A New Perspective on Borane Chemistry: The Nucleophilicity of the B−H Bonding Pair Electrons

A number of recently discovered nucleophilic boron compounds, such as boryl anions and borylenes, are breaking the rules regarding boron and boron‐containing compounds and their reputation as Lewis acids/electrophiles. In a similar fashion, the B?H bonding pair electrons in boranes also show nucleophilicity which is ascribed to the lower electronegativity of boron relative to that of hydrogen. However, this nucleophilicity of the B?H bond has received far less attention. Explorations of the nucleophilicity of the B?H bonding pair electrons have led to the formation of B?H?B bonded units and B?H???H?Y dihydrogen bonds, based on which new chemistry has been uncovered, including the elucidation of the mechanism of formation of aminodiborane (ADB), the diammoniate of diborane (DADB), and lithium or sodium salts of octahydrotriborates (B₃H₈^?), as well as the development of more convenient and straightforward synthetic routes to these reagents. Moreover, the recognition of the nucleophilic properties of the B?H bonding pair electrons will also help to more deeply understand the different mechanisms operating in hydroboration reactions.     

Highly Emissive Boron Ketoiminate Derivatives as a New Class of Aggregation‐Induced Emission Fluorophores

A series of boron ketoiminate derivatives that exhibited clear aggregation‐induced emission (AIE) characteristics (in THF: Φ_PL≤0.01; in the solid state: Φ_PL=0.30–0.76) were prepared by the reactions of 1,3‐enaminoketone derivatives with boron trifluoride–diethyl etherate. The structures and optical properties were investigated by UV‐visible spectroscopy, photoluminescent (PL) spectroscopy, and X‐ray single‐crystal measurements. These results indicate that the AIE characteristics were derived from molecular motions of the boron‐chelating rings with a boron? nitrogen (B? N) bond. Furthermore, the optical properties were controllable by steric hindrance of the substituted groups on the nitrogen atom.     

Mercuration ofB(9)-substitutedmeta- andortho-carboranes

The mercuration of substituted R₂C₂B₁₀H₉X-9 type carboranes (where R=m-H, X= Cl, Br, I, Me; R=o-H, X=Me) was studied. It was found that mercury atoms add to the boron atoms in position 10 ofmeta-carboranes and in position 12 ofortho-carboranes,i.e., to the boron atoms adjacent to the boron atom bonded to the X substituent. Symmetrical (R₂C₂B₁₀H₈X)₂Hg type derivatives were obtained. It was shown that they can be used as starting materials in transmetallation reactions.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 743–747, April, 1995.This work was carried out with financial support from the Russian Foundation for Basic Research (Project No 93-03-18654) and the International Science Foundation (Grant NDM 000).     

Large-Scale and Facile Preparation of Pure Ammonia Borane through Displacement Reactions

Ammonia borane (AB) is the most widely studied hydride for hydrogen storage in addition to being a useful reducing agent. Attempts to synthesize pure AB through simple displacement reactions date back to the 1960s; but have been thwarted by the formation of the diammoniate of diborane (DADB), an ionic byproduct. Based on our recent characterization of the formation mechanism of DADB, we have developed a large-scale synthesis of pure AB by both increasing the basicity of the Lewis base of the borane carrier and using a nonpolar solvent to limit the formation of an intermediate, the ammonia diborane (AaDB). Conditions were optimized for the preparation of pure AB by two displacement reactions, either ammonia with dimethylsulfide borane or ammonia with dimethylaniline borane in toluene at room temperature. These procedures are also suitable for preparation of other amine boranes which had the same problem of forming ionic byproducts during displacement reactions.     

The first C2 selective halide substitution reaction of 2,3-epoxy alcohols by the use of (CH3O)3B-MX (X=I, Br, Cl) system

The first C2 selective halide substitution reactions of 2,3-epoxy alcohols have been realized by the use of the (CH₃O)₃B-MX (X=I, Br, Cl) system, which proceed through novel endo-mode epoxide-opening of intramolecular boron chelates to afford the corresponding C2 halohydrin derivatives with high regio- and stereoselectivity.     

The reaction of (trifluoromethyl)dialkylaminoboranes with HF,HCl and HBr. X-Ray structure investigation of the amineboranes (CF3)2B(X)NHMe2, X  F and OH

The reactions of CF₃B(NMe₂)₂ (I) and (CF₃)₂BNMe₂ (II) with HX (X  F, Cl and Br) have been investigated. Additions with preservation of the BC bonds to yield species with tetracoordinate boron, along with some BN cleavage, were observed. While I formed boronium salts CF₃B(X)(NHMe₂)₂⁺X⁻ with X  Cl and Br, CF₃BF₂ · NHMe₂ (V) was obtained with HF. On the other hand, reactions of II with HX yielded the 1 : 1 adducts (CF₃)₂B(X) · HNMe₂ in each case. Of these, the species with X  F (VI) and X  OH (IX) (obtained by hydrolysis) were examined by single crystal X-ray diffraction. Surprisingly, no difference was found between the average BC bond lengths of these borates (VI 1.612(8), IX 1.624(4) Å) and that of II. The implications of this observation for BCF₃ bonding are discussed.     

Large‐Scale and Facile Preparation of Pure Ammonia Borane through Displacement Reactions

Ammonia borane (AB) is the most widely studied hydride for hydrogen storage in addition to being a useful reducing agent. Attempts to synthesize pure AB through simple displacement reactions date back to the 1960s; but have been thwarted by the formation of the diammoniate of diborane (DADB), an ionic byproduct. Based on our recent characterization of the formation mechanism of DADB, we have developed a large‐scale synthesis of pure AB by both increasing the basicity of the Lewis base of the borane carrier and using a nonpolar solvent to limit the formation of an intermediate, the ammonia diborane (AaDB). Conditions were optimized for the preparation of pure AB by two displacement reactions, either ammonia with dimethylsulfide borane or ammonia with dimethylaniline borane in toluene at room temperature. These procedures are also suitable for preparation of other amine boranes which had the same problem of forming ionic byproducts during displacement reactions.     

Synthesis of Boron(III)‐Coordinated Subchlorophins and Their Peripheral Modifications

A pyrrole‐cleaving modification to transform boron(III) meso ‐triphenylsubporphyrin into boron(III) meso ‐triphenylsubchlorophin has been developed. Boron(III) subchlorophins thus synthesized show absorption and fluorescence spectra that are roughly similar to those of boron(III) subchlorins, but B ‐methoxy boron(III) subchlorophin showed considerably intensified fluorescence and a small Stokes shift. Peripheral modification reactions of B ‐phenyl boron(III) subchlorophin such as regioselective nitration with Cu(NO₃)₂⋅3 H₂O, ipso ‐substitution reactions of boron(III) α‐nitrosubchlorophin with CsF and CsCl, and Pd‐catalyzed cross‐coupling reactions of boron(III) α‐chlorosubchlorophin with arylacetylenes, have been also explored to tune the optical properties of subchlorophins.     

Formation of an Isolable Divinylborinium Ion through Twofold 1,2‐Carboboration between a Diarylborinium Ion and Diphenylacetylene

Borinium ions, that is, two‐coordinate boron cations, are the most electron‐deficient isolable boron compounds. As borinium ions have only four formal valence electrons on boron, they should show a strong tendency to accept electron pairs on the boron atom to fill its valence shell. Thus chemical reactions of borinium ions are expected to give products in which the coordination number of boron is increased from two to three or four. However, contrary to this expectation, we found that the dimesitylborinium ion (Mes₂B⁺) undergoes twofold 1,2‐carboboration reactions with two equivalents of diphenylacetylene to yield an unprecedented borinium ion ( 1 ⁺) with two substituted vinyl groups on the boron center. NMR spectroscopy and X‐ray diffraction analysis of 1 ⁺, together with electronic‐structure calculations, revealed that the positive charge is delocalized over the entire π‐conjugated system. The fact that the chemical transformation of a borinium ion gives rise to a different borinium ion without a change in the coordination number is remarkable and should provide new insight into the chemistry of the Group 13 elements.     

Fine‐Tuning the Nucleophilic Reactivities of Boron Ate Complexes Derived from Aryl and Heteroaryl Boronic Esters

Boron ate complexes derived from thienyl and furyl boronic esters and aryllithium compounds have been isolated and characterized by X‐ray crystallography. Products and mechanisms of their reactions with carbenium and iminium ions have been analyzed. Kinetics of these reactions were monitored by UV/Vis spectroscopy, and the influence of the aryl substituents, the diol ligands (pinacol, ethylene glycol, neopentyl glycol, catechol), and the counterions on the nucleophilic reactivity of the boron ate complexes were examined. A Hammett correlation confirmed the polar nature of their reactions with benzhydrylium ions, and the correlation lg k(20 °C)=s_N(E+N) was employed to determine the nucleophilicities of the boron ate complexes and to compare them with those of other borates and boronates. The neopentyl and ethylene glycol derivatives were found to be 10⁴ times more reactive than the pinacol and catechol derivatives.     

Boron‐Based Catalysts for C−C Bond‐Formation Reactions

Because the construction of the C?C bond is one of the most significant reactions in organic chemistry, the development of an efficient strategy has attracted much attention throughout the synthetic community. Among various protocols to form C?C bonds, organoboron compounds are not just limited to stoichiometric reagents, but have also made great achievements as catalysts because of the easy modification of the electronic and steric impacts on the boron center. This review presents recent developments of boron‐based catalysts applied in the field of C?C bond‐formation reactions, which are classified into four kinds on the basis of the type of boron catalyst: 1) highly Lewis acidic borane, B(C₆F₅)₃; 2) organoboron acids, RB(OH)₂, and their ester derivatives; 3) borenium ions, (R₂BL)X; and 4) other miscellaneous kinds.     

A Vicinal Electrophilic Diborylation Reaction Furnishes Doubly Boron‐Doped Polycyclic Aromatic Hydrocarbons

Ten examples of unsymmetrically benzannulated, boron‐doped polycyclic aromatic hydrocarbons (B‐PAHs) were prepared by a one‐pot protocol using 4,5‐dichloro‐1,2‐bis(trimethylsilyl)benzene ( 1 ), BBr₃, and selected PAHs—among them anthracene, benzo[a ]pyrene, biphenylene, and fluoranthene. After mesitylation at the boron centers, the resulting air‐ and water‐stable products were investigated by ¹H/¹¹B{¹H}/¹³C{¹H} NMR spectroscopy, X‐ray crystallography, cyclic voltammetry, and UV/Vis absorption/emission spectroscopy. The experiments were augmented by DFT calculations. Most of the B‐PAHs are brightly luminescent (Φ _PL up to 90 %) and undergo reversible reduction at moderate half‐wave potentials. The two chloro substituents of 1 are not only mandatory for accomplishing efficient diborylation, but can subsequently be used for Stille‐type coupling reactions to introduce 2‐thienyl moieties. Alternatively, Cl/H exchange is achievable with HSiEt₃ in a quantitative, Pd‐catalyzed transformation.     

Heterogeneous versus Homogeneous Copper(II) Catalysis in Enantioselective Conjugate‐Addition Reactions of Boron in Water

We have developed Cu^II‐catalyzed enantioselective conjugate‐addition reactions of boron to α,β‐unsaturated carbonyl compounds and α,β,γ,δ‐unsaturated carbonyl compounds in water. In contrast to the previously reported Cu^I catalysis that required organic solvents, chiral Cu^II catalysis was found to proceed efficiently in water. Three catalyst systems have been exploited: cat. 1: Cu(OH)₂ with chiral ligand L1 ; cat. 2: Cu(OH)₂ and acetic acid with ligand L1 ; and cat. 3: Cu(OAc)₂ with ligand L1 . Whereas cat. 1 is a heterogeneous system, cat. 2 and cat. 3 are homogeneous systems. We tested 27 α,β‐unsaturated carbonyl compounds and an α,β‐unsaturated nitrile compound, including acyclic and cyclic α,β‐unsaturated ketones, acyclic and cyclic β,β‐disubstituted enones, acyclic and cyclic α,β‐unsaturated esters (including their β,β‐disubstituted forms), and acyclic α,β‐unsaturated amides (including their β,β‐disubstituted forms). We found that cat. 2 and cat. 3 showed high yields and enantioselectivities for almost all substrates. Notably, no catalysts that can tolerate all of these substrates with high yields and high enantioselectivities have been reported for the conjugate addition of boron. Heterogeneous cat. 1 also gave high yields and enantioselectivities with some substrates and also gave the highest TOF (43 200 h^?1) for an asymmetric conjugate‐addition reaction of boron. In addition, the catalyst systems were also applicable to the conjugate addition of boron to α,β,γ,δ‐unsaturated carbonyl compounds, although such reactions have previously been very limited in the literature, even in organic solvents. 1,4‐Addition products were obtained in high yields and enantioselectivities in the reactions of acyclic α,β,γ,δ‐unsaturated carbonyl compounds with diboron 2 by using cat. 1, cat. 2, or cat. 3. On the other hand, in the reactions of cyclic α,β,γ,δ‐unsaturated carbonyl compounds with compound 2 , whereas 1,4‐addition products were exclusively obtained by using cat. 2 or cat. 3, 1,6‐addition products were exclusively produced by using cat. 1. Similar unique reactivities and selectivities were also shown in the reactions of cyclic trienones. Finally, the reaction mechanisms of these unique conjugate‐addition reactions in water were investigated and we propose stereochemical models that are supported by X‐ray crystallography and MS (ESI) analysis. Although the role of water has not been completely revealed, water is expected to be effective in the activation of a borylcopper(II) intermediate and a protonation event subsequent to the nucleophilic addition step, thereby leading to overwhelmingly high catalytic turnover.     

Syntheses of Halogenated Polyhedral Phosphaboranes: Crystal Structure of conjuncto‐3,3′‐(closo‐1,2‐P2B4Br3)2

Co‐pyrolysis of B₂Br₄ with PBr₃ at 480 °C gave, in addition to the main product closo‐1,2‐P₂B₄Br₄, conjuncto‐3,3′‐(1,2‐P₂B₄Br₃)₂ ( 1 ) and the twelve‐vertex closo‐1,7‐P₂B₁₀Br₁₀ ( 2 ), both in low yields. X‐ray structure determination for 1 [triclinic, space‐group P1 with a = 7.220(2) Å, b = 7.232(2) Å, c = 8.5839(15) Å, α = 97.213(15)°, β = 96.81(2)°, γ = 94.07(2)° and Z = 1] confirmed that 1 adopts a structure consisting of two symmetrically boron–boron linked distorted octahedra with the bridging boron atoms in the 3,3′‐positions and the phosphorus atoms in the 1,2‐positions. The intercluster 2e/2c B–B bond length is 1.61(3) Å. The shortest boron–boron bond within the cluster framework is 1.68(2) Å located between the boron atoms antipodal to the phosphorus atoms. The icosahedral phosphaborane 2 was characterized by ¹¹B‐¹¹B COSY NMR spectroscopy showing cross peaks indicative for the isomer with the phosphorus atoms in 1,7‐positions. Both the X‐ray data of 1 and the NMR spectroscopic data of 1 and 2 give further evidence for the influence of an antipodal effect of heteroatoms to cross‐cage boron atoms and, vice versa, of an additional shielding of the phosphorus atoms caused by B‐Hal substitution at the boron positions trans to phosphorus.     

Reduction of Dioxygen by Radical/B(p‐C6F4X)3 Pairs to Give Isolable Bis(borane)superoxide Compounds

Triplet dioxygen was reduced by TEMPO or trityl radicals in the presence of two molar equivalents of the strong B(p ‐C₆F₄X)₃ (X: F or H) boron Lewis acids under mild conditions to give the bis(borane)superoxide systems 2 . The sensitive radical anion species were isolated and characterized by methods including X‐ray crystal structure analysis and EPR spectroscopy.     

A 1,1‐Carboboration Route to Bora‐Nazarov Systems

Hydroboration of the conjugated enynes 1 a and 1 b with Piers’ borane [HB(C₆F₅)₂] gave the respective dienylboranes trans‐ 2 c and trans‐ 2 d . Their photolysis resulted in the formation of the dihydroborole products 3 c and 3 d . Both were converted to their pyridine adducts 5 c and 5 d , respectively. Compounds 3 c and 5 c,d were characterized by X‐ray diffraction. The reaction of the bis(enynyl)boranes 6 a and 6 b with B(C₆F₅)₃ resulted in the formation of the dihydroboroles 7 a and 7 b , respectively. This reaction is thought to proceed by 1,1‐carboboration of one of the enynyl substituents at boron to generate the dienylborane intermediates 8 a / 8 b , followed by thermally induced bora‐Nazarov ring‐closure and subsequent stabilizing 1,2‐pentafluorophenyl group migration from boron to carbon. Compound 7 a was characterized by X‐ray diffraction and solid‐state ¹¹B NMR spectroscopy.     

Nuclear reaction analysis of boron and oxygen in silicon

The main characteristics of analysis by the nuclear method at low energy are summarized. The experimental set-up, and especially the target chamber arrangement are shown. Nuclear analyses of boron and oxygen in a silicon substrate are presented; sensitivity, accuracy and competing reactions have been studied. The analysis of boron and oxygen is possible, although these elements give interfering reactions. Results concerning homogeneity in the sample surface (for diffused or implanted boron), a diffusion study (SiO₂−Si exchange) and profile measurements are presented.     

Ligand-free Suzuki-Miyaura Cross-Coupling Reactions of Aryltriazenes with Arylboronic Acids

The boron trifluoride induced Suzuki‐Miyaura cross‐coupling of aryltriazenes with arylboronic acids catalyzed by Pd(OAc)₂ without added ligands has been achieved for the first time. The reactions performed at room temperature under an argon atmosphere give biaryls in good to excellent yields. It is noteworthy that the reactions were conducted under mild and ligand‐free conditions.     

Quantum mechanics investigation on initial decomposition of ammonia borane in glyme

The chemical kinetics of ammonia borane (AB) in glyme solution is studied using quantum mechanics (QM) based calculations along with experimental results available in the literature. The primary objective of this study is to propose a detailed reaction mechanism that explains the formation of species observed during AB decomposition for temperatures ranging from 323 to 368 K. The quantum mechanics investigation uses transition state theory to identify the relevant reaction pathways. Intrinsic reaction coordinate calculations use the identified transition‐state structure to link the reactants to the products. These calculations were performed using the Gaussian 09 program package, including the solvation model based on density (SMD) with acetonitrile as the solvent. Thermodynamic properties of species at equilibrium or at transition states were computed using the G4(MP2) compound method. Sensitivity analysis was performed using a species conservation model to identify reactions and species that play a critical role. This study confirms the previous experimental observation regarding the initiation of decomposition of AB in glyme. It also elucidates the role of DADB, ammonium borohydride salt ([BH₄]⁻[NH₄]⁺) and BH₂NH₂ in hydrogen release and intermediates formed during initial phase of AB decomposition. This work shows how QM calculations along with experimental results can contribute to our understanding of the complex chemical kinetics involved during AB dehydrogenation.     

Facile solid-phase synthesis of the diammoniate of diborane and its thermal decomposition behavior

The recent mechanistic finding of the hydrogen release pathways from ammonia borane (AB) has sparked new interest in the chemistry and properties of the diammoniate of diborane (DADB), an ionic isomer of AB. We herein report a facile one-step solid-phase synthesis route of DADB using inexpensive starting materials. Our study found that mechanically milling a 1?:?1 NaBH(4)/NH(4)F powder mixture causes the formation of crystalline DADB via a NH(4)BH(4) intermediate. The produced DADB can be readily separated from the sodium fluoride (NaF) by-product by a purification procedure using liquid ammonia at -78 °C. The thermal decomposition behavior of DADB was studied using synchronous thermal analyses, particularly in comparison with AB. It was found that the decomposition steps and products of DADB are similar to those of AB. But meanwhile, DADB exhibits a series of advantages over AB that merit its potential hydrogen storage application, such as lower dehydrogenation temperature, free of foaming and lack of an induction period in the thermal decomposition process. Our study further found that the volatile non-hydrogen products from DADB can be effectively suppressed by milling with MgH(2).