Transition Metal Lewis Acids Research Articles

Ring-Opening of 1,3-Imidazole Based Mesoionic Carbenes (iMICs) and Ring-Closing Clicks: Facile Access to iMIC-Compounds.

Herein, ring-opening of mesoionic carbenes (iMICs) (iMIC=[ArC{N(Dipp)}2 C(SiMe3 )C:) (Dipp=2,6-iPr2 C6 H3 , Ar=Ph, 4-Me2 NC6 H4 or 4-PhC6 H4 ) based on an 1,3-imidazole scaffold to yield N-ethynylformimidamide (eFIM) derivatives as crystalline solids (eFIM={(Dipp)N=C(Ar)N(Dipp)}C≡CSiMe3 ) is reported. eFIMs are thermally stable under inert gas atmosphere and show moderate air stability (t1/2= 3 h for Ar=Ph). eFIMs are excellent surrogates of iMICs, which generally have a limited shelf-life, and readily undergo ring-closing click reactions with a variety of main-group as well as transition metal Lewis acids to form hitherto challenging iMIC-compounds in good to excellent yields. In addition to the relevance of eFIMs in the synthesis of iMIC-compounds, quantification of the stereoelectronic properties of a representative iMIC (Ar=Ph) by experimental and theoretical methods suggests remarkably σ-donor property and steric profile of these new ligand sets.

Feature Analysis in High-Dimensional Data: Structure-Activity Relationships of Lewis Acid-Transition-Metal Complex-Catalyzed H2 Activation.

Lewis acid-transition metal (LA-TM) catalysts have been proven to have an advantage in catalyzing hydrogen activation. Herein, a high-dimensional structure-activity relationship study is performed for LA-TM-catalyzed hydrogen activation by density functional theory calculations. The DPB-Ni complex is taken as the representative catalyst, and the explored Lewis acid sites and transition-metal centers include B, Al, Ga and Ni, Pd, Pt, respectively. Totally, four general hydrogen activation mechanisms are systematically studied among the nine catalytic systems. The Ga-Ni system undergoes the lowest free energy of activation (11.0 kcal/mol), which is considered to be the optimal combination of the Lewis acid site and transition-metal center. Furthermore, more than 100 parameters are used to analyze the structure-activity relationship, including the physical structure, the bond order, the atom charge, and many other properties. Key parameters of important structures are dug out to show a high correlation with the activity of the LA-TM systems, including the M-H2 distance, the H-H bond length, the second-order perturbation stabilization energy of M-H2, the bond order of the LA-TM, and so on. The multivariable analysis indicates that the feature related to the basic elemental properties and the global feature codetermine the activity of the catalyst. In the LA-TM system, the combination of IpLA/IpTM (Ip, the first ionization energy, the feature related to basic elemental properties) and the chemical hardness (the global feature) can better explain the activity of the catalyst. The IpLA/IpTM reflects the difficulty of breaking the LA-TM bond, affecting the reaction site of activating hydrogen. The hardness reflects the stability and reactivity of LA-TM-RC complexes. The above two features with the addition of the LA-TM bond length (the local feature) can better reflect the activity of the LA-TM system-catalyzed H2 activation. The feature combinations and the method of multidimensional data analysis should be informative guidance for the rational design of efficient LA-TM catalysts for H2 activation.

Coordination chemistry of pnictogenylboranes towards group 6 transition metal Lewis acids

AbstractThe coordination behavior of different pnictogenylboranes towards group 6 metal Lewis acids is investigated. The resulting complexes with phosphanylboranes [(CO)4M(PH2BH2 ⋅ NMe3)2] (M=Cr, Mo, W; 1–3), arsanylboranes ([(CO)4M(AsH2BH2 ⋅ NMe3)2] (M=Cr, Mo, W; 4–6) and tBu‐substituted phosphanylboranes [(CO)4M(tBuPHBH2 ⋅ NMe3)2] (M=Cr, W; 7–8) are fully characterized by multinuclear NMR spectroscopy, single crystal X‐ray diffraction and IR spectroscopy. The systematic nature of the approach of the synthesis and the high purity of the compounds enable a comparative investigation of the coordination behavior of pnictogenylboranes. The influences of the metal center, the pnictogen atom and the substituent at the pnictogen atom on the coordination behavior of pnictogenylboranes are compared.

Total Synthesis of Pentaketide Ansamycin Microansamycin H.

Herein, we report the first total synthesis of pentaketide ansamycin microansamycin H. Key to our success was the endo-selective epoxide-opening O-alkylation to construct the elusive seven-membered benzoxepane core. Due to the electron-rich disposition of the aromatic substrate, our pivotal transformation was hindered by competing electrophilic aromatic substitution at multiple C-based nucleophilic sites that generated kinetically favored products. Judicious choices of transition metal Lewis acid promoters biased toward the formation of the desired oxepane.

Insights into H2 Activation and Styrene Hydrogenation by Nickel–Borane and Nickel–Alane Bifunctional Catalysts

Lewis acid–transition metal bifunctional complexes have recently emerged as a new class of catalysts. The nickel–borane complex, Ni[(Mes)B(o-Ph2PC6H4)2], has been reported as an efficient catalyst for H2 activation and styrene hydrogenation. Here, we performed density functional theory calculations to investigate the cooperation between nickel and a group 13 element (Z = B and Al), including the effect of its substituent (R = Mes, Ph, and C6F5 in Ni[(R)Z(o-Ph2PC6H4)2]), on H2 activation and styrene hydrogenation. We found that H2 activation by the nickel–borane complex is dominated by charge transfer from the σ-bonding orbital of H2 to the p-based vacant orbital of boron, whereas H2 activation by the nickel–alane complex is governed by charge transfer from the d-based orbital of Ni to the σ*-antibonding orbital of H2. The resulting trans-dihydride nickel–alane complex has higher negative charges on both terminal and bridging hydrogen atoms than the corresponding nickel–borane complex. This accounts for the lower energy barriers observed for the nickel–alane complex toward both H2 activation and the subsequent hydrogenation of styrene. While the C6F5 electron-withdrawing substituent on boron of the nickel–borane complex facilitates H2 activation and styrene hydrogenation better than the phenyl or mesityl substituent, the substituent on aluminum does not affect the reactivity of the nickel–alane complex. As H2 activation and styrene hydrogenation by nickel–alane complexes proceed with lower energy barriers than those by nickel–borane complexes, the nickel–alane complex with [(R)Z(o-Ph2PC6H4)2] ligand scaffold should be more reactive than the nickel–borane counterpart. Insights into the role of Lewis acid in this Z-type σ-acceptor ligand scaffold will assist with the development of metal–ligand bifunctional catalysts.

Metal Effect Meets Volcano Plots: A DFT Study on Tris(phosphino)borane-Transition Metal Complexes Catalyzed H2 Activation.

Bifunctional transition metal complexes are of particular interest in metal-ligand cooperative activation of small molecules. As a novel type of bifunctional catalyst, Lewis acid transition metal (LA-TM) complexes have attracted increasing interest in hydrogen activation and storage. To advance the catalyst design, herein the metal effect of LA-TM complexes on the hydrogen activation has been systematically studied with a series of tris(phosphino)borane (TPB) complexes with V, Cr, Mn, Fe, Co, and Ni as metal centers. The metal effect not only influences the mechanism of hydrogen activation, but also notably casts a volcano plot for the activity. TPB complexes of V, Cr, Mn, Fe, and Co tend to activate H2 through a stepwise mechanism, while TPB-Ni prefers a synergetic mechanism for H2 activation. More importantly, the metal effect significantly influences the activity of H2 activation and the formation of the LA-H-TM bridging hydride. The trend of changes in the LA-H-TM structures, the second-order perturbation stabilization energies, and the Laplacian bond orders, along with different metals (from V to Ni), are all interestingly constitute volcano plots for the performance of TPB-TM complexes catalyzed H2 activation. TPB-Mn and TPB-Fe are found to be the optimal catalysts among the discussed TPB-TM complexes. The volcano plots disclosed for the metal effects should be informative and instructive for homogeneous and heterogeneous LA-TM catalysts development.

Coordination Behavior of a P4 -Butterfly Complex towards Transition Metal Lewis Acids: Preservation versus Rearrangement.

The reactivity of the P4 butterfly complex [{Cp’’’Fe(CO)2}2(μ,η1:1‐P4)] (1, Cp’’’=η5‐C5H2 tBu3) towards divalent Co, Ni and Zn salts is investigated. The reaction with the bromide salts leads to [{Cp’’’Fe(CO)2}2(μ3,η2:1:1‐P4){MBr2}] (M=Co (2Co), Ni (2Ni), Zn (2Zn)) in which the P4 butterfly scaffold is preserved. The use of the weakly ligated Co complex [Co(NCCH3)6][SbF6]2, results in the formation of [{(Cp’’’Fe(CO)2)2(μ3,η4:1:1‐P4)}2Co][SbF6]3 (3), which represents the second example of a homoleptic‐like octaphospha‐metalla‐sandwich complex. The formation of the threefold positively charged complex 3 occurs via redox processes, which among others also enables the formation of [{Cp’’’Fe(CO)2}4(μ5,η4:1:1:1:1‐P8){Co(CO)2}][SbF6] (4), bearing a rare octaphosphabicyclo[3.3.0]octane unit as a ligand. On the other hand, the reaction with [Zn(NCCH3)4][PF6]2 yields the spiro complex [{(Cp’’’Fe(CO)2)2(μ3,η2:1:1‐P4)}2Zn][PF6]2 (5) under preservation of the initial structural motif.

Lewis Acid Transition-Metal-Catalyzed Hydrogen Activation: Structures, Mechanisms, and Reactivities.

As a new type of bifunctional catalyst, the Lewis acid transition-metal (LA-TM) catalysts have been widely applied for hydrogen activation. This study presents a mechanistic framework to understand the LA-TM-catalyzed H2 activation through DFT studies. The mer(trans)-homolytic cleavage, the fac(cis)-homolytic cleavage, the synergetic heterolytic cleavage, and the dissociative heterolytic cleavage should be taken as general mechanisms for the field of LA-TM catalysis. Four typical LA-TM catalysts, the Z-type κ4 -L3 B-Rh complex tri(azaindolyl)borane-Rh, the X-type κ3 -L2 B-Co complex bis-phosphino-boryl (PBP)-Co, the η2 -BC-type κ3 -L2 B-Pd complex diphosphine-borane (DPB)-Pd, and the Z-type κ2 -LB-Pt complex (boryl)iminomethane (BIM)-Pt are selected as representative models to systematically illustrate their mechanistic features and explore the influencing factors on mechanistic variations. Our results indicate that the tri(azaindolyl)borane-Rh catalyst favors the synergetic heterolytic mechanism; the PBP-Co catalyst prefers the mer(trans)-homolytic mechanism; the DPB-Pd catalyst operates through the fac(cis)-homolytic mechanism, whereas the BIM-Pt catalyst tends to undergo the dissociative heterolytic mechanism. The mechanistic variations are determined by the coordination geometry, the LA-TM bonding nature, the electronic structure of the TM center, and the flexibility or steric effect of the LA ligands. The presented mechanistic framework should provide helpful guidelines for LA-TM catalyst design and reaction developments.

A Simple and Effective Method of Determining Lewis Acidity by Using Fluorescence

Summary The strength of a Lewis acid is an important parameter in establishing its utility for practical applications. Here, we report a universal fluorescence-based method for accurately determining the strength of a range of Lewis acids. Utilizing a fluorescent dithienophosphole oxide architecture, we are able to emulate the structural motif of the widely employed Gutmann-Beckett method. This allows us to precisely correlate the optical response to the strength of a Lewis acid. In comparison to commonly used methods that employ 31P NMR chemical shifts or quantum-chemical calculations, our method strongly suggests that using fluorescent Lewis acid-base adducts (FLAs) is more reliable. Moreover, our method provides a direct measure of the Lewis acidity of a given solution, independent of the Lewis base, with direct implications to the overall utility of a Lewis acid in practical applications.

Highly Efficient Synthesis of Substituted 3,4-Dihydropyrimidin-2-(1H)-ones (DHPMs) Catalyzed by Hf(OTf)₄: Mechanistic Insights into Reaction Pathways under Metal Lewis Acid Catalysis and Solvent-Free Conditions.

In our studies on the catalytic activity of Group IVB transition metal Lewis acids, Hf(OTf)4 was identified as a highly potent catalyst for ”one-pot, three-component” Biginelli reaction. More importantly, it was found that solvent-free conditions, in contrast to solvent-based conditions, could dramatically promote the Hf(OTf)4-catalyzed formation of 3,4-dihydro-pyrimidin-2-(1H)-ones. To provide a mechanistic explanation, we closely examined the catalytic effects of Hf(OTf)4 on all three potential reaction pathways in both “sequential bimolecular condensations” and “one-pot, three-component” manners. The experimental results showed that the synergistic effects of solvent-free conditions and Hf(OTf)4 catalysis not only drastically accelerate Biginelli reaction by enhancing the imine route and activating the enamine route but also avoid the formation of Knoevenagel adduct, which may lead to an undesired byproduct. In addition, 1H-MMR tracing of the H-D exchange reaction of methyl acetoacetate in MeOH-d4 indicated that Hf(IV) cation may significantly accelerate ketone-enol tautomerization and activate the β-ketone moiety, thereby contributing to the overall reaction rate.

Hafnium Triflate as a Highly Potent Catalyst for Regio- and Chemoselective­ Deprotection of Silyl Ethers

As a Group IVB transition metal Lewis acid, hafnium triflate [Hf(OTf)4] exhibited exceptionally high potency in desilylations. Since the amounts of Hf(OTf)4 required for the deprotection of 1°, 2°, 3° alkyl and aryl tert-butyldimethylsilyl (TBS) ethers are significantly different, ranging from 0.05 mol% to 3 mol%, regioselective deprotection of TBS could be easily implemented. Moreover, chemoselective cleavage of different silyl ethers or removal of TBS in the presence of most hydroxyl protecting groups was also accomplished. NMR analyses of silyl products from TBS deprotection indicated that Hf(OTf)4-catalyzed desilylation may proceed via different mechanisms, depending on the solvent used.

Homogeneously catalyzed hydrogenation and dehydrogenation reactions – From a mechanistic point of view

Abstract Homogeneously catalyzed hydrogenation/dehydrogenation reactions represent not only one of the most synthetically important chemical transformations, but also a promising way to renewably utilize the hydrogen energy. In order to rationally design efficient homogeneous catalysts for hydrogenations/dehydrogenations, it is of fundamental importance to understand their reaction mechanisms in detail. With this aim in mind, we herein provide a brief overview of the mechanistic understanding and related catalyst design strategies. Hydrogenations and dehydrogenations represent the reverse process of each other, and involve the activation/release of H2 and the insertion/elimination of hydride as major steps. The mechanisms discussed in this chapter include the cooperation (bifunctional) mechanism and the non-cooperation mechanisms. Non-cooperation mechanisms usually involve single-site transition metal (TM) catalysts or transition metal hydride (TM-H) catalysts. Cooperation mechanisms usually operate in the state-of-the-art bifunctional catalysts, including Lewis-base/transition-metal (LB-TM) catalysts, Lewis-acid/transition-metal (LA-TM) catalysts, Lewis-acid/Lewis-base (LA-LB; the so-called frustrated Lewis pairs - FLPs) catalysts, newly developed ambiphilic catalysts, and bimetallic transition-metal/transition-metal (TM-TM) catalysts. The influence of the ligands, the electronic structure of the metal, and proton shuttle on the reaction mechanism are also discussed to improve the understanding of the factors that can govern mechanistic preferences. The content presented in this chapter should both inspire experimental and theoretical chemists concerned with homogeneously catalyzed hydrogenation and dehydrogenation reactions, and provide valuable information for future catalyst design.

Elucidating metal hydride reactivity using late transition metal boryl and borane hydrides: 2c–2e terminal hydride, 3c–2e bridging hydride, and 3c–4e bridging hydride

A general trend for the hydrogenation reactivity of metal hydride(s): 3c–4e bridging hydride > 2c–2e terminal hydride > 3c–2e bridging hydride.

Boron-Based Lewis Acid Transition Metal Complexes as Potential Bifunctional Catalysts

Lewis acid-transition metal (LA-TM) complexes, with the LA site functions as an electron acceptor and the TM center functions as an electron donor, have been emerging as a new type of bifunctional catalysts lately, different from traditional transition metal bifunctional catalysts. Due to their rapid developments recently, the boron-based LA-TM complexes, which are divided into three major types, the sp 3 , sp 2 , and sp boron-based complexes, are reviewed in this paper according to their binding features. Reactions promoted by this new type of LA-TM bifunctional catalysts have been surveyed, including migration reactions, activation of H-H/E-H/E-E bonds, hydrogenations, hydrosilylations, and transfer dehydrogenation reactions etc . This overview of boron-based LA-TM complexes could provide valuable information to explore the new horizon in LA-TM bifunctional catalysis.

The effect of auxiliary ligand on the mechanism and reactivity: DFT study on H2 activation by Lewis acid–transition metal complex (tris(phosphino)borane)Fe(L)

The crucial role of the auxiliary ligand in hydrogen activation is revealed by DFT studies for the LA–TM ferraboratrane complex.

General H2 Activation Modes for Lewis Acid–Transition Metal Bifunctional Catalysts

A general mechanism for H2 activation by Lewis acid–transition metal (LA-TM) bifunctional catalysts has been presented via density functional theory (DFT) studies on a representative nickel borane system, (PhDPBPh)Ni. There are four typical H2 activation modes for LA-TM bifunctional catalysts: (1) the cis homolytic mode, (2) the trans homolytic mode, (3) the synergetic heterolytic mode, and (4) the dissociative heterolytic mode. The feature of each activation mode has been characterized by key transition state structures and natural bond orbital analysis. Among these four typical modes, (PhDPBPh)Ni catalyst most prefers the synergetic heterolytic mode (ΔG‡ = 29.7 kcal/mol); however the cis homolytic mode cannot be totally disregarded (ΔG‡ = 33.7 kcal/mol). In contrast, the trans homolytic mode and dissociative heterolytic mode are less feasible (ΔG‡ = ∼42 kcal/mol). The general mechanistic picture presented here is fundamentally important for the development and rational design of LA-TM catalysts in the f...

ChemInform Abstract: One‐Pot Protocol to Functionalized Benzopyrrolizidine Catalyzed Successively by Rh2(OAc)4 and Cu(OTf)2: A Transition Metal—Lewis Acid Catalysis Relay.

AbstractA new method for the efficient synthesis of structurally unique N‐heterocycle benzopyrrolizidine is developed.

One-pot protocol to functionalized benzopyrrolizidine catalyzed successively by Rh2(OAc)4 and Cu(OTf)2: a transition metal-Lewis acid catalysis relay.

4-N-allylarylpropylamino-1-sulfonyl triazoles are converted to structurally unique benzopyrrolizidinyl sulfonamides in a one-pot operation. Intramolecular capture of rhodium carbene with arylamino nitrogen gives rise to the formation of an ammonium ylide immediate. A [2,3]- or [1,2]-rearrangement occurs to give a 2-allylpyrrolidinyl-2-carbimine intermediate which undergoes Cu(OTf)2 catalyzed aza-Friedel-Crafts cyclization to finish a highly functionalized tricyclic system decorated with a synthetically difficult quaternary carbon center, a sulfonamide group, and an allyl segment.

Oligomer Saccharide Reduction During Dilute Acid Pretreatment Co-Catalyzed with Lewis Acids on Corn Stover Biomass

The dilute sulfuric acid pretreatment of lignocellulosic biomass is a well understood process that significantly enhances the yield of glucose after enzymatic saccharification. The goal of this research was to perform a systematic study to evaluate the yield of fermentable sugars during dilute sulfuric acid pretreatment that is co-catalyzed with the transition metal Lewis acid salts: AlCl3, FeCl2, FeCl3, and La(OTf)3. All Lewis acids apart from FeCl2 reduced the presence of xylo-oligomers by a large margin when compared to the non-co-catalyzed control sample pretreatments. The presence of these xylo-oligomers acts as inhibitors during enzymatic saccaharification step. The Lewis acids AlCl3, FeCl3, and La(OTf)3 were also able to marginally increase the overall enzymatic digestibility specifically for corn stover pretreated at 160°C with 10 mM of Lewis acids. The hard Lewis acid such as AlCl3 increased the formation inhibitory products such as furfural and 5-hydroxymethylfurfural (HMF). There was good correlation between reduction of xylo-oligomers and increased concentration furfural with increase in Lewis acid hardness.

Metal-catalyzed one-pot synthesis of tetrazines directly from aliphatic nitriles and hydrazine.

There is rapidly growing interest in the use of 1,2,4,5-tetrazines as bioorthogonal coupling agents.[1-3] Recent applications of tetrazine cycloadditions include intracellular small molecule imaging, genetically targeted protein tagging, post-synthetic DNA labeling, nanoparticle based clinical diagnostics, and in-vivo imaging.[4-7] In addition, tetrazines have seen significant use in materials science[8,9], coordination chemistry, [10,11] and specialty explosives research.[12,13] They are also valuable synthetic intermediates, and have been elegantly deployed on route to several natural product syntheses.[14-16] Despite the promise of tetrazines, the lack of convenient synthetic methods is a significant roadblock to their broader use and study by the scientific community.[17] Here we report that Lewis acid transition metal catalysts, most notably divalent nickel and zinc salts, can catalyze the formation of 1,2,4,5-tetrazines directly from nitriles. To our knowledge, this is the first method utilizing homogenous catalysis to directly synthesize tetrazines from a wide range of unactivated aliphatic nitriles and hydrazine. Symmetric and asymmetric tetrazines were conveniently prepared from multiple precursors including alkyl nitriles, aromatic nitriles, and formamidine salts. This methodology should greatly improve the accessibility of tetrazines and lead to further exploration of their applications, particularly with respect to bioorthogonal conjugations.