Palladium-catalyzed, direct boronic acid synthesis from aryl chlorides: a simplified route to diverse boronate ester derivatives

Cobalt-Catalyzed Enantioselective Reductive Arylation, Heteroarylation, and Alkenylation of Michael Acceptors via an Elementary Mechanism of 1,4-Addition

Cobalt complexes with chiral quinox ligands effectively promote the enantioselective conjugate addition of enones using aryl, heteroaryl, and alkenyl halides and sulfonates. Additionally, a cobalt complex with a strongly donating diphosphine, BenzP*, successfully catalyzes the asymmetric reductive arylation and alkenylation of α,β-unsaturated amides. Both catalytic systems show broad scopes and tolerance of sensitive functional groups. Both reactions can be scaled up with low loadings of cobalt catalysts. Experimental results and density functional theory (DFT) calculations suggest a new mechanism of elementary 1,4-addition of aryl cobalt(I) complexes.

Evolution of a Synthetic Strategy toward the Syntheses of Bis-tetrahydroisoquinoline Alkaloids

ConspectusThe bis-tetrahydroisoquinoline (bis-THIQ) natural products represent a medicinally important class of isoquinoline alkaloids that exhibit broad biological activities with particularly potent antitumor properties, as exemplified by the two U.S. FDA approved molecules trabectidin and lurbinectedin. Accordingly, other members within the bis-THIQ family have emerged as prime targets for synthetic chemists, aiming to innovate an orthogonal chemical production of these compounds. With the ability of these complementary strategies to reliably and predictably manipulate molecular structures with atomic precision, this should allow the preparation of synthetic derivatives not existing in nature as new drug leads in the development of novel medicines with desired biological functions.Beyond the biological perspective, bis-THIQ natural products also possess intricate and unique structures, serving as a source of intellectual stimulation for synthetic organic chemists. Within our laboratory, we have developed an integrated program that combines reaction development and target-directed synthesis, leveraging the architecturally complex molecular framework of bis-THIQ natural products as a driving force for the advancement of novel reaction methodologies. In this Account, we unveil our synthetic efforts in a comprehensive story, describing how our synthetic strategy toward bis-THIQ natural products, specifically jorunnamycin A and jorumycin, has evolved over the course of our studies through our key transformations comprising (a) the direct functionalization of isoquinoline N-oxide to prepare the bis-isoquinoline (bis-IQ) intermediate, (b) the diastereoselective and enantioselective isoquinoline hydrogenation to forge the pentacyclic skeleton of the natural product, and (c) the late-stage oxygenation chemistry to adjust the oxidation states of the A- and E-rings. First, we detail our plan in utilizing the aryne annulation strategy to prepare isoquinoline fragments for the bis-THIQ molecules. Faced with unpromising results in the direct C-H functionalization of isoquinoline N-oxide, we lay out in this Account our rationale behind the design of each isoquinoline coupling partner to overcome these challenges. Additionally, we reveal the inspiration for our hydrogenation system, the setup of our pseudo-high-throughput screening, and the extension of the developed hydrogenation protocols to other simplified isoquinolines.In the context of non-natural bis-THIQ molecules, we have successfully adapted this tandem coupling/hydrogenation approach in the preparation of perfluorinated bis-THIQs, representing the first set of electron-deficient non-natural analogues. Finally, we include our unsuccessful late-stage oxygenation attempts prior to the discovery of the Pd-catalyzed C-O cross-coupling reaction. With this full disclosure of the chemistry developed for the syntheses of bis-THIQs, we hope our orthogonal synthetic tactics will provide useful information and serve as an inspiration for the future development of bis-THIQ pharmaceuticals.

A Simple Nickel Metal-Organic Framework-Catalyzed Borylation of Aryl Chlorides and Bromides

We report a recyclable and efficient catalyst system based on a nickel-benzene tricarboxylic acid metal-organic framework (Ni-BTC MOF) for the borylation of aryl halides, including aryl chlorides, with bis(pinacolato)diboron, affording aryl boronate esters in high yields (up to >99% yield) with high selectivity. This protocol demonstrates broad functional group tolerance. Catalyst can be recyclable up to four times, and gram-scale reactions further highlights the usefulness of this method. In situ EPR experiments confirmed the formation of catalytically active Ni(I) species.

Decarbonylative borylation of aryl anhydrides via rhodium catalysis

Decarbonylative borylation of aryl anhydrides by rhodium catalysis has been reported. A base-free system with Rh(PPh3)3Cl as a catalyst enables the efficient synthesis of various arylboronate esters from readily available aryl anhydrides. The reaction involves the cleavage of C(O)-O bonds and the formation of C-B bonds. The experimental results demonstrated that compared with carboxylic acids, amides, and esters, anhydrides have higher reactivity in the decarbonylative borylation reaction under the current conditions. Furthermore, compared with the reported palladium-catalyzed borylation reaction of aryl anhydrides, the present rhodium-catalyzed method has the advantages of a shorter reaction time and a lower reaction temperature.

Pd‐Catalyzed Difluoromethylations of Aryl Boronic Acids, Halides, and Pseudohalides with ICF 2 H Generated ex Situ

An expedient ex‐situ generation of difluoroiodomethane (DFIM) and its immediate use in a Pd‐catalyzed difluoromethylation of aryl boronic acids and ester derivatives in a two‐chamber reactor is reported. Heating a solution of bromodifluoroacetic acid with sodium iodide in sulfolane proved to be effective for the generation of near stoichiometric amounts of DFIM for the ensuing catalytic coupling step. A two‐step difluoromethylation of aryl (pseudo)halides with tetrahydroxydiboron as a low‐cost reducing agent, both promoted by Pd catalysis, proved effective to install this fluorine‐containing C₁ group onto several pharmaceutically relevant molecules. Finally, the method proved adaptable to deuterium incorporation by simply adding D₂O to the DFIM‐generating chamber.

Chemoselective Primary Amination of Aryl Boronic Acids by PIII/PV═O-Catalysis: Synthetic Capture of the Transient Nef Intermediate HNO

A catalytic approach to intercept the transient HNO for a chemoselective primary amination of arylboronic acids is reported. A phosphetane-based catalyst operating within P^III/P^V═O redox cycling is shown to capture HNO, generated in situ by Nef decomposition of 2-nitropropane, to selectively install the primary amino group at aryl C_sp2 centers. The method furnishes versatile primary arylamines from arylboronic acid substrates with the preservation of otherwise reactive functional groups.

Development of Methods to the Synthesis of β-Boryl Acyls, Imines and Nitriles

Organoboron compounds are highly important and versatile synthetic intermediates for the preparation of a wide range of organic molecules. Organoboron compounds have drawn significant attention among organic chemists due to their Lewis acidic property, non-toxicity, and commercial availability. Over the last several decades, there has been a substantial development of new organoboron compounds, useful in organic synthesis. Among all other organoboron compounds, β-boryl carbonyl compounds are the important ones. The β-boryl compounds have appeared as promising intermediates for various synthetic transformations. The 1,4-conjugate addition of diboron reagents to carbon-carbon double bond in the presence of different transition-metal catalysts has been extensively reported by various research groups across the globe. This mini-review outlines the numerous racemic as well as asymmetric β-borylation methods developed to date.

A General, Multimetallic Cross-Ullmann Biheteroaryl Synthesis from Heteroaryl Halides and Heteroaryl Triflates

Despite their importance to medicine and materials science, the synthesis of biheteroaryls by cross-coupling remains challenging. We describe here a new, general approach to biheteroaryls: the Ni- and Pd-catalyzed multimetallic cross-Ullmann coupling of heteroaryl halides with triflates. An array of 5-membered, 6-membered, and fused heteroaryl bromides and chlorides, as well as aryl triflates derived from heterocyclic phenols, proved to be viable substrates in this reaction (62 examples, 63 ± 17% average yield). The generality of this approach to biheteroaryls was further demonstrated in 96-well plate format at 10 μmol scale. An array of 96 possible products provided >90% hit rate under a single set of conditions. Further, low-yielding combinations could be rapidly optimized with a single "Toolbox Plate" of ligands, additives, and reductants.

Transmetalation of boronic acids and their derivatives: mechanistic elucidation and relevance to catalysis

The Suzuki-Miyaura reaction (the cross-coupling reaction of boronic acids with organic halides catalysed by Pd complexes) has been recognised as a useful synthetic organic reaction that forms a C(sp²)-C(sp²) bond. The catalytic cycle of the reaction involves the transmetalation of aryl- and alkenylboronic acids with Pd(II) complexes. It migrates the aryl and alkenyl groups of boronic acid to Pd and produces a Pd-C bond. Many studies have investigated the mechanism of transmetalation. They elucidated the mechanism of the organometallic reaction and its role as a fundamental step in catalytic reactions. This perspective reviews studies on the transmetalation of aryl- and alkenylboronic acids with Pd(II) complexes. Emphasis was laid on the structures and chemical properties of the intermediate Pd complexes and the effects of OH^- on the pathways of the catalytic Suzuki-Miyaura reaction. The reactions of arylboronic acids with Rh(I)-OH complexes were investigated, which are relevant to the mechanism of Rh-catalysed addition of aryl boronic acids to enones and aldehydes. Recent studies on the transmetalation of boronic acids with other late transition metals such as Fe(II), Co(I), Pt(II), Au(III), and Au(I) are presented with the related catalytic reactions and their utilisation in the synthesis of aromatic molecules and π-conjugated materials.

Economical and Readily Accessible Preparation of o,o-Disubstituted Arylboronates through Palladium-Catalyzed Borylation of Haloarenes

Miyaura borylation, that is, palladium-catalyzed cross-coupling between bromoarenes and diboron, offers a versatile method for preparing arylboronates; however, a costly and inaccessible catalyst has been required for synthesizing highly congested arylboronates with the method. Here the Pd(OAc)₂-tri(4-methoxyphenyl)phosphine catalyst was found to work as an efficient catalyst for the sterically demanding borylation. A broad range of o,o-disubstituted bromoarenes were converted into the corresponding arylboronates in high yields by using the palladium catalyst with Cs₂CO₃ in EtOAc at 80 °C.

Palladium-Catalyzed Decarbonylative Borylation of Aryl Anhydrides

A palladium-catalyzed base-free decarbonylative borylation of aryl anhydrides has been developed. Catalyst system consisting of Pd(OAc)₂/dppb enables readily available aryl anhydrides to be employed as electrophiles for the synthesis of versatile arylboronate esters via O-C(O) bond activation and decarbonylation. This method is characterized by an excellent functional group tolerance and broad substrate scope, using bench stable aryl anhydrides as aryl electrophiles in C-B bond formation. Mechanistic studies and functionalization of late-stage pharmaceutical molecules are disclosed.

Five Step Total Synthesis of Lythranidine

A concise synthesis of the alkaloid lythranidine is reported. The strategy exploits the target's local C₂ symmetry by adopting a two directional synthetic approach, first in an acyclic environment, then in a cyclic system and finally in a bridged macrocyclic domain. The latter phase of the synthesis, which installs all four stereocenters, involves a thermodynamically controlled, twofold intermolecular/transannular aza-Michael addition and a twofold hydride reduction. The synthesis is one third of the length of the most step-economic previous approach, providing access to gram quantities of the natural product. The broad-spectrum nature of the synthesis is demonstrated through the preparation of three diastereomeric analogues of the natural product.

Ligand-Enabled δ-C(sp3 )-H Borylation of Aliphatic Amines

Directed C-H functionalization has been realized as a complimentary technique to achieve borylation at a distal position of aliphatic amines. Here, we demonstrated the oxidative borylation at the distal δ-position of aliphatic amines using various borylating agents, a palladium catalyst, and a rightly tuned ligand in the presence of a cheap oxidant. Moreover, an organopalladium δ-C(sp³ )-H-activated intermediate has been isolated and crystallographically characterized to get mechanistic insight.

Investigation of interaction between boronic acids and sugar: effect of structural change of sugars on binding affinity using steady state and time resolved fluorescence spectroscopy and molecular docking

Binding interactions of boronic acid derivatives viz. 2-Methylphenylboronic acid (B1) and 3-Methoxyphenylboronic acid (B2) with mono saccharides (arabinose, fructose and galactose) and disaccharides (sucrose, lactose and maltose) in aqueous condition at pH 7.4 by means of fluorescence spectroscopy is reported in the present investigation. Sugar sensing as well as continuous glucose monitoring (CGM) plays a significant role in diabetes regulation. Sugar sensors mediated through enzymes have their own drawbacks, which led to encouragement to search for designing new sensors through alternate approaches. Among many, fluorescence-based sensors are drawing more attention. Boronic acid-based fluorescence sensors have the capacity to bind reversibly with diols, which makes their demand high in applications. Addition of sugar reduces fluorescence intensities. Change in intensities is associated to cleavage of intermolecular hydrogen bonding which leads in reduced stability of boronate ester. Lineweaver-Burk and Benesi-Hildebrand equation is used for analysing data. Mono sugars are estimated to have higher binding constants. Mutarotation leads to structural changes in saccharides which play a key role in binding interactions. Sugars in furanose form are found to be highly favoured for binding. Molecular docking of B1 and B2 with proteins with PDB ID: 2IPL and 2IPM being periplasmic was done with the help of Schrodinger Maestro 11.2 version. GLIDE scores terms are used for expressing binding affinity.

Transition-Metal-Free Borylation of Aryl Bromide Using a Simple Diboron Source

In this study, we developed a simple transition-metal-free borylation reaction of aryl bromides. Bis-boronic acid (BBA), was used, and the borylation reaction was performed using a simple procedure at a mild temperature. Under mild conditions, aryl bromides were converted to arylboronic acids directly without any deprotection steps and purified by conversion to trifluoroborate salts. The functional group tolerance was considerably high. The mechanism study suggested that this borylation reaction proceeds via a radical pathway.

Understanding the binding interaction between phenyl boronic acid P1 and sugars: determination of association and dissociation constants using S-V plots, steady-state spectroscopic methods and molecular docking

Continuous monitoring of glucose and sugar sensing plays a vital role in diabetes control. The drawbacks of the present enzyme-based sugar sensors have encouraged the investigation into alternate approaches to design new sensors. The popularity of fluorescence sensors is due to their ability to bind reversibly to compounds containing diol. In this study we investigated the binding ability of phenyl boronic acid P1 for monosaccharides and disaccharides (sugars) in aqueous medium at physiological pH 7.4 using steady-state fluorescence and absorbance. P1 fluorescence was quenched due to formation of esters with sugars. Absorbance and fluorescence measurements led to results that indicated that the sugars studied could be ordered in terms of their affinity to P1, as stated: sucrose > lactose > galactose > xylose > ribose > arabinose. In each case, the slope of modified Stern-Volmer plots was nearly 1, indicating the presence of only a single binding site in boronic acids for sugars. Docking studies were carried out using Schrodinger Maestro v.11.2 software. The binding affinity of phenyl boronic acid P1 with periplasmic protein (PDB ID 2IPM and 2IPL) was estimated using GlideScore.

Visible Light-Induced Borylation of C-O, C-N, and C-X Bonds

Boronic acids are centrally important functional motifs and synthetic precursors. Visible light-induced borylation may provide access to structurally diverse boronates, but a broadly efficient photocatalytic borylation method that can effect borylation of a wide range of substrates, including strong C-O bonds, remains elusive. Herein, we report a general, metal-free visible light-induced photocatalytic borylation platform that enables borylation of electron-rich derivatives of phenols and anilines, chloroarenes, as well as other haloarenes. The reaction exhibits excellent functional group tolerance, as demonstrated by the borylation of a range of structurally complex substrates. Remarkably, the reaction is catalyzed by phenothiazine, a simple organic photocatalyst with MW < 200 that mediates the previously unachievable visible light-induced single electron reduction of phenol derivatives with reduction potentials as negative as approximately - 3 V versus SCE by a proton-coupled electron transfer mechanism. Mechanistic studies point to the crucial role of the photocatalyst-base interaction.

Copper-catalyzed mild desulfonylation of vinyl sulfonyl molecules

The first Cu-catalyzed chemical selective desulfonylation of vinyl sulfonyl molecules to olefins was developed using B₂pin₂–water as the reductant.

TriQuinoline

The bottom-up synthesis of structurally well-defined motifs of graphitic materials is crucial to understanding their physicochemical properties and to elicit new functions. Herein, we report the design and synthesis of TriQuinoline (TQ) as a molecular model for pyridinic-nitrogen defects in graphene sheets. TQ is a trimer of quinoline units concatenated at the 2- and 8-positions in a head-to-tail fashion, whose structure leads to unusual aromatisation behaviour at the final stage of the synthesis. The central atomic-sized void endows TQ with high proton affinity, which was confirmed empirically and computationally. TQ•H⁺ is a two-dimensional cationic molecule that displays both π-π and CH-π contact modes, culminating in the formation of the ternary complex ([12]cycloparaphenylene(CPP) ⊃ (TQ•H⁺/coronene)) that consists of TQ•H⁺, coronene (flat), and [12]cycloparaphenylene ([12]CPP) (ring). The water-miscibility of TQ•H⁺ allows it to serve as an efficient DNA intercalator for e.g. the inhibition of topoisomerase I activity.

A Monophosphine Ligand Derived from Anthracene Photodimer: Synthetic Applications for Palladium-Catalyzed Coupling Reactions

Herein, we present an air-stable dianthracenyl monophosphine ligand (diAnthPhos) which can be prepared in two steps from commercially available anthracene derivatives. The ligand exhibits excellent efficiency for palladium-catalyzed coupling reactions. In particular, Miyaura borylation of heterocycle-containing electrophiles can be facilitated employing the diAnthPhos ligand with a broad substrate scope and low catalyst loading. The valuable synthetic utility of the new ligand is further demonstrated by a one-pot Miyaura borylation/Suzuki coupling protocol for heteroaryl-containing substrates.

Novel Carbazole-Based N-Heterocyclic Carbene Ligands to Access Synthetically Relevant Stilbenes in Pd-Catalyzed Coupling Processes

A series of new carbazole-based N-heterocyclic carbene (NHC) ligands have been synthesized in a simple and facile synthetic route and subsequently used in a Pd/carbazole-based NHC catalytic system, which was found to be effective in catalyzing Heck reactions to provide substituted stilbene derivatives in good yields. Several bioactive stilbenes, including pterostilbene, pinosylvin, trimethoxy resveratrol, and resveratrol, were synthesized in good yields, and a 10 mmol scale-up was also performed for trimethoxy resveratrol. The synthetic application was also extended by performing a double-tandem chemoselective Heck reaction followed by Miyaura borylation in a one-pot procedure to give single-step access to synthetically useful stilbenyl boronate esters. Similarly, a unique triple-tandem protocol of a chemoselective Heck reaction/Miyaura borylation/Suzuki-Miyaura coupling reaction sequence was performed for the one-pot modification of biologically relevant molecules.

LiCl-Accelerated Multimetallic Cross-Coupling of Aryl Chlorides with Aryl Triflates

While the synthesis of biaryls has advanced rapidly in the past decades, cross-Ullman couplings of aryl chlorides, the most abundant aryl electrophiles, have remained elusive. Reported here is the first general cross-Ullman coupling of aryl chlorides with aryl triflates. The selectivity challenge associated with coupling an inert electrophile with a reactive one is overcome using a multimetallic strategy with the appropriate choice of additive. Studies demonstrate that LiCl is essential for effective cross-coupling by accelerating the reduction of Ni(II) to Ni(0) and counteracting autoinhibition of reduction at Zn(0) by Zn(II) salts. The modified conditions tolerate a variety of functional groups on either coupling partner (42 examples), and examples include a three-step synthesis of flurbiprofen.

Miyaura borylation/Suzuki-Miyaura coupling (MBSC) sequence of 4-bromo-2,4'-bithiazoles with halides: straightforward access to a heterocylic cluster of d-series of thiopeptide GE2270

Herein, palladium-catalyzed Miyaura borylation of 4-bromo-2,4'-bithiazoles followed by Suzuki-Miyaura cross-coupling reaction (named the MBSC process) with (hetero)aryl- and alkenyl halides is reported. This methodology offers rapid access to various 2',4-disubstituted 2,4'-bithiazole features including naturally-occurring 4-alkenylated and 4-pyridinylated 2,4'-bithiazoles. To prove its application, a concise approach for the synthesis of a heterocyclic cluster of the thiopeptide d-series antibiotic GE2270 is reported through a late-stage MBSC strategy.

Organoboron chemistry comes to light: recent advances in photoinduced synthetic approaches to organoboron compounds

Photoinduced synthetic approaches to organoboron compounds have attracted significant attention in the recent years. Photochemical activation of organic molecules enables generation of reactive intermediates from a variety of precursors, resulting in borylation methods with improved and broader substrate scopes. The review summarizes recent developments in the area of photoinduced reactions of organoboron compounds with an emphasis on borylation of haloarenes, amine derivatives, and redox-active esters of carboxylic acids, as well as photoinduced rearrangements of organoboron compounds and photoinduced synthesis of organoboron compounds from alkenes and alkynes.

Recent advances in the borylative transformation of carbonyl and carboxyl compounds

The recent advances in the borylative transformation of carbonyl and carboxyl compounds are summarized.

Copper-catalysed borylation of aryl chlorides

The first example of a Cu-catalysed borylation of a wide range of aryl chlorides with different electronic and steric properties is mediated by a readily prepared NHC-stabilised Cu catalyst and KO^tBu. The aryl chlorides are converted into their corresponding arylboronic esters using B₂pin₂ or B₂neop₂ as the boron reagent.

Alkylammoniotrifluoroborate functionalized polystyrenes: polymeric pre-catalysts for the metal-free borylation of heteroarenes

Three polymeric versions of ansa-N,N-dialkylammoniumtrifluoroborate ambiphilic molecules based on the styrene motif (poly(1-NMe2H+-2-BF3--4-styrene) (P-Me), poly(1-NEt2H+-2-BF3--4-styrene) (P-Et) and poly(1-piperidinyl-H+-2-BF3--4-styrene) (P-Pip)) were synthesized, characterized and tested as heterogeneous pre-catalysts for the borylation of electron-rich heteroarenes. These heterogeneous versions of previously reported pre-catalysts show similar reactivity patterns and represent the first examples of solid-supported FLP metal-free catalysts for the C-H borylation of heteroarenes.

Decarboxylative Suzuki–Miyaura coupling of (hetero)aromatic carboxylic acids using iodine as the terminal oxidant

A novel methodology for the decarboxylative Suzuki–Miyaura-type coupling of carboxylic acids with arylboronic acids has been developed.

Computational and experimental investigation on the BCl3 promoted intramolecular amination of alkenes and alkynes

The BCl₃ promoted aminoboration of alkenes and alkynes was investigated both computationally and experimentally, leading to the discovery of a metal-free hydroamination of alkynes.