Improvement in the Palladium-Catalyzed Miyaura Borylation Reaction by Optimization of the Base: Scope and Mechanistic Study

Pd-Catalyzed Miyaura Borylation and Telescopic Borylation/Suzuki-Miyaura Cross-Coupling Processes in Deep-Eutectic Solvents

We report an efficient procedure to carry out palladium-catalyzed Miyaura borylation reactions of (hetero)aromatic halides and triflates in choline chloride (ChCl)-based deep eutectic solvents (DESs). The procedure employs bis(pinacolato)diboron as a boron source and a catalyst prepared in situ from readily available Pd2(dba)3 and the phosphine ligand XPhos. Reactions proceed well in different ChCl-based DESs, among which the best results were provided by environmentally friendly and biodegradable mixtures with glycerol and glucose. The reaction tolerates both EDG and EWG substituents on the substrates and can be run on different halides (chloride, bromide, iodide) and pseudohalides (triflate). Furthermore, for several substrates, the catalyst loading can be reduced to 1 mol % Pd (0.5% mol Pd2(dba)3) without compromising the reaction yield. Moreover, we show that the Miyaura borylation protocol in DES can be combined with a subsequent Suzuki-Miyaura cross-coupling reaction in a one-pot procedure, allowing access to various biaryl products and demonstrating its synthetic utility by preparing the precursors of two compounds with reported applications in the photovoltaics sector. Finally, two green metrics (E-factor and EcoScale) of the new one-pot procedure in DES were calculated and compared with literature values to assess the potential advantages in terms of waste reduction, safety, and energy consumption.

Catalytic 2-Ethylhexanoic Acid Promotes Mild Miyaura Borylations

The Miyaura borylation of aryl and heteroaryl chlorides and bromides using a combination of potassium carbonate and 5 mol % 2-ethylhexanoic acid at 25 °C is reported. The in situ generation of a catalytic amount of potassium 2-ethylhexanoate under these conditions avoids the need for special handling of stoichiometric quantities of hygroscopic potassium 2-ethylhexanoate during the reaction setup as well as difficulties in removing the resulting carboxylic acid during product isolation.

Palladium-Catalyzed ipso-Borylation of Aryl Halides Promoted by Lewis Acid-Mediated Electrophilic Activation of Aryl(halo)palladium(II) Complex

Palladium-catalyzed ipso-borylation of aryl halides, well-known as Miyaura borylation, is one of the reliable synthetic methods for organoborons. This reaction involves base-mediated nucleophilic activation of diboron that enables transmetalation of an aryl(halo)palladium(II) intermediate with a diboron. As an alternative, herein, we have established Lewis acid-mediated conditions for borylating (pseudo)haloarenes that require no external base. The electrophilic activation of the aryl(halo)palladium(II) intermediate via dehalogenation with Lewis acidic zinc complexes promotes the borylation.

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.

Iron-Catalyzed, Directed Benzylic Borylation

Herein, we describe an iron-catalyzed benzylic C-H borylation reaction. The reaction targets primary and secondary C(sp³)-H bonds to deliver high-value boronic esters under mild conditions with short (7-8 min) reaction times. Functional groups are well tolerated, and complete site selectivity is observed in the presence of multiple benzylic C-H bonds.

Protoboration of Alkynes and Miyaura Borylation Catalyzed by Low Loadings of Palladium

The versions of Miyaura borylation and protoboration of alkynes catalyzed by low loadings of palladium (400 mol ppm = 0.04 mol %) have been developed. These transformations have a broad substrate scope, good functional-group compatibility, and gram-scale synthetic ability.

Palladium-catalyzed solid-state borylation of aryl halides using mechanochemistry

This study describes the solid-state palladium-catalyzed cross-coupling between aryl halides and bis(pinacolato)diboron using ball milling. The reactions were completed within 10 min for most aryl halides to afford a variety of synthetically useful arylboronates in high yields. Notably, all experimental operations could be performed in air, and did not require the use of large amounts of dry and degassed organic solvents. The utility of this method was further demonstrated by gram-scale synthesis under solvent-free, mechanochemical conditions.

Inorganometallics (Transition Metal-Metalloid Complexes) and Catalysis

While the formation and breaking of transition metal (TM)–carbon bonds plays a pivotal role in the catalysis of organic compounds, the reactivity of inorganometallic species, that is, those involving the transition metal (TM)–metalloid (E) bond, is of key importance in most conversions of metalloid derivatives catalyzed by TM complexes. This Review presents the background of inorganometallic catalysis and its development over the last 15 years. The results of mechanistic studies presented in the Review are related to the occurrence of TM–E and TM–H compounds as reactive intermediates in the catalytic transformations of selected metalloids (E = B, Si, Ge, Sn, As, Sb, or Te). The Review illustrates the significance of inorganometallics in catalysis of the following processes: addition of metalloid–hydrogen and metalloid–metalloid bonds to unsaturated compounds; activation and functionalization of C–H bonds and C–X bonds with hydrometalloids and bismetalloids; activation and functionalization of C–H bonds with vinylmetalloids, metalloid halides, and sulfonates; and dehydrocoupling of hydrometalloids. This first Review on inorganometallic catalysis sums up the developments in the catalytic methods for the synthesis of organometalloid compounds and their applications in advanced organic synthesis as a part of tandem reactions.

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.

Protodeboronation of (Hetero)Arylboronic Esters: Direct versus Prehydrolytic Pathways and Self-/Auto-Catalysis

The kinetics and mechanism of the base-catalyzed hydrolysis (ArB(OR)₂ → ArB(OH)₂) and protodeboronation (ArB(OR)₂ → ArH) of a series of boronic esters, encompassing eight different polyols and 10 polyfluoroaryl and heteroaryl moieties, have been investigated by in situ and stopped-flow NMR spectroscopy (¹⁹F, ¹H, and ¹¹B), pH-rate dependence, isotope entrainment, ²H KIEs, and KS-DFT computations. The study reveals the phenomenological stability of boronic esters under basic aqueous-organic conditions to be highly nuanced. In contrast to common assumption, esterification does not necessarily impart greater stability compared to the corresponding boronic acid. Moreover, hydrolysis of the ester to the boronic acid can be a dominant component of the overall protodeboronation process, augmented by self-, auto-, and oxidative (phenolic) catalysis when the pH is close to the pK_a of the boronic acid/ester.

Construction of Axially Chiral Arylborons via Atroposelective Miyaura Borylation

Compared with the well-developed centrally chiral boron chemistry, C-B axially chiral chemistry remains elusive and challenging. Herein we report the first atroposelective Miyaura borylation of bromoarenes with unsymmetrical diboron reagents for the direct catalytic synthesis of optically active atropisomeric arylborons. This reaction features broad substrate scope and produces axially chiral arylborons with high yields and good enantioselectivities.

An evaluation of palladium-based catalysts for the base-free borylation of alkenyl carboxylates

Palladium catalysis can achieve the base-free borylation of alkenyl carboxylates, enabling direct access to functionalized enones and heterocycles.