Lewis Acidic Boron Research Articles

Bis(oxazolinyl)phenylborane: A Lewis acid-containing ligand for methide abstraction-based coordination to aluminum(III)

A compound that contains a Lewis acidic boron center and coordinating oxazoline groups, bis(4,4-dimethyl-2-oxazolinyl)phenylborane (PhB(Ox(Me2))2; 1), has been prepared and spectroscopically characterized. Solvent dependent 15N and 11B NMR spectroscopic properties and solid-state 11B NMR measurements provide support for intermolecular interactions involving Lewis acid and base sites. The bifunctional nature of oxazolinylborane 1 is demonstrated by its reaction with (AlMe3)2, which proceeds via methide abstraction by the boron and oxazoline coordination to aluminum to yield [(kappa(2)-PhMeB(Ox(Me2))2AlMe2] (2). Compound 2 contains a planar six-membered chelate ring, in contrast to related bis(pyrazolyl)boratoaluminum compounds that are puckered. Additionally, compound 2 and related bidentate tris(oxazolinyl)phenylborato dimethylaluminum are inert toward aluminum-methyl bond protonolysis. This robust nature suggested the possibility of using these oxazolinylboratoaluminum compounds in catalytic reactions, as is demonstrated by lactide ring-opening polymerization.

An ab initio molecular orbital study of the complexes formed between silicon tetrafluoride and some Lewis bases

The structures, interaction energies and vibrational spectra of the electron donor–acceptor complexes formed between silicon tetrafluoride and the common Lewis bases ammonia, water, phosphine and hydrogen sulphide have been determined by means of a series of ab initio molecular orbital calculations. The results confirm the trends observed in our previous studies of the complexes formed between the same four bases and the Lewis acids boron trifluoride and sulphur dioxide. The analogous complexes of silicon tetrafluoride with hydrogen fluoride and hydrogen chloride were also examined and, consistent with our earlier findings concerning the adducts of boron trifluoride and sulphur dioxide with these two diatomic species, were found to be very weakly-bound, if at all.

Selective Reduction of Carbonyl Compounds with B-Trifluoromethanesulfonyldiisopinocampheylborane in Ethyl Ether

In the previous reports, 1-4 we showed that Lewis acidity of trisubstituted borane derivatives, a series of new MeerweinPonndorf-Verley (MPV) type borane reagents, plays a role in part on their reactivity toward the reduction of carbonyl compounds: for example, the strong electron-withdrawing effect of fluorine substituent of B-trifluoroacetoxydiisopinocampheylborane (Ipc2BO2CF3) makes it much more reactive than Ipc2BOAc itself. This phenomenon strongly indicates the reaction proceeds by the activation of carbonyl group through coordination of Lewis acidic boron atom followed by hydride transfer from β-hydrogen source to the carbonyl acceptor via the six-membered transition state, quite similar to the mechanism which is generally accepted in the original MPV reaction using aluminum isopropoxide. 5 From such mechanistic point of view, we extended to design other derivative B-trifluoromethanesulfonyldiisopinocampheylborane (Ipc2BOSO2CF3). Evidently, trifluoromethanesulfonyl group is one of the strongest electron-withdrawing ones. Accordingly, we examined the general reducing characteristics of the reagent in order to understand its nature and explore its role in organic synthesis.

Complexation of Nitrous Oxide by Frustrated Lewis Pairs

Frustrated Lewis pairs comprised of a basic yet sterically encumbered phosphine with boron Lewis acids bind nitrous oxide to give intact PNNOB linkages. The synthesis, structure, and bonding of these species are described.

Polyfluorene Derivatives with Hydroxyl and Carboxyl Substitution: Electrosynthesis and Characterization

Novel high-quality poly(9-hydroxyfluorene) (PHF), poly(9-fluorenecarboxylic acid) (PFCA), and poly(9-hydroxyl-9-fluorenecarboxylic acid) (PHFCA) films can be easily electrodeposited by low-potential anodic oxidation of fluorene derivatives, which contain either the electron-withdrawing carboxyl group (-COOH) or the electron-donating hydroxyl group (-OH), using a midstrength Lewis acid boron trifluoride diethyl etherate (BFEE) as the solvent and supporting electrolyte. The complexing reaction between the -COOH or -OH groups at the C(9) position of these fluorene derivatives and BFEE increased the ionic conductivity of the BFEE system as an electrolyte significantly. The 13C NMR spectra clearly demonstrates that the chemical shift of carbons at the C(9) and C(14) positions has made a low-field shift. This change in the chemical shift of the carbon atoms confirms the formation of complexion cations when FCA or HFCA was mixed with BFEE. The as-formed polymer films showed good redox behavior and thermal stabil...

Periodic Trends and Index of Boron Lewis Acidity

Lewis acidity is customarily gauged by comparing the relative magnitude of coordinate covalent bonding energies, where the Lewis acid moiety is varied and the Lewis base is kept constant. However, the prediction of Lewis acidity from first principles is sometimes contrary to that suggested by experimental bond energies. Specifically, the order of boron trihalide Lewis acidities predicted from substituent electronegativity arguments is opposite to that inferred by experiment. Contemporary explanations for the divergence between theory, computation, and experiment have led to further consternation. Due to the fundamental importance of understanding the origin of Lewis acidity, we report periodic trends for 21 boron Lewis acids, as well as their coordinate covalent bond strengths with NH(3), utilizing ab initio, density functional theory, and natural bond orbital analysis. Coordinate covalent bond dissociation energy has been determined to be an inadequate index of Lewis acid strength. Instead, acidity is measured in the manner originally intended by Lewis, which is defined by the valence of the acid of interest. Boron Lewis acidity is found to depend upon substituent electronegativity and atomic size, differently than for known Brønsted-Lowry periodic trends. Across the second period, stronger substituent electronegativity correlates (R(2) = 0.94) with increased Lewis acidity. However, across the third period, an equal contribution from substituent electronegativity and atomic radii is correlated (R(2) = 0.98) with Lewis acidity. The data suggest that Lewis acidity depends upon electronegativity solely down group 14, while equal contribution from both substituent electronegativity and atomic size are significant down groups 16 and 17. Originally deduced from Pauling's electronegativities, boron's substituents determine acidity by influencing the population of its valence by withdrawing electron density. However, size effects manifest differently than previously considered, where greater sigma bond (not pi bond) orbital overlap between boron and larger substituents increase the electron density available to boron's valence, thereby decreasing Lewis acidity. The computed electronegativity and size effects of substituents establish unique periodic trends that provide a novel explanation of boron Lewis acidity, consistent with first principle predictions. The findings resolve ambiguities between theory, computation, and experiment and provide a clearer understanding of Lewis acidity.

Fluoride Ion Recognition by Chelating and Cationic Boranes

Because of the ubiquity of fluoride ions and their potential toxicity at high doses, researchers would like to design receptors that selectively detect this anion. Fluoride is found in drinking water, toothpaste, and osteoporosis drugs. In addition, fluoride ions also can be detected as an indicator of uranium enrichment (via hydrolysis of UF(6)) or of the chemical warfare agent sarin, which releases the ion upon hydrolysis. However, because of its high hydration enthalpy, the fluoride anion is one of the most challenging targets for anion recognition. Among the various recognition strategies that are available, researchers have focused a great deal of attention on Lewis acidic boron compounds. These molecules typically interact with fluoride anions to form the corresponding fluoroborate species. In the case of simple triarylboranes, the fluoroborates are formed in organic solvents but not in water. To overcome this limitation, this Account examines various methods we have pursued to increase the fluoride-binding properties of boron-based receptors. We first considered the use of bifunctional boranes, which chelate the fluoride anion, such as 1,8-diborylnaphthalenes or heteronuclear 1-boryl-8-mercurio-naphthalenes. In these molecules, the neighboring Lewis acidic atoms can cooperatively interact with the anionic guest. Although the fluoride binding constants of the bifunctional compounds exceed those of neutral monofunctional boranes by several orders of magnitude, the incompatibility of these systems with aqueous media limits their utility. More recently, we have examined simple triarylboranes whose ligands are decorated by cationic ammonium or phosphonium groups. These cationic groups increase the electrophilic character of these boranes, and unlike their neutral analogs, they are able to complex fluoride in aqueous media. We have also considered cationic boranes, which form chelate complexes with fluoride anions. Our work demonstrates that Coulombic and chelate effects are additive and can be combined to boost the anion affinity of Lewis acidic hosts. The boron compounds that we have investigated present a set of photophysical and electrochemical properties that can serve to signal the fluoride-binding event. We can also apply this approach to cyanide complexation and are continuing our investigations in that area.

Stepwise Oxygenation of Pinacolborane by a Rhodiumperoxo Complex: Detection of an Intermediate Metal Borate and Perborate

Acting as go-between: A rhodium borate and perborate have been identified as intermediates in the rhodium-mediated oxygenation of pinacolborane (HBpin; see scheme). The rhodium(III)–peroxo complex reacts with the Lewis acidic boron compound HBpin by oxygen transfer from the rhodium center to HBpin to give a rhodium(I) species. The reaction sequence might play a crucial role in the homocoupling of boronic acid.

ELECTROCHEMICAL POLYMERIZATION OF FLUORENE HALIDE IN BORON TRIFLUORIDE DIETHYL ETHERATE SOLUTION

Poly(9-bromofluorene)(PBF) films and poly(9,9-dichlorofluorene)(PDCF) films were electro-synthesized successfully by direct anodic oxidation of 9-bromofluorene(BF) and 9,9-dichlorofluorene(DCF) using a middle-strong Lewis acid boron trifluoride diethyl etherate(BFEE) as the solvent and the supporting electrolyte containing 0.02 mol/L BF and 0.05 mol/L DCF,respectively.The oxidation potential onset of BF and DCF in pure BFEE was 1.45 V versus SCE and 1.67 V versus SCE,respectively,which were much lower than those determined in acetonitrile containing 0.1 mol L-1 Bu_4NBF_4(BF,1.95 V versus SCE and DCF,2.19 V versus SCE).For spectral analysis,polymers were dedoped with 25% ammonia for 3 days.Then,they were dried at 60℃ for 24 h.For characterization of the structure and properties of as-formed polymers,UV-Vis,fluorescence,FT-IR,and 1H-NMR spectra of the polymers were determined.In the FT-IR spectra of BF or PBF,the peak at 650 cm-1 was the characteristic absorption of the C—Br bond.Three peaks at 819,759,737 cm-1 suggested that PBF was 1,2,4-trisubstituted.Similarly,the peak at 802 cm-1 was the characteristic absorption of the C—Cl bond in the IR spectra of DCF and PDCF.Additionally,the peaks at 867,731,829 cm-1 indicated that PDCF was also 1,2,4-trisubstituted.FT-IR and 1H-NMR spectral investigations indicated that electrochemical polymerization of BF and DCF mainly occurred at 2,7 positions.According to the UV-Vis spectra of the monomers,the UV-Vis spectra of PBF showed red-shfited absorption at 334 nm,and the PDCF showed broader absorption at 290 nm,342 nm,indicating higher conjugation length of backbone of the polymer.In the fluorescence spectra,the emission peaks of BF and DCF monomers appeared at 334 nm and 338 nm,respectively while those of PBF and PDCF in DMSO appeared at 396 nm and 537 nm,implying the blue-light-emitting property of PBF and the green-light-emitting property of PDCF.The main reason contributing to different light-emitting properties of PBF and PDCF was the electron-withdrawing effect of Cl and Br atoms.The electrical conductivity of PBF and PDCF films was measured to be nearly 10-1 S/cm.These semi-conducting and emitting properties will be greatly helpful for the applications of PBF and PDCF films.

Aziridinyl Vinyl Ketones from the Asymmetric Catalytic Aziridination Reaction

AbstractUnder the aegis of boron Lewis acids, prepared from either the vanol or vapol ligand, vinyl aziridinyl ketones can be obtained with a high degree of asymmetric induction from the catalytic asymmetric aziridination reaction (AZ) of imines and vinyl diazomethyl ketones. The products have potential as synthons for five‐carbon chiral amines. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Silylated Initiators for the Efficient Preparation of Borane-End-Functionalized Polymers via ATRP

Borane-end-functionalized polymers have been prepared through (i) atom transfer radical polymerization (ATRP) of styrene using a silylated initiator, 1-bromo-1-(4-trimethylsilylphenyl)ethane (PEB-SiMe3), (ii) exchange of the SiMe3 end groups with BBr3, and (iii) modification of the Lewis acidic boron centers with nucleophiles. Ditelechelic α,ω-borane-functionalized polymers were obtained through atom transfer radical coupling (ATRC) of the silane-end-functionalized polymers followed by exchange with BBr3 and subsequent reaction with nucleophiles. Self-assembly of the borane-functionalized mono- and ditelechelic polymers into supramolecular structures by means of reversible noncovalent and covalent interactions including Lewis acid−base complexation and boroxine formation has been explored.

Lewis acidity enhancement of organoboranes via oxidation of appended ferrocene moieties

The Lewis acidity of boron in diboradiferrocene 1 is strongly enhanced through oxidation of the iron atoms as evident from examination of X-ray structural parameters of the mixed-valent cation 1(+)PF(6) and further confirmed from the strong complexation of MeCN to the dication in 2(2+)(I(3))(2).

Synthesis and molecular structure of 4,4,5,5-tetramethyl-2-(1-(phenylsulfonyl)propan-2-yl)-1,3,2-dioxaborolane

The compound 4,4,5,5-tetramethyl-2-(1-(phenylsulfonyl)propan-2-yl)-1,3,2-dioxabo- rolane has been prepared by the rhodium catalyzed hydroboration of allyl phenyl sulfone and characterized by a single crystal X-ray diffraction study. The title compound crystallized in the monoclinic space group P21/c, with cell parameters a=16.0728(8) A, b=9.8884(5) A, and c=10.1926(5) A, β=92.067(1)°, Z=4, and V=1618.90(14) A3. The structure was solved by direct methods and refined to a final R=0.0449 for 3627 reflections with I>2σ(I). No significant intramolecular or intermolecular interaction is observed with the Lewis acidic boron atom.

Reaction of Azole Heterocycles with Tris(dimethylamino)borane, a New Method for the Construction of Tripodal Borate‐Centred Ligands

Reaction of 2-mercapto-1-methylimidazole (methimazole) with tris(dimethylamino)borane, B(NMe2)3, provides the tetrahedral dimethylamine adduct of tris(methimazolyl)borane, [(Me2HN)B(methimazolyl)3]. By contrast, imidazole, 2-methylimidazole, 2-chloroimidazole and benzimidazole provide the homoleptic tetra-azolyl systems H[B(azolyl)4], and the same product is obtained even when a substoichiometric quantity of the heterocyle is employed. The change in reaction outcome is correlated with the variation of basic pKa for the heterocycles. A simple acid-base reaction with elimination of HNMe2 is proposed for the reaction with the weakly basic, but more strongly acidic, methimazole. However, for the more strongly basic imidazoles, initial coordination of the heterocycle imine nitrogen to the weakly Lewis acidic boron centre in B(NMe2)3 to form the tetrahedral adduct [(azole)B(NMe2)3] is proposed. The greater availability of the NMe2 lone pairs in this species results in increased basicity and a rapid reaction with further heterocycle to provide the observed H[B(azolyl)4] products. For 2-nitroimidazole, the low basicity (and increased N-H acidity) results in the formation of [(HNMe2)B(2-nitroimidazolyl)3] on reaction with B(NMe2)3, analogous to the product formed with methimazole. Both [(HNMe2)B(methimazolyl)3] and H[B(benzimidazolyl)4] have been structurally characterised by single crystal X-ray crystallography. This chemistry has been exploited to provide a new synthesis of borate-centred tripod ligands, whereby N-methylimidazole is used to activate B(NMe2)3 to reaction with methimazole to form the new ligand [(N-methylimidazole)B(methimazolyl)3] in good yield and a complex of this ligand with Ru(II) has been structurally characterised.

Hybrid Lewis Acid/Hydrogen-Bond Donor Receptor for Fluoride

[reaction: see text] To verify if hydrogen-bond donor groups can assist fluoride binding at the boron center of triaryl boranes, o-(dimesitylboryl)trifluoroacetanilide has been synthesized. Reaction of this new borane with [n-Bu(4)N][F] in acetone affords the corresponding fluoroborate complex whose stability constant exceeds that of [Mes(3)BF](-) by at least 2 orders of magnitude. Presumably, the higher fluoride affinity of o-(dimesitylboryl)trifluoroacetanilide results from the cooperativity of the Lewis acidic boron center and the hydrogen-bond donor trifluoroacetamide group.

Electrochemical Polymerization of Fluoranthene and Characterization of Its Polymers

A novel inherently conducting polymer, high-quality polyfluoranthene (PFA) film with electrical conductivity of 10(-2) S cm(-1), was first synthesized electrochemically by direct anodic oxidation of fluoranthene in a middle strong Lewis acid-boron trifluoride diethyl etherate. The oxidation potential onset of fluoranthene in this medium was measured to be only 1.07 V vs SCE, which was much lower than that in acetonitrile + 0.1 mol L(-1) tetrabutylammonium tetrafluoroborate (1.68 V vs SCE). This PFA film showed good redox activity and stability even in concentrated sulfuric acid. Moreover, the fluorescence properties of PFA were greatly improved in comparison with those of the monomer. Dedoped PFA films were partly soluble in polar solvents such as CH(2)Cl(2), acetone, tetrahedrofuran, and dimethyl sulfoxide. The structure and morphology of the polymer were investigated by UV-vis spectroscopy, infrared spectroscopy, and scanning electron microscopy, respectively. The results of quantum chemistry calculations of fluoranthene monomer and (1)H NMR spectroscopy of dedoped PFA films indicated that the polymerization mainly occurred at C((3)), C((4)), C((13)), and C((14)) positions.

A novel conducting polymer polyfluoranthene: Electrosyntheses and characterization

High quality polyfluoranthene (PFA) films, a novel inherently conducting polymer with electrical conductivity of 10−2 S cm−1, were synthesized electrochemically by direct anodic oxidation of fluoranthene in a middle strong Lewis acid boron trifluoride diethyl etherate (BFEE). PFA films obtained from this medium showed good redox activity and stability even in concentrated sulfuric acid. Moreover, the fluorescence properties of PFA were greatly improved in comparison with those of the monomer. Dedoped PFA films were partly soluble in polar solvent such as CH2Cl2, acetone, tetrahedrofuran, dimethyl sulfoxide. Quantum chemistry calculations of fluoranthene monomer and 1H NMR spectroscopy of dedoped PFA films were used to reach deep insight into the polymer structure. The polymerization mainly occurred at C (3), C (4), C (13) and C (14) positions.

Lewis Acid Mediated Reactions of Aldehydes with Styrene Derivatives: Synthesis of 1,3-Dihalo-1,3-diarylpropanes and 3-Chloro-1,3-diarylpropanols

[reaction: see text] The reactions of aryl aldehydes with styrene derivatives, mediated by various boron Lewis acids, were investigated. 1,3-Dihalo-1,3-diarylpropanes were obtained in high yields with boron trihalides, while 3-chloro-1,3-diarylpropanols were obtained in good to excellent yields with phenylboron dichloride. Reactions involving nonenolizable aliphatic aldehydes, trans-cinnamaldehyde, and beta-substituted styrenes were also investigated for the first time.

Tris(pentafluorophenyl)borane: A Special Boron Lewis Acid for Special Reactions

AbstractFor Abstract see ChemInform Abstract in Full Text.

A Family of Main-Chain Polymeric Lewis Acids: Synthesis and Fluorescent Sensing Properties of Boron-Modified Polythiophenes

A new family of main-chain organoborane polymeric Lewis acids (PTh-BAr) that contain Lewis acidic boron groups embedded into a polythiophene backbone has been prepared under mild conditions through tin-boron exchange reaction. When phenyl and pentafluorophenyl groups are attached to boron, blue and green luminescence is observed, respectively, while the attachment of ferrocenyl substituents leads to a characteristic red color. The incorporation of readily accessible highly Lewis acidic groups into the conjugated polymer backbone provides an opportunity for sensing of Lewis basic substrates. For instance, treatment of the polymer containing phenyl substituents on boron with pyridine leads to efficient quenching of the fluorescence, while the polymer containing ferrocenyl groups changes color from dark red to light orange.