Olefin–Borane “van der Waals Complexes”: Intermediates in Frustrated Lewis Pair Addition Reactions

Mechanistic Insight Into the Reactivity of Frustrated Lewis Pairs: Liquid-State NMR Studies

Frustrated Lewis pairs (FLPs) have been widely investigated as promising catalysts due to their metal-free feature and ability to activate small molecules. Over the last few years, the structure, dynamics and interactions between the Lewis centers and their effects on the reactivity with different substrates have been studied. Nuclear magnetic resonance (NMR) is a powerful tool in studying the reaction intermediates, kinetics and mechanism of frustrated Lewis pairs (FLPs). Various NMR experiments have been applied to precisely determine the association or cooperativity of FLPs and one or two-dimensional spectra were obtained. Herein, insights coming from NMR spectroscopy for FLPs are presented, the structure and reactivity of FLPs in solution are described, and their effects on the kinetics and mechanism of different substrates are also illustrated in this review.

Skeletal rearrangement of a boron-containing annulenic molecule into a macrocycle bridged by an electronically stabilized boron cation

An annulenic molecule containing a three-coordinate chloroborane moiety, which exhibits a borane-olefin proximity effect, undergoes a skeletal rearrangement upon chloride abstraction, to generate a three-dimensional macrocyclic molecule featuring a borocenium (η5-cyclopentadienyl-B+-R) structure.

Evidence for the encounter complex in frustrated Lewis pair chemistry

Frustrated Lewis Pairs (FLPs) are combinations of bulky Lewis acids and bases that can carry out small-molecule activation and catalysis. Mechanistically, the reaction of the acid, base and substrate involves the collision of three distinct molecules, and so the pre-association of the acid and base to form an encounter complex has been proposed. This article will examine the evidence for the formation of this encounter complex, focusing on the archetypal main-group combinations P(^tBu)₃/B(C₆F₅)₃ and PMes₃/B(C₆F₅)₃ (Mes = mesityl), and includes quantum chemical calculations, molecular dynamics simulations, NMR spectroscopic measurements and neutron scattering. Furthermore, the recent discovery that the associated acid and base can absorb a photon to promote single-electron transfer has enabled the encounter complex to also be studied by UV-Vis spectroscopy, EPR spectroscopy, transient absorption spectroscopy, and resonance Raman spectroscopy. These data all support the notion that the encounter complex is only weakly held together and in low concentration in solution. The insights that these studies provide underpin the exciting transformations that can be promoted by FLPs. Finally, some observations and unanswered questions are provided to prompt further study in this field.

Metal-Free Phosphorus-Directed Borylation of C(sp2 )-H Bonds

Spectacular progress has recently been achieved in transition metal-catalyzed C-H borylation of phosphines as well as directed electrophilic C-H borylation. As shown here, P-directed electrophilic borylation provides a new, straightforward, and efficient access to phosphine-boranes. It operates under metal-free conditions and leverages simple, readily available substrates. It is applicable to a broad range of backbones (naphthyl, biphenyl, N-phenylpyrrole, binaphthyl, benzyl, naphthylmethyl) and gives facile access to various substitution patterns at boron (by varying the boron electrophile or post-derivatizing the borane moiety). NMR monitoring supports the involvement of P-stabilized borenium cations as key intermediates. DFT calculations reveal the existence and stabilizing effect of π-arene/boron interactions in the (biphenyl)(i-Pr)₂ P→BBr₂ ⁺ species.

Bringing out the potential of organoboron compounds by designing the chemical bonds and spaces around boron

Since the structures, reactivity and properties of organoboron compounds stem from the electron deficiency and low electronegativity of boron, the design of the chemical bonds attached to boron as well...

Observation of Borane-Olefin Proximity Interaction Governing the Structure and Reactivity of Boron-Containing Macrocycles

While attractive interactions between borane and olefin have been postulated to trigger various boron-mediated organic transformations, proximity structures of these functional groups, other than the formation of weak van der Waals complexes, have never been directly observed. Here we show that a close intramolecular borane-olefin interaction operates in macrocyclic systems containing borane and olefinic groups obtained by multi-step 1,2-carboboration between a strained alkyne and 9-borafluorene derivatives. Depending on Lewis acidity of the borane moiety and the size of the macrocycles, the magnitude of interaction changes, resulting in different reaction modes. The whole picture of the multi-step reactions has been revealed experimentally with theoretical supports. The present finding may not only provide a deeper understanding of the fundamental boron-mediated interaction but also lead to the development of new organic transformations involving molecular activation by boranes.

1,2-Carbopentafluorophenylation of Alkynes: The Metallomimetic Pull-Push Reactivity of Tris(pentafluorophenyl)borane

We report the novel single-step 1,2-dicarbofunctionalization of an arylacetylene with an allylsilane and tris(pentafluorophenyl)borane [B(C₆ F₅ )₃ ] involving C-C bond formation with C-H bond scission at the β-position to the silicon atom of an allylsilane and B→C migration of a C₆ F₅ group. The 1,2-carbopentafluorophenylation occurs smoothly without the requirement for a catalyst or heating. Mechanistic studies suggest that the metallomimetic "pull-push" reactivity of B(C₆ F₅ )₃ imparts consecutive electrophilic and nucleophilic characteristics to the benzylic carbon of the arylacetylene. Subsequent photochemical 6π-electrocyclization affords tetrafluoronaphthalenes, which are important in the pharmaceutical and materials sciences. Owing to the unique reactivity of B(C₆ F₅ )₃ , the 1,2-carbopentafluorophenylation using 2-substituted furan proceeded with ring opening, and the reaction using silyl enolates formed a C-C bond with C-O bond scission at the silyloxy-substituted carbon.

A BH Borenium-Derived Thioxoborane, Its Persulfide, and Their Li+-Induced Reactions with Alkynes and with Carbon Dioxide

Insertion of sulfur into the B-H bond of the BH borenium salt [IMes(C₆F₅)BH]⁺ followed by deprotonation gave the thioxoborane IMes(C₆F₅)B═S. Subsequent treatment with additional sulfur gave the corresponding boron persulfide, a NHC-stabilized boradithiirane. The B═S compound reacted with carbon dioxide in the presence of the lithium salt Li[B(C₆F₅)₄] by formal [2+2] cycloaddition to give a boron thiocarbonate-type product. The boron persulfide formally inserted phenyl acetylene into the B-S bond in the presence of Li[B(C₆F₅)₄] to give the respective five-membered heterocycle.

A recyclable metal–organic framework for ammonia vapour adsorption

The incorporation of sterically hindered boron centers into MOFs allows the recyclable adsorption of NH₃ vapour from aqueous solution.

Borane-induced ring closure reaction of oligomethylene-linked bis-allenes

The trimethylene-linked bis-allene 3a reacts with Piers' borane [HB(C₆F₅)₂] by a hydroboration/allylboration sequence to generate the cyclization product 5a. Its pyridine adduct was isolated and characterized by X-ray diffraction. Compound 5a undergoes a typical frustrated Lewis pair 1,2-P/B alkene addition reaction with PPh₃ to give the heterobicyclic bridged olefinic zwitterionic product 9a. The tetramethylene-linked bis-allene 3b and its phenylene annulated analogue 3c react with HB(C₆F₅)₂ to give the analogous seven-membered ring products 5b,c under mild conditions. The cyclization product 5a undergoes a series of sequential allylboration reactions with two equivalents of allene followed by ring-closure to give the four-component coupling product 12a. It undergoes FLP addition to an exo-methylene group upon treatment with PPh₃. Compound 12a is oxidatively converted to the boron-free alcohol.

Formation of Active Cyclic Five-membered Frustrated Phosphane/Borane Lewis Pairs and their Cycloaddition Reactions

The cyclic five-membered frustrated phosphane/borane Lewis pairs 11 a, b featuring the bulky octaethylhydrindacenyl- (Eind) substituent or its mono-bromo derivative (^Br Eind) at phosphorus are monomeric at room temperature. The reactive frustrated Lewis pairs (FLPs) cleave dihydrogen. The cyclic FLP 11 b (^Br Eind) undergoes 1,2-P/B addition to ethylene to give the zwitterionic heteronorbornane derivative 14 b. It reacts similarly with the carbon-carbon double bond of norbornene. With a variety of organic π-reagents, the cyclic FLP 11 b often undergoes reaction sequences reminiscent of the Alder-Rickert reaction: the cycloaddition reaction is followed by rapid cycloreversion to form new five-membered heterocyclic FLP products with extrusion of ethene. Reactions of 11 b with benzaldehyde or with acetylenes follow this reaction pattern.

Halogenoborane mediated allene cyclooligomerization

Allene reacts with the strongly electrophilic halogenoboranes XB(C₆F₅)₂ (X: Cl or Br) by forming a mixture of 1,3,5-trimethylenecyclohexane and the stoichiometric halogeno-borated tetramerization products.

The halogenoboranes XB(C₆F₅)₂ (X: Cl, Br) react with allene to give a mixture of the cyclotrimer 1,3,5-trimethylenecyclohexane (1) and the related halogenoborylated cyclotetramerization products 3a and 3b. Alkyl-substituted allenes were catalytically cyclotrimerized metal-free by XB(C₆F₅)₂ (X: Cl, Br) to give cis,trans-2,4,6-trialkyl-1,3,5-trimethylenecyclohexanes under mild conditions.

Metallomimetic Chemistry of Boron

The study of main-group molecules that behave and react similarly to transition-metal (TM) complexes has attracted significant interest in recent decades. Most notably, the attractive idea of replacing the all-too-often rare and costly metals from catalysis has motivated efforts to develop main-group-element-mediated reactions. Main-group elements, however, lack the electronic flexibility of TM complexes that arises from combinations of empty and filled d orbitals and that seem ideally suited to bind and activate many substrates. In this review, we look at boron, an element that despite its nonmetal nature, low atomic weight, and relative redox staticity has achieved great milestones in terms of TM-like reactivity. We show how in interelement cooperative systems, diboron molecules, and hypovalent complexes the fifth element can acquire a truly metallomimetic character. As we discuss, this character is powerfully demonstrated by the reactivity of boron-based molecules with H₂, CO, alkynes, alkenes and even with N₂.

Carbonyl and olefin hydrosilylation mediated by an air-stable phosphorus(iii) dication under mild conditions

The readily-accessible, air-stable Lewis acid [(terpy)PPh][B(C6F5)4]21 is shown to mediate the hydrosilylation of aldehydes, ketones, and olefins. The utility and mechanism of these hydrosilylations are considered.

A theoretical investigation on boron–ligand cooperation to activate molecular hydrogen by a frustrated Lewis pair and subsequent reduction of carbon dioxide

Activation of molecular hydrogen by a B/N frustrated Lewis pair.

Borane-Induced Dimerization of Arylallenes

A series of arylallenes react with HB(C₆ F₅ )₂ in a 2:1 molar ratio to give the tail-to-tail 1,6-diaryl-2-boryl-hexa-1,5-diene coupling products. The reaction of the phenylallene substrate with HB(C₆ F₅ )₂ was shown to initially give the 2-boryl-3,4-diphenyl-1,5-hexadiene head-to-head coupling product which then rearranged, by a thermally induced Cope rearrangement, into the final product.

Mechanistic insights into the catalytic carbonyl hydrosilylation by cationic [CpM(CO)2(IMes)]+ (M = Mo, W) complexes: the intermediacy of η1-H(Si) metal complexes

The ionic S_N2-type mechanistic pathway initiated by silane end-on coordination on the metal centers, forming η¹-H(Si) Mo/W complexes, is the preferred reaction pathway for the two cationic cyclopentadienyl molybdenum/tungsten complexes, [CpM(CO)₂(IMes)]⁺ (M = Mo, W) in catalyzing carbonyl hydrosilylation.

A model for C–F activation by electrophilic phosphonium cations

The electrophilic phosphonium cation (EPC) salt [C₁₀H₆(CF₃)PF(C₆F₅)₂][B(C₆F₅)₄] 4 exhibited structural and spectroscopic features evidencing an interaction between the CF₃ and fluorophosphonium units. It thus models a key step in the proposed mechanism of main group C–F activation.

Contrasting Frustrated Lewis Pair Reactivity with Selenium- and Boron-Based Lewis Acids

The activation of π-bonds in diynyl esters has been investigated by using soft and hard Lewis acids. In the case of the soft selenium Lewis acid PhSeCl, sequential activation of the alkyne bonds leads initially to an isocoumarin (1 equiv PhSeCl) and then to a tetracyclic conjugated structure with the isocoumarin subunit fused to a benzoselenopyran (3 equiv PhSeCl). Conversely, the reaction with the hard Lewis acidic borane B(C6 F5 )3 initiates a cascade reaction to yield a complex π-conjugated system containing phthalide and indene subunits.

An Ethylene-Bridged Phosphane/Borane Frustrated Lewis Pair Featuring the -B(Fxyl)2 Lewis Acid Component

Hydroboration of dimesitylvinylphosphane with bis[3,5-bis(trifluoromethyl)phenyl]borane [HB(Fxyl)2 ] gave the intramolecular ethylene-bridged P/B frustrated Lewis pair (FLP) Mes2 PCH2 CH2 B(Fxyl)2 . The new compound underwent a variety of typical FLP reactions such as P/B-addition to the carbonyl group of p-chloro-benzaldehyde. Cooperative N,N-addition to nitric oxide gave the respective persistent P/B FLPNO(.) radical, which readily reacted with 1,4-cyclohexadiene by H-atom abstraction to yield the corresponding P/B FLPNOH product. The B(Fxyl)2 -containing FLP reacted as a template for the HB(C6 F5 )2 reduction of carbon monoxide to the formyl stage to give the respective FLP(η(2) -formylborane) product. Most products were characterized by single-crystal X-ray crystal structure analysis.

Frustrated Lewis Pair-Catalyzed Cycloisomerization of 1,5-Enynes via a 5-endo-dig Cyclization/Protodeborylation Sequence

The first frustrated Lewis pair-catalyzed cycloisomerization of a series of 1,5-enynes was developed. The reaction proceeds via the π-activation of the alkyne and subsequent 5-endo-dig cyclization with the adjacent alkene. The presence of PPh3 was of utmost importance on the one hand to prevent side reactions (for example, 1,1-carboboration) and on the other hand for the efficient protodeborylation to achieve the catalytic turnover. The mechanism is explained on the basis of quantum-chemical calculations, which are in full agreement with the experimental observations.

Microfluidic Separation of Ethylene and Ethane Using Frustrated Lewis Pairs

Separation of gaseous olefins and paraffins is one of the most important separation processes in the industry. Development of new cost-effective technologies aims at reducing the high energy consumption during the separation process. Here, we took advantage of the reaction of frustrated Lewis pairs (FLPs) with ethylene to achieve reactive extraction of ethylene from ethylene-ethane mixtures. The extraction was studied using a microfluidic platform, which enabled a rapid, high-throughput assessment of reaction conditions to optimize gas separation efficiency. A separation factor of 7.3 was achieved for ethylene from a 1:1 volume ratio mixture of ethylene and ethane, which corresponded to an extracted ethylene purity of 88 %. The results obtained in the microfluidic studies were validated using infrared spectroscopy. This work paves the way for further development of the FLPs and optimization of reaction conditions, thereby maximizing the separation efficiency of olefins from their mixtures with paraffins.

Cyclopropanation/Carboboration Reactions of Enynes with B(C6F5)3

Stoichiometric reaction of B(C6F5)3 with 1,6-enynes is shown to proceed via initial cyclopropanation and formal 1,1-carboboration. Depending on the substitution on the alkene moiety, subsequent ring-opening of the cyclopropane affords either cyclopentane or cyclohexane derivatives in which the C6F5 and B(C6F5)2 adopt a 1,4-positioning. Mechanistically, this transformation involves π-activation of the alkyne moiety, which triggers cyclopropanation, followed by carboboration. Both the cyclopropanation and subsequent ring-opening are shown to be stereospecific. Both cyclopropanation and 1,4-carboborated products were employed as Lewis acid components in frustrated Lewis pair activation of H2 and CO2.