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In 2001, our curiosity to understand the stereochemistry of C−H metalation with Pd prompted our first studies in Pd(II)‐catalyzed asymmetric C−H activation (RSC Research appointment: 020 7451 2545, Grant: RG 36873, Dec. 2002). We identified four central challenges: 1. poor reactivity of simple Pd salts with native substrates; 2. few strategies to control site selectivity for remote C−H bonds; 3. the lack of chiral catalysts to achieve enantioselectivity via asymmetric C−H metalation, and 4. low practicality due to limited coupling partner scope and the use of specialized oxidants. These challenges necessitated new strategies in catalyst and reaction development. For reactivity, we developed approaches to enhance substrate–catalyst affinity together with novel bifunctional ligands which participate in and accelerate the C−H cleavage step. For site‐selectivity, we introduced the concept of systematically modulating the distance and geometry between a directing template, catalyst, and substrate to selectively access remote C−H bonds. For enantioselectivity, we devised predictable stereomodels for catalyst‐controlled enantioselective C−H activation based on the participation of bifunctional ligands. Finally, for practicality, we have developed varied catalytic manifolds for Pd(II) to accommodate diverse coupling partners while employing practical oxidants such as simple peroxides. These advances have culminated in numerous C−H activation reactions, setting the stage for broad industrial applications. [ABSTRACT FROM AUTHOR]

1,3‐Difunctionalized cyclobutanes are an emerging scaffold in medicinal chemistry that can confer beneficial pharmacological properties to small‐molecule drug candidates. However, the diastereocontrolled synthesis of these compounds typically requires complicated synthetic routes, indicating a need for novel methods. Here, we report a sequential C−H/C−C functionalization strategy for the stereospecific synthesis of cis‐γ‐functionalized cyclobutyl ketones from readily available cyclobutyl aryl ketones. Specifically, a bicyclo[1.1.1]pentan‐2‐ol intermediate is generated from the parent cyclobutyl ketone via an optimized Norrish‐Yang procedure. This intermediate then undergoes a ligand‐enabled, palladium‐catalyzed C−C cleavage/functionalization to produce valuable cis‐γ‐(hetero)arylated, alkenylated, and alkynylated cyclobutyl aryl ketones, the benzoyl moiety of which can subsequently be converted to a wide range of functional groups including amides and esters. [ABSTRACT FROM AUTHOR]

Cyclic organic molecules are common among natural products and pharmaceuticals1,2. In fact, the overwhelming majority of small-molecule pharmaceuticals contain at least one ring system, as they provide control over molecular shape, often increasing oral bioavailability while providing enhanced control over the activity, specificity and physical properties of drug candidates3–5. Consequently, new methods for the direct site and diastereoselective synthesis of functionalized carbocycles are highly desirable. In principle, molecular editing by C–H activation offers an ideal route to these compounds. However, the site-selective C–H functionalization of cycloalkanes remains challenging because of the strain encountered in transannular C–H palladation. Here we report that two classes of ligands—quinuclidine-pyridones (L1, L2) and sulfonamide-pyridones (L3)—enable transannular γ-methylene C–H arylation of small- to medium-sized cycloalkane carboxylic acids, with ring sizes ranging from cyclobutane to cyclooctane. Excellent γ-regioselectivity was observed in the presence of multiple β-C–H bonds. This advance marks a major step towards achieving molecular editing of saturated carbocycles: a class of scaffolds that are important in synthetic and medicinal chemistry3–5. The utility of this protocol is demonstrated by two-step formal syntheses of a series of patented biologically active small molecules, prior syntheses of which required up to 11 steps6.Quinuclidine-pyridone and sulfonamide-pyridone ligands enable transannular γ-methylene C–H arylation of cycloalkane carboxylic acids with a range of ring sizes, bringing us closer to molecular editing of saturated carbocycles. [ABSTRACT FROM AUTHOR]

In 2001, our curiosity to understand the stereochemistry of C-H metalation with Pd prompted our first studies in Pd(II)-catalyzed asymmetric C-H activation (RSC Research appointment: 020 7451 2545, Grant: RG 36873, Dec. 2002). We identified four central challenges: 1. poor reactivity of simple Pd salts with native substrates; 2. few strategies to control site selectivity for remote C-H bonds; 3. the lack of chiral catalysts to achieve enantioselectivity via asymmetric C-H metalation, and 4. low practicality due to limited coupling partner scope and the use of specialized oxidants. These challenges necessitated new strategies in catalyst and reaction development. For reactivity, we developed approaches to enhance substrate-catalyst affinity together with novel bifunctional ligands which participate in and accelerate the C-H cleavage step. For site-selectivity, we introduced the concept of systematically modulating the distance and geometry between a directing template, catalyst, and substrate to selectively access remote C-H bonds. For enantioselectivity, we devised predictable stereomodels for catalyst-controlled enantioselective C-H activation based on the participation of bifunctional ligands. Finally, for practicality, we have developed varied catalytic manifolds for Pd(II) to accommodate diverse coupling partners while employing practical oxidants such as simple peroxides. These advances have culminated in numerous C-H activation reactions, setting the stage for broad industrial applications., (© 2024 Wiley‐VCH GmbH.)

The achievement of sufficient substrate-metal catalyst affinity is a fundamental challenge for the development of synthetically useful C-H activation reactions of weakly coordinating native substrates. While hydrogen bonding has been harnessed to bias site selectivity in existing C(sp 2 )-H activation reactions, the potential for designing catalysts with hydrogen bond donors (HBDs) to enhance catalyst-substrate affinity and, thereby, facilitate otherwise unreactive C(sp 3 )-H activation remains to be demonstrated. Herein, we report the discovery of a ligand scaffold containing a remote amide motif that can form a favorable meta -macrocyclic hydrogen bonding interaction with the aliphatic acid substrate. The utility of this ligand scaffold is demonstrated through the development of an unprecedented C(sp 3 )-H bromination of α-tertiary and α-quaternary free carboxylic acids, which proceeds in exceedingly high mono -selectivity. The geometric relationship between the NHAc hydrogen bond donor and the coordinating quinoline ligand is crucial for forming the meta -macrocyclophane-like hydrogen bonding interaction, which provides a guideline for the future design of catalysts employing secondary interactions.

The ability to differentiate and selectively activate remote C-H bonds represents a perennial challenge in the field of C-H activation. Since its first report in 2012, a now-established "directing template" (DT) approach remains demonstrably effective for the functionalization of remote C-H bonds. As selectivity is hypothesized to be principally determined by the optimal positioning of the reactive catalyst to a target C-H bond, a DT's spatial factors are particularly important toward achieving high selectivity, though a systematic study on its requisite factors remain unelucidated. Through an in-depth analysis of 119 structurally unique published remote DTs, this report summarizes the key factors that are central toward achieving high selectivity at defined aryl positions, which are experimentally corroborated through the development of new aliphatic meta and para-selective DTs for electronically unbiased arenes. These empirical rules, which summarize key distance and geometric factors, are expected to be useful tools for the future development of site-selective arene C-H activation as well as other reactions that rely on covalent/noncovalent DT-mediated remote regioselection.

Enolate alkylation and conjugate addition into an α,β-unsaturated system have served as long-standing strategic disconnections for the installation of α- or β-substituents on carbonyl-containing compounds. At the onset of our efforts to develop C-H activation reactions for organic synthesis, we set our eye toward developing asymmetric β-C-H activation reactions of aliphatic acids with the perspective that this bond-forming event could serve as a more flexible retrosynthetic surrogate for both canonical carbonyl-related asymmetric transformations.In this Account, we describe our early efforts using strongly coordinating chiral oxazolines to probe reaction mechanism and the stereochemical nature of the C-H cleavage transition state. The characterization of key reactive intermediates through X-ray crystallography and computational studies suggested a transition state with C-H and Pd-OAc bonds being approximately coplanar for optimum interaction. We then moved forward to develop more practical, weakly coordinating monodentate amide directing groups, a necessary advance toward achieving the β-C-H activation of weakly coordinating native carboxylic acids. Throughout this journey, gradual deconvolution between a substrate's directing effect and its intimate interplay with ligand properties has culminated in the design of new ligand classes that ultimately allowed the competency of native carboxylic acids in β-C-H activation. These efforts established the importance of ligand acceleration in Pd-catalyzed C-H activation, where the substrate's weak coordination is responsible for positioning the catalyst for C-H cleavage, while the direct participation from the bifunctional ligand is responsible for enthalpically stabilizing the C-H cleavage transition state.Building upon these principles, we developed five classes of chiral ligands (MPAA, MPAQ, MPAO, MPAThio, MPAAM) to enable enantioselective β-C-H activation reactions, including carbon-carbon and carbon-heteroatom bond formation. The accumulated data from our developed enantioselective C-H activation reactions indicate that ligands possessing point chirality are most effective for imparting stereoinduction in the C-H activation step, the application of which enabled the desymmetrization and subsequent C-H functionalization of enantiotopic carbon and protons across a range of weakly coordinating arylamides and, more recently, free carboxylic acids. Progress in ligand design, in conjunction with the enabling nature of alkali metal countercations, led to the realization of a suite of β-methyl and now methylene C(sp 3 )-H activation reactions. These advancements also enabled the use of economical oxidants, such as peroxides and molecular oxygen, to facilitate catalyst turnover. In the future, continued progress in designing more efficient bifunctional chiral ligands is likely to provide a myriad of enantioselective β-C-H activation reactions of readily available native substrates.

A precisely designed chiral squaramide derivative is shown to promote the highly enantioselective addition of trimethylsilyl bromide (TMSBr) to a broad variety of 3-substituted and 3,3-disubstituted oxetanes. The reaction provides direct and general access to synthetically valuable 1,3-bromohydrin building blocks from easily accessed achiral precursors. The products are readily elaborated both by nucleophilic substitution and through transition-metal-catalyzed cross-coupling reactions. The enantioselective catalytic oxetane ring opening was employed as part of a three-step, gram-scale synthesis of pretomanid, a recently approved medication for the treatment of multidrug-resistant tuberculosis. Heavy-atom kinetic isotope effect (KIE) studies are consistent with enantiodetermining delivery of bromide from the H-bond-donor (HBD) catalyst to the activated oxetane. While the nucleophilicity of the bromide ion is expected to be attenuated by association to the HBD, overall rate acceleration is achieved by enhancement of Lewis acidity of the TMSBr reagent through anion abstraction.

Chiral urea derivatives are shown to catalyze enantioselective tail-to-head cyclization reactions of neryl chloride analogues. Experimental data are consistent with a mechanism in which π-participation by the nucleophilic olefin facilitates chloride ionization and thereby circumvents simple elimination pathways. Kinetic and computational studies support a cooperative mode of catalysis wherein two molecules of the urea catalyst engage the substrate and induce enantioselectivity through selective transition state stabilization.