On the Mechanism of Hydrogen Activation by Frustrated Lewis Pairs

We report herein a comprehensive theoretical study of the thermodynamics and kinetics of molecular hydrogen activation by frustrated Lewis pairs (FLPs). A series of intermolecularly combined boranes (Lewis acids) and phosphines (Lewis bases), with experimentally established different reactivities towards H₂, have been subjected to DFT and (SCS‐)MP2 calculations, and analyzed in terms of their structural properties, the energetics of association of the FLPs, and the kinetics of their interactions with H₂ and hydrogenation to the ion‐pair products. The analysis included the following steps: 1) assessment of the ability/inability of the Lewis species to preorganize into FLPs with an optimum arrangement of the acid and base sites for preconditioning the reaction with H₂, 2) comprehension of the different thermodynamics of hydrogenation of the selected FLPs by comparing the Gibbs energies of the overall reactions, and 3) estimation of the mechanism of the activation of H₂ by identifying the reaction steps and the associated kinetic barriers. The results of our studies correlate well with experimental findings and have clarified the reasons for the observed different reactivities of the investigated systems, ranging from reversible or nonreversible activation to no reaction with H₂. The derived predictions could assist the future design of Lewis acid–base systems with desired properties and applicability as metal‐free hydrogenation catalysts.     

In Situ Formation of Frustrated Lewis Pairs in a Water‐Tolerant Metal‐Organic Framework for the Transformation of CO2

Frustrated Lewis pairs (FLPs) consist of sterically hindered Lewis acids and Lewis bases, which provide high catalytic activity towards non‐metal‐mediated activation of “inert” small molecules, including CO₂ among others. One critical issue of homogeneous FLPs, however, is their instability upon recycling, leading to catalytic deactivation. Herein, we provide a solution to this issue by incorporating a bulky Lewis acid‐functionalized ligand into a water‐tolerant metal‐organic framework (MOF), named SION‐105 , and employing Lewis basic diamine substrates for the in situ formation of FLPs within the MOF. Using CO₂ as a C1‐feedstock, this combination allows for the efficient transformation of a variety of diamine substrates into benzimidazoles. SION‐105 can be easily recycled by washing with MeOH and reused multiple times without losing its identity and catalytic activity, highlighting the advantage of the MOF approach in FLP chemistry.     

受阻路易斯酸碱对在高分子聚合上的应用

受阻路易斯酸碱对(frustrated Lewis pairs,FLPs)是大位阻的路易斯酸和大位阻的路易斯碱在溶液中受空间位阻因素影响而不能形成配位键所得到的组合。 在这种特殊的组合中,路易斯酸和路易斯碱未能被中和淬灭,依旧保持着的反应活性。 而当H₂等小分子靠近时,FLPs可以将H₂的化学键异裂,进而得到一个阳离子和一个阴离子。 这种独特的反应特性使得FLPs在催化加氢、小分子气体活化、烯烃聚合和开环聚合等方面展现出了一些具有新特性的研究思想和方法。 尤其是在烯烃聚合和开环聚合中,FLPs具有很强的催化活性。 本文简要介绍了FLPs的发展历史及其在小分子活化中的应用,并重点介绍了其在高分子催化领域中的应用。     

Formation of Thermally Robust Frustrated Lewis Pairs by Electrocyclic Ring Closure Reactions

The phosphorus/boron‐substituted hexatriene systems 6 undergo thermally induced electrocyclic ring closure to yield the cyclohexadiene‐derived P/B frustrated Lewis pairs (FLPs) 7 . Subsequent TEMPO oxidation gives the phenylene‐bridged FLPs 8 . Both systems activate dihydrogen and the thermally robust FLPs undergo carbon–carbon coupling reactions at a mesityl group upon treatment with dimethyl acetylenedicarboxylate at elevated temperatures.     

Carbene‐9‐BBN Ring Expansions as a Route to Intramolecular Frustrated Lewis Pairs for CO2 Reduction

Reactions of phosphine‐derived carbenes with 9‐borabicyclo[3.3.1]nonane (9‐BBN) result in ring‐expansion reactions to generate novel intramolecular frustrated Lewis pairs (FLPs). These FLPs effect the catalytic reduction of CO₂ in the presence of boranes to give BOB and methoxy‐borate species.     

Aromaticity‐promoted CO2 Capture by P/N‐Based Frustrated Lewis Pairs: A Theoretical Study

Carbon dioxide (CO₂, a common combustion pollutant) releasing continuously into the atmosphere is primarily responsible for the rising atmospheric temperature. Therefore, CO₂ sequestration has been an indispensable area of research for the past several decades. On the other hand, the concept of aromaticity is often employed in designing chemical reactions and metal‐free frustrated Lewis pairs (FLPs) have proved ideal reagents to achieve CO₂ reduction. However, considering FLP and aromaticity together is less developed in CO₂ capture. Here we report theoretical investigations on the aromaticity‐promoted CO₂ activation, involving heterocyclopentadiene‐bridged P/N‐FLPs. The calculations reveal that furan‐ and pyrrole‐bridged P/N‐FLPs can make CO₂ capture both thermodynamically and kinetically favorable (with activation energies of 5.4–7.7 kcal mol^?1) due to the aromatic stabilization of the transition states and products. Our findings could open an avenue to the design of novel FLPs for CO₂ capture.     

Frustrated Lewis Pair Chemistry: Searching for New Reactions

Frustrated Lewis pair chemistry has taken a steep development in the recent years. It offers possibilities of developing new variants of known reactions and of finding new chemical transformations. This is demonstrated and described by the recently developed FLP‐formylborane chemistry, which has led to the formation of the unique (η²‐formylborane)FLP adducts and opened a way of preparing a genuine formylborane compound, which shows an interesting follow‐up chemistry. FLPs have helped finding phosphorus analogues of the enamine Stork reaction and the Claisen reaction. These reactions lead to new organophosphorus compounds and they make new phosphane/borane systems available. P/B FLPs add to a variety of small main group element oxides. They undergo 1,2‐addition reactions to CO₂, SO₂ and other heterocumulenes and they feature unique 1,1‐addition reactions to carbon monoxide, to isonitriles and even to nitric oxide (NO), the latter yielding examples of a new class of persistent nitroxide radicals, the FLPNO nitroxyls. Eventually, some remarkable radical reactions of FLPs and related compounds are briefly mentioned.     

Metal‐Free Hydrogenation of Unsaturated Hydrocarbons Employing Molecular Hydrogen

The metal‐free activation of hydrogen by frustrated Lewis pairs (FLPs) is a valuable method for the hydrogenation of polarized unsaturated molecules ranging from imines, enamines, and silyl enol ethers to heterocycles. However, one of the most important applications of hydrogenation technology is the conversion of unsaturated hydrocarbons into alkanes or alkenes. Despite the fast development of the FLP chemistry, such reactions proved as highly challenging. This Minireview provides an overview of the basic concepts of FLP chemistry, the challenge in the hydrogenation of unsaturated hydrocarbons, and first solutions to this central transformation.     

Frustrated Lewis Pairs: Metal‐free Hydrogen Activation and More

Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form “classical” Lewis acid/Lewis base adducts. Rather, both the unquenched Lewis acidity and basicity of such sterically “frustrated Lewis pairs (FLPs)” is available to carry out unusual reactions. Typical examples of frustrated Lewis pairs are inter‐ or intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C₆F₅)₂ components. Many examples of such frustrated Lewis pairs are able to cleave dihydrogen heterolytically. The resulting H⁺/H^? pairs (stabilized for example, in the form of the respective phosphonium cation/hydridoborate anion salts) serve as active metal‐free catalysts for the hydrogenation of, for example, bulky imines, enamines, or enol ethers. Frustrated Lewis pairs also react with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three‐component reactions, offering new strategies for synthetic chemistry.     

Insight into the relative reactivity of "frustrated Lewis pairs" and stable carbenes in activating H2 and CH4: a comparative computational study

Computational study has been conducted to gain insight into the relative reactivity of stable carbenes (1 and 2) and typical frustrated Lewis pairs (FLPs, 3-6) in activating H(2) and CH(4). For the FLP H(2) activations, despite the quite different basicities of the Lewis base components, they have comparable reactivities. The unexpected relative reactivity can be attributed to the following two factors: (i) the vacant carbene C: p(π) orbital, which is important when carbene works alone but does not participate in the FLP activation; and (ii) the electrostatic interaction between the Lewis base center and the approaching H atom which plays an important role and can either favor or disfavor a reaction. These explanations are also applicable to methane activations. The study brings two messages to the experimentalists for constructing FLPs: (i) it is recommended to use P- and N-centered Lewis bases to construct FLPs for H(2) activation because using more reactive components does not benefit the activation; and (ii) the FLPs are less reactive in activating CH(4) than H(2). In addition, using more reactive carbenes as Lewis bases in FLPs does not necessarily benefit the methane activation.     

Effects of Ionic Liquids on the Thermodynamics of Hydrogen Activation by Frustrated Lewis Pairs: A Density Functional Theory Study**

Nowadays, hydrogen activation by frustrated Lewis pairs (FLPs) and their applications are one of the emerging research topics in the field of catalysis. Previous studies have shown that the thermodynamics of this reaction is determined by electronic structures of FLPs and solvents. Herein, we investigated systems consisting of typical FLPs and ionic liquids (ILs), which are well known by their large number of types and excellent solvent effects. The density functional theory (DFT) calculations were performed to study the thermodynamics for H₂ activation by both inter- and intra-molecular FLPs, as well as the individual components. The results show that the computed overall Gibbs free energies in ILs are more negative than that computed in toluene. Through the thermodynamics partitioning, we find that ILs favor the H−H cleavage elemental step over the elemental steps of proton attachment, hydride attachment and zwitterionic stabilization. Moreover, the results show that these effects are strongly dependent on the type of FLPs, where intra-molecular FLPs are more affected compared to the inter-molecular FLPs.     

Asymmetric Hydrogenation of Ketones and Enones with Chiral Lewis Base Derived Frustrated Lewis Pairs

The concept of frustrated Lewis pairs (FLPs) has been widely applied in various research areas, and metal‐free hydrogenation undoubtedly belongs to the most significant and successful ones. In the past decade, great efforts have been devoted to the synthesis of chiral boron Lewis acids. In a sharp contrast, chiral Lewis base derived FLPs have rarely been disclosed for the asymmetric hydrogenation. In this work, a novel type of chiral FLP was developed by simple combination of chiral oxazoline Lewis bases with achiral boron Lewis acids, thus providing a promising new direction for the development of chiral FLPs in the future. These chiral FLPs proved to be highly effective for the asymmetric hydrogenation of ketones, enones, and chromones, giving the corresponding products in high yields with up to 95 % ee. Mechanistic studies suggest that the hydrogen transfer to simple ketones likely proceeds in a concerted manner.     

Frustrated Lewis pairs beyond the main group: synthesis, reactivity, and small molecule activation with cationic zirconocene-phosphinoaryloxide complexes

The extension of the frustrated Lewis pair (FLP) concept to the transition series with cationic zirconocene-phosphinoaryloxide complexes is demonstrated. Such complexes mimic the reactivity of main group FLPs in reactions such as heterolytic hydrogen cleavage, CO(2) activation, olefin and alkyne addition, and ring-opening of tetrahydrofuran. The interplay between sterics and electronics is shown to have an important role in determining the reactivity of these compounds with hydrogen in particular. The Zr-H species generated from the heterolytic activation of hydrogen is shown to undergo insertion reactions with both CO(2) and CO. Crucially, these transition metal FLPs are markedly more reactive than main group systems in many cases, and in addition to the usual array of reactions they demonstrate unprecedented reactivity in the activation of small molecules. This includes S(N)2 and E2 reactions with alkyl chlorides and fluorides, enolate formation from acetone, and the cleavage of C-O bonds to facilitate S(N)2 type reactions with noncyclic dialkyl ethers.     

Functionalization of Intramolecular Frustrated Lewis Pairs by 1,1‐Carboboration with Conjugated Enynes

The vicinal P/B frustrated Lewis pair (FLP) Mes₂PCH₂CH₂B(C₆F₅)₂ undergoes 1,1‐carboboration reactions with the Me₃Si‐substituted enynes to give ring‐enlarged functionalized C₃‐bridged P/B FLPs. These serve as active FLPs in the activation of dihydrogen to give the respective zwitterionic [P]H⁺/[B]H^? products. One such product shows activity as a metal‐free catalyst for the hydrogenation of enamines or a bulky imine. The ring‐enlarged FLPs contain dienylborane functionalities that undergo “bora‐Nazarov”‐type ring‐closing rearrangements upon photolysis. A DFT study had shown that the dienylborane cyclization of such systems itself is endothermic, but a subsequent C₆F₅ migration is very favorable. Furthermore, substituted 2,5‐dihydroborole products are derived from cyclization and C₆F₅ migration from the photolysis reaction. In the case of the six‐membered annulation product, a subsequent stereoisomerization reaction takes place and the resultant compound undergoes a P/B FLP 1,2‐addition reaction with a terminal alkyne with rearrangement.     

An Unprecedented Ga/P Frustrated Lewis Pair: Synthesis,Characterization, and Reactivity

Frustrated Lewis pairs (FLPs) represent a new paradigm of main-group chemistry. The Lewis acidic centers in FLP chemistry are typically B and Al atoms in the studies reported over the past decade, and most of them are tri-coordinated with strong electron-withdrawing groups. Herein, a Ga/P system is reported which contains an unprecedented four-coordinated Lewis acidic Ga center. This Ga/P species performs classical addition reactions toward heterocumulenes, alkyne, diazomethane, and transition metal complex. Regioselective formation of the products can be rationalized by DFT calculations. The penta-coordinated gallium atom center in these products is rare in the FLP chemistry. This study enriches the diversity of FLPs and demonstrates that a four-coordinated Lewis acidic site with a donor-acceptor bond can also be FLP active.     

A New Mode of Chemical Reactivity for Metal‐Free Hydrogen Activation by Lewis Acidic Boranes

We herein explore whether tris(aryl)borane Lewis acids are capable of cleaving H₂ outside of the usual Lewis acid/base chemistry described by the concept of frustrated Lewis pairs (FLPs). Instead of a Lewis base we use a chemical reductant to generate stable radical anions of two highly hindered boranes: tris(3,5‐dinitromesityl)borane and tris(mesityl)borane. NMR spectroscopic characterization reveals that the corresponding borane radical anions activate (cleave) dihydrogen, whilst EPR spectroscopic characterization, supported by computational analysis, reveals the intermediates along the hydrogen activation pathway. This radical‐based, redox pathway involves the homolytic cleavage of H₂, in contrast to conventional models of FLP chemistry, which invoke a heterolytic cleavage pathway. This represents a new mode of chemical reactivity for hydrogen activation by borane Lewis acids.     

Organophosphine syntheses via activation of the phosphorus‐silicon bond of silylphosphines

This paper describes the recent advances in the syntheses of organophosphines by forming several types of phosphorus‐carbon bonds via activation of the phosphorus‐silicon bond of silylphosphines. In this account, the activation methods are classified into three types: nucleophile‐induced activations, reactions with Lewis acid‐activated electrophiles, and transition metal catalyzed reactions. Nucleophile‐induced activations of silylphosphines, especially by a fluoride, generated a reactive phosphide equivalent for selective formation of a P‐C bond. Silylphosphines also reacted selectively with Lewis acid‐activated electrophiles. The Lewis acid mediated/catalyzed additions and substitutions, to form sp³‐carbon‐phosphorus bonds including an asymmetric reaction, are described. Several important types of transition metal catalyzed reactions of silylphosphines are also mentioned in this account. Selective formation of a P‐C bond is achieved through these activations to produce a variety of functional organophosphines including optically active ones. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 236–245; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200900011     

Infrared Spectroscopy Evidence of Weak Interactions in Frustrated Lewis Pairs Formed by Tris(pentafluorophenyl)borane

Frustrated Lewis pairs (FLPs) have been widely investigated as promising catalysts due to their metal-free feature and ability to activate small molecules. Since their discovery, many works have been investigating how these Lewis pairs (intermolecular pairs) are held together in an encounter complex. This prompted several studies based on theoretical investigations, but experimental ones are limited yet. In this communication we show evidence of weak intermolecular interactions between Lewis acids and Lewis bases, distinguishing the Lewis adduct from FLPs, by probing fluorine-carbon vibrational modes using infrared spectroscopy. The main evidence is based on the band shifts occurring in FLPs due to weak hydrogen bonds between the hydrogen atoms of the Lewis base and the fluorine atoms of Lewis acid.     

Understanding the C−F Bond Activation Mediated by Frustrated Lewis Pairs: Crucial Role of Non-covalent Interactions

The activation of a single C−F bond in di- and trifluoromethyl groups by frustrated Lewis pairs (FLPs) has been computationally explored by means of Density Functional Theory calculations. It is found that in this activation reaction the FLP partners exhibit a peculiar cooperative action, which is markedly different from related FLP-mediated processes, and where non-covalent interactions established between the Lewis base and the substrate play a decisive role. In addition, the process proceeds through the intermediacy of a hypervalent species featuring a pentacoordinate carbon atom, which is rare in the chemistry of FLPs. The physical factors controlling this process as well as the bonding situation of these hypervalent intermediates have been quantitatively analyzed in detail by using state-of-the-art computational methods to not only rationalize the mechanism of the transformation but also to guide experimentalists towards the realization of these so far elusive hypervalent systems.