A Valuable, Inexpensive CuI/N-Heterocyclic Carbene Catalyst for the Selective Diboration of Styrene

Phosphine-catalyzed 1,2-cis-diboration of 1,3-butadiynes

Trialkyl phosphines PMe3 and PEt3 catalyze the 1,2-cis-diboration of 1,3-butadiynes to give 1,2-diboryl enynes. The products were utilized to synthesize 1,1,2,4-tetraaryl enynes using a Suzuki-Miyaura protocol and can readily undergo proto-deborylation.

Wingtip-Flexible N-Heterocyclic Carbenes: Unsymmetrical Connection between IMes and IPr

IMes (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and IPr (IPr=1,3- bis(2,6-diisopropylphenyl)imidazol-2-ylidene) represent by far the most frequently used N-heterocyclic carbene ligands in homogeneous catalysis, however, despite numerous advantages, these ligands are limited by the lack of steric flexibility of catalytic pockets. We report a new class of unique unsymmetrical N-heterocyclic carbene ligands that are characterized by freely-rotatable N-aromatic wingtips in the imidazol-2-ylidene architecture. The combination of rotatable N-CH2 Ar bond with conformationally-fixed N-Ar linkage results in a highly modular ligand topology, entering the range of geometries inaccessible to IMes and IPr. These ligands are highly reactive in Cu(I)-catalyzed β-hydroboration, an archetypal borylcupration process that has had a transformative impact on the synthesis of boron-containing compounds. The most reactive Cu(I)-NHC in this class has been commercialized in collaboration with MilliporeSigma to enable broad access of the synthetic chemistry community. The ligands gradually cover %Vbur geometries ranging from 37.3 % to 52.7 %, with the latter representing the largest %Vbur described for an IPr analogue, while retaining full flexibility of N-wingtip. Considering the modular access to novel geometrical space in N-heterocyclic carbene catalysis, we anticipate that this concept will enable new opportunities in organic synthesis, drug discovery and stabilization of reactive metal centers.

Selective, Transition Metal-free 1,2-Diboration of Alkyl Halides, Tosylates, and Alcohols

Defunctionalization of readily available feedstocks to provide alkenes for the synthesis of multifunctional molecules represents an extremely useful process in organic synthesis. Herein, we describe a transition metal‐free, simple and efficient strategy to access alkyl 1,2‐bis(boronate esters) via regio‐ and diastereoselective diboration of secondary and tertiary alkyl halides (Br, Cl, I), tosylates, and alcohols. Control experiments demonstrated that the key to this high reactivity and selectivity is the addition of a combination of potassium iodide and N,N‐dimethylacetamide (DMA). The practicality and industrial potential of this transformation are demonstrated by its operational simplicity, wide functional group tolerance, and the late‐stage modification of complex molecules. From a drug discovery perspective, this synthetic method offers control of the position of diversification and diastereoselectivity in complex ring scaffolds, which would be especially useful in a lead optimization program.

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.

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.

Palladium-Catalyzed Remote Diborylative Cyclization of Dienes with Diborons via Chain Walking

A novel method for catalytic remote bismetalation of alkene substrates by the addition of dimetal reagents is accomplished by using chain walking. In the presence of a palladium catalyst, the reaction of various 1,n-dienes and diborons were converted into cyclopentane derivatives with two boryl groups at remote positions via facile regioselective transformation of an unactivated sp³ C-H bond to a C-B bond. Sequential construction of three distant bonds, which is difficult to achieve by any method, was accomplished for the reactions of 1,n-dienes (n ≥ 7).

Alkoxide activation of tetra-alkoxy diboron reagents in C-B bond formation: a decade of unpredictable reactivity

Any attempt to facilitate a new generation of C-B bonds represents a useful tool in organic synthesis. In addition, if that approach highlights the nucleophilic character of boryl moieties in the absence of transition metal complexes, the challenge to create new reactive platforms becomes an opportunity. We have been deeply involved in the experimental and theoretical validation of C-B bond formation by means of alkoxide activation of tetra-alkoxy diboron reagents and here is presented a convenient guide to understand the concept and the applications.

Stereoselective Diboration of Spirocyclobutenes: A Platform for the Synthesis of Spirocycles with Orthogonal Exit Vectors

The diastereo- and enantioselective diboration of spirocyclobutenes provides a platform for the rapid preparation of a wide variety of chiral spirocyclic building blocks. The chemoselective functionalization of the carbon-boron bond in the products, including a stereospecific sp³ -sp² Suzuki-Miyaura cross-coupling reaction, provides a powerful tool to control the directionality and the nature of the exit vectors in the spirocyclic framework.

Mononuclear Cu Complexes Based on Nitrogen Heterocyclic Carbene: A Comprehensive Review

During the last decade, organometallic, coordination, and catalytic chemistry of the three-dimensional metals such as copper (Cu) has been greatly affected by the emergence of nitrogen heterocyclic carbene (NHC) complexes. The NHCs, and in particular the mononuclear Cu^I-based ones, have been proven vastly useful in several applications such as in biosynthesis, catalysis, photochemistry, etc. This review tries to thoroughly describe a series of mononuclear Cu^I NHC complexes and their subcategories such as heteroleptics, and bidentate and tridentate heteroatom complexes, and give some detailed insights on their development, emergence, and applications. A brief outlook is also disclosed to enable other researchers to further develop a platform for future advances and studies in the field of Cu^I-based NHCs.

Regio- and Stereoselective Synthesis of 1,1-Diborylalkenes via Brønsted Base-Catalyzed Mixed Diboration of Alkynyl Esters and Amides with BpinBdan

The NaOtBu-catalyzed mixed 1,1-diboration of terminal alkynes using the unsymmetrical diboron reagent BpinBdan (pin = pinacolato; dan = 1,8-diaminonaphthalene) proceeds in a regio- and stereoselective fashion affording moderate to high yields of 1,1-diborylalkenes bearing orthogonal boron protecting groups. It is applicable to gram-scale synthesis without loss of yield or selectivity. The mixed 1,1-diborylalkene products can be utilized in Suzuki-Miyaura cross-coupling reactions which take place selectivly at the C-B site. DFT calculations suggest the NaOtBu-catalyzed mixed 1,1-diboration of alkynes occurs through deprotonation of the terminal alkyne, stepwise addition of BpinBdan to the terminal carbon followed by protonation with tBuOH. Experimentally observed selective formation of (Z)-diborylalkenes is supported by our theoretical studies.

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.

Stereo- and regio-selective synthesis of silicon-containing diborylalkenes via platinum-catalyzed mono-lateral diboration of dialkynylsilanes

A highly chemoselective platinum-catalyzed mono-lateral diboration of dialkynylsilanes for the construction of silicon-tethered alkynyl diborylalkenes is described, in which tris(4-methoxyphenyl)phosphine was found to be an effective ligand for the cis-addition of diboron agents to the silicon-tethered alkynes, and the chiral ligand (AFSi-Phos)-mediated diboration of dialkynylsilanes resulted in the desymmetric construction of silicon-stereogenic centers with promising enantioselectivity.

Atom-Economical Ni-Catalyzed Diborylative Cyclization of Enynes: Preparation of Unsymmetrical Diboronates

We report a Ni-catalyzed diborylative cyclization of enynes that affords carbo- and heterocycles containing both alkyl- and alkenylboronates. The reaction is fully atom-economical, shows a broad scope, and employs a powerful and inexpensive catalytic Ni-based system. The reaction mechanism seems to involve activation of the enyne by Ni(0) through oxidative cyclometalation of the enyne prior to diboron reagent activation. An unprecedented dinuclear bis(organometallic) Ni(I) intermediate complex was isolated.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Copper-Catalyzed Trifunctionalization of Alkynes: Rapid Formation of Oxindoles Bearing Geminal Diboronates

A copper-catalyzed trifunctionalization of alkynes that provides rapid access to oxindoles bearing a geminal diboronate side chain, highlighted by the simultaneous formation of one C-C bond and two C-B bonds, is reported. This new reaction features simple reaction conditions (ligand-free catalysis), a general substrate scope, and excellent chemoselectivity. Mechanistic study revealed a reaction sequence constituted by, in the order, borylation, intramolecular cross-coupling, hydroboration, which has been rarely documented.

Copper-catalysed, diboron-mediated cis-dideuterated semihydrogenation of alkynes with heavy water

Methods to incorporate deuterium atoms into organic molecules are valuable for the pharmaceutical industry. Here, we found that diboron reagents can efficiently mediate the transfer of two D atoms from heavy water directly onto alkynes through copper-catalysed cis-selective semihydrogenation. Avoiding the use of costly and flammable D2 gas, this safe and practical process can proceed with excellent chemoselectivity and stereoselectivity. Utilizing the present method as the key step, the formal asymmetric total synthesis of d2-deuterium-labeled cis-combretastatin A4 is demonstrated. Mechanistic studies suggest that monoborylation of alkynes is the key step for this semihydrogenation process.

Copper-boryl mediated organic synthesis

Organoboron compounds are valuable synthetic intermediates that find application in a diverse variety of processes including both C-X and C-C bond-forming transformations. This has been achieved by using a variety of boron derivatives. Of these, boronate esters are probably the most versatile and, reflecting this, methods for the generation of boronate esters are of considerable current interest. Given the mild reaction conditions, good functional group tolerance, and low cost of the metal catalyst, the use of copper-boryl reagents is particularly attractive. In this review, methodologies in copper-boryl chemistry are discussed and the many different transformations possible are surveyed.

Facile reactions of gold(i) complexes with tri(tert-butyl)azadiboriridine

Direct structural evidence for group 11 metal-mediated B-B bond activation was obtained from reactions of tri(tert-butyl)azadiboriridine (1) with AuCl(L) complexes. The AuCl(SMe2) reaction afforded [η2-B,B-B(tBu)N(tBu)B(tBu)]AuCl (2) by ligand displacement. More donating phosphines as co-ligands led to B-B bond cleavage accompanied by either halide or L migration to form boron-gold complexes 3 (L = PPh3) and 4 (L = PMe3). A similar product 5, which is isostructural to 4, was obtained by the addition of dimethylaminopyridine (DMAP) to 2-4. Complexes 2-5 constitute rare examples of metal complexes bearing two Lewis acidic centres. The effect of the boryl ligand was demonstrated in the formation of a gold(i) complex 6 bearing a 5-membered heterocycle from 3 and tert-butylisonitrile. Plausible reaction mechanisms that led to these complexes and their bonding situation were explored computationally at the DFT level.

Carbohydrate/DBU Cocatalyzed Alkene Diboration: Mechanistic Insight Provides Enhanced Catalytic Efficiency and Substrate Scope

A mechanistic investigation of the carbohydrate/DBU cocatalyzed enantioselective diboration of alkenes is presented. These studies provide an understanding of the origin of stereoselectivity and also reveal a strategy for enhancing reactivity and broadening the substrate scope.

Nickel-catalyzed synthesis of 1,1-diborylalkanes from terminal alkenes

Organoboron compounds play an irreplaceable role in synthetic chemistry and the related transformations based on the unique reactivity of C-B bond are potentially the most efficient methods for the synthesis of organic molecules. The synthetic importance of multiboron compounds in C-C bond formation and function transformation reactions is growing and the related borations of activated or nonactivated alkenes have been developed recently. However, introducing directly two boron moieties into the terminal sites of alkenes giving 1,1-diborylalkanes in a catalytic fashion has not been explored yet. Here we describe a synthetic strategy of 1,1-diborylalkanes via a Ni-catalyzed 1,1-diboration of readily available terminal alkenes. This methodology shows high level of chemoselectivity and regioselectivity and can be used to convert a large variety of terminal alkenes, such as vinylarenes, aliphatic alkenes and lower alkenes, to 1,1-diborylalkanes.1,1-diborylalkanes are useful building blocks in synthetic chemistry. Here, the authors present a highly chemo- and regioselective Ni-catalyzed reaction for the synthesis of 1,1-diborylalkanes from a wide variety of readily available terminal alkenes.

Substrate-Assisted, Transition-Metal-Free Diboration of Alkynamides with Mixed Diboron: Regio- and Stereoselective Access to trans-1,2-Vinyldiboronates

A substrate-assisted diboration of alkynamides using the unsymmetrical pinacolato-1,8-diaminonaphthalenato diboron (pinBBdan) is described. The transition-metal-free reaction proceeds in a regio- and stereoselective fashion to exclusively afford trans-vinyldiboronates in good to excellent yields. Notably, Bdan and Bpin are installed on the α- and β-carbon atoms, respectively.

Theoretical study on homolytic B–B cleavages of diboron(4) compounds

The B–B BDEs of diboron(4) compounds and the Pt–B and Cu–B BDEs of their corresponding complexes were investigated by SOGGA11-X method.

Catalytic and catalyst-free diboration of alkynes

The present review provides a comprehensive summary of the catalytic and catalyst-free diboration of alkynes.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B-C bonds and other processes.

Homolytic Cleavage of a B-B Bond by the Cooperative Catalysis of Two Lewis Bases: Computational Design and Experimental Verification

Density functional theory (DFT) investigations revealed that 4-cyanopyridine was capable of homolytically cleaving the B-B σ bond of diborane via the cooperative coordination to the two boron atoms of the diborane to generate pyridine boryl radicals. Our experimental verification provides supportive evidence for this new B-B activation mode. With this novel activation strategy, we have experimentally realized the catalytic reduction of azo-compounds to hydrazine derivatives, deoxygenation of sulfoxides to sulfides, and reduction of quinones with B2 (pin)2 at mild conditions.

Copper-Catalyzed trans-Hydroboration of Terminal Aryl Alkynes: Stereodivergent Synthesis of Alkenylboron Compounds

A Cu-catalyzed highly Z-stereoselective hydroboration of alkynes with 1,8-naphthalenediaminatoborane (HB(dan)) is developed. DPEphos (bis[(2-diphenylphosphino)phenyl]ether)-ligated Cu catalysts produced alkenylboron compounds from terminal alkynes with excellent Z-stereoselectivity. In contrast, using a SIPr-CuCl complex as the precatalyst exclusively produced E-hydroboration products at mild conditions. Both catalytic procedures form alkenylboron products stereocomplementary to each other, constituting stereodivergent hydroboration of alkynes through Cu catalysis. Deuterium labeling and isomerization studies support the Z-selective hydroboration via trans-addition of the boron reagent to terminal alkynes as opposed to precedent noble-metal-catalyzed trans-hydroborations.