Palladium (II)-Catalyzed C-H Activation with Bifunctional Ligands: From Curiosity to Industrialization.

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.

Macrocyclization via remote meta-selective C-H olefination using a practical indolyl template.

The synthesis of macrocyclic compounds with different sizes and linkages remains a great challenge via transition metal-catalysed intramolecular C-H activation. Herein, we disclose an efficient macrocyclization strategy via Pd-catalysed remote meta-C-H olefination using a practical indolyl template. This approach was successfully employed to access macrolides and coumarins. In addition, the intermolecular meta-C-H olefination also worked well and was exemplified by the synthesis of antitumor drug belinostat from inexpensive and readily available benzenesulfonyl chloride. Notably, catalytic copper acetate and molecular oxygen were used in place of silver salts as oxidants. Furthermore, for the first time, the formation of a macrocyclophane cyclopalladated intermediate was detected through in situ Fourier-transform infrared monitoring experiments and ESI-MS.

Remote C-H Olefination of Heterocyclic Biaryls Enabled by Reversibly Bound Templates.

Remote C-H functionalization of heterocyclic biaryls will be of great importance in synthesis and medicinal chemistry. Through adjusting the geometric relationship of the directing atom and target C-H bonds, two new catalytic templates have been developed to enable the functionalization of the more hindered ortho-C-H bonds of heterobiaryls bearing directing heteroatom at the meta- or para-positions, affording unprecedented site-selectivity. The use of template chaperone also overcomes product inhibition and renders the directing templates catalytic. The utility of this protocol was demonstrated by olefination of heterocyclic biaryls with various substituents, overriding conventional steric and electronic effects. These ortho-C-H olefinated heterobiaryls are sterically hindered and can often be challenging to prepare through aryl-aryl coupling reactions.

Ruthenium-Catalyzed Ligand-Enabled Regiodivergent Difluoroallylation of Aryl C-H Bonds.

The gem-difluoroallyl group is a sought-after structural motif commonly found in pharmaceutical compounds. Despite its appeal, achieving a controlled synthesis of both α,α- and γ,γ-difluoroallylated compounds has proven to be a challenging task. This study presents a new approach to difluoroallylation, which utilizes a regiodivergent C-H bond reaction catalyzed by ruthenium catalysis. This method enables the meta and ortho C-H α,α- and ortho C-H γ,γ-difluoroallylation of arenes using 3-bromo-3,3-difluoropropenes.

Formal γ-C-H Functionalization of Cyclobutyl Ketones: Synthesis of cis-1,3-Difunctionalized Cyclobutanes.

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.

Molecular editing of aza-arene C-H bonds by distance, geometry and chirality.

Direct molecular editing of heteroarene carbon-hydrogen (C-H) bonds through consecutive selective C-H functionalization has the potential to grant rapid access into diverse chemical spaces, which is a valuable but often challenging venture to achieve in medicinal chemistry1. In contrast to electronically biased heterocyclic C-H bonds2-9, remote benzocyclic C-H bonds on bicyclic aza-arenes are especially difficult to differentiate because of the lack of intrinsic steric/electronic biases10-12. Here we report two conceptually distinct directing templates that enable the modular differentiation and functionalization of adjacent remote (C6 versus C7) and positionally similar (C3 versus C7) positions on bicyclic aza-arenes through careful modulation of distance, geometry and previously unconsidered chirality in template design. This strategy enables direct C-H olefination, alkynylation and allylation at adjacent C6 and C7 positions of quinolines in the presence of a competing C3 position that is spatially similar to C7. Notably, such site-selective, iterative and late-stage C-H editing of quinoline-containing pharmacophores can be performed in a modular fashion in different orders to suit bespoke synthetic applications. This Article, in combination with previously reported complementary methods, now fully establishes a unified late-stage 'molecular editing' strategy to directly modify bicyclic aza-arenes at any given site in different orders.

Empirical Guidelines for the Development of Remote Directing Templates through Quantitative and Experimental Analyses.

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.

Distal γ-C(sp3 )-H Olefination of Ketone Derivatives and Free Carboxylic Acids.

Reported herein is the distal γ-C(sp3 )-H olefination of ketone derivatives and free carboxylic acids. Fine tuning of a previously reported imino-acid directing group and using the ligand combination of a mono-N-protected amino acid (MPAA) and an electron-deficient 2-pyridone were critical for the γ-C(sp3 )-H olefination of ketone substrates. In addition, MPAAs enabled the γ-C(sp3 )-H olefination of free carboxylic acids to form diverse six-membered lactones. Besides alkyl carboxylic acids, benzylic C(sp3 )-H bonds also could be functionalized to form 3,4-dihydroisocoumarin structures in a single step from 2-methyl benzoic acid derivatives. The utility of these protocols was demonstrated in large scale reactions and diversification of the γ-C(sp3 )-H olefinated products.

Ligand-Enabled Monoselective ß-C(sp3)-H Acyloxylation of Free Carboxylic Acids Using a Practical Oxidant.

The development of C-H activation reactions that use inexpensive and practical oxidants remains a significant challenge. Until our recent disclosure of the ß-lactonization of free aliphatic acids, the use of peroxides in C-H activation reactions directed by weakly coordinating native functional groups was unreported. Herein, we report C(sp3)-H ß-acetoxylation and γ-, δ-, and ε-lactonization reactions of free carboxylic acids enabled by a novel cyclopentane-based mono-N-protected ß-amino acid ligand. Notably, tert-butyl hydrogen peroxide is used as the sole oxidant for these reactions. This reaction has several key advantages over other C-H activation protocols: (1) exclusive monoselectivity was observed in the presence of two α-methyl groups; (2) aliphatic carboxylic acids containing α-hydrogens are compatible with this protocol; (3) lactonization of free acids, affording γ-, δ-, or ε-lactones, has been achieved for the first time.

Rational Development of Remote C-H Functionalization of Biphenyl: Experimental and Computational Studies.

A simple and efficient nitrile-directed meta-C-H olefination, acetoxylation, and iodination of biaryl compounds is reported. Compared to the previous approach of installing a complex U-shaped template to achieve a molecular U-turn and assemble the large-sized cyclophane transition state for the remote C-H activation, a synthetically useful phenyl nitrile functional group could also direct remote meta-C-H activation. This reaction provides a useful method for the modification of biaryl compounds because the nitrile group can be readily converted to amines, acids, amides, or other heterocycles. Notably, the remote meta-selectivity of biphenylnitriles could not be expected from previous results with a macrocyclophane nitrile template. DFT computational studies show that a ligand-containing Pd-Ag heterodimeric transition state (TS) favors the desired remote meta-selectivity. Control experiments demonstrate the directing effect of the nitrile group and exclude the possibility of non-directed meta-C-H activation. Substituted 2-pyridone ligands were found to be key in assisting the cleavage of the meta-C-H bond in the concerted metalation-deprotonation (CMD) process.

Merging C(sp3)-H activation with DNA-encoding.

DNA-encoded library (DEL) technology has the potential to dramatically expedite hit identification in drug discovery owing to its ability to perform protein affinity selection with millions or billions of molecules in a few experiments. To expand the molecular diversity of DEL, it is critical to develop different types of DNA-encoded transformations that produce billions of molecules with distinct molecular scaffolds. Sequential functionalization of multiple C-H bonds provides a unique avenue for creating diversity and complexity from simple starting materials. However, the use of water as solvent, the presence of DNA, and the extremely low concentration of DNA-encoded coupling partners (0.001 M) have hampered the development of DNA-encoded C(sp3)-H activation reactions. Herein, we report the realization of palladium-catalyzed C(sp3)-H arylation of aliphatic carboxylic acids, amides and ketones with DNA-encoded aryl iodides in water. Notably, the present method enables the use of alternative sets of monofunctional building blocks, providing a linchpin to facilitate further setup for DELs. Furthermore, the C-H arylation chemistry enabled the on-DNA synthesis of structurally-diverse scaffolds containing enriched C(sp3) character, chiral centers, cyclopropane, cyclobutane, and heterocycles.

Recent advances in transition metal-catalyzed C(sp2)-H nitration.

Nitroaromatic compounds play an important role in organic synthesis, medicinal chemistry and chemical industry. Among the recently reported synthetic methods to access nitroarenes, transition metal-catalyzed C(sp2)-H activation/nitration has become one of the most attractive protocols, showing high regioselectivity, excellent functional group tolerance and step-economy. In this review, we discuss advances in direct C(sp2)-H nitration for the synthesis of nitroaromatic compounds and mechanistic insights over the past decade. We hope this will provide valuable information for researchers interested in the rapidly developing field of metal-catalyzed C(sp2)-H nitration.

Highly Stereoselective Assembly of Polycyclic Molecules from 1,6-Enynes Triggered by Rhodium(III)-Catalyzed C-H Activation.

An Rh(III)-catalyzed C-H activation of pyrazolones with 1,6-enynes was investigated. The regioselectivity of the C-H activation/alkyne insertion is readily solved by using symmetric enyne coupling partners, and a C-H activation-triggered cascade reaction is realized, which involves alkyne insertion, tautomerization, and double cyclization to offer a class of structurally complex polycyclic architectures. This cascade reaction tolerates a broad substrate scope in high regioselectivity and stereospecificity and furnishes three new chemical bonds and four chiral centers in a single operation. Various derivatizations of the polycyclic scaffolds are conducted, providing products with ample space for further functional transformations.

Ligand-promoted ruthenium-catalyzed meta C-H chlorination of arenes using N-chloro-2,10-camphorsultam.

A practical meta C-H chlorination protocol is established via a Ru(0)-catalyzed ortho-metalation strategy. The use of N-chloro-2,10-camphorsultam as a new chlorinating agent is crucial for the success of the current reaction and an N-heterocyclic carbene (NHC) ligand could significantly enhance the reactivity of the catalytic transformation. The mechanistic studies reveal that an unusual ortho C-H ruthenation relay process with ortho chlorination of the C-Ru bond is probably involved.

Total synthesis of (±)-tanshinol B, tanshinone I, and (±)-tanshindiol B and C.

A concise and efficient approach was established for the divergent total synthesis of (±)-tanshindiol B and C and tanshinone I from a ubiquitous ene intermediate in 1-2 steps. This critical intermediate was derived from (±)-tanshinol B, which was synthesized in 50% overall yield over 3 steps using an ultrasound-promoted cycloaddition as a key step. Compared to a previously reported strategy, our approach is more step-economic, thus greatly improving the synthetic efficiency. The bioactivity evaluation indicates that the diol stereochemistry of the tanshidiols has an impact on the EZH2 inhibitory activity.

PMes3-Promoted Ruthenium-Catalyzed Meta C-H Nitration of 6-Arylpurines.

To address the challenge of meta nitration of 6-arylpurine substrates, a versatile ruthenium catalyzed meta C-H nitration is developed. The use of a sterically hindered phosphine ligand is crucial for the catalytic efficiency, and a broad class of 6-arylpurines and nucleosides were found suitable for this process providing corresponding exclusively meta nitrated products in good yields.

Difluorination of Furonaphthoquinones.

An unprecedented difluorination reaction was developed based on the furonaphthoquinone skeleton of natural products tanshinones and their analogues. By using Selectfluor as the fluorinating source and H2O as the hydroxyl source, a wide range of unique polycyclic α,α-difluoro ß,ß-dihydroxyl para-quinone products were achieved with yields up to 90%. The mechanistic studies revealed that the reaction might undergo tandem multiple electrophilic and nucleophilic substitutions, as well as cleavages of C-O and C-C bonds. This approach not only provides a new method to synthesis of α,α-difluoro ketones, but also affords a series of unique chemotypes for biological activity screening.

Rhodium-catalyzed redox-neutral coupling of phenidones with alkynes.

A switchable synthesis of N-substituted indole derivatives from phenidones via rhodium-catalyzed redox-neutral C-H activation has been achieved. In this protocol, we firstly disclosed that the reactivity of Rh(iii) catalysis could be enhanced through employing palladium acetate as an additive. Some representative features include external oxidant-free, applicable to terminal alkynes, short reaction time and operational simplicity. The utility of this method is further showcased by the economical synthesis of potent anticancer PARP-1 inhibitors.

Monomeric Octahedral Ruthenium(II) Complex Enabled meta-C-H Nitration of Arenes with Removable Auxiliaries.

A removable oxime-assisted meta-C-H nitration of arenes is reported. Mechanistic investigations and DFT calculations reveal a new monomeric octahedral ruthenium(II) complex is responsible for the meta-selective nitration. Dioxygen as a cooxidant is crucial for achieving high conversion and good yields. Moreover, the utility of the present reaction protocol is further showcased by the late-stage modification of the clinical CNS drugs Diazepam and Fluvoxamine.

Cobalt-Catalyzed Carbonylation of C(sp2)-H Bonds with Azodicarboxylate as the Carbonyl Source.

A novel and efficient approach for the C(sp2)-H bond carbonylation of benzamides has been developed using stable and inexpensive Co(OAc)2·4H2O as the catalyst and the commercially available and easily handling azodicarboxylates as the nontoxic carbonyl source. A broad range of substrates bearing diverse functional groups were tolerated. This is the first example where cobalt-catalyzed C(sp2)-H bond carbonylation occurs with azodicarboxylate as the carbonyl source.