A Combined “Electrochemical–Frustrated Lewis Pair” Approach to Hydrogen Activation: Surface Catalytic Effects at Platinum Electrodes

Herein, we extend our “combined electrochemical–frustrated Lewis pair” approach to include Pt electrode surfaces for the first time. We found that the voltammetric response of an electrochemical–frustrated Lewis pair (FLP) system involving the B(C₆F₅)₃/[HB(C₆F₅)₃]^? redox couple exhibits a strong surface electrocatalytic effect at Pt electrodes. Using a combination of kinetic competition studies in the presence of a H atom scavenger, 6‐bromohexene, and by changing the steric bulk of the Lewis acid borane catalyst from B(C₆F₅)₃ to B(C₆Cl₅)₃, the mechanism of electrochemical–FLP reactions on Pt surfaces was shown to be dominated by hydrogen‐atom transfer (HAT) between Pt, [Pt?H] adatoms and transient [HB(C₆F₅)₃] ^? electrooxidation intermediates. These findings provide further insight into this new area of combining electrochemical and FLP reactions, and proffers additional avenues for exploration beyond energy generation, such as in electrosynthesis.     

Cyclisation versus 1,1‐Carboboration: Reactions of B(C6F5)3 with Propargyl Amides

A series of propargyl amides were prepared and their reactions with the Lewis acidic compound B(C₆F₅)₃ were investigated. These reactions were shown to afford novel heterocycles under mild conditions. The reaction of a variety of N‐substituted propargyl amides with B(C₆F₅)₃ led to an intramolecular oxo‐boration cyclisation reaction, which afforded the 5‐alkylidene‐4,5‐dihydrooxazolium borate species. Secondary propargyl amides gave oxazoles in B(C₆F₅)₃ mediated (catalytic) cyclisation reactions. In the special case of disubstitution adjacent to the nitrogen atom, 1,1‐carboboration is favoured as a result of the increased steric hindrance (1,3‐allylic strain) in the 5‐alkylidene‐4,5‐dihydrooxazolium borate species.     

Borane Adducts of Hydrazoic Acid and Organic Azides: Intermediates for the Formation of Aminoboranes

The reaction of HN₃ with the strong Lewis acid B(C₆F₅)₃ led to the formation of a very labile HN₃?B(C₆F₅)₃ adduct, which decomposed to an aminoborane, H(C₆F₅)NB(C₆F₅)₂, above ?20 °C with release of molecular nitrogen and simultaneous migration of a C₆F₅ group from boron to the nitrogen atom. The intermediary formation of azide–borane adducts with B(C₆F₅)₃ was also demonstrated for a series of organic azides, RN₃ (R=Me₃Si, Ph, 3,5‐(CF₃)₂C₆H₃), which also underwent Staudinger‐like decomposition along with C₆F₅ group migration. In accord with experiment, computations revealed rather small barriers towards nitrogen release for these highly labile azide adducts for all organic substituents except R=Me₃Si (m.p. 120 °C, T_dec=189 °C). Hydrolysis of the aminoboranes provided C₆F₅‐substituted amines, HN(R)(C₆F₅), in good yields.     

D–π–A Triarylboron Compounds with Tunable Push–Pull Character Achieved by Modification of Both the Donor and Acceptor Moieties

The push–pull character of a series of donor–bithienyl–acceptor compounds has been tuned by adopting triphenylamine or 1,1,7,7‐tetramethyljulolidine as a donor and B(2,6‐Me₂‐4‐RC₆H₂)₂ (R=Me, C₆F₅ or 3,5‐(CF₃)₂C₆H₃) or B[2,4,6‐(CF₃)₃C₆H₂]₂ as an acceptor. Ir‐catalyzed C?H borylation was utilized in the derivatization of the boryl acceptors and the tetramethyljulolidine donor. The donor and acceptor strengths were evaluated by electrochemical and photophysical measurements. In solution, the compound with the strongest acceptor, B[2,4,6‐(CF₃)₃C₆H₂]₂ ((FMes)₂B), has strongly quenched emission, while all other compounds show efficient green to red (Φ_F=0.80–1.00) or near‐IR (NIR; Φ_F=0.27–0.48) emission, depending on solvent. Notably, this study presents the first examples of efficient NIR emission from three‐coordinate boron compounds. Efficient solid‐state red emission was observed for some derivatives, and interesting aggregation‐induced emission of the (FMes)₂B‐containing compound was studied. Moreover, each compound showed a strong and clearly visible response to fluoride addition, with either a large emission‐color change or turn‐on fluorescence.     

On the mechanism of intramolecular nitrogen‐atom hopping in the carbon chain of C6N radical: A Plausible 3c−4e crossover Long‐Bond

Linear isomers of C₆N radical differ in the position of the nitrogen atom in the carbon chain of C₆N. Reaction routes, involving intramolecular nitrogen atom insertion at varying position in the carbon chain of C₆N, are analyzed for the isomerisation between linear isomers of C₆N. Through an automated and systematic search performed with global reaction route mapping of the potential energy surface, thermal isomerisation pathways for C₆N radical are proposed based on the computations carried out at CASSCF/aug‐cc‐pVTZ, and CCSD(T)/6‐311++G(d,p)//B3LYP/6‐311++G(d,p) levels of the theory. Notably, a high lying linear isomer, centrosymmetric with respect to the nitrogen atom, is observed to be stabilized by a unique crossover three center‐four electron π long bond between the carbon atoms that are spatially separated by a nitrogen atom in a natural bond orbital. This long bond is concluded to be responsible for the predicted thermal isomerisation to be more feasible than the dissociation during the isomerisation pathway of a linear isomer of C₆N. © 2014 Wiley Periodicals, Inc.     

Stable Borane Adducts of Alcoholates and Carboxylates

Six adducts of B(C₆F₅)₃ and archetypical alcoholates and carboxylates, were prepared and isolated as crystalline sodium crown ether salts, [Na(15‐crown‐5)][CH₃O · B(C₆F₅)₃] ( 1 ), [Na(15‐crown‐5)][CH₃CH₂O · B(C₆F₅)₃] ( 2 ), [Na(15‐crown‐5)][HCO₂ · B(C₆F₅)₃] ( 3 ), [Na(15‐crown‐5)][(H₃CCO₂ · B(C₆F₅)₃] ( 4 ), [Na(15‐crown‐5)][(F₃CCO₂ · B(C₆F₅)₃] ( 5 ), and [Na₂(15‐crown‐5)₃][C₂O₄ · 2 B(C₆F₅)₃] ( 6 ). All compounds were fully characterized by multinuclear NMR‐ and IR spectroscopy, ESI MS spectrometry, and X‐ray crystallography.     

Reactivity of an Intramolecular Fluorophosphonium Fluoroborate

The reactions of the intramolecular frustrated Lewis pair‐adduct Ph₂PC(p‐Tol)?C(C₆F₅)B(C₆F₅)₂(CNtBu) with XeF₂ gave Ph₂P(F)C(p‐Tol)?C(C₆F₅)B(F)(C₆F₅)₂ ( 3 ). This species reacts with two equivalents of Al(C₆F₅)₃?C₇H₈ producing the salt, [Ph₂P(F)C(p‐Tol)?C(C₆F₅)B(C₆F₅)₂][F(Al(C₆F₅)₃)₂] ( 4 ), whereas reaction with HSiEt₃/B(C₆F₅)₃ gave Ph₂P(F)C(p‐Tol)?C(H)B(C₆F₅)₃ ( 5 ). The photolysis of 3 resulted in aromatization affording the phenanthralene derivative Ph₂P(F)C(p‐Tol(o‐C₆F₄))?CB(F)(C₆F₅)₂ ( 6 ).     

Lewis Acid Coordination Redirects S‐Nitrosothiol Signaling Output

S‐Nitrosothiols (RSNOs) serve as air‐stable reservoirs for nitric oxide in biology. While copper enzymes promote NO release from RSNOs by serving as Lewis acids for intramolecular electron‐transfer, redox‐innocent Lewis acids separate these two functions to reveal the effect of coordination on structure and reactivity. The synthetic Lewis acid B(C₆F₅)₃ coordinates to the RSNO oxygen atom, leading to profound changes in the RSNO electronic structure and reactivity. Although RSNOs possess relatively negative reduction potentials, B(C₆F₅)₃ coordination increases their reduction potential by over 1 V into the physiologically accessible +0.1 V vs. NHE. Outer‐sphere chemical reduction gives the Lewis acid stabilized hyponitrite dianion trans‐[LA‐O‐N=N‐O‐LA]^2? [LA=B(C₆F₅)₃], which releases N₂O upon acidification. Mechanistic and computational studies support initial reduction to the [RSNO‐B(C₆F₅)₃] radical anion, which is susceptible to N?N coupling prior to loss of RSSR.     

B(C6F5)3/Chiral Phosphoric Acid Catalyzed Ketimine–Ene Reaction of 2‐Aryl‐3H‐indol‐3‐ones and α‐Methylstyrenes

The enantioselective ketimine–ene reaction is one of the most challenging stereocontrolled reaction types in organic synthesis. In this work, catalytic enantioselective ketimine–ene reactions of 2‐aryl‐3H‐indol‐3‐ones with α‐methylstyrenes were achieved by utilizing a B(C₆F₅)₃/chiral phosphoric acid (CPA) catalyst. These ketimine–ene reactions proceed well with low catalyst loading (B(C₆F₅)₃/CPA=2 mol %/2 mol %) under mild conditions, providing rapid and facile access to a series of functionalized 2‐allyl‐indolin‐3‐ones with very good reactivity (up to 99 % yield) and excellent enantioselectivity (up to 99 % ee). Theoretical calculations reveal that enhancement of the acidity of the chiral phosphoric acid by B(C₆F₅)₃ significantly reduces the activation free energy barrier. Furthermore, collective favorable hydrogen‐bonding interactions, especially the enhanced N?H???O hydrogen‐bonding interaction, differentiates the free energy of the transition states of CPA and B(C₆F₅)₃/CPA, thereby inducing the improvement of stereoselectivity.     

C−F Bond Activation by a Saturated N‐Heterocyclic Carbene: Mesoionic Compound Formation and Adduct Formation with B(C6F5)3

The reaction of SIPr, [1,3‐bis(2,6‐diisopropylphenyl)‐imidazolin‐2‐ylidene] ( 1 ), with C₆F₆ led to the formation of an unprecedented mesoionic compound ( 2 ). The formation of 2 is made accessible by deprotonation of the SIPr backbone with simultaneous elimination of HF. The C?F bond para to the imidazolium ring in 2 is only of 1.258(4) Å, which is the one of the shortest structurally authenticated C?F bonds known to date. The liberation of HF during the reaction is unequivocally proved by the addition of one more equivalent of SIPr, which leads to the imidazolium salt with the HF₂^? anion. To functionalize 2 , the latter reacted with B(C₆F₅)₃ to give an unusual donor–acceptor compound, where the fluoride atom from the C₆F₅ moiety coordinates to B(C₆F₅)₃ and the carbanion moiety remains unaffected. Such coordination susceptibility of the fluoride atom of a nonmetallic system to a main‐group Lewis acid (F_non‐metal→BR₃) is quite unprecedented.     

Stannylium ions, a tin(II) arene complex, and a tin dication stabilized by weakly coordinating anions

The reactivity of aryl‐substituted stannylenes, Ar₂Sn ( 4 ), towards silylarenium borates, [R₃SiArH][B(C₆F₅)₄] ( 3 ), was investigated. The reaction with 2,3,4‐trimethyl‐6‐tert‐butylphenyl (mebp)‐substituted stannylene gave silyl‐substituted stannylium ions 2 a , b , which were characterized by NMR spectroscopy supported by the results of quantum‐mechanical computations of molecular structures and magnetic properties. The tri‐iso‐propylphenyl‐substituted stannylium ions 2 c , d undergo a decomposition reaction in toluene to give the dicationic tin–arene complex [Sn(C₇H₈)₃]²⁺ ( 5 ) in the form of the [B(C₆F₅)₄] salt in high yields. The 5 [B(C₆F₅)₄]₂ salt was identified by single crystal X‐ray diffraction analysis and by Mössbauer spectroscopy. The bonding situation was investigated by using natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) calculations. The substitution of the weakly coordinating borate anion by the carboranate [CB₁₁H₆Br₆]^? results in replacement of the toluene ligands and formation of tin(II) carboranate with only weak Sn²⁺–anion interactions as suggested by the solid‐state structure of the isolated salt.     

The B(C6F5)3 Boron Lewis Acid Route to Arene‐Annulated Pentalenes

4,5‐Dimethyl‐1,2‐bis(1‐naphthylethynyl)benzene ( 12 ) undergoes a rapid multiple ring‐closure reaction upon treatment with the strong boron Lewis acid B(C₆F₅)₃ to yield the multiply annulated, planar conjugated π‐system 13 (50 % yield). In the course of this reaction, a C₆F₅ group was transferred from boron to carbon. Treatment of 12 with CH₃B(C₆F₅)₂ proceeded similarly, giving a mixture of 13 (C₆F₅‐transfer) and the product 15 , which was formed by CH₃‐group transfer. 1,2‐Bis(phenylethynyl)benzene ( 8 a ) reacts similarly with CH₃B(C₆F₅)₂ to yield a mixture of the respective C₆F₅‐ and CH₃‐substituted dibenzopentalenes 10 a and 16 . The reaction is thought to proceed through zwitterionic intermediates that exhibit vinyl cation reactivities. Some B(C₆F₅)₃‐substituted species ( 26 , 27 ) consequently formed by in situ deprotonation upon treatment of the respective 1,2‐bis(alkynyl)benzene starting materials ( 24 , 8 ) with the frustrated Lewis pair B(C₆F₅)₃/P(o‐tolyl)₃. The overall formation of the C₆F₅‐substituted products formally require HB(C₆F₅)₂ cleavage in an intermediate dehydroboration step. This was confirmed in the reaction of a thienylethynyl‐containing starting material 21 with B(C₆F₅)₃, which gave the respective annulated pentalene product 23 that had the HB(C₆F₅)₂ moiety 1,4‐added to its thiophene ring. Compounds 12 – 14 , 23 , and 26 were characterized by X‐ray diffraction.     

Effects of tris(pentafluorophenyl)borane on the activation of a metal alkyl‐free Ni‐based catalyst in the polymerization of 1,3‐butadiene

The activation of a metal alkyl‐free Ni‐based catalyst with B(C₆F₅)₃ was investigated in the polymerization of 1,3‐butadiene. A catalyst of bis(1,5‐cyclooctadiene)nickel (Ni(COD)₂)/B(C₆F₅)₃ was found to have high catalytic activity and 1,4‐cis stereoregularity. The catalyst was also found to provide polybutadiene having a molecular weight (M_w) of up to 117,000, even in the absence of AlR₃ and MAO. Variations in the mol ratio of B(C₆F₅)₃ to Ni affected catalytic activity, 1,4‐cis stereoregularity, and the M_w of polybutadiene, while the molecular weight distribution (MWD) of polybutadiene showed little correlation with the mol ratio of B(C₆F₅)₃ to Ni. The use of other borane compounds such as B(C₆H₅)₃, BEt₃, and BF₃ etherate in place of B(C₆F₅)₃ clearly showed the two main functions of B(C₆F₅)₃ in the present catalyst. The high Lewis acidity of B(C₆F₅)₃ enabled it to activate catalytic complexes, thus inducing the polymerization. The steric bulkiness of B(C₆F₅)₃ suppressed chain transfer reactions, contributing to the production of polybutadiene with a high M_w. Kinetic studies showed that the catalyst had an induction period, possibly due to the time needed for the formation of catalytic complexes starting from Ni(COD)₂. A plot of ?ln (1?X), where X is the fractional conversion, as a function of time resulted in a linear relationship, showing that the present catalyst system followed first‐order kinetics with respect to monomer concentration. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1164–1173, 2004     

Synthesis,Structure, and Reactivity of Diazene Adducts: Isolation of iso‐Diazene Stabilized as a Borane Adduct

This work describes the synthesis and full characterization of a series of GaCl₃ and B(C₆F₅)₃ adducts of diazenes R¹?N?N?R² (R¹=R²=Me₃Si, Ph; R¹=Me₃Si, R²=Ph). Trans‐Ph?N?N?Ph forms a stable adduct with GaCl₃, whereas no adduct, but instead a frustrated Lewis acid–base pair is formed with B(C₆F₅)₃. The cis‐Ph?N?N?Ph ? B(C₆F₅)₃ adduct could only be isolated when UV light was used, which triggers the isomerization from trans‐ to cis‐Ph?N?N?Ph, which provides more space for the bulky borane. Treatment of trans‐Ph?N?N?SiMe₃ with GaCl₃ led to the expected trans‐Ph?N?N?SiMe₃ ? GaCl₃ adduct but the reaction with B(C₆F₅)₃ triggered a 1,2‐Me₃Si shift, which resulted in the formation of a highly labile iso‐diazene, Me₃Si(Ph)N?N; stabilized as a B(C₆F₅)₃ adduct. Trans‐Me₃Si?N?N?SiMe₃ forms a labile cis‐Me₃Si?N?N?SiMe₃ ? B(C₆F₅)₃ adduct, which isomerizes to give the transient iso‐diazene species (Me₃Si)₂N?N ? B(C₆F₅)₃ upon heating. Both iso‐diazene species insert easily into one B?C bond of B(C₆F₅)₃ to afford hydrazinoboranes. All new compounds were fully characterized by means of X‐ray crystallography, vibrational spectroscopy, CHN analysis, and NMR spectroscopy. All compounds were further investigated by DFT and the bonding situation was assessed by natural bond orbital (NBO) analysis.     

Borane‐Stabilized Isomeric Dimers of the Phosphaethynolate Anion

The reactions of the phosphaethynolate anion ([PCO]⁻) with a range of boranes were explored. BPh₃ and [PCO]⁻ form a dimeric anion featuring P−B bonds and is prone to dissociation at room temperature. The more Lewis acidic borane B(C₆F₅)₃ yields a less symmetric dimer of [PCO]⁻ with P−B and P−O bonds. Less sterically demanding HB(C₆F₅)₂ and H₂B(C₆F₅) boranes form a third isomer with [PCO]⁻ featuring both boranes bound to the same phosphorus atom. Despite the unexpected thermodynamic preference for P‐coordination, computational data illustrate that electronic and steric features impact the binding modes of the resulting dianionic dimers.     

Decorating Tetrathiafulvalene (TTF) with Fluorinated Phenyls through Sulfur Bridges: Facile Synthesis,Properties, and Aggregation through Fluorine Interactions

Tetrathiafulvalene derivatives ( TTF1 – TTF9 ) bearing fluorinated phenyl groups attached through the sulfur bridges have been synthesized by employing a copper‐mediated C–S coupling reaction of C₆H_5?xF_xI (x=1, 2, 5) and a zinc‐thiolate complex, (TBA)₂[Zn(DMIT)₂] (TBA=tetrabutyl ammonium, DMIT=1,3‐dithiole‐2‐thione‐4,5‐dithiolate), as the key step. Particularly, the selective synthesis of C₆F₅‐substituted ( TTF8 ) and C₆F₄‐fused ( TTF9 ) TTFs from C₆F₅I is disclosed. The physicochemical properties and crystal structures of these TTFs are fully investigated by UV/Vis absorption spectra, cyclic voltammetry, molecular orbital calculation, and single‐crystal X‐ray diffraction. The exchange of hydrogen versus fluorine on the peripheral phenyl groups show a notable influence on both the electronic and crystallographic natures of the resulting TTFs: 1) lowering both the HOMO and the LUMO energy levels, 2) modulating the electrochemical properties by regioselective and/or the degree of fluorination, 3) enhancing the driving forces of stacking by multiple fluorine interactions (F???S, C?F???π/π_F, C?F???F?C, and C?F???H). This work indicates that the decoration with fluorinated phenyls holds promise to produce functional TTFs with novel electronic and aggregation features.     

B(C6F5)3‐Catalyzed Transfer Hydrogenation of Imines and Related Heteroarenes Using Cyclohexa‐1,4‐dienes as a Dihydrogen Source

The strong boron Lewis acid tris(pentafluorophenyl)borane, B(C₆F₅)₃, is shown to abstract a hydride from suitably donor‐substituted cyclohexa‐1,4‐dienes, eventually releasing dihydrogen. This process is coupled with the FLP‐type (FLP=frustrated Lewis pair) hydrogenation of imines and nitrogen‐containing heteroarenes that are catalyzed by the same Lewis acid. The net reaction is a B(C₆F₅)₃‐catalyzed, i.e., transition‐metal‐free, transfer hydrogenation using easy‐to‐access cyclohexa‐1,4‐dienes as reducing agents. Competing reaction pathways with or without the involvement of free dihydrogen are discussed.     

[Re{NB(C6F5)3}(Et2dtc)2]2 – Dimerisierung infolge der Ausbildung einer Nitridobrücke

[Re{NB(C₆F₅)₃}(Et₂dtc)₂]₂ – Dimerization as a Consequence of the Formation of a Nitrido Bridge The title compound is formed from [ReN(Et₂dtc)₂] with five‐coordinate Re atom upon reaction with B(C₆F₅)₃. As a consequence of the formation of a nitrido bridge between Re and B the structural trans influence of the nitrido ligand decreases and its trans position which is not occupied in the edduct becomes available for co‐ordination. The dimer is built up by two [Re{NB(C₆F₅)₃}(Et₂dtc)₂] units which are linked by weak bonds between the metal and each one sulphur atom of the neighbouring unit (Re–S: 2.856(6) and 2.835(6) Å, respectively).     

Autoinduced Catalysis and Inverse Equilibrium Isotope Effect in the Frustrated Lewis Pair Catalyzed Hydrogenation of Imines

The frustrated Lewis pair (FLP)‐catalyzed hydrogenation and deuteration of N‐benzylidene‐tert‐butylamine ( 2 ) was kinetically investigated by using the three boranes B(C₆F₅)₃ ( 1 ), B(2,4,6‐F₃‐C₆H₂)₃ ( 4 ), and B(2,6‐F₂‐C₆H₃)₃ ( 5 ) and the free activation energies for the H₂ activation by FLP were determined. Reactions catalyzed by the weaker Lewis acids 4 and 5 displayed autoinductive catalysis arising from a higher free activation energy (2 kcal mol^?1) for the H₂ activation by the imine compared to the amine. Surprisingly, the imine reduction using D₂ proceeded with higher rates. This phenomenon is unprecedented for FLP and resulted from a primary inverse equilibrium isotope effect.