Development of 3,5-Di-tert-butylphenol as a Model Substrate for Biomimetic Aerobic Copper Catalysis

We develop 3,5-di-tert­butylphenol as a strategic substrate for the evaluation of biomimetic Cu2–O2 complexes intended to mimic the activity of tyrosinase. We describe a practical and scalable synthesis and validate its use in an aerobic ortho-oxygenation catalyzed by N,N′-di-tert-butylethylenediamine and [Cu(CH3CN)4]PF6.

Synthesis of a 1,3-Bridged Macrobicyclic Enyne via Chemoselective Cycloisomerization Using Palladium-Catalyzed Alkyne-Alkyne Coupling

A unique intramolecular Pd-catalyzed alkyne-alkyne coupling is presented. This transformation generates a strained, 1,3-bridged, macrocyclic enyne. The process was readily executed on gram scale, and the structure of the product was elucidated via X-ray crystallographic analysis. A mechanistic rationale for the observed chemoselectivity is provided.

Asymmetric synthesis of chiral β-alkynyl carbonyl and sulfonyl derivatives via sequential palladium and copper catalysis

We present a full account detailing the development of a sequential catalysis strategy for the synthesis of chiral β-alkynyl carbonyl and sulfonyl derivatives. A palladium-catalyzed cross coupling of terminal alkyne donors with acetylenic ester, ketone, and sulfone acceptors generates stereodefined enynes in high yield. These compounds are engaged in an unprecedented, regio- and enantioselective copper-catalyzed conjugate reduction. The process exhibits a high functional group tolerance, and this enables the synthesis of a broad range of chiral products from simple, readily available alkyne precursors. The utility of the method is demonstrated through the elaboration of the chiral β-alkynyl products into a variety of different molecular scaffolds. Its value in complex molecule synthesis is further validated through a concise, enantioselective synthesis of AMG 837, a potent GPR40 receptor agonist.

A Catalyst-Controlled Aerobic Coupling of ortho-Quinones and Phenols Applied to the Synthesis of Aryl Ethers

ortho-Quinones are underutilized six-carbon-atom building blocks. We herein describe an approach for controlling their reactivity with copper that gives rise to a catalytic aerobic cross-coupling with phenols. The resulting aryl ethers are generated in high yield across a broad substrate scope under mild conditions. This method represents a unique example where the covalent modification of an ortho-quinone is catalyzed by a transition metal, creating new opportunities for their utilization in synthesis.

Redox-promoted associative assembly of metal-organic materials

We develop an associative synthesis of metal-organic materials that combines solid-state metal oxidation and coordination-driven self-assembly into a one-step, waste-free transformation. The methodology hinges on the unique reactivity of ortho-quinones, which we introduce as versatile oxidants for mechanochemical synthesis. Our strategy opens a previously unexplored route to paramagnetic metal-organic materials from elementary metals.

A bio-inspired synthesis of oxindoles by catalytic aerobic dual C-H functionalization of phenols

We report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C–N bond formation to the reduction of oxygen.

Nitrogen-containing heterocycles are fundamentally important to the function of pharmaceuticals, agrochemicals and materials. Herein, we report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C–N bond formation to the reduction of molecular oxygen (O₂). Our method is inspired by the biosynthesis of melanin pigments (melanogenesis), but diverges from the biosynthetic polymerization. Mechanistic analysis reveals the involvement of Cu^II-semiquinone radical intermediates, which enable dehydrogenative carbon–heteroatom bond formation that avoids a catechol/quinone redox couple. This mitagates the deleterious polarity reversal that results from phenolic dearomatization, and enables a high-yielding phenolic C–H functionalization under catalytic aerobic conditions. Our work highlights the broad synthetic utility and efficiency of forming C–N bonds via a catalytic aerobic dearomatization of phenols, which is currently an underdeveloped transformation.

A Biomimetic Mechanism for the Copper-Catalyzed Aerobic Oxygenation of 4-tert-Butylphenol

Controlling product selectivity during the catalytic aerobic oxidation of phenols remains a significant challenge that hinders reaction development. This work provides a mechanistic picture of a Cu-catalyzed, aerobic functionalization of phenols that is selective for phenoxy-coupled ortho-quinones. We show that the immediate product of the reaction is a Cu(II)-semiquinone radical complex and reveal that ortho-oxygenation precedes oxidative coupling. This complex is the resting state of the Cu catalyst during turnover at room temperature. A mechanistic study of the formation of this complex at low temperatures demonstrates that the oxygenation pathway mimics the dinuclear Cu enzyme tyrosinase by involving a dinuclear side-on peroxodicopper(II) oxidant. Unlike the enzyme, however, the rate-limiting step of the ortho-oxygenation reaction is the self-assembly of the oxidant from Cu(I) and O2. We provide details for all steps in the cycle and demonstrate that turnover is contingent upon proton-transfer events that are mediated by a slight excess of ligand. Finally, our knowledge of the reaction mechanism can be leveraged to diversify the reaction outcome. Thus, uncoupled ortho-quinones are favored in polar, coordinating media, highlighting unusually high levels of chemoselectivity for a catalytic aerobic oxidation of a phenol.

A bio-inspired total synthesis of tetrahydrofuran lignans

Lignan natural products comprise a broad spectrum of biologically active secondary metabolites. Their structural diversity belies a common biosynthesis, which involves regio- and chemoselective oxidative coupling of propenyl phenols. Attempts to replicate this oxidative coupling have revealed significant challenges for controlling selectivity, and these challenges have thus far prevented the development of a unified biomimetic route to compounds of the lignan family. A practical solution is presented that hinges on oxidative ring opening of a diarylcyclobutane to intercept a putative biosynthetic intermediate. The effectiveness of this approach is demonstrated by the first total synthesis of tanegool in 4 steps starting from ferulic acid, as well as a concise synthesis of the prototypical furanolignan pinoresinol.

A biomimetic catalytic aerobic functionalization of phenols

The importance of aromatic C-O, C-N, and C-S bonds necessitates increasingly efficient strategies for their formation. Herein, we report a biomimetic approach that converts phenolic C-H bonds into C-O, C-N, and C-S bonds at the sole expense of reducing dioxygen (O2) to water (H2O). Our method hinges on a regio- and chemoselective copper-catalyzed aerobic oxygenation to provide ortho-quinones. ortho-Quinones are versatile intermediates, whose direct catalytic aerobic synthesis from phenols enables a mild and efficient means of synthesizing polyfunctional aromatic rings.

Asymmetric synthesis of chiral cycloalkenone derivatives via palladium catalysis

The palladium-catalyzed oxidative desymmetrization of meso dibenzoates yields γ-benzoyloxy cycloalkenones in good yields and with excellent levels of enantioselectivity. These compounds serve as precursors to a broad range of substituted cycloalkenones, including well-established synthetic building blocks and elaborated cycloalkanone derivatives. The ability to prepare both enantiomers of the oxidative desymmetrization products enables a unified strategy toward stereochemically diverse epoxyquinoid natural products.

A new strategy for the synthesis of chiral β-alkynyl esters via sequential palladium and copper catalysis

A new strategy for the synthesis of chiral β-alkynyl esters which relies on sequential Pd and Cu catalysis is reported. Terminal alkynes bearing aryl, alkyl, and silyl groups can be employed without prior activation yielding a wide range of important chiral building blocks. The reaction sequence utilizes a robust Pd(II)-catalyzed hydroalkynylation of ynoates with terminal alkynes providing geometrically pure ynenoates which are readily reduced by CuH. In contrast to previous reports, where additions to ynenoates proceed with marginal preference for the 1,6-pathway, this conjugate reduction occurs with high 1,4-selectivity yielding β-alkynyl esters with excellent levels of enantioselectivity. Importantly, the method tolerates a wide range of functionality, including allylic carbonates and carbamates, and thus allows for rapid elaboration of the β-alkynyl esters into a variety of chiral, substituted heterocycles.

An atom-economic synthesis of nitrogen heterocycles from alkynes

A robust route to 2,4-disubstituted pyrrole heterocycles relying upon a cascade reaction is reported. The reaction benefits from operational simplicity: it is air and moisture tolerant and is performed at ambient temperature. Control over the reaction conditions provides ready access to isopyrroles, 2,3,4-trisubstituted pyrroles, and 3-substituted pyrollidin-2-ones.