Ion Pair-Directed C–H Activation on Flexible Ammonium Salts: meta-Selective Borylation of Quaternized Phenethylamines and Phenylpropylamines

Repurposing a supramolecular iridium catalyst via secondary Zn⋯O[double bond, length as m-dash]C weak interactions between the ligand and substrate leads to ortho-selective C(sp2)-H borylation of benzamides with unusual kinetics

The iridium-catalyzed C-H borylation of benzamides typically leads to meta and para selectivities using state-of-the-art iridium-based N,N-chelating bipyridine ligands. However, reaching ortho selectivity patterns requires extensive trial-and-error screening via molecular design at the ligand first coordination sphere. Herein, we demonstrate that triazolylpyridines are excellent ligands for the selective iridium-catalyzed ortho C-H borylation of tertiary benzamides and, importantly, we demonstrate the almost negligible effect of the first coordination sphere in the selectivity, which is so far unprecedented in iridium C-H bond borylations. Remarkably, the activity is dramatically enhanced by exploiting a remote Zn⋯O[double bond, length as m-dash]C weak interaction between the substrate and a rationally designed molecular-recognition site in the catalyst. Kinetic studies and DFT calculations indicate that the iridium-catalyzed C-H activation step is not rate-determining, this being unique for remotely controlled C-H functionalizations. Consequently, a previously established supramolecular iridium catalyst designed for meta-borylation of pyridines is now compatible with the ortho-borylation of benzamides, a regioselectivity switch that is counter-intuitive regarding precedents in the literature. In addition, we highlight the role of the cyclohexene additive in avoiding the formation of undesired side-products as well as accelerating the HBpin release event that precedes the catalyst regeneration step, which is highly relevant for the design of powerful and selective iridium borylating catalysts.

Photo-induced tungsten-catalyzed cascade synthesis of pyrrolo[2,1-a]isoquinoline-1,3-dicarboxylate and its reaction mechanism

A pyrrolo[2,1-a]isoquinoline core structure is prevalent in marine and other natural products. This article describes a tungsten-catalyzed [3+2] cycloaddition aromatization of dihydroisoquinoline ester and maleic anhydride or an acrylate. The photochemical reaction tolerates a range of functional groups such as ester, cyano, ketone, bromide, and alkene. It is shown that the cycloaddition-aromatization of 2-substitued acrylate catalyzed by a tungsten photocatalyst can be used to evaluate the leaving ability of the leaving group. Experiments done to determine the reaction mechanism revealed that the formation of an ion-pair intermediate generated in situ from dihydroisoquinoline ester and (Z)-4-methoxy-4-oxobut-2-enoic acid via the solvolysis of maleic anhydride with methanol is crucial for the cascade process to occur. The key cycloadduct acid intermediate derived from [3+2] cycloaddition was isolated and determined by X-ray crystallography.

Ir-Catalyzed Ortho-Selective C-H Borylation of Difluoromethyl Arenes

The difluoromethyl group (CF2H) has received great attention due to its distinct properties in recent years. Herein, we report a new strategy for postmodification of difluoromethyl compounds. Ortho-selective C-H borylation of difluoromethyl arenes is achieved by a cyclometalated mesoionic carbene-Ir complex. The regioselectivity is controlled by a hydrogen bond between CF2H and the boryl group via the outer-sphere direction.

Non-Covalent Interaction-Controlled Site-Selective C-H Transformations

Site-selective C-H transformations are important to obtain desired compounds as single products in a highly efficient manner. However, it is generally difficult to achieve such transformations because organic substrates contain many C-H bonds with similar reactivities. Therefore, the development of practical and efficient methods for controlling site selectivity is highly desirable. The most frequently used strategy is "directing group method". Although this method is highly effective and promotes site-selective reactions, it has several limitations. Our group recently reported other methods to achieve site-selective C-H transformations using non-covalent interactions between a substrate and a reagent or a catalyst and a substrate (non-covalent method). In this personal account, the background of site-selective C-H transformations, our reaction design to achieve site-selective C-H transformations, and recently reported reactions are explained.

Controlling Reactivity and Selectivity in the Nondirected C-H Activation of Arenes with Palladium

ConspectusAromatic structures are widespread motifs throughout organic chemistry, and C-H activation has been recognized as a major tool for enabling their sustainable and efficient functionalization. Through C-H activation, arenes can be modified without the need for prefunctionalization, leading to inherent atom- and step-economic advantages over traditional methods. However, for the development of synthetically useful methods, several hurdles have to be overcome. The strength of C-H bonds necessitates the development of sufficiently reactive catalysts, while the presence of multiple C-H bonds within a substrate poses challenges in terms of site-selectivity. Traditionally these challenges have been addressed by substrate control. By attaching different directing groups (DGs), the reactivity of the respective arene was significantly enhanced and the DG guided the metal in close proximity to specific C-H bonds, resulting in high site-selectivity. However, the introduction and removal of the DG add additional steps to the synthetic sequence, and the scope of the reaction is limited to a specific substrate class. The development of complementary nondirected methods that can be applied to a broad range of arenes without the necessity to carry a specific functional group that coordinates to Pd (referred to as simple arenes) is therefore highly desirable. However, the intrinsically lower reactivity of such substrates and the absence of a selectivity-determining DG pose significant challenges that can be solved only by the development of highly efficient catalysts. Consequently, the field of nondirected C-H activation, especially with respect to Pd-catalyzed methods, remained comparatively underdeveloped when we initiated our research program in 2017. At that time, state-of-the-art methods required the arene to be used in large excess, precluding its use in late-stage functionalization. Since organopalladium species are among the most versatile synthetic intermediates, we realized that developing a system, which can effectively and selectively activate C-H bonds in simple arenes with the arene as the limiting reagent, would be a powerful tool in synthetic organic chemistry. This account summarizes our groups' research toward the development and application of catalytic systems offering this desired reactivity and focuses explicitly on Pd-catalyzed nondirected C-H functionalization reactions of arenes, where the arene is employed as a limiting reagent. After an introduction that summarizes the state of Pd-catalyzed C-H activation of arenes before 2017 and the associated challenges, experimental and mechanistic details about the development of the first arene-limited, nondirected C-H functionalization of simple arenes with palladium will be discussed. This reactivity was enabled by the identification and combination of two complementary ligands, an N-heterocycle and an amino acid-derived ligand. Afterward we will discuss the expansion of this dual-ligand approach to further arene-limited transformations. Finally, we describe two methodologies that originated from the observations we made during our studies, namely, the late-stage deuteration of simple arenes and a highly selective olefination method that uses noncovalent interactions to induce meta selectivity.

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.

Amino Acid-Derived Ionic Chiral Catalysts Enable Desymmetrizing Cross-Coupling to Remote Acyclic Quaternary Stereocenters

Synthetic application of asymmetric catalysis relies on strategic alignment of bond construction to creation of chirality of a target molecule. Remote desymmetrization offers distinctive advantages of spatial decoupling of catalytic transformation and generation of a stereogenic element. However, such spatial separation presents substantial difficulties for the chiral catalyst to discriminate distant enantiotopic sites through a reaction three or more bonds away from a prochirality center. Here, we report a strategy that establishes acyclic quaternary carbon stereocenters through cross-coupling reactions at distal positions of aryl substituents. The new class of amino acid-derived ionic chiral catalysts enables desymmetrizing (enantiotopic-group-selective) Suzuki-Miyaura reaction, Sonogashira reaction, and Buchwald-Hartwig amination between diverse diarylmethane scaffolds and aryl, alkynyl, and amino coupling partners, providing rapid access to enantioenriched molecules that project substituents to widely spaced positions in the three-dimensional space. Experimental and computational investigations reveal electrostatic steering of substrates by the C-terminus of chiral ligands through ionic interactions. Cooperative ion-dipole interactions between the catalyst's amide group and potassium cation aid in the preorganization that transmits asymmetry to the product. This study demonstrates that it is practical to achieve precise long-range stereocontrol through engineering the spatial arrangements of the ionic catalysts' substrate-recognizing groups and metal centers.

Strategies That Utilize Ion Pairing Interactions to Exert Selectivity Control in the Functionalization of C-H Bonds

Electrostatic attraction between two groups of opposite charge, typically known as ion-pairing, offers unique opportunities for the design of systems to enable selectivity control in chemical reactions. Catalysis using noncovalent interactions is an established and vibrant research area, but it is noticeable that hydrogen bonding interactions are still the main interaction of choice in system design. Opposite charges experience the powerful force of Coulombic attraction and have the ability to exert fundamental influence on the outcome of reactions that involve charged reagents, intermediates or catalysts. In this Perspective, we will examine how ion-pairing interactions have been used to control selectivity in C-H bond functionalization processes. This broad class of reactions provides an interesting and thought-provoking lens through which to examine the application of ion-pairing design strategies because it is one that encompasses great mechanistic diversity, poses significant selectivity challenges, and perhaps most importantly is of immense interest to synthetic chemists in both industry and academia. We survey reactions that proceed via radical and ionic mechanisms alongside those that involve transition metal catalysis and will deal with control of site-selectivity and enantioselectivity. We anticipate that as this emerging area develops, it will become an ever-more important design strategy for selectivity control.

Recent Trends in Group 9 Catalyzed C–H Borylation Reactions: Different Strategies To Control Site-, Regio-, and Stereoselectivity

Organoboron compounds continue contributing substantially to advances in organic chemistry with their increasing role as both synthetic intermediates and target compounds for medicinal chemistry. Particularly attractive methods for their synthesis are based on the direct borylation of C–H bonds of available starting materials since no additional pre-functionalization steps are required. However, due to the high abundance of C–H bonds with similar reactivity in organic molecules, synthetically useful C–H borylation protocols demand sophisticated strategies to achieve high regio- and stereoselectivity. For this purpose, selective transition-metal-based catalysts have been developed, with group 9 centered catalysts being among the most commonly utilized. Recently, a multitude of diverse strategies has been developed to push the boundaries of C–H borylation reactions with respect to their regio- and enantioselectivity. Herein, we provide an overview of approaches for the C–H borylation of arenes, alkenes, and alkanes based on group 9 centered catalysts with a focus on the recent literature. Lastly, an outlook is given to assess the future potential of the field.

1 Introduction

1.1 Mechanistic Considerations

1.2 Selectivity Issues in C–H Borylation

1.3 Different Modes of Action Employing Directing Group Strategies in C–H Borylation

1.4 Scope and Aim of this Short Review

2 Trends in C–H Borylation Reactions

2.1 Photoinduced Catalysis

2.2 Transfer C–H Borylation

2.3 Lewis Acid Mediated C–H Borylation

2.4 Directed Metalation

2.5 Miscellaneous C–H Borylation Reactions

2.6 Electrostatic Interactions

2.7 Hydrogen Bonding

3 Conclusion and Outlook

Understanding the Regioselectivity of Ion-Pair-Assisted Meta-Selective C(sp2)-H Activation in Conformationally Flexible Arylammonium Salts

The lack of directionality and the long-range nature of Coulomb interactions have been a bottleneck to achieve chemically precise C-H activation using ion-pairs. Recent report by Phipps and co-workers of the ion-pair-directed regioselective Iridium-catalyzed borylation opens a new direction toward harnessing noncovalent interactions for C-H activation. In this article, the mechanism and specific role of ion-pairing are investigated using density functional theory (DFT). Computational studies reveal that meta C-H activation is kinetically more favorable than the para analogue due to stronger electrostatic interactions between the ion-pairs in closer proximity [d(NMe₃⁺···SO₃^-)_TSP1m = 3.93 Å versus d(NMe₃⁺···SO₃^-)_TSP1p = 4.30 Å]. The electrostatic interactions overwhelm the Pauli repulsion and distortion interactions incurred in bringing the oppositely charged ions in close contact for the rate-limiting meta transition state (TSP1_m). Multiple linear regression shows that the free energies of activation correlate well with descriptors like the charge densities on the meta carbon and Ir atom along with that on the cation and anion with R² = 0.74. Tuned range-separated DFT calculations demonstrate accurately the localization of charge separation in the reactant complex and transition state for the meta selectivity.

Metal-catalysed C-H bond activation and borylation

Transition metal-catalysed direct borylation of hydrocarbons via C-H bond activation has received a remarkable level of attention as a popular reaction in the synthesis of organoboron compounds owing to their synthetic versatility. While controlling the site-selectivity was one of the most challenging issues in these C-H borylation reactions, enormous efforts of several research groups proved instrumental in dealing with selectivity issues that presently reached an impressive level for both proximal and distal C-H bond borylation reactions. For example, in the case of ortho C-H bond borylation reactions, innovative methodologies have been developed either by the modification of the directing groups attached with the substrates or by creating new catalytic systems via the design of new ligand frameworks. Whereas meta and para selective C-H borylations remained a formidable challenge, numerous innovative concepts have been developed within a very short period of time by the development of new catalytic systems with the employment of various noncovalent interactions. Moreover, significant advancements have occurred for aliphatic C(sp³)-H borylations as well as enantioselective borylations. In this review article, we aim to discuss and summarize the different approaches and findings related to the development of directed proximal ortho, distal meta/para, aliphatic (racemic and enantioselective) borylation reactions since 2014. Additionally, considering the C-H borylation reaction as one of the most important mainstream reactions, various applications of this C-H borylation reaction toward the synthesis of natural products, therapeutics, and applications in materials chemistry will be summarized in the last part of this review article.

para-Selective C-H Borylation of Aromatic Quaternary Ammonium and Phosphonium Salts

Aromatic ammonium and phosphonium salts are important synthetic intermediates and multifunctional materials, but para-selective functionalization of the aromatic salts remains a challenge. Here we develop an ionic ligand based on our newly designed "biphenyl-phenanthroline" skeleton and realize the Ir-catalyzed para-selective C-H borylation of seven types of aromatic quaternary ammonium and phosphonium salts. Gram-scale transformation, late-stage elaboration for drug molecule, and diversification of borylated products demonstrate the potential utility of this reaction. The mechanistic studies and computational analysis elucidate the origin of para-selectivity.

Development of Cobalt Catalysts for the meta-Selective C(sp2)–H Borylation of Fluorinated Arenes

Cobalt precatalysts for the meta-selective borylation of fluorinated arenes are described. Initial screening and stoichiometric reactivity studies culminated in the preparation of a cobalt alkyl precatalyst supported by the sterically protected terpyridine (5,5″-Me₂^ArTpy = 4'-(4-N,N'-dimethylaminophenyl)-5,5″-dimethyl-2,2':6',2″-terpyridine). Under the optimized conditions, borylation with this precatalyst afforded up to 16 turnovers and near-exclusive meta regioselectivity with a range of substituted fluoroarenes in cyclopentyl methyl ether solvent at room temperature. Deuterium kinetic isotope effects of 2.9(2) at 23 °C support a turnover-limiting and selectivity-determining C(sp²)-H activation step, and stoichiometric C-H activation experiments provided insights into the identity of the C-H activating intermediate in catalysis. Analysis of the relevant Co-C and C-H bond thermodynamics support that the thermodynamics of C-H activation favor ortho-to-fluorine selectivity, providing additional, indirect support for kinetic control of C-H activation as the origin of meta selectivity.

Site-Selective C-H Functionalization of Arenes Enabled by Noncovalent Interactions

The direct metal-catalyzed C-H functionalization of arenes has emerged as a powerful tool for streamlining the synthesis of complex molecular scaffolds. However, despite the different chemical environments, the energy values of all C-H bonds are within a fairly narrow range; hence, the regioselective C-H bond functionalization poses a great challenge. The use of covalently bound directing groups is to date the most exploited approach to achieve regioselective C-H functionalization of arenes. However, the required installation and removal of those groups is a serious drawback. Recently, new strategies for regioselective metal-catalyzed distal C-H functionalization of arenes based on noncovalent forces (hydrogen bonds, Lewis acid-base interactions, ionic or electrostatic forces, etc.) have been developed to tackle these issues. Nowadays, these approaches have already showcased impressive advances. Therefore, the aim of this mini-review is to cover chronologically how these groundbreaking strategies evolved over the past decade.

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.

Remote steric control for undirected meta-selective C-H activation of arenes

Regioselective functionalization of arenes remains a challenging problem in organic synthesis. Steric interactions are often used to block sites adjacent to a given substituent, but they do not distinguish the remaining remote sites. We report a strategy based on remote steric control, whereby a roof-like ligand protects the distant para site in addition to the ortho sites, and thereby enables selective activation of meta carbon-hydrogen (C-H) bonds in the absence of ortho or para substituents. We demonstrate this concept for iridium-catalyzed meta-selective borylation of various monosubstituted arenes, including complex drug molecules. This strategy has the potential to expand the toolbox of C-H bond functionalization to previously nondifferentiable reaction sites.

Distal Ionic Substrate-Catalyst Interactions Enable Long-Range Stereocontrol: Access to Remote Quaternary Stereocenters through a Desymmetrizing Suzuki-Miyaura Reaction

Spatial distancing of a substrate's reactive group and nonreactive catalyst-binding group from its pro-stereogenic element presents substantial hurdles in asymmetric catalysis. In this context, we report a desymmetrizing Suzuki-Miyaura reaction that establishes chirality at a remote quaternary carbon. The anionic, chiral catalyst exerts stereocontrol through electrostatic steering of substrates, even as the substrate's reactive group and charged catalyst-binding group become increasingly distanced. This study demonstrates that precise long-range stereocontrol is achievable by engaging ionic substrate-ligand interactions at a distal position.

Ir-catalyzed proximal and distal C-H borylation of arenes

Over the past two decades, the C-H bond activation and functionalization reaction has been known as a prevailing method for the construction of carbon-carbon and carbon-heteroatom bonds using various transition metal catalysts. In this context, the iridium-catalyzed C-H bond activation and borylation reaction is one of the most valued methods. However, the major challenge in these borylation reactions is how to control the proximal (ortho) and distal (meta and para) selectivity. Interestingly, while so many approaches are now available for the proximal ortho selective borylation of arenes, borylation at the distal meta and or para position of arenes remains still challenging. Only a few approaches have been reported so far in the literature employing iridium catalysis. In this feature article, we have demonstrated some of the recent discoveries from our laboratories for the proximal (ortho) and distal (meta and para) selective borylation reactions. Moreover, some of the recent catalyst engineering discoveries for the selective proximal ortho borylation reactions for a diverse class of substrates have also been discussed. The discussion part of several other pioneering reports is limited due to the lack of scope of this feature article.

Toolbox for Distal C-H Bond Functionalizations in Organic Molecules

Transition metal catalyzed C-H activation has developed a contemporary approach to the omnipresent area of retrosynthetic disconnection. Scientific researchers have been tempted to take the help of this methodology to plan their synthetic discourses. This paradigm shift has helped in the development of industrial units as well, making the synthesis of natural products and pharmaceutical drugs step-economical. In the vast zone of C-H bond activation, the functionalization of proximal C-H bonds has gained utmost popularity. Unlike the activation of proximal C-H bonds, the distal C-H functionalization is more strenuous and requires distinctly specialized techniques. In this review, we have compiled various methods adopted to functionalize distal C-H bonds, mechanistic insights within each of these procedures, and the scope of the methodology. With this review, we give a complete overview of the expeditious progress the distal C-H activation has made in the field of synthetic organic chemistry while also highlighting its pitfalls, thus leaving the field open for further synthetic modifications.

Transient Directing Groups in Metal-Organic Cooperative Catalysis

The direct functionalization of C-H bonds is among the most fundamental chemical transformations in organic synthesis. However, when the innate reactivity of the substrate cannot be utilized for the functionalization of a given single C-H bond, this selective C-H bond functionalization mostly relies on the use of directing groups that allow bringing the catalyst in close proximity to the C-H bond to be activated and these directing groups need to be installed before and cleaved after the transformation, which involves two additional undesired synthetic operations. These additional steps dramatically reduce the overall impact and the attractiveness of C-H bond functionalization techniques since classical approaches based on substrate pre-functionalization are sometimes still more straightforward and appealing. During the past decade, a different approach involving both the in situ installation and removal of the directing group, which can then often be used in a catalytic manner, has emerged: the transient directing group strategy. In addition to its innovative character, this strategy has brought C-H bond functionalization to an unprecedented level of usefulness and has enabled the development of remarkably efficient processes for the direct and selective introduction of functional groups onto both aromatic and aliphatic substrates. The processes unlocked by the development of these transient directing groups will be comprehensively overviewed in this review article.

Enzyme-like Supramolecular Iridium Catalysis Enabling C-H Bond Borylation of Pyridines with meta-Selectivity

The use of secondary interactions between substrates and catalysts is a promising strategy to discover selective transition metal catalysts for atom-economy C-H bond functionalization. The most powerful catalysts are found via trial-and-error screening due to the low association constants between the substrate and the catalyst in which small stereo-electronic modifications within them can lead to very different reactivities. To circumvent these limitations and to increase the level of reactivity prediction in these important reactions, we report herein a supramolecular catalyst harnessing Zn⋅⋅⋅N interactions that binds to pyridine-like substrates as tight as it can be found in some enzymes. The distance and spatial geometry between the active site and the substrate binding site is ideal to target unprecedented meta-selective iridium-catalyzed C-H bond borylations with enzymatic Michaelis-Menten kinetics, besides unique substrate selectivity and dormant reactivity patterns.

New horizons for catalysis disclosed by supramolecular chemistry

The adoption of a supramolecular approach in catalysis promises to address a number of unmet challenges, ranging from activity (unlocking of novel reaction pathways) to selectivity (alteration of the innate selectivity of a reaction, e.g. selective functionalization of C-H bonds) and regulation (switch ON/OFF, sequential catalysis, etc.). Supramolecular tools such as reversible association and recognition, pre-organization of reactants and stabilization of transition states upon binding offer a unique chance to achieve the above goals disclosing new horizons whose potential is being increasingly recognized and used, sometimes reaching the degree of ripeness for practical use. This review summarizes the main developments that have opened such new frontiers, with the aim of providing a guide to researchers approaching the field. We focus on artificial supramolecular catalysts of defined stoichiometry which, under homogeneous conditions, unlock outcomes that are highly difficult if not impossible to attain otherwise, namely unnatural reactivity or selectivity and catalysis regulation. The different strategies recently explored in supramolecular catalysis are concisely presented, and, for each one, a single or very few examples is/are described (mainly last 10 years, with only milestone older works discussed). The subject is divided into four sections in light of the key design principle: (i) nanoconfinement of reactants, (ii) recognition-driven catalysis, (iii) catalysis regulation by molecular machines and (iv) processive catalysis.

Iridium-Catalysed C-H Borylation of Heteroarenes: Balancing Steric and Electronic Regiocontrol

The iridium-catalysed borylation of aromatic C-H bonds has become the preferred method for the synthesis of aromatic organoboron compounds. The reaction is highly efficient, tolerant of a broad range of substituents and can be applied to both carbocyclic and heterocyclic substrates. The regioselectivity of C-H activation is dominated by steric considerations and there have been considerable efforts to develop more selective processes for less constrained substrates. However, most of these have focused on benzenoid-type substrates and in contrast, heteroarenes remain much desired but more challenging substrates with the position and/or nature of the heteroatom(s) significantly affecting reactivity and regioselectivity. This review will survey the borylation of heteroarenes, focusing on the influence of steric and electronic effects on regiochemical outcome and, by linking to current mechanistic understandings, will provide insights to what is currently possible and where further developments are required.

Mechanistic Origins of Regioselectivity in Cobalt-Catalyzed C(sp2)-H Borylation of Benzoate Esters and Arylboronate Esters

Synthetic and mechanistic investigations into the C(sp²)-H borylation of various electronically diverse arenes catalyzed by bis(phosphine)pyridine (^iPrPNP) cobalt complexes are reported. Borylation of various benzoate esters and arylboronate esters gave remarkably high selectivities for the position para to the functional group; in both cases, this regioselectivity was found to override the ortho to fluorine regioselectivity previously reported for (^iPrPNP)Co borylation catalysts which arises from thermodynamic control of C(sp²)-H oxidative addition. Mechanistic studies support two distinct pathways that result in para-to-ester and para-to-boronate ester regioselectivity by thermodynamic and kinetic control, respectively, of C(sp²)-H oxidative addition. Borylation of a particularly electron-deficient fluorinated arylboronate ester resulted in acceleration of C(sp²)-H oxidative addition and concomitant inversion of regioselectivity, demonstrating that subtle changes in the relative rates of individual steps of the catalytic cycle can enable unique and switchable site selectivities.

Noncovalent interactions in Ir-catalyzed remote C–H borylation: a recent update

This highlight provides a recent update on noncovalent interaction enabled Ir-catalyzed remote C–H borylation, with a special emphasis on the corresponding enantioselective variant.

Beyond hydrogen bonding: recent trends of outer sphere interactions in transition metal catalysis

The implementation of interactions beyond hydrogen bonding in the 2^nd coordination sphere of transition metal catalysts is rare. However, it has already shown great promise in last 5 years, providing new tools to control the activity and selectivity as here reviewed.

Systematic Variation of Ligand and Cation Parameters Enables Site-Selective C-C and C-N Cross-Coupling of Multiply Chlorinated Arenes through Substrate-Ligand Electrostatic Interactions

Use of attractive noncovalent interactions between ligand and substrate is an emerging strategy for controlling positional selectivity. A key question relates to whether fine control on molecules with multiple, closely spaced reactive positions is achievable using typically less directional electrostatic interactions. Herein, we apply a 10-piece "toolkit" comprising of two closely related sulfonated phosphine ligands and five bases, each possessing varying cation size, to the challenge of site-selective cross-coupling of multiply chlorinated arenes. The fine tuning provided by these ligand/base combinations is effective for Suzuki-Miyaura coupling and Buchwald-Hartwig coupling on a range of isomeric dichlorinated and trichlorinated arenes, substrates that would produce intractable mixtures when typical ligands are used. This study develops a practical solution for site-selective cross-coupling to generate complex, highly substituted arenes.

Recent Developments in Enantioselective Transition Metal Catalysis Featuring Attractive Noncovalent Interactions between Ligand and Substrate

Enantioselective transition metal catalysis is an area very much at the forefront of contemporary synthetic research. The development of processes that enable the efficient synthesis of enantiopure compounds is of unquestionable importance to chemists working within the many diverse fields of the central science. Traditional approaches to solving this challenge have typically relied on leveraging repulsive steric interactions between chiral ligands and substrates in order to raise the energy of one of the diastereomeric transition states over the other. By contrast, this Review examines an alternative tactic in which a set of attractive noncovalent interactions operating between transition metal ligands and substrates are used to control enantioselectivity. Examples where this creative approach has been successfully applied to render fundamental synthetic processes enantioselective are presented and discussed. In many of the cases examined, the ligand scaffold has been carefully designed to accommodate these attractive interactions, while in others, the importance of the critical interactions was only elucidated in subsequent computational and mechanistic studies. Through an exploration and discussion of recent reports encompassing a wide range of reaction classes, we hope to inspire synthetic chemists to continue to develop asymmetric transformations based on this powerful concept.

Achieving Site-Selectivity for C-H Activation Processes Based on Distance and Geometry: A Carpenter's Approach

The ability to differentiate between highly similar C-H bonds in a given molecule remains a fundamental challenge in organic chemistry. In particular, the lack of sufficient steric and electronic differences between C-H bonds located distal to functional groups has prevented the development of site-selective catalysts with broad scope. An emerging approach to circumvent this obstacle is to utilize the distance between a target C-H bond and a coordinating functional group, along with the geometry of the cyclic transition state in directed C-H activation, as core molecular recognition parameters to differentiate between multiple C-H bonds. In this Perspective, we discuss the advent and recent advances of this concept. We cover a wide range of transition-metal-catalyzed, template-directed remote C-H activation reactions of alcohols, carboxylic acids, sulfonates, phosphonates, and amines. Additionally, we review eminent examples which take advantage of non-covalent interactions to achieve regiocontrol. Continued advancement of this distance- and geometry-based differentiation approach for regioselective remote C-H functionalization reactions may lead to the ultimate realization of molecular editing: the freedom to modify organic molecules at any site, in any order.

Enantioselective remote C-H activation directed by a chiral cation

Chiral cations have been used extensively as organocatalysts, but their application to rendering transition metal-catalyzed processes enantioselective remains rare. This is despite the success of the analogous charge-inverted strategy in which cationic metal complexes are paired with chiral anions. We report here a strategy to render a common bipyridine ligand anionic and pair its iridium complexes with a chiral cation derived from quinine. We have applied these ion-paired complexes to long-range asymmetric induction in the desymmetrization of the geminal diaryl motif, located on a carbon or phosphorus center, by enantioselective C-H borylation. In principle, numerous common classes of ligand could likewise be amenable to this approach.