Hydroboration Of Alkynes Research Articles

Location‐Specific Microenvironment Modulation Around Single‐Atom Metal Sites in Metal–Organic Frameworks for Boosting Catalysis

AbstractDespite coordination environment of catalytic metal sites has been recognized to be of great importance in single‐atom catalysts (SACs), a significant challenge remains in the understanding how the location‐specific microenvironment in the higher coordination sphere influences their catalysis. Herein, a series of Cu‐based SACs, namely Cu1/UiO‐66‐X (X=‐NO2, ‐H, and ‐NH2), are successfully constructed by anchoring single Cu atoms onto the Zr‐oxo clusters of metal–organic frameworks (MOFs), i.e., UiO‐66‐X. The ‐X functional groups dangling on the MOF linkers could be regarded as location‐specific remote microenvironment to regulate electronic properties of the single Cu atoms. Remarkably, they exhibit significant differences in the catalysis toward the hydroboration of alkynes. The activity follows the order of Cu1/UiO‐66‐NO2 > Cu1/UiO‐66 > Cu1/UiO‐66‐NH2 under identical reaction conditions, where Cu1/UiO‐66‐NO2 showcases the phenylacetylene conversion of 92 %, ~3.5 times higher efficiency than that of Cu1/UiO‐66‐NH2. Experimental and calculation results jointly support that the Cu electronic structure is modulated by the location‐specific microenvironment, thereby regulating the product desorption and promoting the catalysis.

Location-Specific Microenvironment Modulation Around Single-Atom Metal Sites in Metal-Organic Frameworks for Boosting Catalysis.

Despite coordination environment of catalytic metal sites has been recognized to be of great importance in single-atom catalysts (SACs), a significant challenge remains in the understanding how the location-specific microenvironment in the higher coordination sphere influences their catalysis. Herein, a series of Cu-based SACs, namely Cu1/UiO-66-X (X = -NO2, -H, and -NH2), are successfully constructed by anchoring single Cu atoms onto the Zr-oxo clusters of metal-organic frameworks (MOFs), i.e. UiO-66-X. The -X functional groups dangling on the MOF linkers could be regarded as location-specific remote microenvironment to regulate electronic properties of the single Cu atoms. Remarkably, they exhibit significant differences in the catalysis toward the hydroboration of alkynes. The activity follows the order of Cu1/UiO-66-NO2 ˃ Cu1/UiO-66 ˃ Cu1/UiO-66-NH2 under identical reaction conditions, where Cu1/UiO-66-NO2 showcases the phenylacetylene conversion of 92%, ~3.5 times higher efficiency than that of Cu1/UiO-66-NH2. Experimental and calculation results jointly support that the Cu electronic structure is modulated by the location-specific microenvironment, thereby regulating the product desorption and promoting the catalysis.

Atomically precise copper clusters with dual sites for highly chemoselective and efficient hydroboration.

The hydroboration of alkynes into vinylboronate esters is a vital transformation, but achieving high chemoselectivity of targeted functional groups and an appreciable turnover number is a considerable challenge. Herein, we develop two dynamically regulating dual-catalytic-site copper clusters (Cu4NC and Cu8NC) bearing N-heterocyclic thione ligands that endow Cu4NC and Cu8NC catalysts with performance. In particular, the performance of microcrystalline Cu4NC in hydroboration is characterized by a high turnover number (77786), a high chemoselectivity, high recovery and reusability under mild conditions. Mechanistic studies and density functional theory calculations reveal that, compared with the Cu8NC catalyst, the Cu4NC catalyst has a lower activation energy for hydroboration, accounting for its high catalytic activity. This work reveals that precisely constructed cluster catalysts with dual catalytic sites may provide a way to substantially improve catalytic properties by fully leveraging synergistic interactions and dynamic ligand effects, thus promoting the development of cluster catalysts.

Modulating Lewis acidity with boronamides and boronates: effect on behavior in borylation and cross-coupling

Abstract Modulation of boron-Lewis acidities with cyclic boronamide and boronate moieties, which decisively affects the regiochemical outcome of the copper-catalyzed hydroboration of terminal alkynes, is described. The direct Suzuki–Miyaura coupling of strongly Lewis acidity-diminished R–B(dan), previously believed to be inert toward the reaction, is also reported.

NiII, PdII and PtII pincer complexes of 2-(diphenylphosphanyl)-N-(2-(diphenyl-phosphanyl)benzyl)benzamide: synthesis, reactivity and catalytic studies.

In this article, the synthesis of bis(phosphine), o-Ph2PC6H4C(O)N(H)CH2C6H4PPh2-o (1) (hereafter referred to as "PCNHCP" and its anionic form as "PCNCP") and its group 10 metal chemistry and catalytic studies are described. PCNHCP (1) on reaction with NiCl2(DME) and PdCl2(COD) afforded pincer complexes, [MCl{(PCNCP)κ3-P,N,P}] (M = Ni, 2; Pd, 3). A similar reaction of 1 with PtCl2(COD) yielded a chelate complex, [PtCl2{(PCNHCP)κ2-P,P}] (4), which on further treatment with LiHMDS produced the 1,2-azaphospholene-phosphine complex, [PtCl(Ph){(o-P(Ph)C6H4CONCH2C6H4PPh2-o)κ2-P,P}] (5) via P-C/P-N bond metathesis. Passing dry HCl gas through the solution of 5 resulted in benzene elimination to form [PtCl2{(o-P(Ph)C6H4CONCH2C6H4PPh2-o)κ2-P,P}] (6). Treatment of 1 with PtCl2(COD) and Pt(Cl)(Me)(COD) in the presence of a base resulted in pincer complexes [PtX{(PCNCP)κ3-P,N,P}] (X = Cl, 7; Me, 8). Nickel complex 2 catalyzed the Suzuki-Miyaura cross coupling reaction between bromobenzene and phenyl boronic acid to give the corresponding biphenyls in good yield. The platinum complex 5 showed good catalytic activity towards regio- and stereoselective hydroboration of terminal alkynes. Both the catalytic reactions were performed under mild reaction conditions with a very low catalyst loading.

Ligand-Controlled Regiodivergent Asymmetric Cascade Hydroboration and Hydroallylation of Terminal Alkynes

Ligand-Controlled Regiodivergent Asymmetric Cascade Hydroboration and Hydroallylation of Terminal Alkynes

Hydroboration of Terminal Alkynes Catalyzed by a Mn(I) Alkyl PCP Pincer Complex Following Two Diverging Pathways.

A stereo- and regioselective Mn(I)-catalyzed hydroboration of terminal alkynes with pinacolborane (HBPin) is described. The hydroboration reaction is highly Z-selective in the case of aryl alkynes and E-selective in the case of aliphatic alkynes. The reaction requires no additives or solvents and proceeds with a catalyst loading of 1 mol % at 50-70 °C. The most active precatalyst is the bench-stable alkyl Mn(I) complex cis-[Mn(PCP-iPr)(CO)2(CH2CH2CH3)]. The catalytic process is initiated by the migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate. This species undergoes C-H and B-H bond cleavage of the alkyne (aromatic alkynes) and HBPin (in the case of aliphatic alkynes) forming the active Mn(I) alkynyl and boryl catalysts [Mn(PCP-iPr)(CO)(C≡CR)] and [Mn(PCP-iPr)(CO)(BPin)], respectively. A broad variety of aromatic and aliphatic alkynes was efficiently and selectively borylated. Mechanistic insights are provided based on experimental data and DFT calculations. The functionalized alkenes can be used for further applications in cross-coupling reactions.

Reagent- and Ligand-Dependent Mechanistic Trichotomy in Fe-Catalyzed Borylative Cyclizations of Enynes.

Based on some very recent results on Fe-catalyzed boron-source-dependent regiodivergent hydroborations of internal alkynes, we report here a boron-source-dependent but also ligand-dependent mechanistic trichotomy in borylative cyclizations. The choice of ligand plus boron source allows the synthesis of three isomeric borylative cyclization products starting from a common substrate and using the same precatalyst ((Ph3P)2Fe(CO)(NO)H) and sets the stage for the development of a unifying concept in Fe-catalyzed borylative cyclizations.

Copper Catalyzed Borylation of Alkynes: An Experimental Mechanistic Study.

The copper catalyzed hydroboration of alkynes with B2pin2 was studied by in detail studies of individual relevant steps along the catalytic pathway. A number of reaction steps were retraced by in situ NMR spectroscopy as well as central intermediates and side-products were isolated and comprehensively characterized. A copper boryl complex is central to the catalytic process by inserting the terminal alkyne substrate into the B-Cu bond. The selectivity of this step - depending on the NHC auxiliary ligand - determines the α/β selectivity observed in the product. The latter complex is protonated by the auxiliary alcohol reagent resulting in hydroboration product formation and formation of a Cu alkoxido complex. Reaction of the latter with B2pin2 results in the regeneration of the central copper boryl complex. This alcoholysis step depends on the acidity of the alcohol, in particular on the relative acidity of the alcohol vs. the alkyne substrate. A number of side reactions leading to the hydrogenation product of the alkyne substrate and a bis hydroborated product were identified and studied in some detail. It is concluded that the performance of a particular catalytic system depends crucially on the relative acidities of the reagents and generalizations may be difficult.

Alkylaluminium complexes supported by Schiff base as pre-catalysts in hydroboration of alkynes and nitriles

Dinuclear Schiff base aluminium complexes (C1, C2) were synthesized from salicylaldehyde derivatives and 2-aminopyridine. Complexes C1 and C2 were characterized by NMR and single-crystal X-ray diffraction. Subsequently, the catalytic activity of C1 as a pre-catalyst for the hydroboration of alkynes and nitriles was investigated. It exhibited good functional group tolerance towards both terminal alkynes and nitriles under solvent-free and low-load conditions. The potential mechanism for the C1-catalyzed reaction of alkynes and nitriles with HBpin are proposed by the NMR spectroscopy of the intermediates.

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.

Gallium Hydride-Catalyzed Selective Hydroboration of Unsaturated Organic Substrates.

The first examples of tetrasubstituted conjugated bis-guanidinate (CBG) supported monomeric and thermally stable gallium dihalides [LGaX2], (X=Cl (Ga-Cl), I (Ga-I)) and dihydride (Ga-H) [LGaH2] (where L={(ArHN)(ArN)-C=N-C=(NAr)(NHAr)}; Ar=2,6-Et2-C6H3) compounds are reported. The reaction of in situ generated LLi with 1.0 equiv. GaX3 (X=Cl, I) afforded compounds Ga-Cl and Ga-I. The reaction between Ga-Cl and Li[HBEt3] in benzene yielded the dihydride compound Ga-H. All reported compounds (Ga-Cl, Ga-I, and Ga-H) were characterized by NMR, HRMS, and single-crystal X-ray diffraction studies. Ga-H was probed for the hydroboration of carbodiimides (CDI), isocyanates, and isothiocyanates with HBpin. Compound Ga-H was also found effective for the catalytic hydroboration of imines, nitriles, alkynes, esters, and formates, affording the corresponding products in quantitative yields. Stoichiometric reactions with a CDI were performed to establish the catalytic cycle.

Heterovalent Metal Pair Sites on Metal-Organic Framework Ordered Macropores for Multimolecular Co-Activation.

The precise design of catalytic metal centers with multiple chemical states to facilitate sophisticated reactions involving multimolecular activation is highly desirable but challenging. Herein, we report an ordered macroporous catalyst with heterovalent metal pair (HMP) sites comprising CuII-CuI on the basis of a microporous metal-organic framework (MOF) system. This macroporous HMP catalyst with proximity heterovalent dual copper sites, whose distance is controlled to ∼2.6 Å, on macropore surface exhibits a co-activation behavior of ethanol at CuII and alkyne at CuI, and avoids microporous restriction, thereby promoting additive-free alkyne hydroboration reaction. The desired yield enhances dramatically compared with the pristine MOF and ordered macroporous MOF both with solely isovalent CuII-CuII sites. Density functional theory calculations reveal that the Cu-HMP sites can stabilize the Bpin-CuII-CuI-alkyne intermediate and facilitate C-B bond formation, resulting in a smooth alkyne hydroboration process. This work provides new perspectives to design multimolecular activation catalysts for sophisticated matter transformations.

Regio‐ and Stereo‐Selective Synthesis of (E)‐β‐Alkenyl Boronates by Phosphene‐Free CuI‐Catalyzed Hydroboration and Carboboration of Alkynes and Access to Allyl‐Vinyl Ethers

AbstractA convenient and operationally trivial protocol to access synthetically versatile vinylboronates through CuI catalyzed hydroboration and carboboration of alkynes utilizing bis(pinacolato)diboron (B2pin2) as the boron precursor in a 1, 10‐phenanthroline (phen) ligand environment is reported. This is a highly efficient and practical method for the synthesis of regio‐ and stereoselective (E)‐β‐vinylboronates from simple aromatic and aliphatic alkynes. These organoboron compounds have further been utilized for the synthesis of allyl‐vinyl ethers, which are potential starting materials for the Claisen rearrangement via an improved CuII mediated coupling of allyl alcohols and vinylboronates. The vinylboronates were also subjected to Suzuki‐Miyaura cross coupling reaction, allowing access to the corresponding tri‐ and tetra‐substituted alkenes with good isolated yields.

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.

Defect-engineering improves the activity of Metal-Organic frameworks for catalyzing hydroboration of Alkynes: A combination of experimental investigation and Density functional theory calculations

Defect-engineered metal–organic frameworks (DEMOFs) are emerging advanced materials. The construction of DEMOFs is of great significance; however, DEMOF-based catalysis remains unexplored. (E)-vinylboronates, an important building block for asymmetric synthesis, can be synthesized via the hydroboration of alkynes. However, the lack of high-performance catalysts considerably hinders their synthesis. Herein, a series of DEHKUST-1 (HKUST = Hong Kong University of Science and Technology) (Da–f) catalysts with missing occupation of linkers at Cu nodes were designed by partially replacing benzene-1,3,5-tricarboxylate (H3BTC) with defective connectors of pyridine-3,5-dicarboxylate (PYDC) to efficiently promote the hydroboration of alkynes. Results showed that the Dd containing 0.8 doping ratio of PYDC exhibited remarkable catalytic activity than the defect-free HKUST-1. This originated from the improved accessibility for reactants towards the Lewis acid active Cu sites of DEHKUST-1 due to the presence of plenty of rooms next to the Cu sites and enhanced coordination ability in such ‘defective’ HKUST-1. Dd had high selectivity (>99 %) and yield (>96 %) for (E)-vinylboronates and extensive functional group compatibility for terminal alkynes. Density functional theory (DFT) calculations were performed to elucidate the mechanism of hydroboration. Compared with that of defect-free HKUST-1, the low energy barrier of DEHKUST-1 can be attributed to the lower coordination number of Cu sites and enhanced accessibility of Cu active sites towards reagents.

Alumina-Promoted Copper-Catalyzed Hydroboration of Alkynes

AbstractAlumina was found to activate Cu(I) catalysts in hydroboration of alkynes. This allowed to develop a simple and efficient protocol for hydroboration of alkynes with B2Pin2 using inexpensive CuCl/PPh3 catalytic system. The approach does not require moisture-sensitive activators and stepwise preparation of ‘activated’ Cu complex. The developed conditions were applied to the preparation of β-styrylboronates from terminal and internal alkynes with excellent regio- and stereoselectivity. The procedure can also be extended to other terminal alkynes by the appropriate choice of ligand.

Regio- and chemoselective hydroboration of terminal alkynes with pinacolborane catalyzed by organo rare earth metal complexes

We herein report the regio- and chemoselective hydroboration of terminal alkynes with pinacolborane by a series of constrained-geometry catalyst-type or CGC-type rare earth metal monoalkyl complexes.

Efficient and selective external activator-free cobalt catalyst for hydroboration of terminal alkynes enabled by BiPyPhos.

Herein, a robust catalyst system, composed of a bipyridine-based diphosphine ligand (BiPyPhos) and a cobalt precursor Co(acac)2, is successfully developed and applied in the hydroboration of terminal alkynes, exclusively affording various versatile β-E-vinylboronates in high yields at room temperature.

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.