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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.

Site-selective functionalization of C-H bonds will ultimately afford chemists transformative tools for editing and constructing complex molecular architectures. Towards this goal, it is essential to develop strategies to activate C-H bonds that are distal from a functional group. In this context, distinguishing remote C-H bonds on adjacent carbon atoms is an extraordinary challenge due to the lack of electronic or steric bias between the two positions. Herein, we report the design of a catalytic system leveraging a remote directing template and a transient norbornene mediator to selectively activate a previously inaccessible remote C-H bond that is one bond further away. The generality of this approach has been demonstrated with a range of heterocycles, including a complex anti-leukaemia agent and hydrocinnamic acid substrates.

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.

C-H activation and functionalization are on the forefront of modern synthetic chemistry. Imagine if any C-H bond of a molecule could be converted to a C-X bond, where X is the target functionality-this would alter the synthetic blueprints for complex target molecules since it would provide novel disconnections in retrosynthetic analysis. Collaborations between many experimental and computational groups have led to rapid developments of new C-H functionalization methods. Our groups represent an example of this; we were brought together as part of the NSF-supported Center for Selective C-H Functionalization. Many examples of experimental-computational synergy for selective Pd(II)-catalyzed C-H activation of aryl and alkyl groups are described in this Account. We describe computations by the Houk group made in response to experimental stimuli by the Yu group. The first section discusses the experimental and computational investigations of oxazoline-directed stereoselective Pd(II)-catalyzed C(sp3)-H bond activation that occurs through the concerted metalation-deprotonation (CMD) pathway involving a monomeric Pd(II) complex. The second section involves two types of bidentate ligands, mono-N-protected amino acid (MPAA) and acetyl-protected aminoethyl quinoline (APAQ) ligands that promote the C-H activation reactions with the ligand as the internal base. In the MPAA-assisted remote C-H bond activation, the basic dianionic amidate ligand participates in the deprotonation of a specific C-H bond. This mechanism accounts for the improved reactivity and selectivity in C-H activation reactions with MPAA ligands. The chiral APAQ ligands enable asymmetric palladium insertion into prochiral C-H bonds on a single methylene carbon center. The dianionic amidate of the APAQ ligand acts as an intramolecular base to deprotonate the methylene C-H asymmetrically and facilitate chiral Pd-C bond formation. The origins of the dramatic differences of five-membered (relatively inactive) and six-membered (highly active) chelation in β-methylene C(sp3)-H activation reactions by a Pd(II) catalyst were explained with density functional theory (DFT) calculations. This is mainly due to the steric repulsions between the ArF group of the substrate and the quinoline group of the ligand. The steric repulsion between the ArF group of the substrate and the quinoline group of the APAQ ligand destabilizes the five-membered chelate transition structure, increasing the energy of the transition state. The third section discusses a mechanism involving a Pd-Ag heterodimeric complex intermediate in the template-directed, Pd(II)-catalyzed remote meta functionalization of toluene derivatives and benzoic acid derivatives. The nitrile directing group of the template coordinates with Ag while the Pd is placed adjacent to the meta-C-H bond in the transition state, leading to the observed high meta selectivity. The selective activation of remote meta-C-H bonds at various distances can be achieved by tuning the template. The dual role of AgOAc as both an oxidant and part of the heteronuclear active species in the mechanism involving PdAg(OAc)3 was determined by DFT calculations and is in accord with literature information about complexes. For the systems discussed in these three sections, the similarity is that they all proceed via the CMD mechanism. The differences lie in the proton acceptors and the active Pd species. Common CMD involves a monomeric Pd mechanism with acetate as the proton acceptor. Both MPAA and APAQ ligands react via monomeric Pd mechanisms with a ligand moiety (the amidate oxygen) as the proton acceptor. Nitrile-containing template-mediated meta-C-H activations proceed via a Pd-Ag heterodimeric mechanism, still with acetate as the proton acceptor. The interaction between our two groups, experts in experiment and computation, and the discoveries made possible by that interplay are highlighted in this Account.

A conformationally flexible template for the meta-C-H olefination of benzoic acids was designed through both experimental and computational efforts. The newly designed template favors a silver-palladium heterodimer low barrier transition state, and demonstrates that it is feasible to lengthen templates so as to achieve meta-selectivity when the distance between the functional handle of the native substrate and target C-H bond decreases. Analysis of the ortho-, meta-, and para-C-H cleavage transition states determined that the new template conformation optimizes the interaction between the nitrile and palladium-silver dimer in the meta-transition state, enabling palladium to cleave meta-C-H bonds with moderate-to-good yields and generally high regioselectivity. Regioselectivity is governed exclusively by the template, and kinetic experiments reveal that there is a 4-fold increase in rate in the presence of monoprotected amino acid ligands. Using a Boltzmann distribution of all accessible C-H activation transition states, it is possible to computationally predict meta-selectivity in a number of investigated templates with reasonable accuracy. Structural and distortion energies reported may be used for the further development of templates for meta-C-H activation of hitherto unexplored arene substrates.

Cytochrome P450 enzymes are known to catalyse bimodal oxidation of aliphatic acids via radical intermediates, which partition between pathways of hydroxylation and desaturation1,2. Developing analogous catalytic systems for remote C–H functionalization remains a significant challenge3–5. Here, we report the development of Cu(I)-catalysed bimodal dehydrogenation/lactonization reactions of synthetically common N-methoxyamides through radical abstractions of the γ-aliphatic C–H bonds. The feasibility of switching from dehydrogenation to lactonization is also demonstrated by altering reaction conditions. The use of a readily available amide as both radical precursor and internal oxidant allows for the development of redox-neutral C–H functionalization reactions with methanol as the sole side product. These C–H functionalization reactions using a Cu(I) catalyst with loading as low as 0.5 mol.% is applied to the diversification of a wide range of aliphatic acids including drug molecules and natural products. The exceptional compatibility of this catalytic system with a wide range of oxidatively sensitive functionality demonstrates the unique advantage of using a simple amide substrate as a mild internal oxidant.Use of N-methoxyamides as oxidants enables controllable, redox-neutral, green catalysis of bimodal dehydrogenation/lactonization reactions with methanol as the only by-product. [ABSTRACT FROM AUTHOR]

Effective differentiation of prochiral carbon-hydrogen (C-H) bonds on a single methylene carbon via asymmetric metal insertion remains a challenge. Here, we report the discovery of chiral acetyl-protected aminoethyl quinoline ligands that enable asymmetric palladium insertion into prochiral C-H bonds on a single methylene carbon center. We apply these palladium complexes to catalytic enantioselective functionalization of β-methylene C-H bonds in aliphatic amides. Using bidentate ligands to accelerate C-H activation of otherwise unreactive monodentate substrates is crucial for outcompeting the background reaction driven by substrate-directed cyclopalladation, thereby avoiding erosion of enantioselectivity. The potential of ligand acceleration in C-H activation is also demonstrated by enantioselective β-C-H arylation of simple carboxylic acids without installing directing groups.

Functionalization of the [beta]-C-H bonds of aliphatic acids is emerging as a valuable synthetic disconnection that complements a wide range of conjugate addition reactions.sup.1-5. Despite efforts for [beta]-C-H functionalization in carbon-carbon and carbon-heteroatom bond-forming reactions, these have numerous crucial limitations, especially for industrial-scale applications, including lack of mono-selectivity, use of expensive oxidants and limited scope.sup.6-13. Notably, the majority of these reactions are incompatible with free aliphatic acids without exogenous directing groups. Considering the challenge of developing C-H activation reactions, it is not surprising that achieving different transformations requires independent catalyst design and directing group optimizations in each case. Here we report a Pd-catalysed [beta]-C(sp.sup.3)-H lactonization of aliphatic acids enabled by a mono-N-protected [beta]-amino acid ligand. The highly strained and reactive [beta]-lactone products are versatile linchpins for the mono-selective installation of diverse alkyl, alkenyl, aryl, alkynyl, fluoro, hydroxyl and amino groups at the [beta] position of the parent acid, thus providing a route to many carboxylic acids. The use of inexpensive tert-butyl hydrogen peroxide as the oxidant to promote the desired selective reductive elimination from the Pd(iv) centre, as well as the ease of product purification without column chromatography, render this reaction amenable to tonne-scale manufacturing. Pd-catalysed [beta]-C(sp.sup.3)-H lactonization of aliphatic acids enabled by a mono-N-protected [beta]-amino acid and tert-butyl hydrogen peroxide is reported, achieving high [beta]-position selectivity without the use of a directing group., Author(s): Zhe Zhuang [sup.1] , Jin-Quan Yu [sup.1] Author Affiliations: (1) Department of Chemistry, The Scripps Research Institute, La Jolla, USA Main Alkyl carboxylic acids are ubiquitous and inexpensive reagents [...]

γ‐methylene C(sp3)−H functionalization of linear free carboxylic acids remains a significant challenge. Here in we report a Pd(II)‐catalyzed tandem γ‐arylation and γ‐lactonization of aliphatic acids enabled by a L,X‐type CarboxPyridone ligand. A wide range of γ‐arylated γ‐lactones are synthesized in a single step from aliphatic acids in moderate to good yield. Arylated lactones can readily be converted into disubstituted tetrahydrofurans, a prominent scaffold amongst bioactive molecules. [ABSTRACT FROM AUTHOR]

Enantioselective carbon-hydrogen (C-H) activation reactions by asymmetric metallation could provide new routes for the construction of chiral molecules.sup.1,2. However, current methods are typically limited to the formation of five- or six-membered metallacycles, thereby preventing the asymmetric functionalization of C-H bonds at positions remote to existing functional groups. Here we report enantioselective remote C-H activation using a catalytic amount of a chiral norbornene as a transient mediator, which relays initial ortho-C-H activation to the meta position. This was used in the enantioselective meta-C-H arylation of benzylamines, as well as the arylation and alkylation of homobenzylamines. The enantioselectivities obtained using the chiral transient mediator are comparable across different classes of substrates containing either neutral [sigma]-donor or anionic coordinating groups. This relay strategy could provide an alternative means to remote chiral induction, one of the most challenging problems in asymmetric catalysis.sup.3,4. Remote, enantioselective C-H activation reactions can be achieved by relaying ortho-C-H activation to the meta position, using a chiral norbornene as the mediator., Author(s): Hang Shi [sup.1] , Alastair N. Herron [sup.1] , Ying Shao [sup.1] , Qian Shao [sup.1] , Jin-Quan Yu [sup.1] Author Affiliations: (1) Department of Chemistry, The Scripps Research [...]

Author(s): Peng Wang [1]; Pritha Verma [1]; Guoqin Xia [1]; Jun Shi [2]; Jennifer X. Qiao [3]; Shiwei Tao [2]; Peter T. W. Cheng [2]; Michael A. Poss [3]; Marcus [...]

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. [ABSTRACT FROM AUTHOR]