AgSbF6-Catalyzed anti-Markovnikov hydroboration of terminal alkynes

Radical hydroboration for the synthesis of organoboron compounds

Organoboron compounds demonstrate diverse applications in the fields of organic synthesis, materials science, and medicinal chemistry. Compared to the conventional hydroboration reaction, radical hydroboration serves as an alternative approach for the synthesis of organoborons via different mechanisms. In radical hydroboration, a boryl radical is initially generated from homolytic cleavage of a B-H or a B-B bond, which is then added to an unsaturated double bond to deliver a carbon radical. Subsequent hydrogen atom transfer or reduction of the carbon radical to form a carbanion followed by protonation gave the final product. Over the past few years, numerous efforts have been made for efficient synthesis of boryl radicals and the expansion of substrate scope of the radical hydroboration reaction. Here, we discuss the recent advancement of radical hydroboration and its associated mechanisms. Numerous radical hydroboration strategies employing N-heterocyclic carbene borane, bis(pinacolato)diboron and pinacolborane as the boron source were illustrated. Thermochemical, photochemical and electrochemical strategies for the generation of boryl radicals were also discussed in detail.

Coinage metal amido and thiolate SNS complexes: consequences of catalyst speciation in Cu(I)-catalysed carbonyl hydroboration

Three new IPr-Ag- and -Au-SNS amido and thiolate complexes were synthesized and compared to their previously reported Cu analogues as carbonyl hydroboration catalysts (IPr = bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Although these complexes showed no catalytic activity, treatment of the IPr-Ag-SNS amido complex with pinacolborane released the N-borylated ligand, S^MeN^BpinS^Me, (L1-Bpin). This finding led us to reinvestigate the IPr-Cu-SNS amido precatalyst, revealing that immediate loss of L1-Bpin converts our catalyst system to [CuH(IPr)]₂.

An ionic Fe-based metal-organic-framework with 4'-pyridyl-2,2':6',2''-terpyridine for catalytic hydroboration of alkynes

An ionic metal-organic-framework (MOF) containing nanoscale channels was readily assembled from ditopic 4'-pyridyl-2,2':6',2''-terpyridine (pytpy) and a simple iron(ii) salt. X-ray structural analysis revealed a two-dimensional grid-like framework assembled by classic octahedral (pytpy)2FeII cations as linkers (with pytpy as a new ditopic pyridyl ligand) and octa-coordinate FeCl2 centers as nodes. The layer-by-layer assembly of the 2-D framework resulted in the formation of 3-D porous materials consisting of nano-scale channels. The charges of the cationic framework were balanced with anionic Cl3FeOFeCl3 in its void channels. The new Fe-based MOF material was employed as a precatalyst for syn-selective hydroboration of alkynes under mild, solvent-free conditions in the presence of an activator, leading to the synthesis of a range of trans-alkenylboronates in good yields. The larger scale applicability and recyclability of the new MOF catalyst was further explored. This represents a rare example of an ionic MOF material that can be utilized in hydroboration catalysis.

A simple synthesis of surfactant-free polycrystalline CuO nanoparticles supported on carbon nanofibers for regioselective hydroboration of alkynes

Copper oxide nanoparticles (CuO NPs) with a clean surface supported on carbon nanofibers (CNFs) in one-pot were prepared by simple solid-state grinding and aging followed by thermal treatment, yielding CuO nanoparticles with high (23.8 wt%) and uniform CuO loading in the absence of surfactants and solvent. The CuO NPs on CNFs (CuO/CNF) showed excellent catalytic activity in transforming a wide variety of alkynes into alkenyl boronates using B2Pin2 as a boron source in the absence of ligand and additives with high regioselectivity under mild conditions. Moreover, the present CuO/CNF catalyst was recyclable and possesses excellent chemoselectivity. The developed green synthetic approach is expected to offer exciting opportunities for designing monometallic or bimetallic nanoparticle catalysts on different supports applied in heterogeneous catalysis.

Silver Catalyzed One-Pot Three-Component Synthesis of α-Aminonitriles and Biologically Relevant α-Amino-phosphonates

A simple silver salt (AgSbF6) catalyzed aminophosphonylation and Strecker reaction have been developed and successfully applied to a wide range of substrates (>55 substrates). This solvent-, ligand-, and base-free one-pot three component protocol operates effectively at room temperature to provide diversified α-aminophosphonates and α-aminonitriles, which gave access to the respective α-amino amides. Importantly, the present catalyst system is also capable to produce the rarely reported and biologically relevant aminophosphonates (having anti-leishmanial activity). Further, the mechanistic studies reveal that the present phosphonylation protocol follows a radical pathway.

Simple silver(I)-salt catalyzed selective hydroboration of isocyanates, pyridines, and quinolines

AgSbF₆ has been established as an effective catalyst for the hydroboration of structurally and electronically diverse isocyanates under ligand- and solvent-free conditions which selectively yielded either N-boryl formamides or N-boryl methylamines under different conditions. Further, various N-heterocycles can be selectively hydroborated using this simple catalytic system; pyridine derivatives undergo preferential 1,4 hydroboration whereas the formation of tetrahydroquinoline (after hydrolysis) via complete heterocycle hydrogenation was observed for quinolines.

Hexamethyldisilazane Lithium (LiHMDS)-Promoted Hydroboration of Alkynes and Alkenes with Pinacolborane

Lithium-promoted hydroboration of alkynes and alkenes using commercially available hexamethyldisilazane lithium as a precatalyst and HBpin as a hydride source has been developed. This method will be appealing for organic synthesis because of its remarkable substrate tolerance and good yields. Mechanistic studies revealed that the hydroboration proceeds through the in situ-formed BH₃ species, which acts to drive the turnover of the hydroboration of alkynes and alkenes.

The transition metal-catalysed hydroboration reaction

This review covers the development of the transition metal-catalysed hydroboration reaction, from its beginnings in the 1980s to more recent developments including earth-abundant catalysts and an ever-expanding array of substrates.

Cobalt-Catalyzed Hydroboration of Terminal and Internal Alkynes

A novel methodology to access synthetically versatile vinylboronic esters through a ligand-controlled cobalt-catalyzed hydroboration of terminal and internal alkynes is reported. The approach relies on the in situ reduction of Co(II) by H-BPin in the presence of bisphosphine ligands generating catalytically active Co(I) hydride complexes. This procedure avoids the use of stoichiometric amounts of base, and no boron-containing byproducts are generated which is translated into high functional group tolerance and atom economy.

Tropylium-Promoted Hydroboration Reactions: Mechanistic Insights Via Experimental and Computational Studies

Hydroboration reaction of alkynes is one of the most synthetically powerful tools to access organoboron compounds, versatile precursors for cross-coupling chemistry. This type of reaction has traditionally been mediated by transition-metal or main group catalysts. Herein, we report a novel method using tropylium salts, typically known as organic oxidants and Lewis acids, to promote the hydroboration reaction of alkynes. A broad range of vinylboranes can be easily accessed via this metal-free protocol. Similar hydroboration reactions of alkenes and epoxides can also be efficiently catalyzed by the same tropylium catalysts. Experimental studies and DFT calculations suggested that the reaction follows an uncommon mechanistic pathway, which is triggered by the hydride abstraction of pinacolborane with tropylium ion. This is followed by a series of in situ counterion-activated substituent exchanges to generate boron intermediates that promote the hydroboration reaction.

Stereoselective Rhodium-Catalyzed Isomerization of Stereoisomeric Mixtures of Arylalkenes

A new efficient method for the synthesis of a high ratio of E-alkenes from E/Z mixtures of alkenes with B2pin2 in the presence of a rhodium catalyst is described. This reaction features mild reaction conditions, broad functional group tolerance, and highly great application potential.

Wet carbonate-promoted radical arylation of vinyl pinacolboronates with diaryliodonium salts yields substituted olefins

Since the landmark work of Heck, Negishi and Suzuki on Pd-catalyzed crossing coupling reactions, innovative discovery of new reactions forming C-C bonds and constructing functional olefins via nonmetal catalysts remains an imperative area in organic chemistry. Herein, we report a transition-metal-free arylation method of vinyl pinacolboronates with diaryliodonium salts to form C(sp²)-C(sp²) bond and provide trans-arylvinylboronates. The resulting vinylboronates can further react with the remaining aryl iodides (generated from diaryliodonium salts) via Suzuki coupling to afford functional olefins, offering an efficient use of aryliodonium salts. Computational mechanistic studies suggest radical-pair pathway of the diaryliodonium salts promoted by the multi-functional wet carbonate.

Catalyst-free and solvent-free hydroboration of alkynes

The hydroboration of alkynes with pinacolborane (HBpin) under catalyst- and solvent-free conditions was demonstrated.

Catalytic Hydroboration of Aldehydes, Ketones, and Alkenes Using Potassium Carbonate: A Small Key to Big Transformation

An efficient transition-metal-free protocol for the hydroboration of aldehydes and ketones (reduction) was developed. The hydroboration of a wide range of aldehydes and ketones with pinacolborane (HBpin) under the K₂CO₃ catalyst has been studied. The reaction system is practical and reliable and proceeds under extremely mild and operationally simple conditions. No prior preparation of the complex metal catalyst was required, and hydroboration occurred stoichiometrically. Further, the chemoselective reduction of aldehydes over ketones was carried out. Moreover, we demonstrated the use of K₂CO₃ as an efficient catalyst for the hydroboration of alkenes. The operational simplicity, inexpensive and transition-metal-free catalyst, and the applicability to gram-scale synthesis strengthen its potential applications for hydroboration (reduction) at an industrial scale.

Catalyst-Free and Solvent-Free Facile Hydroboration of Imines

A facile process for the catalyst-free and solvent-free hydroboration of aromatic as well as heteroaromatic imines is reported. This atom-economic methodology is scalable, compatible with sterically and electronically diverse imines, displaying excellent tolerance towards various functional groups, and works efficiently at ambient temperature in most of the cases, affording secondary amines in good to excellent yield after hydrolysis.

Base-catalysed reductive relay hydroboration of allylic alcohols with pinacolborane to form alkylboronic esters

An unprecedented base-catalysed reductive relay hydroboration of allylic alcohols is described. Commercially available nBuLi was found to be a robust transition metal-free initiator for this protocol, affording various boronic esters in high yield and selectivity. Mechanistically, this methodology involves a one-pot three-step successive process (dehydrocoupling/allylic hydride substitution/anti-Markovnikov hydroboration).

Silver-Catalyzed anti-Markovnikov Hydroboration of C-C Multiple Bonds

A simple silver salt (AgOAc)-catalyzed anti-Markovnikov-selective hydroboration of alkenes, 1,3-dienes, and alkynes with pinacolborane (HBpin) has been described. This strategy provides an efficient and practical method to access various alkyl-, allyl-, and ( E)-alkenylboronate esters in good to excellent yields with regio- and stereoselectivity under ligand- and base-free conditions.

Lithium diisobutyl-tert-butoxyaluminum hydride (LDBBA) catalyzed hydroboration of alkynes and imines with pinacolborane

Lithium diisobutyl-tert-butoxyaluminum hydride (LDBBA) catalyzed hydroboration of alkynes and imines with pinacolborane (HBpin) was demonstrated.

nBuLi-promoted anti-Markovnikov selective hydroboration of unactivated alkenes and internal alkynes

A novel anti-Markovnikov selective hydroboration of alkenes and alkynes with HBpin using ⁿBuLi as an effective initiator is described.

Highly efficient and selective hydroboration of terminal and internal alkynes catalysed by a cobalt(ii) coordination polymer

Selective hydroboration of terminal and internal alkynes is catalysed using a Co^II-coordination polymer precatalyst with TOFs of up to 47 520 h⁻¹.