Catalytic, contra-Thermodynamic Positional Alkene Isomerization

Total Synthesis of (-)-Illisimonin A Enabled by Pattern Recognition and Olefin Transposition

We report an asymmetric total synthesis of (-)-illisimonin A, a sesquiterpene natural product with neurotrophic activity. Illisimonin A possesses a unique cage-like 5/5/5/5/5 pentacyclic scaffold containing a trans-pentalene and a norbornane, two highly strained and challenging structural motifs. It also contains seven contiguous fully substituted stereocenters, including three all-carbon quaternary centers, two of which are adjacent. Our synthesis exploits a pattern recognition strategy to identify a 5,6-fused bicyclic intermediate derived from (S)-carvone in two steps as the starting point and leverages five sequential olefin transposition reactions to decorate the bicyclic carbocycle. Other key steps include a tandem Mukaiyama hydration-translactonization to form the γ-butyrolactone and an intramolecular aldol cyclization to close the cage and finally deliver (-)-illisimonin A in 16 steps.

Co(II)-catalyzed isomerization of enals using hydrogen atom transfer

To develop a process for the synthesis of an enal acetate, the so called C5-acetate that is important in the industrial preparation of vitamin A, we show here that Co(II)-based hydrogen atom transfer catalysts under H2 promote such isomerizations in high yield with low catalyst loading (0.1 mol%). D-labelling studies suggest the enal isomerization process and catalyst activation by H2 to be reversible.

Deconjugative Photoisomerization of Cyclic Enones

The deconjugative isomerization of α,β-unsaturated carbonyl compounds enables regioisomeric products to be forged with simultaneous Umpolung of alkene reactivity. Although highly enabling, the endergonic nature of the net process coupled with governing regioselectivity outcomes, renders it challenging. Innovations in the positional isomerization of linear species, often by light-triggered activation, have re-energized this area. However, the deconjugative isomerization of cyclic enones is underdeveloped and often associated with impractical reaction conditions, limited substrate scopes, and a lack of mechanistic clarity. Herein, we report an operationally simple photochemical isomerization of cyclic enones using near-UV (372 nm) irradiation with catalytic amounts of Brønsted acid (HCl). This platform enables exocyclic deconjugative isomerization of a diverse array of enones including α-isophorone (a key intermediate in a variety of industrial processes), terpenoids and steroids. Mechanistic studies reveal the pivotal role of the solvent as a key mediator in the isomerization, where sequential hydrogen atom transfer (HAT) and reverse-HAT (RHAT) are proposed to be operational.

Enantioselective Cobalt-Catalyzed Remote Hydroboration of Alkenylboronates

Heteroatomic groups in alkenes typically direct thermodynamically favored chain walking of C═C bonds toward themselves, thereby facilitating C-H bond functionalization near the heteroatoms. We present herein an efficient cobalt-catalyzed contra-thermodynamic remote hydroboration of alkenylboronates with pinacolborane to synthesize chiral 1,n-diboronates. This protocol features a broad substrate scope, high functional group tolerance, and excellent enantioselectivity. Mechanistic studies indicate the involvement of a chain-walking process. Gram-scale reactions and various product derivatizations further highlight its practicality.

Two-Step Constitutional Isomerization of Saturated Cyclic Amines Using Borane Catalysis

The prevalence of saturated azacycles within pharmaceuticals, natural products, and agrochemicals has prompted the development of many methods that modify their periphery. In contrast, technologies that interconvert distinct saturated azacyclic frameworks, which would uniquely facilitate access to underexplored chemical space, are highly limited. Existing approaches for modifying the core of azacycles usually require either the installation of reactive functionality, which must later be removed in subsequent steps, or the use of tailored substrates, limiting applicability to drug discovery. Herein, we report a borane-catalyzed contraction of saturated N-hydroxy azacycles. This transformation is uniquely enabling, allowing reorganization of the connectivity of the substrate without altering the molecular formula and generating products without vestigial functionality derived from auxiliary groups. The outcome of the reductive Stieglitz-type contraction can be attributed to a key stereoelectronic interaction enforced by geometric constraints, the mechanism of which we investigate using density functional theory. The method developed here enables the rapid late-stage reorganization of bioactive molecules featuring cyclic and linear amines. Overall, a general platform for saturated amine constitutional isomerization has been achieved.

Application of Hydroxyaromatic Aldehydes in Ultra-Efficient and Metal-Free Photocatalytic E→Z Isomerization of Olefin

The strategy of photocatalyzed E→Z isomerization of olefins to access thermodynamically less stable Z-alkenes has recently received considerable attention. Here, we have discovered a sensitizer of hydroxyaromatic aldehyde that can rapidly achieve olefin E→Z isomerization under blue light irradiation. Notably, 2-hydroxybenzene-1,3,5-tricarbaldehyde, when assisted by blue light, can achieve efficient and selective conversion within just 5 minutes (Z/E=92 : 8). The reaction can be successfully scaled up to gram scale, and exhibits remarkable reactivity toward various derivatives of ethyl cinnamate (27 examples) and other olefins. Furthermore, the former can be directly cyclized by a hydroxyl derivative to produce 4-substituted coumarin. The prominent preponderance of this method includes being metal-free, efficient, convenient, no by-products and achieving high selectivity. Correlation of sensitizer triplet energy (ET) and preliminary mechanistic experiments indicate that the accomplishment of this reaction is based on the selective excitation mechanism.

Photoinduced Pd-Catalyzed Direct Sulfonylation of Allylic C-H Bonds

Allylic sulfones are valuable motifs due to their medicinal and biological significance and their versatile chemical reactivities. While direct allylic C-H sulfonylation represents a straightforward and desirable approach, these methods are primarily restricted to terminal alkenes, leaving the engagement of the internal counterparts a formidable challenge. Herein we report a photocatalytic approach that accommodates both cyclic and acyclic internal alkenes with diverse substitution patterns and electronic properties. Importantly, the obtained allylic sulfones can be readily diversified into a wide range of products, thus enabling formal alkene transposition and all-carbon quaternary center formation through the sequential C-H functionalization.

Cobalt-Hydride-Catalyzed Alkene-Carboxylate Transposition (ACT) of Allyl Carboxylates

The alkene-carboxylate transposition (ACT) of allyl carboxylates is one of the most atom-economic and synthetically reliable transformations in organic chemistry, as allyl carboxylates are versatile synthetic intermediates. Classic ACT transformations, including [3,3]-sigmatropic rearrangement and transition metal-catalyzed allylic rearrangement, typically yield 1,2-alkene/1,3-acyloxy shifted products through a two-electron process. However, position-altered ACT to produce distinct 1,3-alkene/1,2-acyloxy shifted products remains elusive. Here, we report the first cobalt-hydride-catalyzed ACT of allyl carboxylates, enabling access to these unprecedented 1,3-alkene/1,2-acyloxy shifted products via a 1,2-radical migration (RaM) strategy. This transformation demonstrates broad functional group tolerance, is suitable for late-stage modification of complex molecules, and is amenable to gram-scale synthesis. It also expands the reaction profiles of both allyl carboxylates and cobalt catalysis. Preliminary experimental and computational studies suggest a mechanism involving metal-hydride hydrogen atom transfer (MHAT) and the 1,2-RaM process. This reaction is expected to serve as the basis for the development of versatile Co-H-catalyzed transformations of allyl carboxylates, generating a wide array of valuable building blocks for synthetic, medicinal, and materials chemistry.

Remote Catalytic C(sp3)-H Alkylation via Relayed Carbenoid Transfer upon Olefin Chain Walking

Transition metal carbenes have emerged as versatile intermediates for various types of alkylations. While reactions of metal carbene species with alkenes have been extensively studied, most examples focus on cyclopropanation and allylic C-H insertion. Herein, we present the first example of a catalytic strategy for the carbene-involved regioselective remote C-H alkylation of internal olefins by synergistically combining two iridium-mediated reactivities of olefin chain walking and carbenoid migratory insertion. The present method, utilizing sulfoxonium ylides as a bench-stable robust carbene precursor, was found to be effective for a series of olefins tethered with alkyl chains, heteroatom substituents, and complex biorelevant moieties. Combined experimental and computational studies revealed that reversible iridium hydride-mediated olefin chain walking proceeds to lead to a terminal alkyl-Ir intermediate, which then forms a carbenoid species for the final migratory insertion, resulting in regioselective terminal-alkylated products.

Dual-Catalytic Structural Isomerisation as a Route to α-Arylated Ketones

Isomerisation reactions provide streamlined routes to organic compounds which are otherwise hard to directly synthesise. The most common forms are positional, geometrical or stereochemical isomerisations which involve the relocation of a double bond or a change in relative location of groups in space. In contrast, far fewer examples of structural (or constitutional) isomerisation exist where the connectivity between atoms is altered. The development of platforms capable of such rearrangement poses a unique set of challenges because chemical bonds must be selectively cleaved, and new ones formed without overall addition or removal of atoms. Here, we show that a dual catalytic system can enable the structural isomerisation of readily available allylic alcohols into more challenging-to-synthesise α-arylated ketones via a H-atom transfer initiated semi-pinacol rearrangement. Key to our strategy is the combination of a cobalt catalyst and photocatalyst under reductive, protic conditions which allows intermediates to propagate catalytic turnover. By providing an unusual disconnection to structural motifs which are difficult to access through direct arylation, we anticipate inspiring other advanced catalytic isomerisation strategies that will further retrosynthetic logic for complex molecule synthesis.

(Z)-Selective Isomerization of 1,1-Disubstituted Alkenes by Scandium-Catalyzed Allylic C-H Activation

The isomerization of 1,1-disubstituted alkenes through 1,3-hydrogen shift is an atom-efficient route for synthesizing trisubstituted alkenes, which are important moieties in many natural products, pharmaceuticals, and organic materials. However, this reaction often encounters regio- and stereoselectivity challenges, typically yielding E/Z-mixtures of the alkene products or thermodynamically favored (E)-alkenes. Herein, we report the (Z)-selective isomerization of 1,1-disubstituted alkenes to trisubstituted (Z)-alkenes via the regio- and stereospecific activation of an allylic C-H bond. The key to the success of this unprecedented transformation is the use of a sterically demanding half-sandwich scandium catalyst in combination with a bulky quinoline compound, 2-tert-butylquinoline. Deuterium-labeling experiments and density functional theory (DFT) calculations have revealed that 2-tert-butylquinoline not only facilitates the C═C bond transposition through hydrogen shuttling but also governs the regio- and stereoselectivity due to the steric hindrance of the tert-butyl group. This protocol enables the synthesis of diverse (Z)-configured acyclic trisubstituted alkenes and endocyclic trisubstituted alkenes from readily accessible 1,1-disubstituted alkenes. It offers an efficient and selective route for preparing a new family of synthetically challenging (Z)-trisubstituted alkenes with broad substrate scope, 100% atom efficiency, high regio- and stereoselectivity, and an unprecedented reaction mechanism.

Radical-triggered translocation of C-C double bond and functional group

Multi-site functionalization of molecules provides a potent approach to accessing intricate compounds. However, simultaneous functionalization of the reactive site and the inert remote C(sp3)-H poses a formidable challenge, as chemical reactions conventionally occur at the most active site. In addition, achieving precise control over site selectivity for remote C(sp3)-H activation presents an additional hurdle. Here we report an alternative modular method for alkene difunctionalization, encompassing radical-triggered translocation of functional groups and remote C(sp3)-H desaturation via photo/cobalt dual catalysis. By systematically combining radical addition, functional group migration and cobalt-promoted hydrogen atom transfer, we successfully effectuate the translocation of the carbon-carbon double bond and another functional group with precise site selectivity and remarkable E/Z selectivity. This redox-neutral approach shows good compatibility with diverse fluoroalkyl and sulfonyl radical precursors, enabling the migration of benzoyloxy, acetoxy, formyl, cyano and heteroaryl groups. This protocol offers a resolution for the simultaneous transformation of manifold sites.

Synthesis of non-canonical amino acids through dehydrogenative tailoring

Amino acids are essential building blocks in biology and chemistry. Whereas nature relies on a small number of amino acid structures, chemists desire access to a vast range of structurally diverse analogues1-3. The selective modification of amino acid side-chain residues represents an efficient strategy to access non-canonical derivatives of value in chemistry and biology. While semisynthetic methods leveraging the functional groups found in polar and aromatic amino acids have been extensively explored, highly selective and general approaches to transform unactivated C-H bonds in aliphatic amino acids remain less developed4,5. Here we disclose a stepwise dehydrogenative method to convert aliphatic amino acids into structurally diverse analogues. The key to the success of this approach lies in the development of a selective catalytic acceptorless dehydrogenation method driven by photochemical irradiation, which provides access to terminal alkene intermediates for downstream functionalization. Overall, this strategy enables the rapid synthesis of new amino acid building blocks and suggests possibilities for the late-stage modification of more complex oligopeptides.

The sugar cube: Network control and emergence in stereoediting reactions

Stereochemical editing strategies have recently enabled the transformation of readily accessible substrates into rare and valuable products. Typically, site selectivity is achieved by minimizing kinetic complexity by using protecting groups to suppress reactivity at undesired sites (substrate control) or by using catalysts with tailored shapes to drive reactivity at the desired site (catalyst control). We propose "network control," a contrasting paradigm that exploits hidden interactions between rate constants to greatly amplify modest intrinsic biases and enable precise multisite editing. When network control is applied to the photochemical isomerization of hexoses, six of the eight possible diastereomers can be selectively obtained. The amplification effect can be viewed as a mesoscale phenomenon between the limiting regimes of kinetic control in simple chemical systems and metabolic regulation in complex biological systems.

Organic Synthesis Away from Equilibrium: Contrathermodynamic Transformations Enabled by Excited-State Electron Transfer

ConspectusChemists have long been inspired by biological photosynthesis, wherein a series of excited-state electron transfer (ET) events facilitate the conversion of low energy starting materials such as H2O and CO2 into higher energy products in the form of carbohydrates and O2. While this model for utilizing light-driven charge transfer to drive catalytic reactions thermodynamically "uphill" has been extensively adapted for small molecule activation, molecular machines, photoswitches, and solar fuel chemistry, its application in organic synthesis has been less systematically developed. However, the potential benefits of these approaches are significant, both in enabling transformations that cannot be readily achieved using conventional thermal chemistry and in accessing distinct selectivity regimes that are uniquely enabled by excited-state mechanisms. In this Account, we present work from our group that highlights the ability of visible light photoredox catalysis to drive useful organic transformations away from their equilibrium positions, addressing a number of long-standing synthetic challenges.We first discuss how excited-state ET enabled the first general methods for the catalytic anti-Markovnikov hydroamination of unactivated alkenes with alkyl amines. In these reactions, an excited-state iridium(III) photocatalyst reversibly oxidizes secondary amine substrates to their corresponding aminium radical cations (ARCs). These electrophilic N-centered radicals can then react with olefins to furnish valuable tertiary amine products with complete anti-Markovnikov regioselectivity. Notably, some of these products are less thermodynamically stable than their corresponding amine and alkene starting materials. We next present a strategy for light-driven C-C bond cleavage within various aliphatic alcohols mediated by homolytic activation of alcohol O-H bonds by excited-state proton-coupled electron transfer (PCET). The resulting alkoxy radical intermediates then undergo C-C β-scission to ultimately provide isomeric linear carbonyl products that are often higher in energy than their cyclic alcohol precursors. Applications of this chemistry for the light-driven depolymerization of lignin biomass, commercial phenoxy resin, hydroxylated polyolefin derivatives, and thermoset polymers are presented as well. We then describe a method for the contrathermodynamic positional isomerization of highly substituted olefins by means of cooperative photoredox and chromium(II) catalysis. In this work, generation of an allylchromium(III) species that can undergo highly regioselective in situ protodemetalation enables access to a less substituted and thermodynamically less stable positional isomer. Product selectivity in this reaction is determined by the large differential in oxidation potentials between differently substituted olefin isomers. Lastly, we discuss a light-driven deracemization reaction developed in collaboration with the Miller group, wherein a racemic urea substrate undergoes spontaneous optical enrichment upon visible light irradiation in the presence of an iridium(III) chromophore, a chiral Brønsted base, and a chiral peptide thiol. Excellent levels of enantioselectivity are achieved via sequential and synergistic proton transfer (PT) and H atom transfer (HAT) steps. Taken together, these examples highlight the ability of excited-state ET events to enable access to nonequilibrium product distributions across a wide range of catalytic, redox-neutral transformations in which photons are the only stoichiometric reagents.

Direct synthesis of branched amines enabled by dual-catalyzed allylic C─H amination of alkenes with amines

Direct conversion of hydrocarbons into amines represents an important and atom-economic goal in chemistry for decades. However, intermolecular cross-coupling of terminal alkenes with amines to form branched amines remains extremely challenging. Here, a visible-light and Co-dual catalyzed direct allylic C─H amination of alkenes with free amines to afford branched amines has been developed. Notably, challenging aliphatic amines with strong coordinating effect can be directly used as C─N coupling partner to couple with allylic C─H bond to form advanced amines with molecular complexity. Moreover, the reaction proceeds with exclusive regio- and chemoselectivity at more steric hinder position to deliver primary, secondary, and tertiary aliphatic amines with diverse substitution patterns that are difficult to access otherwise.

A Neutral PCNHCP Co(I)-Me Pincer Complex as a Catalyst for N-Allylic Isomerization with a Broad Substrate Scope

Earth-abundant-metal catalyzed double bond transposition offers a sustainable and atom-economical route toward the synthesis of internal alkenes. With an emphasis specifically on internal olefins and ethers, the isomerization of allylic amines has been particularly under represented in the literature. Herein, we report an efficient methodology for the selective isomerization of N-allylic organic compounds, including amines, amides, and imines. The reaction is catalyzed by a neutral PCNHCP cobalt(I) pincer complex and proceeds via a π-allyl mechanism. The isomerization occurs readily at 80-90 °C, and it is compatible with a wide variety of functional groups. The in situ formed enamines could additionally be used for a one-pot inverse-electron-demand Diels-Alder reaction to furnish a series of diversely substituted heterobiaryls, which is further discussed in this report.

Recent advances in the selective semi-hydrogenation of alkyne to (E)-olefins

Considering the potential importance and upsurge in demand, the selective semi-hydrogenation of alkynes to (E)-olefins has attracted significant interest. This article highlights the recent advances in newer technologies and important methodologies directed to (E)-olefins from alkynes developed from 2015 to 2023. Notable features summarised include the catalyst or ligand design and control of product selectivity based on precious and nonprecious metal catalysts for semi-hydrogenation to (E)-olefins. Mechanistic studies for various catalytic transformations, including synthetic application to bioactive compounds, are summarised.

Iridium-Catalyzed Migratory Terminal C(sp3)-H Amidation of Heteroatom-Substituted Internal Alkenes via Olefin Chain Walking

Hydroamination facilitated by metal hydride catalysis is an appealing synthetic approach to access valuable nitrogen-containing compounds from readily available unsaturated hydrocarbons. While high regioselectivity can be achieved usually for substrates bearing polar chelation groups, the reaction involving simple alkenes frequently provides nonselective outcomes. Herein, we report an iridium-catalyzed highly regioselective terminal C(sp3)-H amidation of internal alkenes utilizing dioxazolones as an amino source via olefin chain walking. Most notably, this mechanistic motif of double bond migration to the terminal position operates not only with dialkyl-substituted simple alkenes including styrenes but also with heteroatom-substituted olefins such as enol ethers, vinyl silanes, and vinyl borons, thus representing the first example of the terminal methyl amidation of the latter type of alkenes through a nondissociative chain walking process.

Strain-Inducing Positional Alkene Isomerization

Small, strained ring systems are important pharmacophores in medicinal chemistry and versatile intermediates in organic synthesis. However, the kinetic and thermodynamic instability of many strained organic molecules renders them challenging to prepare. Here, we report a strain-inducing positional alkene isomerization reaction that provides mild and selective access to cyclobutene building blocks from readily obtained cyclobutylidene precursors. This endergonic isomerization relies on the sequential and synergistic action of a decatungstate polyanion photocatalyst and cobaloxime co-catalyst to store potential energy in the form of ring strain. The versatility of the cyclobutene products is demonstrated through diverse subsequent strain-releasing transformations. Mechanistic studies reveal a steric basis for strain-selective product formation.

Late-Stage Molecular Editing Enabled by Ketone Chain-Walking Isomerization

Herein, a method for the isomerization of ketones in a manner akin to the chain-walking reaction of alkenes is described. Widely available and inexpensive pyrrolidine and elemental sulfur are deployed as catalysts to achieve this reversible transformation. Key to the utility of this approach was the elucidation of a stereochemical model to determine the thermodynamically favored product of the reaction and the kinetic selectivity observed. With the distinct selectivity profile of our ketone chain-walking process, the isomerization of various steroids was demonstrated to rapidly access novel steroids with "unnatural" oxidation patterns.

Functional-group translocation of cyano groups by reversible C-H sampling

Chemical transformations that introduce, remove or manipulate functional groups are ubiquitous in synthetic chemistry1. Unlike conventional functional-group interconversion reactions that swap one functionality for another, transformations that alter solely the location of functional groups are far less explored. Here, by photocatalytic, reversible C-H sampling, we report a functional-group translocation reaction of cyano (CN) groups in common nitriles, allowing for the direct positional exchange between a CN group and an unactivated C-H bond. The reaction shows high fidelity for 1,4-CN translocation, frequently contrary to inherent site selectivity in conventional C-H functionalizations. We also report the direct transannular CN translocation of cyclic systems, providing access to valuable structures that are non-trivial to obtain by other methods. Making use of the synthetic versatility of CN and a key CN translocation step, we showcase concise syntheses of building blocks of bioactive molecules. Furthermore, the combination of C-H cyanation and CN translocation allows access to unconventional C-H derivatives. Overall, the reported reaction represents a way to achieve site-selective C-H transformation reactions without requiring a site-selective C-H cleavage step.

Iron-Catalyzed Hydrobenzylation: Stereoselective Synthesis of (-)-Eugenial C

Metal-hydride hydrogen atom transfer (MHAT) has emerged as a useful tool to form quaternary carbons from alkenes via hydrofunctionalization. Methods to date that cross-couple alkenes with sp3 partners rely on heterobimetallic catalysis to merge the two cycles. Here, we report an iron-only cross-coupling via putative MHAT/SH2 steps that solves a key stereochemical problem in the synthesis of meroterpenoid eugenial C and obviates the need for nickel. The concise synthesis benefits from a conformationally locked o,o'-disubstituted benzyl bromide and a locally sourced chiral pool terpene coupling partner.

Electrocatalytic Access to Azetidines via Intramolecular Allylic Hydroamination: Scrutinizing Key Oxidation Steps through Electrochemical Kinetic Analysis

Azetidines are prominent structural scaffolds in bioactive molecules, medicinal chemistry, and ligand design for transition metals. However, state-of-the-art methods cannot be applied to intramolecular hydroamination of allylic amine derivatives despite their underlying potential as one of the most prevalent synthetic precursors to azetidines. Herein, we report an electrocatalytic method for intramolecular hydroamination of allylic sulfonamides to access azetidines for the first time. The merger of cobalt catalysis and electricity enables the regioselective generation of key carbocationic intermediates, which could directly undergo intramolecular C-N bond formation. The mechanistic investigations including electrochemical kinetic analysis suggest that either the catalyst regeneration by nucleophilic cyclization or the second electrochemical oxidation to access the carbocationic intermediate is involved in the rate-determining step (RDS) of our electrochemical protocol and highlight the ability of electrochemistry in providing ideal means to mediate catalyst oxidation.

Molecular Engineering of Metal-Organic Layers for Sustainable Tandem and Synergistic Photocatalysis

Metal-organic layers (MOLs), a monolayered version of metal-organic frameworks (MOFs), have recently emerged as a novel two-dimensional molecular material platform to design multifunctional catalysts. MOLs inherit the intrinsic molecular tunability of MOFs and yet have more accessible and modifiable building blocks. Here we report molecular engineering of six MOLs via modulated solvothermal synthesis between HfCl4 and three photosensitizing ligands followed by postsynthetic modification with two carboxylate-containing cobaloximes for tandem and synergistic photocatalysis. Morphological and structural characterization by transmission electron microscopy and atomic force microscopy and compositional analysis by inductively coupled plasma-mass spectrometry and nuclear magnetic resonance spectroscopy establish the MOLs as flat nanoplates with a periodic lattice structure of hexagonal symmetry. The MOLs efficiently catalyze tandem dehydrogenative coupling reactions and synergistic Heck-type coupling reactions. The most active MOL catalyst was used for the gram-scale synthesis of vesnarinone, a cardiotonic agent, in 80% yield with a turnover number of 400 and in eight consecutive reaction cycles without significant loss of activities.

Photocatalytic Deuterium Atom Transfer Deuteration of Electron-Deficient Alkenes with High Functional Group Tolerance

Due to its mild reaction conditions and unique chemoselectivity, hydrogen atom transfer (HAT) hydrogenation represents an indispensable method for the synthesis of complex molecules. Its analog using deuterium, deuterium atom transfer (DAT) deuteration, is expected to enable access to complex deuterium-labeled compounds. However, DAT deuteration has been scarcely studied for synthetic purposes, and a method that possesses the favorable characteristics of HAT hydrogenations has remained elusive. Herein, we report a protocol for the photocatalytic DAT deuteration of electron-deficient alkenes. In contrast to the previous DAT deuteration, this method tolerates a variety of synthetically useful functional groups including haloarenes. The late-stage deuteration also allows access to deuterated amino acids as well as donepezil-d₂ . Thus, this work demonstrates the potential of DAT chemistry to become the alternative method of choice for preparing deuterium-containing molecules.

Light-empowered contra-thermodynamic stereochemical editing

Creating, conserving and modifying the stereochemistry of organic compounds has been the subject of significant research efforts in synthetic chemistry. Most synthetic routes are designed according to the stereoselectivity-determining step. Stereochemical editing is an alternative strategy, wherein the chiral-defining or geometry-defining steps are independent of the construction of the major scaffold or complexity. It enables late-stage alterations of stereochemistry and can generate isomers from a single compound. However, in many instances, stereochemical editing processes are contra-thermodynamic, meaning the transformation is unfavourable. To overcome this barrier, photocatalysis uses photogenerated radical species and introduces thermochemical biases. A range of synthetically valuable contra-thermodynamic stereochemical editing processes have been invented, including deracemization of chiral molecules, positional alkene isomerization and dynamic epimerization of sugars and diols. In this Review, we highlight the fundamental mechanisms of visible-light photocatalysis and the general reactivity modes of the photogenerated radical intermediates towards contra-thermodynamic stereochemical editing processes.

Energy- and atom-efficient chemical synthesis with endergonic photocatalysis

Endergonic photocatalysis is the use of light to perform catalytic reactions that are thermodynamically unfavourable. While photocatalysis has become a powerful tool in facilitating chemical transformations, the light-energy efficiency of these processes has not gathered much attention. Exergonic photocatalysis does not take full advantage of the light energy input, producing low-energy products and heat, whereas endergonic photocatalysis incorporates a portion of the photon energy into the reaction, yielding products that are higher in free energy than the reactants. Such processes can enable catalytic, atom-economic syntheses of reactive compounds from bench-stable materials. With respect to environmental friendliness and carbon neutrality, endergonic photocatalysis is also of interest to large-scale industrial manufacturing, where better energy efficiency, less waste and value addition are highly sought. We therefore assess here the thermochemistry of several classes of reported photocatalytic transformations to showcase current advances in endergonic photocatalysis and point to their industrial potential.

Bench-Stable Dinuclear Mn(I) Catalysts in E-Selective Alkyne Semihydrogenation: A Mechanistic Investigation

Dinuclear manganese hydride complexes of the form [Mn2 (CO)8 (μ-H)(μ-PR2 )] (R=Ph, 1; R=iPr, 2) were used in E-selective alkyne semi-hydrogenation (E-SASH) catalysis. Catalyst speciation studies revealed rich coordination chemistry and the complexes thus formed were isolated and in turn tested as catalysts; the results underscore the importance of dinuclearity in engendering the observed E-selectivity and provide insights into the nature of the active catalyst. The insertion product obtained from treating 2 with (cyclopropylethynyl)benzene contains a cis-alkenyl bridging ligand with the cyclopropyl ring being intact. Treatment of this complex with H2 affords exclusively trans-(2-cyclopropylvinyl)benzene. These results, in addition to other control experiments, indicate a non-radical mechanism for E-SASH, which is highly unusual for Mn-H catalysts. The catalytically active species are virtually inactive towards cis to trans alkene isomerization indicating that the E-selective process is intrinsic and dinuclear complexes play a critical role. A reaction mechanism is proposed accounting for the observed reactivity which is fully consistent with a kinetic analysis of the rate limiting step and is further supported by DFT computations.

γ C-H Functionalization of Amines via Triple H-Atom Transfer of a Vinyl Sulfonyl Radical Chaperone

A selective, remote desaturation has been developed to rapidly access homoallyl amines from their aliphatic precursors. The strategy employs a triple H-atom transfer (HAT) cascade, entailing (i) cobalt-catalyzed metal-HAT (MHAT), (ii) carbon-to-carbon 1,6-HAT, and (iii) Co-H regeneration via MHAT. A new class of sulfonyl radical chaperone (to rapidly access and direct remote, radical reactivity) enables remote desaturation of diverse amines, amino acids, and peptides with excellent site-, chemo-, and regioselectivity. The key, enabling C-to-C HAT step in this cascade was computationally designed to satisfy both thermodynamic (bond strength) and kinetic (polarity) requirements, and it has been probed via regioselectivity, isomerization, and competition experiments. We have also interrupted this radical transfer dehydrogenation to achieve γ-selective C-Cl, C-CN, and C-N bond formations.

Two-Stage Syntheses of Clionastatins A and B

A concise and divergent synthesis of the polychlorinated marine steroids clionastatin A and B from inexpensive testosterone has been achieved through a unique two-stage chlorination-oxidation strategy. Key features of the two-stage synthesis include (1) conformationally controlled, highly stereoselective dichlorination at C1 and C2 and C4-OH-directed C19 oxygenation followed by a challenging neopentyl chlorination to install three chlorine atoms; (2) desaturation through one-pot photochemical dibromination-reductive debromination and anti-Markovnikov olefin oxidation by photoredox-metal dual catalysis to enhance the oxidation level of the backbone; and (3) Wharton transposition to furnish the D-ring enone. This synthesis proved that the introduction of the C19 chloride in the early stage of the synthesis secured the stability of the backbone against susceptibility to aromatization during the oxidation stage.

Scaffold hopping by net photochemical carbon deletion of azaarenes

Discovery chemists routinely identify purpose-tailored molecules through an iterative structural optimization approach, but the preparation of each successive candidate in a compound series can rarely be conducted in a manner matching their thought process. This is because many of the necessary chemical transformations required to modify compound cores in a straightforward fashion are not applicable in complex contexts. We report a method that addresses one facet of this problem by allowing chemists to hop directly between chemically distinct heteroaromatic scaffolds. Specifically, we show that selective photolysis of quinoline N-oxides with 390-nanometer light followed by acid-promoted rearrangement affords N-acylindoles while showing broad compatibility with medicinally relevant functionality. Applications to late-stage skeletal modification of compounds of pharmaceutical interest and more complex transformations involving serial single-atom changes are demonstrated.

Radical Aza-Heck Cyclization of Imidates via Energy Transfer, Electron Transfer, and Cobalt Catalysis

A radical aza-Heck cyclization has been developed to afford functionally rich products with four contiguous C-heteroatom bonds. This multi-catalytic strategy provides rapid syntheses of dense, medicinally relevant motifs by enabling the conversion of alcohol-derived imidates to heteroatom-rich fragments containing vinyl oxazolines/oxazoles, allyl amines, β-amino alcohols/halides, and combinations thereof. Mechanistic insights of this process show how three distinct photocatalytic cycles cooperate to enable: (1) imidate radical generation by energy transfer, (2) dehydrogenation by Co catalysis, and (3) catalyst turnover by electron transfer.

Regioselective Deaminative Allylation of Aliphatic Amines via Dual Cobalt and Organophotoredox Catalysis

Despite the rapid progress in C-C bond-forming reactions using Katritzky salts, their deaminative allylation remains a challenge. Inspired by the metallaphotoredox-catalyzed allylic substitution regime, here, we report the deaminative allylation of Katritzky salts via cobalt/organophotoredox dual catalysis. This cross-electrophile coupling enables regioselective allylation using a variety of allylic esters, overcoming the substrate limitations of reported protocols. Mechanistic studies indicate the involvement of a π-allyl cobalt complex as a radicalophile that mediates C-C bond formation.

Photocatalytic synthesis of 10-phenanthrenols via intramolecular cycloaromatization under oxidant-free conditions

A novel tandem photocycloaddition/dehydrogenative aromatization with hydrogen evolution of ortho biaryl-appended 1,3-dicarbonyl compounds for the synthesis of 10-phenanthrenol via cobaloxime catalysis is disclosed.