Direct Dehydrogenation for the Synthesis of α,β‐Unsaturated Carbonyl Compounds

Tandem dehydrogenation-olefination-decarboxylation of cycloalkyl carboxylic acids via multifold C-H activation

Dehydrogenation chemistry has long been established as a fundamental aspect of organic synthesis, commonly encountered in carbonyl compounds. Transition metal catalysis revolutionized it, with strategies like transfer-dehydrogenation, single electron transfer and C-H activation. These approaches, extended to multiple dehydrogenations, can lead to aromatization. Dehydrogenative transformations of aliphatic carboxylic acids pose challenges, yet engineered ligands and metal catalysis can initiate dehydrogenation via C-H activation, though outcomes vary based on substrate structures. Herein, we have developed a catalytic system enabling cyclohexane carboxylic acids to undergo multifold C-H activation to furnish olefinated arenes, bypassing lactone formation. This showcases unique reactivity in aliphatic carboxylic acids, involving tandem dehydrogenation-olefination-decarboxylation-aromatization sequences, validated by control experiments and key intermediate isolation. For cyclopentane carboxylic acids, reluctant to aromatization, the catalytic system facilitates controlled dehydrogenation, providing difunctionalized cyclopentenes through tandem dehydrogenation-olefination-decarboxylation-allylic acyloxylation sequences. This transformation expands carboxylic acids into diverse molecular entities with wide applications, underscoring its importance.

Cu2O-catalyzed cascade phosphinylation/cyclization of 2'-aminochalcones for the synthesis of hemi-indigo derivatives

A Cu2O-catalyzed cascade phosphinylation/cyclization reaction of 2'-aminochalcones and diphenylphosphine oxides to produce hemi-indigo derivatives has been developed. This strategy facilitates the sequential formation of a C-P bonds and a C-N bond in a single reaction step. Notably, the approach features one-pot operation, an earth-abundant copper catalyst, readily available starting materials, a broad substrate scope and high compatibility with functional groups, providing 33 compounds in acceptable yields.

A facile synthesis of α,β-unsaturated imines via palladium-catalyzed dehydrogenation

The dehydrogenation adjacent to an electron-withdrawing group provides an efficient access to α,β-unsaturated compounds that serving as versatile synthons in organic chemistry. However, the α,β-desaturation of aliphatic imines has hitherto proven to be challenging due to easy hydrolysis and preferential dimerization. Herein, by employing a pre-fluorination and palladium-catalyzed dehydrogenation reaction sequence, the abundant simple aliphatic amides are amendable to the rapid construction of complex molecular architectures to produce α,β-unsaturated imines. Mechanistic investigations reveal a Pd(0)/Pd(II) catalytic cycle involving oxidative H-F elimination of N-fluoroamide followed by a smooth α,β-desaturation of the in-situ generated aliphatic imine intermediate. This protocol exhibits excellent functional group tolerance, and even the carbonyl groups are compatible without any competing dehydrogenation, allowing for late-stage functionalization of complex bioactive molecules. The synthetic utility of this transformation has been further demonstrated by a diversity-oriented derivatization and a concise formal synthesis of (±)-alloyohimbane.

Potent Immunomodulators Developed from an Unstable Bacterial Metabolite of Vitamin B2 Biosynthesis

Bacterial synthesis of vitamin B2 generates a by-product, 5-(2-oxopropylideneamino)-d-ribityl-aminouracil (5-OP-RU), with potent immunological properties in mammals, but it is rapidly degraded in water. This natural product covalently bonds to the key immunological protein MR1 in the endoplasmic reticulum of antigen presenting cells (APCs), enabling MR1 refolding and trafficking to the cell surface, where it interacts with T cell receptors (TCRs) on mucosal associated invariant T lymphocytes (MAIT cells), activating their immunological and antimicrobial properties. Here, we strategically modify this natural product to understand the molecular basis of its recognition by MR1. This culminated in the discovery of new water-stable compounds with extremely powerful and distinctive immunological functions. We report their capacity to bind MR1 inside APCs, triggering its expression on the cell surface (EC50 17 nM), and their potent activation (EC50 56 pM) or inhibition (IC50 80 nM) of interacting MAIT cells. We further derivatize compounds with diazirine-alkyne, biotin, or fluorophore (Cy5 or AF647) labels for detecting, monitoring, and studying cellular MR1. Computer modeling casts new light on the molecular mechanism of activation, revealing that potent activators are first captured in a tyrosine- and serine-lined cleft in MR1 via specific pi-interactions and H-bonds, before more tightly attaching via a covalent bond to Lys43 in MR1. This chemical study advances our molecular understanding of how bacterial metabolites are captured by MR1, influence cell surface expression of MR1, interact with T cells to induce immunity, and offers novel clues for developing new vaccine adjuvants, immunotherapeutics, and anticancer drugs.

Comprehensive Mechanistic Analysis of Palladium- and Nickel-Catalyzed α,β-Dehydrogenation of Carbonyls via Organozinc Intermediates

Introducing degrees of unsaturation into small molecules is a central transformation in organic synthesis. A strategically useful category of this reaction type is the conversion of alkanes into alkenes for substrates with an adjacent electron-withdrawing group. An efficient strategy for this conversion has been deprotonation to form a stabilized organozinc intermediate that can be subjected to α,β-dehydrogenation through palladium or nickel catalysis. This general reactivity blueprint presents a window to uncover and understand the reactivity of Pd- and Ni-enolates. Within this context, it was determined that β-hydride elimination is slow and proceeds via concerted syn-elimination. One interesting finding is that β-hydride elimination can be preferred to a greater extent than C-C bond formation for Ni, more so than with Pd, which defies the generally assumed trends that β-hydride elimination is more facile with Pd than Ni. The discussion of these findings is informed by KIE experiments, DFT calculations, stoichiometric reactions, and rate studies. Additionally, this report details an in-depth analysis of a methodological manifold for practical dehydrogenation and should enable its application to challenges in organic synthesis.

Photo-Catalyzed Acyl Azolium Promoted Selective α-C(sp3 )-H Acylation of Acetone via HAT: Access to Thermodynamically Less Favoured (Z)-α,β-Unsaturated Ketones

Mono α-acylation of acetone has been achieved for the first time by reacting with bench-stable acyl azolium salts under violet-LED light at room temperature. The intermolecular hydrogen atom transfer (HAT) from acetone to triplet state of azolium salts under violet LED irradiation resulted in thermodynamically less favourable (Z)-α,β-unsaturated ketones with up to 99 : 1 selectivity via C-C bond formation. This compelling protocol access the desired α-C(sp3 )-H acylation product under metal-, ligand- and oxidant-free conditions on a wide range of substrates.

Oxidative Dehydrogenation of Alkanes through Homogeneous Base Metal Catalysis

Preparing valuable olefins from cheap and abundant alkane resources has long been a challenging task in organic synthesis, which mainly suffers from harsh reaction conditions and narrow scopes. Homogeneous transition metals catalyzed dehydrogenation of alkanes has attracted much attention for its excellent catalytic activities under relatively milder conditions. Among them, base metal catalyzed oxidative alkane dehydrogenation has emerged as a viable strategy for olefin synthesis for its usage of cheap catalysts, compatibility with various functional groups, and low reaction temperature. In this review, we discuss recent development of base metal catalyzed alkane dehydrogenation under oxidative conditions and their application in constructing complex molecules.

Copper-Catalyzed Intramolecular Olefinic C(sp2)-H Amidation for the Synthesis of γ-Alkylidene-γ-lactams

Herein, we report the copper-catalyzed dehydrogenative C(sp2)-N bond formation of 4-pentenamides via nitrogen-centered radicals. This reaction provides a straightforward and efficient preparation method for γ-alkylidene-γ-lactams. Notably, we could controllably synthesize α,β-unsaturated- or α,β-saturated-γ-alkylidene-γ-lactams depending on the reaction conditions.

Direct synthesis of alkylated 4-hydroxycoumarin derivatives via a cascade Cu-catalyzed dehydrogenation/conjugate addition sequence

An efficient approach for the direct synthesis of alkylated 4-hydroxycoumarin derivatives via a Cu-catalyzed cascade dehydrogenation/conjugate addition sequence starting from simple saturated ketones and 4-hydroxycoumarins has been developed. This protocol features excellent functional-group tolerance, easy scale-up, and a broad substrate scope including bioactive molecules. More importantly, a series of marketed drugs, such as warfarin, acenocoumarol, coumachlor, and coumafuryl, can be obtained by this method.

Photoinduced Desaturation of Amides by Palladium Catalysis

A photoinduced palladium-catalyzed desaturation method that is suitable for converting the linear amides to their α,β-unsaturated counterparts is reported. The reaction does not require strong base/acid or sulfur/selenium and oxidant reagents and can be carried out at room temperature through a simple one-step operation. The protocol exhibits great scalability and functional group tolerance. The reaction mechanism has been investigated through deuterium labeling experiments, radical clock, radical capture, and kinetic studies. Mechanistic studies suggested a radical pathway involving aryl/alkyl Pd-radical intermediates.

Synthesis of conjugated dienes via palladium-catalysed aerobic dehydrogenation of unsaturated acids and amides

A Pd(II)-catalyzed direct aerobic dehydrogenation of γ,δ-olefinic acids and amides has been demonstrated. The present protocol dehydrogenates the least acidic amides and acids, thus replacing the traditional enolate strategy for dehydrogenation. A broad spectrum of conjugated dienamides and dienoic acids were produced in good to excellent yields. A possible reaction mechanism was proposed and supported by deuterium labelling studies.

Fe-Catalyzed Three-Component Coupling Reaction of α,β,γ,δ-Unsaturated Carbonyl Compounds and Conjugate Dienes with Alkylsilyl Peroxides and Nucleophiles

An Fe(OTf)₂-catalyzed three-component coupling reaction of α,β,γ,δ-unsaturated carbonyl compounds with alkylsilyl peroxides in the presence of certain heteronucleophiles (ROH and indole) is realized under mild reaction conditions. A variety of α,β,γ,δ-diene carbonyl substrates with different substituents were successfully employable via combination with several different alkylsilyl peroxides. This new approach is also applicable to the double functionalization of diene substrates.

Isoindolinone synthesis through Rh/Cu-catalyzed oxidative C-H/N-H annulation of N-methoxy benzamides with saturated ketones

The synthesis of isoindolinones from N-methoxy benzamides and saturated ketones via a bimetallic tandem catalytic annulation has been accomplished. The reaction is catalyzed by a Rh/Cu-cocatalytic system and proceeds via the combination of Cu-catalyzed dehydrogenation of ketones and Rh-catalyzed direct C-H functionalization with the assistance of the N-methoxy amide group which also acts as an oxidant to regenerate the Rh catalyst. This method shows good compatibility with a wide range of substrates and functional groups, and provides an alternative strategy to obtain diverse isoindolinones.

Direct Access to α,β-Unsaturated Ketones via Rh/MgCl2-Mediated Acylation of Vinylsilanes

We report herein the facile and practical construction of α,β-unsaturated ketones via rhodium-catalyzed direct acylation of vinylsilanes with readily available and abundant carboxylic acids. This protocol features access to a diverse array of synthetically useful functionalities with moderate to excellent yields. More importantly, the late-stage functionalization of pharmaceuticals was also realized with synthetically useful yield.

Saegusa Oxidation of Enol Ethers at Extremely Low Pd-Catalyst Loadings under Ligand-free and Aqueous Conditions: Insight into the Pd(II)/Cu(II)-Catalyst System

A highly efficient and practical Pd(II)/Cu(OAc)₂-catalyst system of Saegusa oxidation, which converts enol ethers to the corresponding enals with a number of diverse substrates at extremely low catalyst loadings (500 mol ppm) under ligand-free and aqueous conditions, is described. Its synthetic utility was demonstrated by large-scale applications of the catalyst system to important nature molecules. This work allows Saegusa oxidation to become a highly practical approach to preparing enals and also suggests new insight into the Pd(II)/Cu(II)-catalyst system for dehydrogenation of carbonyl compounds and decreasing Pd-catalyst loadings.

Dehydrogenative Pd and Ni Catalysis for Total Synthesis

The development of novel synthetic methods remains a cornerstone in simplifying complex molecule synthesis. Progress in the field of transition metal catalysis has enabled new mechanistic strategies to achieve difficult chemical transformations, increased the value of abundant chemical building blocks, and pushed the boundaries of creative and strategic route design to improve step economy in multistep synthesis. Methodologies to introduce an olefin into saturated molecules continue to be essential transformations because of the plethora of reactions available for alkene functionalization. Of particular importance are dehydrogenation reactions adjacent to electron-withdrawing groups such as carbonyls, which advantageously provide activated olefins that can be regioselectively manipulated. Palladium catalysis occupies a central role in the most widely adopted carbonyl dehydrogenation reactions, but limits to the scope of these protocols persist.In this Account, we describe our group's contributions to the area of transition-metal-catalyzed dehydrogenation using palladium catalysis and more sustainable and economical nickel catalysis. These metals are used in conjunction with allyl and aryl halides or pseudohalides that serve as oxidants to access a unique mechanistic approach for one-step α,β-dehydrogenation of various electron-withdrawing groups, including ketones, esters, nitriles, amides, carboxylic acids, and electron-deficient heteroarenes. The pivotal reaction parameters that can be modified to influence reaction efficiency are highlighted, including base and oxidant structure as well as ligand and salt additive effects. This discussion is expected to serve as a guide for troubleshooting challenging dehydrogenation reactions and provide insight for future reaction development in this area.In addition to enabling dehydrogenation reactions, our group's allyl-Pd and -Ni chemistry can be used for C-C and C-X bond-forming reactions, providing novel disconnections with practical applications for expediting multistep synthesis. These transformations include a telescoped process for ketone α,β-vicinal difunctionalization; an oxidative enone β-functionalization, including β-stannylation, β-silylation, and β-alkylation; and an oxidative cycloalkenylation between unstabilized ketone enolates and unactivated alkenes. These bond-forming methodologies broaden the range of transformations accessible from abundant ketone, enone, and alkene moieties. Both the dehydrogenation and C-C and C-X bond-forming methodologies have been implemented in our group's total synthesis campaigns to provide step-efficient synthetic routes toward diverse natural products.Through the lens of multistep synthesis, the utility and robustness of our dehydrogenation and dehydrogenative functionalization methodologies can be better appreciated, and we hope that this Account will inspire practitioners to apply our methodologies to their own synthetic challenges.

Tandem Oxidation-Dehydrogenation of (Hetero)Arylated Primary Alcohols via Perruthenate Catalysis

Tandem oxidative-dehydrogenation of primary alcohols to give α,β-unsaturated aldehydes in one pot are rare transformations in organic synthesis, with only two methods currently available. Reported herein is a novel method using the bench-stable salt methyltriphenylphosphonium perruthenate (MTP3), and a new co-oxidant NEMO·PF6 (NEMO = N-ethyl-N-hydroxymorpholinium) which provides unsaturated aldehydes in low to moderate yields. The Ley-Griffith oxidation of (hetero)arylated primary alcohols with N-oxide co-oxidants NMO (NMO = N-methylmorpholine N-oxide)/NEMO, is expanded by addition of the N-oxide salt NEMO·PF6 to convert the intermediate saturated aldehyde into its unsaturated counterpart. The discovery, method development, reaction scope, and associated challenges of this method are highlighted. The conceptual value of late-stage dehydrogenation in natural product synthesis is demonstrated via the synthesis of a polyene scaffold related to auxarconjugatin B.

Quadruple C-H activation coupled to hydrofunctionalization and C-H silylation/borylation enabled by weakly coordinated palladium catalyst

Unlike the well-reported 1,2-difunctionalization of alkenes that is directed by classic pyridine and imine-containing directing groups, oxo-palladacycle intermediates featuring weak Pd-O coordination have been less demonstrated in C-H activated cascade transformations. Here we report a quadruple C-H activation cascade as well as hydro-functionalization, C-H silylation/borylation sequence based on weakly coordinated palladium catalyst. The hydroxyl group modulates the intrinsic direction of the Heck reaction, and then acts as an interrupter that biases the reaction away from the classic β-H elimination and toward C-H functionalization. Mechanistically, density functional theory calculation provides important insights into the key six-membered oxo-palladacycle intermediates, and indicates that the β-H elimination is unfavorable both thermodynamically and kinetically. In this article, we focus on the versatility of this approach, which is a strategic expansion of the Heck reaction.

Combining the Heck reaction with other transformations provides a powerful strategy to access diverse, complex compounds. Here, the authors report a weak coordination dominated Pd(0)-catalyzed quadruple C-H activation followed by hydro-functionalization, C-H silylation, and C-H borylation.