Frustrated Lewis Pair Catalyzed Hydroamination of Terminal Alkynes

A crystalline aluminium-carbon-based ambiphile capable of activation and catalytic transfer of ammonia in non-aqueous media

Despite recent achievements in the field of frustrated Lewis pairs (FLPs) for small molecule activations, the reversible activation and catalytic transformations of N-H-activated ammonia remain a challenge. Here we report on a rare combination of an aluminium Lewis acid and a carbon Lewis base. A so-called hidden FLP consisting of a phosphorus ylide featuring an aluminium fragment in the ortho position of a phenyl ring scaffold is introduced. Although the formation of the Lewis acid/base adduct is observed in the solid state, which at first glance leads to formally quenched FLP reactivity, we show that the title compound readily reacts with non-aqueous ammonia thermoneutrally and splits the N-H bond reversibly at ambient temperature. In addition, NH3 transfer reactions mediated by a main-group catalyst are presented. This proof-of-principle study is expected to initiate further activities in utilizing N-H-activated ammonia as a readily available, atom-economical nitrogen source.

Systematic Assessment of the Catalytic Reactivity of Frustrated Lewis Pairs in C-H Bond Activation

Unreactive C-H bond activation is a new horizon for frustrated Lewis pair (FLP) chemistry. This study provides a systematic assessment of the catalytic reactivity of recently reported intra-molecular FLPs on the activation of typical inert C-H bonds, including 1-methylpyrrole, methane, benzyl, propylene, and benzene, in terms of density functional theory (DFT) calculations. The reactivity of FLPs is evaluated according to the calculated reaction thermodynamic and energy barriers of C-H bond activation processes in the framework of concerted C-H activation mechanisms. As for 1-methylpyrrole, 14 types of N-B-based and 15 types of P-B-based FLPs are proposed to be active. Although none of the evaluated FLPs are able to catalyze the C-H activation of methane, benzyl, or propylene, four types of N-B-based FLPs are suggested to be capable of catalyzing the activation of benzene. Moreover, the influence of the strength of Lewis acid (LA) and Lewis base (LB), and the differences between the influences of LA and LB on the catalytic reactivity of FLPs, are also discussed briefly. This systematic assessment of the catalytic activity of FLPs should provide valuable guidelines to aid the development of efficient FLP-based metal-free catalysts for C-H bond activation.

Expanding the scope of bis(catecholato)germane catalysis: hydrosilylation, hydroboration, Friedel-Crafts alkylation and oligomerization

Various applications of bis(catecholato)germanes in catalysis were examined. The hydrosilylation of benzaldehydes, hydroboration of phenylacetylene derivatives, and Friedel-Crafts alkylation using arylalkenes and either diphenylamine or anisole was achieved. Furthermore, the recently reported oligomerization of α-methylstyrene catalyzed by bis(catecholato)germanes with weak donor ligands was examined further. The formation of a trimer species was observed in DCM. VTNA and Hammett analyses of the oligomerization reaction were conducted and an updated mechanism for bis(catecholato)germane catalysis is proposed.

Atom-economic and stereoselective catalytic synthesis of fully substituted enol esters/carbonates of amides in acyclic systems enabled by boron Lewis acid catalysis

Carboacyloxylation of internal alkynes is emerging as a powerful and straightforward strategy for enol ester synthesis. However, the reported examples come with limitations, including the utilization of noble metal catalysts, the control of regio- and Z/E selectivity, and an application in the synthesis of enol carbonates. Herein, a boron Lewis acid-catalyzed intermolecular carboacyloxylation of ynamides with esters to access fully substituted acyclic enol esters in high yield with generally high Z/E selectivity (up to >96 : 4) is reported. Most importantly, readily available allylic carbonates are also compatible with this difunctionalization reaction, representing an atom-economic, catalytic and stereoselective protocol for the construction of acyclic β,β-disubstituted enol carbonates of amides for the first time. The application of the carboacyloxylation products to decarboxylative allylations provided a ready access to enantioenriched α-quaternary amides. Moreover, experimental studies and theoretical calculations were performed to illustrate the reaction mechanism and rationalize the stereochemistry.

Structure-Reactivity Relationships in Borane-Based FLP-Catalyzed Hydrogenations, Dehydrogenations, and Cycloisomerizations

ConspectusThe activation of molecular hydrogen by main-group element catalysts is an extremely important approach to metal-free hydrogenations. These so-called frustrated Lewis pairs advanced within a short period of time to become an alternative to transition metal catalysis. However, deep understanding of the structure-reactivity relationship is far less developed compared to that of transition metal complexes, although it is paramount for advancing frustrated Lewis pair chemistry.In this Account, we provide detailed insight into how Lewis acidity and Lewis basicity correlate to reactivity. The reactivity of frustrated Lewis pairs will be systematically discussed in context with selected reactions. The influence of major electronic modifications of the Lewis pairs is correlated with the ability to activate molecular hydrogen, to channel reaction kinetics and reaction pathways, or to achieve C(sp³)-H activations.First, we will describe how we entered this emerging field of research after quickly realizing that information was lacking on how the reactivity changes with modification of the frustrated Lewis pair. This led us to the development of a qualitative and quantitative structure-reactivity relationship in metal-free imine hydrogenations. The imine hydrogenation was utilized as the model reaction to experimentally determine the activation parameters of the FLP-mediated hydrogen activation for the first time. This kinetic study revealed autoinduced catalytic profiles when Lewis acids weaker than tris(pentafluorophenyl)borane were applied, opening up to study the Lewis base dependency within one system. With this knowledge of the interplay between Lewis acid strength and Lewis basicity, we developed methods for the hydrogenation of densely functionalized nitroolefins, acrylates, and malonates. Here, the reduced Lewis acidity needed to be counterbalanced by a suitable Lewis base to ensure efficient hydrogen activation. The opposite measure was necessary for the hydrogenation of unactivated olefins. For these, comparably less electron-releasing phosphanes were required to generate strong Brønsted acids by hydrogen activation. These systems displayed highly reversible hydrogen activation even at temperatures as low as -60 °C. A systematic study of these systems enabled the development of acceptorless dehydrocouplings of amines with silanes and dehydrogenations of aza-heterocycles by C(sp³)-H activations. Furthermore, the C(sp³)-H and π-activation was utilized to achieve cycloisomerizations by carbon-carbon and carbon-nitrogen bond formations. Lastly, new frustrated Lewis pair systems featuring weak Lewis bases as active components in the hydrogen activation were developed for the reductive deoxygenation of phosphane oxides and carboxylic acid amides.

Dynamic B/N Lewis Pairs: Insights into the Structural Variations and Photochromism via Light-Induced Fluorescence to Phosphorescence Switching

Ultralong afterglow emissions due to room-temperature phosphorescence (RTP) are of paramount importance in the advancement of smart sensors, bioimaging and light-emitting devices. We herein present an efficient approach to achieve rarely accessible phosphorescence of heavy atom-free organoboranes via photochemical switching of sterically tunable fluorescent Lewis pairs (LPs). LPs are widely applied in and well-known for their outstanding performance in catalysis and supramolecular soft materials but have not thus far been exploited to develop photo-responsive RTP materials. The intramolecular LP M1BNM not only shows a dynamic response to thermal treatment due to reversible N→B coordination but crystals of M1BNM also undergo rapid photochromic switching. As a result, unusual emission switching from short-lived fluorescence to long-lived phosphorescence (rad-M1BNM, τ_RTP =232 ms) is observed. The reported discoveries in the field of Lewis pairs chemistry offer important insights into their structural dynamics, while also pointing to new opportunities for photoactive materials with implications for fast responsive detectors.

Recent Advances in Group 14 and 15 Lewis Acids for Frustrated Lewis Pair Chemistry

Frustrated Lewis pairs (FLP) which rely on the cooperative action of Lewis acids and Lewis bases, played a prominent role in the advancement of main-group catalysis. While the early days of FLP chemistry witnessed the dominance of boranes, there is a growing body of reports on alternative Lewis acids derived from groups 14 and 15. This short review focuses on the discovery of such non-boron candidates reported since 2015.

Catalytic hydroaminations of alkynes: A facile protocol to vinyl- carbazole derivatives via a frustrated Lewis pair mechanism

N-vinylcarbazoles are important skeletons for photoluminescent materials. Herein, a metal-free, B(C6F5)3 mediated carbazolation reaction of alkynes is reported. This provides 24 variants of N-vinylcarbazole derivatives. These N-vinylcarbazole products were obtained...

Boron-Catalyzed Hydroarylation of 1,3-Dienes with Arylamines

Catalytic hydroarylation reactions of conjugated dienes are achieved using tris(pentafluorophenyl)borane as a Lewis acid catalyst under mild reaction conditions. This new protocol shows a broad substrate scope for the highly regioselective functionalization of sterically hindered aniline derivatives. Experimental and extensive density functional theory mechanistic studies show that the complex of residual water and B(C₆F₅)₃ plays a crucial role in the aryl-assisted protonation of conjugated dienes, forming allyl cation intermediates that induce the facile electrophilic aromatic substitution of aniline substrates.

B(C6F5)3-Catalyzed Electron Donor-Acceptor Complex-Mediated Aerobic Sulfenylation of Indoles under Visible-Light Conditions

An efficient B(C₆F₅)₃-catalyzed aerobic oxidative C-S cross-coupling reaction of thiophenol with indoles was developed, affording a wide range of diaryl sulfides in good yields. An electron donor-acceptor complex between B(C₆F₅)₃ and indoles was formed, facilitating the photoinduced single-electron transfer (SET) from indole substrates to the B(C₆F₅)₃ catalyst. This protocol demonstrates a new reaction model using B(C₆F₅)₃ as a single-electron oxidant.

Synergistic Catalysis by Brønsted Acid/Carbodicarbene Mimicking Frustrated Lewis Pair-Like Reactivity

Carbodicarbene (CDC), unique carbenic entities bearing two lone pairs of electrons are well-known for their strong Lewis basicity. We demonstrate herein, upon introducing a weak Brønsted acid benzyl alcohol (BnOH) as a co-modulator, CDC is remolded into a Frustrated Lewis Pair (FLP)-like reactivity. DFT calculation and experimental evidence show BnOH loosely interacting with the binding pocket of CDC via H-bonding and π-π stacking. Four distinct reactions in nature were deployed to demonstrate the viability of proof-of-concept as synergistic FLP/Modulator (CDC/BnOH), demonstrating enhanced catalytic reactivity in cyclotrimerization of isocyanate, polymerization process for L-lactide (LA), methyl methacrylate (MMA) and dehydrosilylation of alcohols. Importantly, the catalytic reactivity of carbodicarbene is uniquely distinct from conventional NHC which relies on only single chemical feature of nucleophilicity. This finding also provides a new spin in diversifying FLP reactivity with co-modulator or co-catalyst.

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.

Electron-deficient boron-based catalysts for C-H bond functionalisation

In contrast to transition metal-catalysed C-H functionalisation, highly efficient construction of C-C and C-X (X = N, O, S, B, Si, etc.) bonds through metal-free catalytic C-H functionalisation remains one of the most challenging tasks for synthetic chemists. In recent years, electron-deficient boron-based catalyst systems have exhibited great potential for C-H bond transformations. Such emerging systems may greatly enrich the chemistry of C-H functionalisation and main-group element catalysis, and will also provide enormous opportunities in synthetic chemistry, materials chemistry, and chemical biology. This article aims to give a timely comprehensive overview to recognise the current status of electron-deficient boron-based catalysis in C-H functionalisation and stimulate the development of more efficient catalytic systems.

An intermolecular FLP System derived from an NHC-coordinated trisilacyclopropylidene

The NHC-coordinated trisilacyclopropylidene (A) is shown to behave as the basic component of an FLP used in combination with the Lewis acid B(C6F4H)3 (i.e. B(2,3,5,6-C6F4H)3). This FLP cleaves dihydrogen highly selectively at room temperature giving rise to the ionic compound [(NHC)SiH(Mes2SiSiMes2)][HB(C6F4H)3] 1 in 90% isolated yield. Further reaction of the FLP with Ph2NH and acetone yielded compounds [(NHC)SiH(Mes2SiSiMes2)][Ph2NB(C6F4H)3] 2 and [(NHC)SiH(Mes2SiSiMes2)][MeC(CH2)OB(C6F4H)3] 3 in 75% and 80% yield, respectively. Reaction of the FLP with N2O results in the oxidation of the silylene center affording [((NHC)SiOB(C6F4H)3)(Mes2SiSiMes2)] 4 in 53% yield. These products are spectroscopically characterized and an X-ray structure of 4 is reported.

A Highly Lewis Acidic Strontium ansa-Arene Complex for Lewis Acid Catalysis and Isobutylene Polymerization

The potential of a dicationic strontium ansa‐arene complex for Lewis acid catalysis has been explored. The key to its synthesis was a simple salt metathesis from SrI₂ and 2 Ag[Al(OR^F)₄], giving the base‐free strontium‐perfluoroalkoxyaluminate Sr[Al(OR^F)₄]₂ (OR^F=OC(CF₃)₃). Addition of an ansa‐arene yielded the highly Lewis acidic, dicationic strontium ansa‐arene complex. In preliminary experiments, the complex was successfully applied as a catalyst in CO₂‐reduction to CH₄ and a surprisingly controlled isobutylene polymerization reaction.

Evidence for Genuine Bimetallic Frustrated Lewis Pair Activation of Dihydrogen with Gold(I)/Platinum(0) Systems

A joint experimental/computational effort to elucidate the mechanism of dihydrogen activation by a gold(I)/platinum(0) metal-only frustrated Lewis pair (FLP) is described herein. The drastic effects on H₂ activation derived from subtle ligand modifications have also been investigated. The importance of the balance between bimetallic adduct formation and complete frustration has been interrogated, providing for the first time evidence for genuine metal-only FLP reactivity in solution. The origin of a strong inverse kinetic isotopic effect has also been clarified, offering further support for the proposed bimetallic FLP-type cleavage of dihydrogen.

Frustrated Lewis pair-catalyzed double hydroarylation of alkynes with N-substituted pyrroles

Metal-free hydroarylation of alkynes with N-substituted pyrroles is shown to be efficiently mediated by B(C₆F₅)₃ to yield variants of dipyrrole methanes. The mechanism is shown to proceed via an FLP addition pathway.

Boron-Catalyzed N-Alkylation of Arylamines and Arylamides with Benzylic Alcohols

A sustainable boron-based catalytic approach for chemoselective N-alkylation of primary and secondary aromatic amines and amides with primary, secondary, and tertiary benzylic alcohols has been presented. The metal-free protocol operates at low catalyst loading, tolerates several functional groups, and generates H₂O as the sole byproduct. Preliminary mechanistic studies were performed to demonstrate the crucial role of boron catalyst for the activation of the intermediate dibenzyl ether and to identify the rate-determining step.

Internal B-O Bond-Facilitated Photoisomerization of Boranes: Ring Expansion Versus Oxyborane Elimination/Intramolecular Diels-Alder Addition

Boron compounds (1-4) containing an internal B-O bond have been found to undergo facile multistructural transformations upon irradiation at 365 or 410 nm, generating rare 8-membered B,O-heterocycles (1c-4c). In addition, 2 and 3 also undergo an intramolecular Diels-Alder addition and oxyborane elimination concomitantly, via intermediates 2b/3b, producing 2d/3d. The pathways to isomer c and product d were found to be a thermal process and a photo process, respectively.

Interaction of Metal Oxido Compounds with B(C6 F5 )3

Lewis acid-base pair chemistry has been placed on a new level with the discovery that adduct formation between an electron donor (Lewis base) and acceptor (Lewis acid) can be inhibited by the introduction of steric demand, thus preserving the reactivity of both Lewis centers, resulting in highly unusual chemistry. Some of these highly versatile frustrated Lewis pairs (FLP) are capable of splitting a variety of small molecules, such as dihydrogen, in a heterolytic and even catalytic manner. This is in sharp contrast to classical reactions where the inert substrate must be activated by a metal-based catalyst. Very recently, research has emerged combining the two concepts, namely the formation of FLPs in which a metal compound represents the Lewis base, allowing for novel chemistry by using the heterolytic splitting power of both together with the redox reactivity of the metal. Such reactivity is not restricted to the metal center itself being a Lewis acid or base, also ancillary ligands can be used as part of the Lewis pair, still with the benefit of the redox-active metal center nearby. This Minireview is designed to highlight the novel reactions arising from the combination of metal oxido transition-metal or rare-earth-metal compounds with the Lewis acid B(C₆ F₅ )₃ . It covers a wide area of chemistry including small molecule activation, hydrogenation and hydrosilylation catalysis, and olefin metathesis, substantiating the broad influence of the novel concept. Future goals of this young and exciting area are briefly discussed.

Unsaturated vicinal frustrated phosphane/borane Lewis pairs as ligands in gold(i) chemistry

The P/B FLP 6, formed by the 1,1-hydroboration of dimesitylphosphino(trimethylsilyl)acetylene with HB(C6F5)2 forms the Au(i)X complexes 9a (X: Cl) and 10a (X: NTf2), respectively. Both show marked AuB interactions. The P/B FLP isomer 8, featuring the bulky SiMe3 substituent at the carbon adjacent to boron forms gold complexes 9b and 10b, both of which show weaker AuB interactions. Complex 10a was employed in the catalytic hydroamination of a series of alkynes with p-toluidine. Complexes 9 and 10 were characterized by X-ray diffraction.

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.

Metal-free gem selective dimerization of terminal alkynes catalyzed by a pyridonate borane complex

The metal-free dimerization of alkynes catalyzed by a pyridonate borane complex via a 1,2-carboration is reported.

Gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes: towards functionalised azoles

Vinyl- and saturated azoles were synthesised through gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes and subsequent hydrogenation.

Computational and experimental investigation on the BCl3 promoted intramolecular amination of alkenes and alkynes

The BCl₃ promoted aminoboration of alkenes and alkynes was investigated both computationally and experimentally, leading to the discovery of a metal-free hydroamination of alkynes.

B(C6F5)3-catalyzed Markovnikov addition of indoles to aryl alkynes: an approach toward bis(indolyl)alkanes

The first example of the metal- and solvent-free B(C6F5)3-catalyzed Markovnikov addition of indoles to aryl alkynes was disclosed. Both N-H and N-protected indoles were tolerated, leading to a wide spectrum of versatile bis(indolyl)alkanes in moderate to good yields with high regioselectivities.

Boron-Catalyzed O-H Bond Insertion of α-Aryl α-Diazoesters in Water

A catalytic, metal-free O-H bond insertion of α-diazoesters in water in the presence of B(C₆F₅)₃· nH₂O (2 mol %) was developed, affording a series of α-hydroxyesters in good to excellent yields. The reaction features easy operation and wide substrate scope, and importantly, no metal is needed as compared with the conventional methods. Significantly, this approach further expands the applications of B(C₆F₅)₃ under water-tolerant conditions.