Understanding the Activation of Air-Stable Ir(COD)(Phen)Cl Precatalyst for C–H Borylation of Aromatics and Heteroaromatics

An Air-Stable, Single-Component Iridium Precatalyst for the Borylation of C-H Bonds on Large to Miniaturized Scales

The functionalization of C-H bonds enables the modification of complex molecules, often with the intention of forming compound libraries. The borylation of aryl C-H bonds is a widely used class of C-H bond functionalization, and conventional catalyst systems for the borylation of C-H bonds consist of an iridium source and an N,N-ligand, in conjunction with pinacolborane, to form the active iridium(III) tris(boryl) catalyst. These multicomponent catalyst systems complicate borylation reactions at large and small scales, due to the air sensitivity of the most common iridium precursor [Ir(cod)OMe]2, and, particularly on small scale, the challenges associated with dispensing multiple components with differing solubilities or that are air-sensitive. We describe the discovery of an air-stable, single-component iridium precatalyst, [(tmphen)Ir(coe)2Cl], that generates the same active iridium(III) tris(boryl) catalyst and reacts with higher turnovers, comparable selectivity, and similar scope to those of known catalyst systems for the borylation of aryl and heteroaryl C-H bonds. We show how the development of this precatalyst enables reactions to be run on submicromole scale in a high-throughput experimentation format in conjunction with ChemBead technology, and with a second diversification step that illustrates the potential to diversify structures by chemical sequences involving catalytic reactions, including C-H bond functionalizations, on submicromole scales in the same reaction vessel.

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.

Noncovalent interaction with a spirobipyridine ligand enables efficient iridium-catalyzed C-H activation

Exploitation of noncovalent interactions for recognition of an organic substrate has received much attention for the design of metal catalysts in organic synthesis. The CH-π interaction is especially of interest for molecular recognition because both the C-H bonds and the π electrons are fundamental properties of organic molecules. However, because of their weak nature, these interactions have been less utilized for the control of organic reactions. We show here that the CH-π interaction can be used to kinetically accelerate catalytic C-H activation of arenes by directly recognizing the π-electrons of the arene substrates with a spirobipyridine ligand. Computation and a ligand kinetic isotope effect study provide evidence for the CH-π interaction between the ligand backbone and the arene substrate. The rational exploitation of weak noncovalent interactions between the ligand and the substrate will open new avenues for ligand design in catalysis.

Unfolding Protein-Based Hapten Coupling via Thiol-Maleimide Click Chemistry: Enhanced Immunogenicity in Anti-Nicotine Vaccines Based on a Novel Conjugation Method and MPL/QS-21 Adjuvants

Vaccines typically work by eliciting an immune response against larger antigens like polysaccharides or proteins. Small molecules like nicotine, on their own, usually cannot elicit a strong immune response. To overcome this, anti-nicotine vaccines often conjugate nicotine molecules to a carrier protein by carbodiimide crosslinking chemistry to make them polymeric and more immunogenic. The reaction is sensitive to conditions such as pH, temperature, and the concentration of reactants. Scaling up the reaction from laboratory to industrial scales while maintaining consistency and yield can be challenging. Despite various approaches, no licensed anti-nicotine vaccine has been approved so far due to the susboptimal antibody titers. Here, we report a novel approach to conjugate maleimide-modified nicotine hapten with a disulfide bond-reduced carrier protein in an organic solvent. It has two advantages compared with other approaches: (1) The protein was unfolded to make the peptide conformation more flexible and expose more conjugation sites; (2) thiol-maleimide "click" chemistry was utilized to conjugate the disulfide bond-reduced protein and maleimide-modified nicotine due to its availability, fast kinetics, and bio-orthogonality. Various nicotine conjugate vaccines were prepared via this strategy, and their immunology effects were investigated by using MPL and QS-21 as adjuvants. The in vivo study in mice showed that the nicotine-BSA conjugate vaccines induced high anti-nicotine IgG antibody titers, compared with vaccines prepared by using traditional condensation methods, indicating the success of the current strategy for further anti-nicotine or other small-molecule vaccine studies. The enhancement was more significant by using MPL and QS-21 than that of traditional aluminum adjuvants.

Aminoboranes via Tandem Iodination/Dehydroiodination for One-Pot Borylation

A rapid synthesis of aminoboranes from amine-boranes utilizing an iodination/dehydroiodination sequence is described. Monomeric aminoboranes are generated exclusively from several substrate adducts, following an E2-type elimination, with the added base playing a critical role in monomer vs dimer formation. Diisopropylaminoborane formed using this methodology has been applied to a one-pot palladium-catalyzed conversion of iodo- and bromoarenes to the corresponding boronates. Additionally, modification of the workup allows for isolation of the boronic acid and recovery of the utilized amine.

Electrostatically Directed Meta-Selective Borylation of Arenes

The constitutional challenge of an electrostatically directed meta-borylation of sterically biased and unbiased substrates is summarized in the present work. The borylation follows an electrostatic interaction between the partially positive and negative charges of the ligand and substrate respectively. Using our developed strategy, it has been demonstrated that a wide range of challenging substrates, especially 4-substituted substrates can selectively be borylated at the meta-position with excellent selectivity. Moreover, unsubstituted substrates are also displayed excellent meta-selectivity. The reaction employs bench stable ligand, proceeds at moderate reaction temperature (40-80 oC) precluding the need to synthesize bulky and sophisticated ligand/template.

Merging Iridium-Catalyzed C-H Borylations with Palladium-Catalyzed Cross-Couplings Using Triorganoindium Reagents

A versatile and efficient method to prepare borylated arenes furnished with alkyl, alkenyl, alkynyl, aryl, and heteroaryl functional groups is developed by merging Ir-catalyzed C-H borylations (CHB) with a chemoselective palladium-catalyzed cross-coupling of triorganoindium reagents (Sarandeses-Sestelo coupling) with aryl halides bearing a boronic ester substituent. Using triorganoindium cross-coupling reactions to introduce unsaturated moieties enables the synthesis of borylated arenes that would be difficult to access through the direct application of the CHB methodology. The sequential double catalyzed procedure can be also performed in one vessel.

Effect of Pincer Methylation on the Selectivity and Activity in (PNP)Cobalt-Catalyzed C(sp2)-H Borylation

Cobalt complexes supported by a tetramethylated PNP pincer ligand (Me₄ ^iPrPNP = 2,6-(ⁱPr₂PCMe₂)₂(C₅H₃N)) have been synthesized and structurally characterized. Examples include cobalt(I)-choride, -methyl, -aryl and -benzofuranyl derivatives. The performance of these compounds was evaluated in the catalytic borylation of fluorinated arenes using B₂Pin₂ as the boron source. While P-C bond cleavage, a known deactivation pathway in [(PNP)Co]-catalyzed borylation was suppressed, the overall activity and selectivity of the borylation of fluoroarenes was reduced as compared to the previously reported [(PNP)Co] catalyst lacking isopropylene spacers. Stoichiometric reactions support an increased barrier for oxidative addition to cobalt(I), a result of the increased steric profile and decreased conformational flexibility of the pincer resulting from methylation distal to the active site. With a more activated substrate such as benzofuran, catalytic borylation with cobalt(I) precatalysts and HBPin was observed. Monitoring the progress of the reaction by NMR spectroscopy revealed the presence of cobalt(III) intermediates during the course of the borylation, supporting a cobalt(I)-(III) redox cycle.

Rhodium and Iridium Mediated C-H and O-H Bond Activation of Two Schiff Base Ligands: Synthesis, Characterization and Catalytic Properties of the Organometallic Complexes

Reaction of [Rh(PPh₃)₃Cl] with two Schiff base ligands, viz. N-(2′-hydroxyphenyl)furan-2-aldimine (H₂L¹) and N-(2′-hydroxyphenyl)thiophene-2-aldimine (H₂L²), in refluxing toluene affords organorhodium complexes of type [Rh(PPh₃)₂(L)Cl] (L = L¹ and L²). Similar reaction with [Ir(PPh₃)₃Cl] yields organoiridium complexes of type [Ir(PPh₃)₂(L) (H)] (L = L¹ and L²). Crystal structures of [Rh(PPh₃)₂(L¹)Cl] and [Ir(PPh₃)₂(L²) (H)] have been determined, where the imine ligands are found to bind to the metal centers as CNO-donors. Structures of [Rh(PPh₃)₂(L²)Cl] and [Ir(PPh₃)₂(L¹) (H)] have been optimized by density functional theory method. Formation of the organometallic complexes is believed to proceed via C-H and O-H bond activation of the imine ligands. All four complexes show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry on the complexes shows an oxidation on the positive side of SCE and a reduction on the negative side. The organoiridium complexes are found to efficiently catalyze Suzuki-type C-C cross coupling reactions.