Hydroboration of Alkynes Catalyzed by Pyrrolide-Based PNP Pincer–Iron Complexes

Regioselective Hydroboration of Unsymmetrical Internal Alkynes Catalyzed by a Cobalt Pincer-NHC Complex

Highly regioselective hydroboration of unsymmetrical internal alkynes remains a significant challenge for synthesizing valuable alkenylboronate esters. Herein, we describe an easily synthesizable pincer NHC-based Co complex as a catalyst for the cis-α selective hydroboration of unactivated internal alkynes and the cis-β selective hydroboration of activated internal alkynes with pinacolborane. The reaction showcases high chemo-, regio-, and stereoselectivity, and the catalyst displays high efficiency and very low loading under base-free reaction conditions. The reaction scope was demonstrated by alkynes having a variety of functional groups. The mechanistic studies suggest a feasible Co-boryl intermediate to explain the unusual regioselectivity.

Ligand-free MnBr2-Catalyzed Chemo- and Stereoselective Hydroboration of Terminal Alkynes

Developing simple and benign protocols for synthesizing alkenylboronates is crucial as they are synthetically valuable compounds in various organic transformations. In this work, we report a straightforward ligand-free protocol for synthesizing alkenylboronates via atom-economical hydroboration of alkynes with HBpin catalyzed by a manganese salt. The reaction shows a high level of chemo and regioselectivity for the terminal alkynes and exclusively produces E-selective alkenylboronates. The hydroboration scope is vast, with the resilience of a range of synthetically beneficial functionalities, such as halides, ether, alkenyl, silyl and thiophenyl groups. This reaction proceeds through the involvement of a metal-hydride intermediate. The developed alkenylboronate can be smoothly converted to useful C-C, C-N and C-I bond-forming reactions.

Cobalt catalyzed practical hydroboration of terminal alkynes with time-dependent stereoselectivity

Stereodefined vinylboron compounds are important organic synthons. The synthesis of E-1-vinylboron compounds typically involves the addition of a B-H bond to terminal alkynes. The selective generation of the thermodynamically unfavorable Z-isomers remains challenging, necessitating improved methods. Here, such a proficient and cost-effective catalytic system is introduced, comprising a cobalt salt and a readily accessible air-stable CNC pincer ligand. This system enables the transformation of terminal alkynes, even in the presence of bulky substituents, with excellent Z-selectivity. High turnover numbers (>1,600) and turnover frequencies (>132,000 h-1) are achieved at room temperature, and the reaction can be scaled up to 30 mmol smoothly. Kinetic studies reveal a formal second-order dependence on cobalt concentration. Mechanistic investigations indicate that the alkynes exhibit a higher affinity for the catalyst than the alkene products, resulting in exceptional Z-selective performance. Furthermore, a rare time-dependent stereoselectivity is observed, allowing for quantitative conversion of Z-vinylboronate esters to the E-isomers.

Tandem manganese catalysis for the chemo-, regio-, and stereoselective hydroboration of terminal alkynes: in situ precatalyst activation as a key to enhanced chemoselectivity

The manganese(ii) complex [Mn(iPrPNP)Cl2] (iPrPNP = 2,6-bis(diisopropylphosphinomethyl)pyridine) was found to catalyze the stereo- and regioselective hydroboration of terminal alkynes employing HBPin (pinacolborane). In the absence of in situ activators, mixtures of alkynylboronate and E-alkenylboronate esters were formed, whereas when NaHBEt3 was employed as the in situ activator, E-alkenylboronate esters were exclusively accessed. Mechanistic studies revealed a tandem C-H borylation/semihydrogenation pathway accounting for the formation of the products. Stoichiometric reactions hint toward reaction of a Mn-H active species with the terminal alkyne as the catalyst entry pathway to the cycle, whereas reaction with HBPin led to catalyst deactivation.

The Complex Reactivity of [(salen)Fe]2(μ-O) with HBpin and Its Implications in Catalysis

We report a detailed study into the method of precatalyst activation during alkyne cyclotrimerization. During these studies we have prepared a homologous series of Fe(III)-μ-oxo(salen) complexes and use a range of techniques including UV-vis, reaction monitoring studies, single crystal X-ray diffraction, NMR spectroscopy, and LIFDI mass spectrometry to provide experimental evidence for the nature of the on-cycle iron catalyst. These data infer the likelihood of ligand reduction, generating an iron(salan)-boryl complex as a key on-cycle intermediate. We use DFT studies to interrogate spin states, connecting this to experimentally identified diamagnetic and paramagnetic species. The extreme conformational flexibility of the salan system appears connected to challenges associated with crystallization of likely on-cycle species.

Nickel boryl complexes and nickel-catalyzed alkyne borylation

The first nickel bis-boryl complexes cis-[Ni( ⁱ Pr₂Im^Me)₂(Bcat)₂], cis-[Ni( ⁱ Pr₂Im^Me)₂(Bpin)₂] and cis-[Ni( ⁱ Pr₂Im^Me)₂(Beg)₂] are reported, which were prepared via the reaction of a source of [Ni( ⁱ Pr₂Im^Me)₂] with the diboron(4) compounds B₂cat₂, B₂pin₂ and B₂eg₂ ( ⁱ Pr₂Im^Me = 1,3-di-iso-propyl-4,5-dimethylimidazolin-2-ylidene; B₂cat₂ = bis(catecholato)diboron; B₂pin₂ = bis(pinacolato)diboron; B₂eg₂ = bis(ethylene glycolato)diboron). X-ray diffraction and DFT calculations strongly suggest that a delocalized, multicenter bonding scheme dictates the bonding situation of the NiB₂ moiety in these square planar complexes, reminiscent of the bonding situation of "non-classical" H₂ complexes. [Ni( ⁱ Pr₂Im^Me)₂] also efficiently catalyzes the diboration of alkynes using B₂cat₂ as the boron source under mild conditions. In contrast to the known platinum-catalyzed diboration, the nickel system follows a different mechanistic pathway, which not only provides the 1,2-borylation product in excellent yields, but also provides an efficient approach to other products such as C-C coupled borylation products or rare tetra-borylated compounds. The mechanism of the nickel-catalyzed alkyne borylation was examined by means of stoichiometric reactions and DFT calculations. Oxidative addition of the diboron reagent to nickel is not dominant; the first steps of the catalytic cycle are coordination of the alkyne to [Ni( ⁱ Pr₂Im^Me)₂] and subsequent borylation at the coordinated and, thus, activated alkyne to yield complexes of the type [Ni(NHC)₂(η²-cis-(Bcat)(R)C[double bond, length as m-dash]C(R)(Bcat))], exemplified by the isolation and structural characterization of [Ni( ⁱ Pr₂Im^Me)₂(η²-cis-(Bcat)(Me)C[double bond, length as m-dash]C(Me)(Bcat))] and [Ni( ⁱ Pr₂Im^Me)₂(η²-cis-(Bcat)(H₇C₃)C[double bond, length as m-dash]C(C₃H₇)(Bcat))].

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.

Density Functional Theory Study on the H2-Acceptorless Dehydrogenative Boration of Alkenes Catalyzed by a Zirconium Complex

For the synthesis of vinyl boronate esters, the direct catalytic H₂-acceptorless dehydrogenative boration of alkenes is one of the promising strategies. In this paper, the density functional theory method was employed to investigate the reaction mechanism of dehydrogenative boration and transfer boration of alkenes catalyzed by a zirconium complex (Cp₂ZrH₂). There are two possible pathways for this reaction: the alkene insertion followed by the dehydrogenative boration (path A) and the alkene insertion after the dehydrogenative boration (path B). The calculated results showed that path A is more favorable than path B, and that the rate-determining step is the C-B coupling step with an energy barrier of 18.7 kcal/mol. The reaction modes of the C-B coupling assisted dehydrogenative boration and the alkene insertion were also discussed. These analyses reveal a novel hydrogen release behavior in dehydrogenative boration and the alkene insertion modes and sequences were proposed to be of importance in the chemoselectivity of this reaction. In addition, the X ligand effect (X = H, Cl) on the catalytic activity of the zirconium complex was explored, indicating that the H ligand could enhance the catalytic activity of the complex for styrene dehydrogenative boration.

Stereoselective Semi-Hydrogenations of Alkynes by First-Row (3d) Transition Metal Catalysts

The chemo- and stereoselective semi-hydrogenation of alkynes to alkenes is a fundamental transformation in synthetic chemistry, for which the use of precious 4d or 5d metal catalysts is well-established. In mankind's unwavering quest for sustainability, research focus has considerably veered towards the 3d metals. Given their high abundancy and availability as well as lower toxicity and noxiousness, they are undoubtedly attractive from both an economic and an environmental perspective. Herein, we wish to present noteworthy and groundbreaking examples for the use of 3d metal catalysts for diastereoselective alkyne semi-hydrogenation as we embark on a journey through the first-row transition metals.

Insertion chemistry of iron(II) boryl complexes

Iron(II) boryl complexes of the pyrrole-based pincer ligand, ^CyPNP (^CyPNP = anion of 2,5-bis(dicyclohexylphophinomethyl)pyrrole) have been synthesized and their insertion reactivity interrogated. Compounds of the type [Fe(BE)(^CyPNP)] (E = pinacholato or catecholato) can be generated by treatment of the precursors, [Fe(OPh)(py)(^CyPNP)] or [FeMe(^CyPNP)], with B₂E₂. The boryl complexes are meta stable, but permit additional reactivity with several unsaturated substrates. Reaction with alkynes, RCCR', leads to rapid insertion into the Fe-B bond to generate stable vinyl boronate complexes of the type [Fe(C{R}C{R'}BE)(^CyPNP)] (R, R' = H, Me, Ph, -CCPh). Each of the compounds is five-coordinate in the solid state by virtue of coordination of one of the oxygen atoms of the boronate ester. Similar reaction with nitriles, RCN (R = Ph, Me), results in facile de-cyanation to produce the correpsonding hydrocarbon complexes, [FeR(^CyPNP)]. In the case of the bulky nitrile 1-AdCN, the insertion intermediate, [Fe(C{Ad}NBpin)(^CyPNP)], has been isolated and structurally characterized. Treatment of the boryl complexes with styrene derivatives results in initial insertion to give an alkylboronate complex followed by either β-H elimination or protonation to give the products of C-H borylation and hydroboration, respectively.

Mechanistic Studies of Alkyne Hydroboration by a Well-Defined Iron Pincer Complex: Direct Comparison of Metal-Hydride and Metal-Boryl Reactivity

Iron-hydride and iron-boryl complexes supported by a pyrrole-based pincer ligand, ^tBuPNP (PNP = anion of 2,5-bis(di-tert-butylphosphinomethyl)pyrrole), were employed for a detailed mechanistic study on the hydroboration of internal alkynes. Several novel complexes were isolated and fully characterized, including iron-vinyl and iron-boryl species, which represent likely intermediates in the catalytic hydroboration pathway. In addition, the products of alkyne insertion into the Fe-B bond have been isolated and structurally characterized. Mechanistic studies of the hydroboration reaction favor a pathway involving an active iron-hydride species, [FeH(^tBuPNP)], which readily inserts alkyne and undergoes subsequent reaction with hydroborane to generate product. The iron-boryl species, [Fe(BR₂)(^tBuPNP)] (R₂ = pin or cat), was found to be chemically competent, although its use in catalysis entailed an induction period whereby the iron-hydride species was generated. Stoichiometric reactions and kinetic experiments were performed to paint a fuller picture of the mechanism of alkyne hydroboration, including pathways for catalyst deactivation and the influence of substrate bulk on catalytic efficacy.

Palladium-catalyzed hydroboration reaction of unactivated alkynes with bis (pinacolato) diboron in water

A highly efficient and mild palladium-catalyzed hydroboration of unactivated internal alkynes in water is described. Both aryl- and alkyl-substituted alkynes proceeded smoothly within the reaction time to afford the desired vinylboronates in moderate to high yields. Bis (pinacolato) diboron was used to afford α- and β-hydroborated products in the presence of HOAc. These reactions showed high reactivities and tolerance, thus providing a promising method for the synthesis of alkenyl boron compounds.

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.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

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.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Organophosphorus and Iron Catalysis: Good Partners for Hydrometalation of Olefins and Alkynes

The last decades have seen an impressive development of iron complexes involving organophosphorus ligands applied in homogeneous catalyzed hydrometalation of olefins and alkynes. Two main topics will be covered in this JOCSynopsis: (i) an overview of the achievements in the area of iron-catalyzed hydrophosphination and then (ii) hydrosilylation, hydroborylation, and hydromagnesiation reactions promoted by catalysts based on organophosphorus ligands and iron.

Reaction Pathways and Redox States in α-Selective Cobalt-Catalyzed Hydroborations of Alkynes

Cobalt(II) alkyl complexes supported by a monoanionic NNN pincer ligand are pre‐catalysts for the regioselective hydroboration of terminal alkynes, yielding the Markovnikov products with α:β‐(E) ratios of up to 97:3. A cobalt(II) hydride and a cobalt(II) vinyl complex appear to determine the main reaction pathway. In a background reaction the highly reactive hydrido species specifically converts to a coordinatively unsaturated cobalt(I) complex which was found to re‐enter the main catalytic cycle.

Z-Selective Alkyne Functionalization Catalyzed by a trans-Dihydride N-Heterocyclic Carbene (NHC) Iron Complex

The Z-selective functionalization of terminal alkynes is a useful transformation in organic chemistry and mainly catalyzed by noble metals. Here, we present the Z-selective hydroboration of terminal alkynes catalyzed by a stable trans-dihydride iron complex [(PC_NHCP)Fe(H)₂N₂)] (2). Overall, the reaction occurs at room temperature and provides near quantitative yields of the Z-vinylboronate ester. Interestingly, the same catalyst could also provide the E-vinylboronate by heating the reaction mixture at slightly elevated temperatures (50 °C). If, however, the reaction is performed in the absence of HBpin, rapid Z-selective alkyne dimerization is observed, which is further discussed in this report.

Nickel(II)-Catalyzed Borylation of Alkenyl Methyl Ethers via C-O Bond Cleavage

A new protocol has been developed for the borylation of conjugated alkenyl methyl ethers using B₂Pin₂ via C-O bond cleavage catalyzed by Ni(II). In this cross-coupling reaction, both E/Z isomers of alkenyl ethers are converted into (E)-alkenyl boronic esters with good reactivity. This transformation exhibits high chemoselectivity in the presence of competitive C-O bonds such as aryl ether, ester, amide, and thioether groups, thus providing a new method for the construction of various alkenyl boronates.

Copper-catalyzed cross-coupling of vinyliodonium salts and diboron reagents to generate alkenyl boronic esters

An efficient approach for the synthesis of alkenyl boronic esters through the copper-catalyzed cross-coupling of vinyliodonium salts and diboron reagents is reported. This method is distinguished by its mild conditions and short reaction time of less than 30 min, which should provide an additional way for the construction of alkenyl boronic esters.

Cobalt-Catalyzed Regio- and Stereoselective Hydroboration of Allenes

An efficient pincer-ligand-based cobalt-complex-catalyzed allene hydroboration affording Z-allylic boronates is described. The reaction demonstrates an excellent regio- as well as Z-stereoselectivity and a wide substrate scope that tolerates many functional groups. Based on solvent-assisted electrospray ionization mass spectrometry (SAESI-MS) studies, a rationale for the cobalt-catalyzed hydroboration involving the highly selective insertion of an allene into the Co-H bond to form Z-allylic cobalt intermediates is proposed.

Zirconium-Catalyzed Synthesis of Alkenylaminoboranes: From a Reliable Preparation of Alkenylboronates to a Direct Stereodivergent Access to Alkenyl Bromides

A simple procedure has been optimized for the preparation of alkenylaminoborane from alkynes using diisopropylaminoborane and HZrCp₂Cl. Coupled with a magnesium-catalyzed dehydrogenation, it allowed for the use of air- and moisture-stable diisopropylamine. This synthesis has been extended to a one-pot sequence leading directly to bromoalkenes with controlled stereochemistry. As such, it provides an easy, scalable, cheap process to access alkenylboronates and both (E)- and (Z)-bromoalkenes from commercially available alkynes.

Highly Chemoselective Hydroboration of Alkynes and Nitriles Catalyzed by Group 4 Metal Amidophosphine-Borane Complexes

We report a series of titanium and zirconium complexes supported by dianionic amidophosphine-borane ligands, synthesized by amine elimination and salt metathesis reactions. The Ti^IV complex [{Ph₂P(BH₃)N}₂C₆H₄Ti(NMe₂)₂] (1) was obtained by the reaction between tetrakis-(dimethylamido)titanium(IV) and the protic aminophosphine-borane ligand [{Ph₂P(BH₃)NH}₂C₆H₄] (LH2) at ambient temperature. Both the heteroleptic zirconium complexes-[η⁵-(C₅H₅)₂Zr{Ph₂P(BH₃)N}₂C₆H₄] (2) and [[{Ph₂P(BH₃)N}₂C₆H₄]ZrCl₂] (3)-and the homoleptic zirconium complex [[{Ph₂P(BH₃)N}₂C₆H₄]₂Zr] (4) were obtained in good yield by the salt metathesis reaction of either zirconocene dichloride [η⁵-(C₅H₅)₂ZrCl₂] or zirconium tetrachloride with the dilithium salt of the ligand [{Ph₂P(BH₃)NLi}₂C₆H₄] (LLi2), which was prepared in situ. The molecular structures of the complexes 1, 2, and 4 in their solid states were confirmed by single-crystal X-ray diffraction analysis. Of these complexes, only titanium complex 1 acts as an effective catalyst for the facile hydroboration of terminal alkynes, yielding exclusive E-isomers. The hydroboration of organic nitriles yielded diborylamines with a broad substrate scope, including broad functional group compatibility. The mechanism of hydroboration occurs through the formation of titanium hydride as an active species.

Copper-Catalyzed Triboration of Terminal Alkynes Using B2 pin2 : Efficient Synthesis of 1,1,2-Triborylalkenes

We report herein the catalytic triboration of terminal alkynes with B2 pin2 (bis(pinacolato)diboron) using readily available Cu(OAc)2 and Pn Bu3 . Various 1,1,2-triborylalkenes, a class of compounds that have been demonstrated to be potential matrix metalloproteinase (MMP-2) inhibitors, were obtained directly in moderate to good yields. The process features mild reaction conditions, a broad substrate scope, and good functional group tolerance. This copper-catalyzed reaction can be conducted on a gram scale to produce the corresponding 1,1,2-triborylalkenes in modest yields. The utility of these products was demonstrated by further transformations of the C-B bonds to prepare gem-dihaloborylalkenes (F, Cl, Br), monohaloborylalkenes (Cl, Br), and trans-diaryldiborylalkenes, which serve as important synthons and have previously been challenging to prepare.

Hydroboration of internal alkynes catalyzed by FeH(CO)(NO)(PPh3)2: a case of boron-source controlled regioselectivity

The Fe–H complex FeH(CO)(NO)(Ph₃P)₂ catalyzes the stereoselective, regiodivergent hydroboration of internal alkynes using either pinacolborane (HBpin) or bis(pinacolato)diboron (B₂pin₂) as a boron source.

Iron catalyzed stereoselective alkene synthesis: a sustainable pathway

Replacing expensive or toxic transition metals with iron has become an important trend. This article summarises the recent progresses of a wide range of Fe-catalyzed reactions for accessing various stereodefined alkenes.

Emergence and Applications of Base Metals (Fe, Co, and Ni) in Hydroboration and Hydrosilylation

Base metal catalysis offers an alternative to reactions, which were once dominated by precious metals in hydrofunctionalization reactions. This review article details the development of some base metals (Fe, Co, and Ni) in the hydroboration and hydrosilylation reactions concomitant with a brief overview of recent advances in the field. Applications of both commercially available metal salts and well-defined metal complexes in catalysis and opportunities to further advance the field is discussed as well.

Catalytic C-H Borylation Using Iron Complexes Bearing 4,5,6,7-Tetrahydroisoindol-2-ide-Based PNP-Type Pincer Ligand

Catalytic C-H borylation has been reported using newly designed iron complexes bearing a 4,5,6,7-tetrahydroisoindol-2-ide-based PNP pincer ligand. The reaction tolerated various five-membered heteroarenes, such as pyrrole derivatives, as well as six-membered aromatic compounds, such as toluene. Successful examples of the iron-catalyzed sp³ C-H borylation of anisole derivatives were also presented.