An axially chiral, electron-deficient borane: synthesis, coordination chemistry, Lewis acidity, and reactivity

An axially chiral dihydroborepine with a binaphthyl backbone and a C(6)F(5) substituent at the boron atom was prepared by transmetalation from the corresponding tin precursor. This novel motif was structurally characterized by X-ray diffraction analysis as its THF and its PhCN Lewis acid/base complex. (1)H NMR measurements at variable temperatures of the former adduct revealed a remarkable dynamic behavior in solution. Several more Lewis pairs with oxygen, nitrogen, carbon, and phosphorus σ-donors were synthesized and analyzed by multinuclear NMR spectroscopy. The determination of the borane's Lewis acidity with the Gutmann-Beckett method attests its substantial Lewis acidity [85% with Et(3) PO as well as 74% with Ph(3) PO relative to the parent B(C(6)F(5))(3)]. Representative examples of Si-H bond activation (carbonyl reduction and dehydrogenative Si-O coupling) are included, demonstrating the chemical stability and the synthetic potential of the new chiral boron-based Lewis acid.     

Promoting the Hydrosilylation of Benzaldehyde by Using a Dicationic Antimony‐Based Lewis Acid: Evidence for the Double Electrophilic Activation of the Carbonyl Substrate

The concomitant activation of carbonyl substrates by two Lewis acids has been investigated by using [1,2‐(Ph₂MeSb)₂C₆H₄]²⁺ ([ 1 ]²⁺), an antimony‐based bidentate Lewis acid obtained by methylation of the corresponding distibine. Unlike the simple stibonium cation [Ph₃MeSb]⁺, dication [ 1 ]²⁺ efficiently catalyzes the hydrosilylation of benzaldehyde under mild conditions. The catalytic activity of this dication is correlated to its ability to doubly activate the carbonyl functionality of the organic substrate. This view is supported by the isolation of [ 1 ‐μ₂‐DMF][OTf]₂, an adduct, in which the DMF oxygen atom bridges the two antimony centers.     

Extending the Scope of the B(C6F5)3‐Catalyzed CN Bond Reduction: Hydrogenation of Oxime Ethers and Hydrazones

The B(C₆F₅)₃‐catalyzed hydrogenation is applied to aldoxime triisopropylsilyl ethers and hydrazones bearing an easily removable phthaloyl protective group. The C?N reduction of aldehyde‐derived substrates (oxime ethers and hydrazones) is enabled by using 1,4‐dioxane as the solvent known to participate as the Lewis‐basic component in FLP‐type heterolytic dihydrogen splitting. More basic ketone‐derived hydrazones act as Lewis bases themselves in the FLP‐type dihydrogen activation and are therefore successfully hydrogenated in nondonating toluene. The difference in reactivity between aldehyde‐ and ketone‐derived substrates is also reflected in the required catalyst loading and dihydrogen pressure.     

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.     

Transfer Hydrocyanation of α‐ and α,β‐Substituted Styrenes Catalyzed by Boron Lewis Acids

A straightforward gram‐scale preparation of cyclohexa‐1,4‐diene‐based hydrogen cyanide (HCN) surrogates is reported. These are bench‐stable but formally release HCN and rearomatize when treated with Lewis acids. For BCl₃, the formation of the isocyanide adduct [(CN)BCl₃]^? and the corresponding Wheland complex was verified by mass spectrometry. In the presence of 1,1‐di‐ and trisubstituted alkenes, transfer of HCN from the surrogate to the C?C double bond occurs, affording highly substituted nitriles with Markovnikov selectivity. The success of this transfer hydrocyanation depends on the Lewis acid employed; catalytic amounts of BCl₃ and (C₆F₅)₂BCl are shown to be effective while B(C₆F₅)₃ and BF₃?OEt₂ are not.     

B(C6F5)3‐Catalyzed Hydrogenation of Oxime Ethers without Cleavage of the NO Bond

The hydrogenation of oximes and oxime ethers is usually hampered by N? O bond cleavage, hence affording amines rather than hydroxylamines. The boron Lewis acid B(C₆F₅)₃ is found to catalyze the chemoselective hydrogenation of oxime ethers at elevated or even room temperature under 100 bar dihydrogen pressure. The use of the triisopropylsilyl group as a protecting group allows for facile liberation of the free hydroxylamines.     

Gas-phase reactions of atomic lanthanide cations with D2O: room-temperature kinetics and periodicity in reactivity.

Reactions of atomic lanthanide cations (excluding Pm+) with D2O have been surveyed in the gas phase using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer to measure rate coefficients and product distributions in He at 0.35+/-0.01 Torr and 295+/-2 K. Primary reaction channels were observed corresponding to O-atom transfer, OD transfer and D2O addition. O-atom transfer is the predominant reaction channel and occurs exclusively with Ce+, Nd+, Sm+, Gd+, Tb+ and Lu+. OD transfer is observed exclusively with Yb+, and competes with O-atom transfer in the reactions with La+ and Pr+. Slow D2O addition is observed with early lanthanide cation Eu+ and the late lanthanide cations Dy+, Ho+, Er+ and Tm+. Higher-order sequential D2O addition of up to five D2O molecules is observed with LnO+ and LnOD+. A delay of more than 50 kcal mol(-1) is observed in the onset of efficient exothermic O-atom transfer, which suggests the presence of kinetic barriers of perhaps this magnitude in the exothermic O-atom transfer reactions of Dy+, Ho+, Er) and Tm+ with D2O. The reaction efficiency for O-atom transfer is seen to decrease as the energy required to promote an electron to make two non-f electrons available for bonding increases. The periodic trend in reaction efficiency along the lanthanide series matches the periodic trend in the electron-promotion energy required to achieve a d1s1 or d2 excited electronic configuration in the lanthanide cation, and also the periodic trends across the lanthanide row reported previously for several alcohols and phenol. An Arrhenius-like correlation is also observed for the dependence of D2O reactivity on promotion energy for early lanthanide cations, and exhibits a characteristic temperature of 2600 K.     

Unimolecular reactivity of strong metal-cation complexes in the gas phase: ethylenediamine-Cu(+)

The gas-phase reactions between ethylenediamine (en) and Cu(+) have been investigated by means of mass spectrometry techniques. The MIKE spectrum reveals that the adduct ions [Cu(+)(H(2)NCH(2)CH(2)NH(2))] spontaneously decompose by loosing H(2), NH(3) and HCu, the loss of hydrogen being clearly dominant. The spectra of the fully C-deuterated species show the loss of HD, NH(3) and CuD but no losses of H(2), D(2), NH(2)D, NHD(2), ND(3) or CuH are observed. This clearly excludes hydrogen exchange between the methylene and the amino groups as possible mechanisms for the loss of ammonia. Conversely, methylene hydrogen atoms are clearly involved in the loss of molecular hydrogen. The structures and bonding characteristics of the Cu(+)(en) complexes as well as the different stationary points of the corresponding potential energy surface (PES) have been theoretically studied by DFT calculations carried out at B3LYP/6-311+G(2df,2p)//B3LYP/6-311G(d,p) level. Based on the topology of this PES the most plausible mechanisms for the aforementioned unimolecular fragmentations are proposed. Our theoretical estimates indicate that Cu(+) strongly binds to en, by forming a chelated structure in which Cu(+) is bridging between both amino groups. The binding energy is quite high (84 kcal mol(-1)), but also the products of the unimolecular decomposition of Cu(+)(en) complexes are strongly bound Cu(+)-complexes.     

Synthesis, structure, and reactivity of a dihydrido borenium cation

Bottled! The employment of hexaphenylcarbodiphosphorane as ancillary ligand has allowed the isolation of the otherwise transient dihydrido borenium cation.     

Structure and Bonding Nature of the Strained Lewis Acid 3‐Methyl‐1‐boraadamantane: A Case Study Employing a New Data‐Analysis Procedure in Gas Electron Diffraction

Base‐free 3‐methyl‐1‐boraadamantane was synthesized by starting from its known THF adduct, transforming it to a butylate‐complex with n‐butyllithium, cleaving the cage with acetyl chloride to give 3‐n‐butyl‐5‐methyl‐7‐methylene‐3‐borabicyclo[3.3.1]nonane and closing the cage again by reacting the latter with dicyclohexylborane. The identity of 3‐methyl‐1‐boraadamantane was proven by ¹H, ¹¹B and ¹³C NMR spectroscopy and elemental analysis. The experimental equilibrium structure of the free 3‐methyl‐1‐boraadamantane molecules has been determined at 100 °C by using gas‐phase electron diffraction. For this structure determination, an improved method for data analysis has been introduced and tested: the structural refinement versus gas‐phase electron diffraction data (in terms of Cartesian coordinates) with a set of quantum‐chemically derived regularization constraints for the complete structure under optimization of a regularization constant, which maximizes the contribution of experimental data while retaining a stable refinement. The detailed analysis of parameter errors shows that the new approach allows obtaining more reliable results. The most important structural parameters are: r_e(B‐C)_av=1.556(5) Å, ${\angle }$ _e(C‐B‐C)_av=116.5(2)°. The configuration of the boron atom is pyramidal with ${\sum \angle }$ (C‐B‐C)=349.4(4)°. The nature of bonding was analyzed further by applying the natural bond orbital (NBO) and atoms in molecules (AIM) approaches. The experimentally observed shortening of the B? C bonds and elongation of the adjacent C? C bonds can be explained by the σ(C‐C)→p(B) hyperconjugation model. Both NBO and AIM analyses predict that the B? C bonds are significantly bent in the direction out of the cage.     

Boron(II) Cations: Interplay between Lewis‐Pair‐Acceptor and Electron‐Donor Properties

Boron(III) cations are widely used as highly Lewis acidic reagents in synthetic chemistry. In contrast, boron(II) cations are extremely rare and their chemistry almost completely unknown. They are both Lewis acids and electron donors, properties that are commonly associated with catalytically active late‐transition‐metal complexes. This double reactivity pattern ensures a rich and diverse chemistry. Herein we report the facile synthesis of several new boron(II) cations starting with a special diborane with two easily exchangeable triflate substituents. By increasing the π‐acceptor character of the neutral σ‐donor reaction partners, first reactions were developed in which the combined Lewis acidity and electron‐donor properties of boron(II) cations are applied for the reduction of organic molecules. The results of our study pave the way for applications of these unusual compounds in synthetic chemistry.     

Catalytic Hydrosilylation of Imines by Aluminum Hydride Cations

In this work, the catalytic activity of electronically unsaturated three coordinated aluminum hydride cations [ L AlH]⁺[HB(C₆F₅)₃]⁻ ( 1 ) and [ L AlH]⁺[B(C₆F₅)₄]⁻ ( 2 ) in hydrosilylation of imines has been disclosed ( L ={(2,6-ⁱPr₂C₆H₃N)P(Ph₂)}₂N). A variety of organo-silanes such as Et₃SiH, MePhSiH₂, PhSiH₃, TMDSO, and PHMS are screened in this endeavour. The amines as products of catalysis were obtained in good to excellent yields after the hydrolysis of silylamine intermediates. Further, a series of controlled experiments systematically designed to investigate the underlying mechanistic pathway through multinuclear NMR analysis showed Lewis adduct formation between cationic aluminum centre and the imine nitrogen, which subsequently undergoes reaction with silane to afford the product. The hydrosilylation of imine performed with Et₃SiH using catalyst 1 with a loading of 2 mol % at 60 °C occurs smoothly. Whereas 2 led to the product formation with Et₃SiH only when used in stoichiometric quantity. Further, to investigate this unique behaviour of 1 NMR investigations were performed and revealed that the anion in 1 competes for hydride delivery and in-situ generates B(C₆F₅)₃ that cooperatively reinforces the catalytic activity of 1 .     

Water compatible gold(III)-catalysed synthesis of unsymmetrical ethers from alcohols

An efficient and broad-scoped method for the preparation of unsymmetrical ethers from alcohols catalysed by the simplest and least expensive gold catalyst, NaAuCl(4), is described for the first time. The procedure enables the etherification of benzylic and tertiary alcohols with moderate to good yields under mild conditions with low catalyst loading. Symmetrical ethers, the usual side products in the etherification of alcohols, were not detected in this case. The formation of the racemic ether from a chiral benzyl alcohol suggests the intermediacy of a carbocation, which has not previously been postulated for gold-catalysed reactions involving alcohols.     

Stoichiometric and Catalytic C−C and C−H Bond Formation with B(C6F5)3 via Cationic Intermediates

This work showcases a new catalytic cyclization reaction using a highly Lewis acidic borane with concomitant C−H or C−C bond formation. The activation of alkyne‐containing substrates with B(C₆F₅)₃ enabled the first catalytic intramolecular cyclizations of carboxylic acid substrates using this Lewis acid. In addition, intramolecular cyclizations of esters enable C−C bond formation as catalytic B(C₆F₅)₃ can be used to effect formal 1,5‐alkyl migrations from the ester functional groups to unsaturated carbon–carbon frameworks. This metal‐free method was used for the catalytic formation of complex dihydropyrones and isocoumarins in very good yields under relatively mild conditions with excellent atom efficiency.