Ir-Catalyzed Ligand-Free Directed C–H Borylation of Arenes and Pharmaceuticals: Detailed Mechanistic Understanding

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.

Iridium-Catalyzed Ortho-Selective C-H Borylation of Aryl Ketones with Transient Imine Ligands

Ortho-selective C-H borylation of aromatic ketones has not been extensively explored. Herein, we report the iridium-catalyzed ortho-selective C-H borylation of aromatic ketones using in situ-formed imine as the ligand. Good compatibility is observed for various substituted acetophenones and other aromatic ketones, and corresponding products are obtained with medium to excellent yields.

Aryl Chloride-Directed Enantioselective C(sp2)-H Borylation Enabled by Iridium Catalysis

We herein report the iridium-catalyzed enantioselective C-H borylation of aryl chlorides. A variety of prochiral biaryl compounds could be well-tolerated, affording a vast array of axially chiral biaryls with high enantioselectivities. The current method exhibits a high turnover number (TON) of 7000, which represents the highest in functional-group-directed asymmetric C-H activation. The high TON was attributed to a weak catalyst-substrate interaction that was caused by mismatched chirality between catalyst and substrate. We also demonstrated the synthetic application of the current method by C-B, ortho-C-H, and C-Cl bond functionalization, including programmed Suzuki-Miyaura coupling for the synthesis of axially chiral polyarenes.

Borylation directed borylation of N-alkyl anilines using iodine activated pyrazaboles

Doubly electrophilic pyrazabole derivatives (pyrazabole = [H2B(μ-C3N2H3)]2) combined with one equiv. of base effect the ortho-borylation of N-alkyl anilines. Initial studies found that the bis(trifluoromethane)sulfonimide ([NTf2]-) pyrazabole derivative, [H(NTf2)B(μ-C3N2H3)]2, is highly effective for ortho-borylation, with this process proceeding through N-H borylation and then ortho C-H borylation. The activation of pyrazabole by I2 was developed as a cheaper and simpler alternative to using HNTf2 as the activator. The addition of I2 forms mono or ditopic pyrazabole electrophiles dependent on stoichiometry. The ditopic electrophile [H(I)B(μ-C3N2H3)]2 was also effective for the ortho-borylation of N-alkyl-anilines, with the primary C-H borylation products readily transformed into pinacol boronate esters (BPin) derivatives. Comparison of borylation reactions using the di-NTf2-and the diiodo-pyrazabole congeners revealed that more forcing conditions are required with the latter. Furthermore, the presence of iodide leads to competitive formation of side products, including [HB(μ-C3N2H3)3BH]+, which are not active for C-H borylation. Using [H(I)B(μ-C3N2H3)]2 and 0.2 equiv. of [Et3NH][NTf2] combines the higher yields of the NTf2 system with the ease of handling and lower cost of the iodide system generating an attractive process applicable to a range of N-alkyl-anilines. This methodology represents a metal free and transiently directed C-H borylation approach to form N-alkyl-2-BPin-aniline derivatives.

Recent Expedition in Pd- and Rh-Catalyzed C(Ar) -B Bond Formations and Their Applications in Modern Organic Syntheses

Transition metal-catalyzed borylation has emerged as a powerful and versatile strategy for synthesizing organoboron compounds. These compounds have found widespread applications in various aspects, including organic synthesis, materials science, and medicinal chemistry. This review provides a concise summary of the recent advances in palladium- and rhodium-catalyzed borylation from 2013 to 2023. The review covers the representative examples of catalysts, substrates scope and reaction conditions, with particular emphasis on the development of catalyst systems, such as phosphine ligands, NHC-carbene, and more. The diverse array of borylative products obtained for further applications in Suzuki-Miyaura coupling, and other transformations, are also discussed. Future directions in this rapidly evolving field, with the goal of designing more efficient, selective borylation methodologies are highlighted, too.

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.

Transition-Metal-Catalyzed Silylation and Borylation of C-H Bonds for the Synthesis and Functionalization of Complex Molecules

The functionalization of C-H bonds in organic molecules containing functional groups has been one of the holy grails of catalysis. One synthetically important approach to the diverse functionalization of C-H bonds is the catalytic silylation or borylation of C-H bonds, which enables a broad array of downstream transformations to afford diverse structures. Advances in both undirected and directed methods for the transition-metal-catalyzed silylation and borylation of C-H bonds have led to their rapid adoption in early-, mid-, and late-stage of the synthesis of complex molecules. In this Review, we review the application of the transition-metal-catalyzed silylation and borylation of C-H bonds to the synthesis of bioactive molecules, organic materials, and ligands. Overall, we aim to provide a picture of the state of art of the silylation and borylation of C-H bonds as applied to the synthesis and modification of diverse architectures that will spur further application and development of these reactions.

Iridium-Catalyzed ortho-Selective Borylation of Aromatic Amides Enabled by 5-Trifluoromethylated Bipyridine Ligands

Iridium-catalyzed borylations of aromatic C-H bonds are highly attractive transformations because of the diversification possibilities offered by the resulting boronates. These transformations are best carried out using bidentate bipyridine or phenanthroline ligands, and tend to be governed by steric factors, therefore resulting in the competitive functionalization of meta and/or para positions. We have now discovered that a subtle change in the bipyridine ligand, namely, the introduction of a CF₃ substituent at position 5, enables a complete change of regioselectivity in the borylation of aromatic amides, allowing the synthesis of a wide variety of ortho-borylated derivatives. Importantly, thorough computational studies suggest that the exquisite regio- and chemoselectivity stems from unusual outer-sphere interactions between the amide group of the substrate and the CF₃ -substituted aryl ring of the bipyridine ligand.

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.