecules. Oxidations of alcohols to aldehydes or ketones are prominent examples. Hydroquinone (H2Q) oxidation to benzoquinone (BQ) is conceptually related to alcohol oxidation, but it is significantly more challenging thermodynamically. The BQ/H2Q redox potential is sufficiently high that BQ is often used as an oxidant in Pd-catalyzed oxidation reactions. A recent report (J. Am Chem. Soc. 2020, 142, 19678-19688) showed that certain ancillary ligands can raise the PdII/0 redox potential sufficiently to reverse this reactivity, enabling (L)PdII(OAc)2 to oxidize hydroquinone to benzoquinone. Here, we investigate the oxidation of tert-butylhydroquinone ( t BuH2Q) and 4-fluorobenzyl alcohol (4FBnOH), mediated by (bc)Pd(OAc)2 (bc = bathocuproine). Although alcohol oxidation is thermodynamically favored over H2Q oxidation by more than 400 mV, the oxidation of t BuH2Q proceeds several orders of magnitude faster than 4FBnOH oxidation. Kinetic and mechanistic studies reveal that these reactions feature different rate-limiting steps. Alcohol oxidation proceeds via rate-limiting β-hydride elimination from a PdII-alkoxide intermediate, while H2Q oxidation features rate-limiting isomerization from an O-to-C-bound PdII-hydroquinonate species. The enhanced rate of H2Q oxidation reflects the kinetic facility of O─H relative to C─H bond cleavage.

Synthesis of substituted (N,C) and (N,C,C) Au(III) complexes: the influence of sterics and electronics on cyclometalation reactions

Cyclometalated Au(III) complexes are of interest due to their catalytic, medicinal, and photophysical properties. Herein, we describe the synthesis of derivatives of the type (N,C)Au(OAc^F)₂ (OAc^F = trifluoroacetate) and (N,C,C)AuOAc^F by a cyclometalation route, where (N,C) and (N,C,C) are chelating 2-arylpyridine ligands. The scope of the synthesis is explored by substituting the 2-arylpyridine core with electron donor or acceptor substituents at one or both rings. Notably, a variety of functionalized Au(III) complexes can be obtained in one step from the corresponding ligand and Au(OAc)₃, eliminating the need for organomercury intermediates, which is commonly reported for similar syntheses. The influence of substituents in the ligand backbone on the resulting complexes was assessed using DFT calculations, ¹⁵N NMR spectroscopy and single-crystal X-ray diffraction analysis. A correlation between the electronic properties of the (N,C) ligands and their ability to undergo cyclometalation was found from experimental studies combined with natural charge analysis, suggesting the cyclometalation at Au(III) to take place via an electrophilic aromatic substitution-type mechanism. The formation of Au(III) pincer complexes from tridentate (N,C,C) ligands was investigated by synthesis and DFT calculations, in order to assess the feasibility of C(sp³)-H bond activation as a synthetic pathway to (N,C,C) cyclometalated Au(III) complexes. It was found that C(sp³)-H bond activation is feasible for ligands containing different alkyl groups (isopropyl and ethyl), although the C-H activation is less energetically favored compared to a ligand containing tert-butyl groups.

Electrochemically Mediated Oxidation of Sensitive Propargylic Benzylic Alcohols

The electrochemical oxidation of sensitive propargylic benzylic alcohols having varying substituents is reported. We describe the preparation and characterization of N-hydroxytetrafluorophthalimide (TFNHPI) and pseudo-high-throughput development of a green electrochemical oxidation protocol for sensitive propargylic benzylic alcohols that employs TFNHPI as a stable electrochemical mediator. The electrochemical oxidation of propargylic benzylic alcohols was leveraged to develop short synthetic pathways for preparing gram quantities of resveratrol natural products such as pauciflorols.

MOF-stabilized perfluorinated palladium cages catalyze the additive-free aerobic oxidation of aliphatic alcohols to acids

Extremely high electrophilic metal complexes, composed by a metal cation and very electron poor σ-donor ancillary ligands, are expected to be privileged catalysts for oxidation reactions in organic chemistry. However, their low lifetime prevents any use in catalysis. Here we show the synthesis of fluorinated pyridine-Pd 2+ coordinate cages within the channels of an anionic tridimensional metal organic framework (MOF), and their use as efficient metal catalysts for the aerobic oxidation of aliphatic alcohols to carboxylic acids without any additive. Mechanistic studies strongly support that the MOF-stabilized coordination cage with perfluorinated ligands unleashes the full electrophilic potential of Pd 2+ to dehydrogenate primary alcohols, without any base, and also to activate O 2 for the radical oxidation to the aldehyde intermediate. This study opens the door to design catalytic perfluorinated complexes for challenging organic transformations, where an extremely high electrophilic metal site is required.

"How Should I Think about Voltage? What Is Overpotential?": Establishing an Organic Chemistry Intuition for Electrochemistry

Redox reactions are ubiquitous in organic synthesis and intrinsic to organic electrosynthesis. The language and concepts used to describe reactions in these domains are sufficiently different to create barriers that hinder broader adoption and understanding of electrochemical methods. To bridge these gaps, this Synopsis compares chemical and electrochemical redox reactions, including concepts of free energy, voltage, kinetic barriers, and overpotential. This discussion is intended to increase the accessibility of electrochemistry for organic chemists lacking formal training in this area.

Syntheses of 1H-Indoles, Quinolines, and 6-Membered Aromatic N-Heterocycle-Fused Scaffolds via Palladium(II)-Catalyzed Aerobic Dehydrogenation under Alkoxide-Free Conditions

Aromatic N-heterocycle-fused scaffolds such as indoles and quinolines are important core structures found in various bioactive natural products and synthetic compounds. Recently, various dehydrogenation methods with the help of alkoxides, known to significantly promote dihydro- or tetrahydro-heterocycles to be oxidized, were developed for the heterocycle synthesis. However, these approaches are sometimes unsuitable due to resulting undesired side reactions such as reductive dehalogenation. Herein, expedient syntheses of 1H-indoles, quinolines, and 6-membered N-heterocycle-fused scaffolds from their hydrogenated forms through palladium(II)-catalyzed aerobic dehydrogenation under alkoxide-free conditions are reported. A total of 48 compounds were successfully synthesized with a wide range of functional groups including halogens (up to 99% yield). These methodologies provide facile routes for various privileged structures possessing aromatic N-heterocycles without the help of alkoxides, in highly efficient manners.

Recent Progress on Photocatalytic Synthesis of Ester Derivatives and Reaction Mechanisms

Esters and their derivatives are distributed widely in natural products, pharmaceuticals, fine chemicals and other fields. Esters are important building blocks in pharmaceuticals such as clopidogrel, methylphenidate, fenofibrate, travoprost, prasugrel, oseltamivir, eszopiclone and fluticasone. Therefore, esterification reaction becomes more and more popular in the photochemical field. In this review, we highlight three types of reactions to synthesize esters using photochemical strategies. The reaction mechanisms involve mainly single electron transfer, energy transfer or other radical procedures.

Sustainable Pd(OAc)2 /Hydroquinone Cocatalyst System for Cis-Selective Dibenzoyloxylation of 1,3-Cyclohexadiene

The 1,4-diacyloxylation of 1,3-cyclohexadiene (CHD) affords valuable stereochemically defined scaffolds for natural product and pharmaceutical synthesis. Existing cis-selective diacyloxylation protocols require superstoichiometric quantities of benzoquinone (BQ) or MnO₂ , which limit process sustainability and large-scale application. In this report, reaction development and mechanistic studies are described that overcome these limitations by pairing catalytic BQ with tert-butyl hydroperoxide as the stoichiometric oxidant. Catalytic quantities of bromide enable a switch from trans to cis diastereoselectivity. A catalyst with a 1:2 Pd:Br ratio supports high cis selectivity while retaining good rate and product yield. Further studies enable replacement of BQ with hydroquinone (HQ) as a source of cocatalyst, avoiding the handling of volatile and toxic BQ in large-scale applications.

Sodium Butylated Hydroxytoluene: A Functional Group Tolerant, Eco-Friendly Base for Solvent-Free, Pd-Catalysed Amination

NaBHT (sodium 2,6-di-tert-butyl-4-methylphenolate), a strong, but hindered and lipophilic base, has been effectively paired with similarly lipophilic, high-reactivity Pd-NHC (N-heterocyclic carbene) catalysts to produce an ideal combination for performing solvent-free (melt) cross-coupling amination. The mild nucleophilicity of NaBHT, coupled with the anti-oxidant properties of its conjugate acid byproduct, BHT means the process seems to have no functional group incompatibilities. Highly effective coupling of base-sensitive and redox-active functional groups was observed in all cases with only 0.1-0.2 mol percent catalyst. Comparisons using the standard base for this reaction, KOtBu, led to poor couplings or complete degradation in most applications - only NaBHT works.

(DiMeIHeptCl)Pd: A Low-Load Catalyst for Solvent-Free (Melt) Amination

(DiMeIHept^Cl)Pd, a hyper-branched N-aryl Pd NHC catalyst, has been shown to be efficient at performing amine arylation reactions in solvent-free ("melt") conditions. The highly lipophilic environment of the alkyl chains flanking the Pd center serves as lubricant to allow the complex to navigate through the paste-like environment of these mixtures. The protocol can be used on a multi-gram scale to make a variety of aniline derivatives, including substrates containing alcohol moieties.

Practical iridium-catalyzed direct α-arylation of N-heteroarenes with (hetero)arylboronic acids by H2O-mediated H2 evolution

Despite the widespread applications of 2-(hetero)aryl N-heteroarenes in numerous fields of science and technology, universal access to such compounds is hampered due to the lack of a general method for their synthesis. Herein, by a H₂O-mediated H₂-evolution cross-coupling strategy, we report an iridium(III)-catalyzed facile method to direct α-arylation of N-heteroarenes with both aryl and heteroaryl boronic acids, proceeding with broad substrate scope and excellent functional compatibility, oxidant and reductant-free conditions, operational simplicity, easy scalability, and no need for prefunctionalization of N-heteroarenes. This method is applicable for structural modification of biomedical molecules, and offers a practical route for direct access to 2-(hetero)aryl N-heteroarenes, a class of potential cyclometalated C^N ligands and N^N bidentate ligands that are difficult to prepare with the existing α-C-H arylation methods, thus filling an important gap in the capabilities of synthetic organic chemistry.

Benzoquinone Cocatalyst Contributions to DAF/Pd(OAc)2-Catalyzed Aerobic Allylic Acetoxylation in the Absence and Presence of a Co(salophen) Cocatalyst

Palladium(II)-catalyzed allylic acetoxylation has been the focus of extensive development and investigation. Methods that use molecular oxygen (O₂) as the terminal oxidant typically benefit from the use of benzoquinone (BQ) and a transition-metal (TM) cocatalyst, such as Co(salophen), to support oxidation of Pd⁰ during catalytic turnover. We previously showed that Pd(OAc)₂ and 4,5-diazafluoren-9-one (DAF) as an ancillary ligand catalyze allylic oxidation with O₂ in the absence of cocatalysts. Herein, we show that BQ enhances DAF/Pd(OAc)₂ catalytic activity, nearly matching the performance of reactions that include both BQ and Co(salophen). These observations are complemented by mechanistic studies of DAF/Pd(OAc)₂ catalyst systems under three different oxidation conditions: (1) O₂ alone, (2) O₂ with cocatalytic BQ, and (3) O₂ with cocatalytic BQ and Co(salophen). The beneficial effect of BQ in the absence of Co(salophen) is traced to synergistic roles of O₂ and BQ, both of which are capable of oxidizing Pd⁰ to Pd^II The reaction of O₂ generates H₂O₂ as a byproduct, which can oxidize hydroquinone to quinone in the presence of Pd^II NMR spectroscopic studies, however, show that hydroquinone is the predominant redox state of the quinone cocatalyst in the absence of Co(salophen), while inclusion of Co(salophen) maintains oxidized quinone throughout the reaction, resulting in better reaction performance.

The Different Facets of Metal-Catalyzed C-H Functionalization Involving Quinone Compounds

Metal-catalysed C-H functionalization has emerged as a powerful platform for the derivatization of quinones, a class of compounds with wide-ranging applications. This review organises and discusses the evolution of this chemistry from early Fujiwara-Moritani reactions, through to modern directing-group assisted C-H functionalization processes, including C-H functionalization reactions directed by the quinone ring itself. Mechanistic details of these reactions are provided to afford insight into how the unique reactivity of quinoidal compounds has been leveraged in each example.