Tetrahedral Boron Atoms Research Articles

Enantioenriched Boron C,N-Chelates via Chirality Transfer.

Molecules stereogenic only at tetrahedral boron atoms show great promise for applications, for example as chiroptical materials, but are scarcely investigated due to their synthetic challenge. Hence, this study reports a two-step synthesis of enantioenriched boron C,N-chelates. First, the diastereoselective complexation of alkyl/aryl borinates with chiral aminoalcohols furnished boron stereogenic heterocycles in up to 86 % yield and d.r. >98 : 2. Treatment of these O,N-complexes with chelate nucleophiles was surmised to transfer the stereoinformation via the ate-complex into the C,N-products. This chirality transfer succeeded by substitution of the O,N-chelates with lithiated phenyl pyridine to give boron stereogenic C,N-chelates in up to 84 % yield and e.r. up to 97 : 3. The chiral aminoalcohol ligands could be recovered after isolation of the C,N-chelates. The chirality transfer tolerated alkyl, alkynyl and (hetero-)aryl moieties at boron and could be further extended by post-modification: transformations such as catalytic hydrogenations or sequential deprotonation/electrophilic trapping were feasible while maintaining the stereochemical integrity of the C,N-chelates. Structural aspects of the boron chelates were studied by variable temperature NMR and X-ray diffraction.

Combustion synthesis and structural characterization of the TiB2–(Na2O·2B2O3–Al2O3) composites

Two TiB2:(Na2B4O7–Al2O3)=3:5 and TiB2:(1.28Na2B4O7–Al2O3)=6:7 compositions, named 5DB and 7DB respectively, were prepared by combustion synthesis in simultaneous mode at 800, 900 and 1000°C. X-ray diffraction, differential scanning calorimetry (DSC), UV–vis and room and high temperature Raman measurements were carried out to investigate the combustion products. The crystalline phase of the 5DB composite yielded at 1000°C (5DB_1000), which represents 62wt%, mostly consists of TiB2 (93wt%) while Ti2O3 accounts for only 7wt%. Also, the Na2Al2B2O7 compound was found to crystallize in the borate glassy matrix of the (5/7) DB composites obtained at the lower combustion temperatures. Proportion of the tetrahedral boron atoms, N4, in the blank and embedding sodium aluminium borate matrices was found from the high frequency Raman spectra using a constrained-width model in order to agree with the literature NMR data. The oxidation stability below 600°C of the TiB2 crystallites in the (5/7) DB composites was proved by in-situ Raman measurements in air.

Preparation of cyclic boramides from salicylaldehydes, ammonium acetate and sodium borohydride

Borohydride reduction of salicylaldehyde imines yields surprisingly a cyclic boramide. This is a fairly stable compound, and X-ray analysis shows it has a tetrahedral boron atom. Mechanistic studies show a reaction pathway through an oxazaborinane intermediate. The reaction also works with halogen substituted salicylaldehydes and for the preparation of non-symmetrical boramides.

Structural investigation in the TiB 2–(Na 2O·B 2O 3·Al 2O 3) system

Composites in the TiB 2–Na 2O·B 2O 3·Al 2O 3 systems, TiB 2–MBA (MB stands for sodium metaborate and A is Al 2O 3), were prepared by self-propagating high-temperature synthesis (SHS), in simultaneous mode. Selection of these compositions was ruled by the interesting properties of both TiB 2 and double borates of alkali metal and aluminum. The structure of the obtained materials was evaluated by micro-Raman spectroscopy, from room temperature up to 600 °C, and X-ray photoelectron spectroscopy (XPS). Formation of the TiB 2 and TiO 2− x B x phases along with TiO 2 as rutile were identified as titanium speciation in the grain phase embedded in a sodium aluminum borate matrix. Integration of the Raman spectra of the grain phases revealed a TiB 2 content of 16.99% and 23.32% for the two composite investigated 2TiB 2·2MBA and 3TiB 2·5MBA. A constrained-width model for the spectral deconvolution of the high-frequency Raman band was forwarded to calculate the proportion of tetrahedral boron atoms (7.424%) in the blank borate matrix Na 2B 2O 4·Al 2O 3 in solid phase.

A New Sodium Borate: Na[B6O8(OH)3]·6H2O·H3BO3

AbstractThe structure of [B6H9NaO14, H3BO3, 6H2O] was determined by single‐crystal X‐ray diffraction and further analyzed by FTIR spectroscopy and differential thermal/thermogravimetric analysis. The asymmetric unit contains Na–O polyhedra (distorted octahedron), [B6O8(OH)3]– fundamental building blocks, one free water molecule and one free H3BO3 molecule. In the hexaborate anion, three B3O3 rings are linked by a common oxygen atom with five trigonal and one tetrahedral boron atoms. The hexaborate group is also linked to the oxygenated environment of the sodium atom by three other six‐membered rings, each of which involve two boron atoms, three oxygen atoms, and sodium as the joint atom.

Substituent Effects on Configurational Stabilities at Tetrahedral Boron Atoms in Intramolecular Borane–Amine Complexes: Structures, Enantiomeric Resolution, and Rates of Enantiomerization of [2-(Dimethylaminomethyl)phenyl]phenylboranes

Abstract The structures and stereochemistries of the five borane–amine complexes, (N–B)-{2-(Me2NCH2)C6H4}BPhX (X = Cl, F, Me, –OCOCF3, and –OCOC2F5), were studied by some spectroscopic methods. The Cl complex prepared by the standard method was converted to each of the other complexes by the treatment with an appropriate reagent (MeLi or AgX). The enantiomers of the two perfluoroacyloxy complexes were resolved by chiral HPLC as a novel intramolecular borane–amine complex with a chiral center at the tetrahedral boron atom, and their chiroptical properties were investigated by optical rotations and CD spectra. The classical kinetics or the saturation transfer measurements revealed that the barriers to enantiomerization via the dissociation of the B–N coordination bond were enhanced from 94 to 116 kJ mol−1 in the following order of X: F < Me < –OCOCF3 ≈ Cl < –OCOC2F5. The substituent effects on the configurational stability, which is closely related to the Lewis acidity of boron atoms, are discussed on the basis of the X-ray structures of the three complexes.

Enantiomeric Resolution of Intramolecular Amine–Borane Complex with a Chiral Boron Center

Abstract The enantiomers of [2-(dimethylaminomethyl)phenyl](pentafluoropropionyloxy)phenylborane were successfully resolved by chiral HPLC as a new type of intramolecular amine–borane complex with a chiral center at the tetrahedral boron atom. The X-ray structure, the barrier to racemization, and the chiroptical properties are reported.

Study of the Reaction of Tridentate Ligands with Ferrocenyl Boronic Acid

AbstractEvaluation of the reactivity of eight tridentate ligands derived from amino alcohols and salicylaldehyde, 2‐hydroxyacetophenone, or 2‐hydroxybenzophenone with ferrocenyl boronic acid has shown that the reaction leads to both monomeric and dimeric ferrocenyl boronates. A coordinative N−B bond is essential to give the tetrahedral boron atoms that are responsible for the formation of heterobicyclic structures containing six, seven and ten members. The purity and identity of all compounds were unequivocally established by analytical and multinuclear (1H, 11B, 13C) magnetic resonance spectroscopic data. Furthermore, the structures of the dimeric (2a) and monomeric (5b) boron complexes were established by X‐ray diffraction analyses. The dimeric compound contains a ten‐membered ring in a boat−chair−boat conformation, while the monomeric compound has a seven‐membered ring with a chair conformation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Synthesis of a New Tridentate Anthracene Ligand Bearing (i-Pr)2P Group at 1,8-Positions: Facile Bond Switch on Tetracoordinate Boron Atom

Abstract 9-Borylanthracene 12 was prepared from 1,8-bis(diisopropylphosphino)-9-bromoanthracene (5) and the X-ray structure of 12 shows a tetrahedral boron atom coordinated by only one of the two (i-Pr)2P groups. However, anthracene protons were observed symmetrically in 1H NMR even at −80 °C, clearly indicating that the intramolecular bond-switching process is taking place very rapidly in solution.

Mechanisms of inversion of bond configuration at the tetrahedral boron atom in five-membered chelate cycles

Mechanisms of inversion of the bond configuration at the tetrahedral boron center in five-membered chelate cycles of the 1,3,2-oxazaborolidine and 1,3,2-oxazaborolidene molecules were studied by theab initio MP2(full)/6-31G** method. It was shown that enantiotopomerization occurs by a dissociative mechanism with the cleavage of the B←N bond and the formation of acyclic intermediates with tricoordinate planar boron atom. The calculated energy barriers to inversion of tetrahedral bond configurations at boron centers in the two chelate complexes are equal to 13.1 and 15.4 kcal mol−1, respectively. In contrast to 1,3,2-oxazaborolidine, internal rotation about the B−O bond in its unsaturated analog makes an appreciable contribution to the reaction coordinate.

Synthesis of tetrakis[(tri‐tert‐butylphosphane)gold(I)]phosphonium salts and crystal structure of the tetraphenylboroxinate salt {[(tBu)3PAu]4P}+ [O(PhBO)2BPh2]—

AbstractThe reaction of (Me2Si)3P with {[(tBu)3PAu]3O}+ BF4– in tetrahydrofuran at —78C affords the compound {[(tBu)3‐PAu]4P}+ BF4— (1a) in high yield. This product is converted into the tetraphenylborate salt (1b) by treatment with Na[BPh4]. The two compounds have been identified on the basis of their analytical and spectroscopic data, but could not be crystallized. In attempts to prepare single crystals of 1b, a crystalline salt {[(tBu)3PAu]4P}+ [O(PhBO)2BPh2]— (1c) has been obtained in very small quantities, probably by partial anion hydrolysis. A single crystal X‐ray diffraction study of 1c CH2Cl2 shows the compound to be built up of cations with a strongly distorted tetrahedral (PAu)4P skeleton and anions with a planar six‐membered boroxine ring. Two of the phenyl groups are attached to two trigonal‐planar boron atoms and the other two phenyl groups to the third (tetrahedral) boron atom of the heterocycle. The origin of the peculiar cation structure is discussed in the light of findings with related ammonium‐ and arsonium‐centered gold clusters.

Structural studies of organoboron compounds. LVII. Synthesis and structure of bicyclic boron-nitrogen betaines. 1-Methyl-5-(3-nitrophenyl)-4,6,9-trioxa-1-azonia-5-boratabicyclo[3.3.1]nonane and 3,3,4-trimethyl-1-(3-nitrophenyl)-2,6,7-trioxa-3-azonia-1-boratabicyclo[2.2.2]octane

The syntheses and structures of the bicyclic boron-nitrogen betaines 1-methyl-5-(3-nitrophenyl)-4,6,9-trioxa-1-azonia-5-boratabicyclo[3.3.1]nonane, 7a, and 3,3,4-trimethyl-1-(3-nitrophenyl)-2,6,7-trioxa-3-azonia-1-boratabicyclo[2.2.2]-octane, 14a, are reported. Crystals of 7a are orthorhombic, a = 20.256(2), b = 37.652(2), c = 6.642(1) Å, Z = 16, space group Fdd2, and those of 14a are orthorhombic, a = 21.402(2), b = 11.334(3), c = 11.248(2) Å, Z = 8, space group Pbca. The structures were solved by direct methods and were refined by full-matrix least-squares procedures to R = 0.038 and 0.045 (Rw = 0.036 and 0.053) for 975 and 1646 reflections with I ≥ 3σ(F2), respectively. The X-ray analyses establish the bicyclo[3.3.1]nonane and bicyclo[2.2.2]octane "cage" structures for 7a and 14a, respectively. Bond lengths involving the tetrahedral boron atom are (N)O—B = 1.519(5), (C)O—B = 1.444(6) and 1.443(6), C(aryl)—B = 1.602(5) Å for 7a; and (N)O—B = 1.530(3), (C)O—B = 1.446(3) and 1.455(3), C(aryl)—B = 1.608(3) Å for 14a. Weak [Formula: see text] interactions are the main feature of the crystal packing arrangements of both compounds.

Structural studies of organoboron compounds L. 4-(2-Hydroxy-2,2-diphenylacetyl)- 6,6-pentamethylene-2-phenyl-1,3-dioxa-4-aza-2-boracyclohexane and 4-[2,2-difluoro-5,5-bis(4-methoxyphenyl)-1,3-dioxa-2-borata-4-cyclopentylidene]- 6,6-pentamethylene-2-phenyl-1,3-dioxa-4-azonia-2-boracyclohexane

The preparation of the organoboron compounds 4-(2-hydroxy-2,2-diphenylacetyl)-6,6-pentamethylene-2-phenyl-1,3-dioxa-4-aza-2-boracyclohexane, 6, 4-(2,2-difluoro-5,5-diphenyl-1,3-dioxa-2-borata-4-cyclopentylidene)-6,6-pentamethylene-2-phenyl-1,3-dioxa-4-azonia-2-boracyclohexane, 14a, and 4-[2,2-difluoro-5,5-bis(4-methoxyphenyl)-1,3-dioxa-2-borata-4-cyclopentylidene]-6,6-pentamethylene-2-phenyl-1,3-dioxa-4-azonia-2-boracyclohexane, 14b, are reported. Crystals of 6 are triclinic, [Formula: see text], a = 8.7348(5), b = 11.0529(6), c = 14.104(1) Å, α = 65.781(5)°, β = 72.147(6)°, γ = 79.928(6)°, Z = 2, ρc = 1.130 g cm−3, and crystals of 14b•1/2 CH2Cl2 are also triclinic, [Formula: see text], a = 11.799(1), b = 13.464(2), c = 10.047(2) Å, α = 103.28(2)°, β = 92.60(2)°, γ = 79.752(8)°, Z = 2, ρc = 1.285 g cm−3. The structures were solved by direct methods and were refined by full-matrix least-squares procedures to final R values of 0.040 and 0.060 for 3881 and 3221 reflections with I ≥ 3σ(I), respectively. Compound 6 has a six-membered cycloboronate structure, bond distances involving the trigonal planar boron atom being (N)O—B = 1.390(2), (C)O—B = 1.345(2), and B—C(phenyl) = 1.557(2) Å. Compound 14b represents a derivative of 6 in which an intramolecularly hydrogen-bonded hydrogen atom has been replaced by a difluoroboron moiety and represents the first structurally characterized compound containing both a phenylboronate and a difluoroboron chelate ring within the same molecule. Bond lengths in 14b involving the trigonal planar boron atom are (N)O—B = 1.403(5), (C)O—B = 1.326(5), and B—C(phenyl) = 1.550(5) Å and those involving the tetrahedral boron atom are O—B = 1.427(5) and 1.547(5), F—B = 1.353(5) and 1.382(5) Å.

Structure of bis-, tris- and tetrakispyrazolylborates in the solid state (sodium and potassium salts of tetrakispyrazolylborate by X-ray crystallography) and in solution ( 1H, 11B, 13C and 15N NMR)

The knowledge of the properties of pyrazolylborates has been considerably improved both in the solid state and in solution. The hydrates of the sodium and the potassium salts of tetrakis(l-pyrazolyl)borate crystallize in the space group C2/ c, both salts being isomorphous. Regular pyrazole rings conform with a pseudosymmetry plane around a distorted tetrahedral boron atom. Chains along the b axis of two different octahedra of coordination, sharing water oxygens, are linked by H bonds. In solution, a careful 1H, 13C, 11B and 15N NMR study was carried out. The applicability of the rules used to assign H(3) versus H(5) and C(3) vcrsus C(5) in the case of pyrazolylborates was reevaluated. Only for tetrakis(1- pyrazolyl)borate were couplings observed with 1B in 1H, 3C and 5N resonances. The regularity of the properties of bis-, tris- and tetrakispyrazolylborates, allows the calculation of the spectral parameters of the still unknown tris-hydrido(1-pyrazolyl)borate, pzBH 3 −. Finally, the electronic properties, as measured by σ p, of the substituents BH 3 −, pzBH 2 −, pz 2BH − and pz 3B − were estimated.

Nitrogen-15 NMR spectroscopy of the catalytic-triad histidine of a serine protease in peptide boronic acid inhibitor complexes.

15N NMR spectroscopy was used to examine the active-site histidyl residue of alpha-lytic protease in peptide boronic acid inhibitor complexes. Two distinct types of complexes were observed: (1) Boronic acids that are analogues of substrates form complexes in which the active-site imidazole ring is protonated and both imidazole N-H protons are strongly hydrogen bonded. With the better inhibitors of the class this arrangement is stable over the pH range 4.0-10.5. The results are consistent with a putative tetrahedral intermediate like complex involving a negatively charged, tetrahedral boron atom covalently bonded to O gamma of the active-site serine. (2) Boronic acids that are not substrate analogues form complexes in which N epsilon 2 of the active-site histidine is covalently bonded to the boron atom of the inhibitor. The proton bound to N delta 1 of the histidine in these histidine-boronate adducts remains strongly hydrogen bonded, presumably to the active-site aspartate. Benzeneboronic acid, which falls in this category, forms an adduct with histidine. In both types of complexes the N-H protons of His-57 exchange unusually slowly as evidenced by the room temperature visibility of the low-field 1H resonances and the 15N-H spin couplings. These results, coupled with the kinetic data of the preceding paper [Kettner, C. A., Bone, R., Agard, D. A., & Bachovchin, W. W. (1988) Biochemistry (preceding paper in this issue)], indicate that occupancy of the specificity subsites may be required to fully form the transition-state binding site. The significance of these findings for understanding inhibitor binding and the catalytic mechanism of serine proteases is discussed.

Hydrogen-containing glass and gas-ceramic microfoams: Raman, XPS, and MAS-NMR results on the structure of precursor SiO 2B 2O 3P 2O 5 glasses

Raman spectroscopy, photoelectron spectroscopy and magic angle spinning nuclear magnetic resonance have been used to analyze the structure of a series of glasses in the system SiO 2B 2O 3P 2O 5 which, on heat treatment, develop a cellular microstructure containing hydrogen gas. Raman spectra show that the addition of 4–21 mol% P 2O 5 to 0.6SiO 2−0.4B 2O 3 (molar) composition systematically disrupts boroxyl rings and converts some of the boron to tetrahedral coordination. The postulated structural species formed has a local structure similar to BPO 4. The Raman spectra also show that P=O bonds are present and suggest that POP linkages occur in the glass with 21 mol% P 2O 5. O1s photoelectron spectroscopy shows that the full width at half maximum does not vary between glass samples containing 4 and 11 mol% P 2O 5, suggesting that the concentration of P=O bonds does not vary between these samples. 11B, 29Si and 31P MASNMR spectra have been collected for two of the glasses (4 and 11 mol% P 2O 5). The 11B results reveal three types of boron-oxygen coordination in the low P 2O 5 glass; regular trigonal; regular tetrahedral; and irregular. Increasing P 2O 5 to 11 mol% selectively destroys the regular trigonal sites, leaving boron in regular tetrahedral sites and in irregular sites. The 29Si NMR spectrum of the 4 mol% P 2O 5 glass has peaks at −90, −95, −105 and −112 ppm indicating that silicon has, primarily, trigonal and tetrahedral boron and other silicon atoms as next nearest neighbors. The loss of the two low fields peaks in the 11 mol% P 2O 5 glass indicates the removal of tetrahedral boron as a next nearest neighbor to silicon. The 31P NMR spectrum of the low P 2O 5 glass has a dominant peak at −41 ppm, similar to branching and middle phosphate groups in compositions with highly electonegative cations. In the 11 mol% P 2O 5 glass, the peak at −33 ppm is consistent with the formation of species with the local coordination of BPO 4. { 1H− 31P} cross-polarization experiments show that molecular hydrogen is likely to be present in the 4 mol% P 2O 5 glass. In the high P 2O 5 glass, hydrogen is preferentially associated with the BPO 4 species and with another phosphate site whose next nearest neighbors are not clearly identifiable. There is no evidence for hydrogen associated with either boron or silicon.

Application of Mössbauer Spectroscopy to a Structural Study of Superionic Conducting AgI–Ag2O–B2O3 Glasses

Abstract A structural study of silver borate glasses containing 20 mol% AgI and 1 mol% 57Fe2O3 has been performed by Mössbauer spectroscopy. A Mössbauer spectrum of these glasses consists of an asymmetric quadrupole doublet with an isomer shift smaller than 0.4 mm s−1 with respect to metallic iron. This indicates that the individual Fe3+ ions are present at the substitutional sites of the tetrahedral boron atoms constituting BO4 units as a network former. An increase in the Ag2O content results in continuous decreases in the isomer shift and the quadrupole splitting when the Ag2O content exceeds about 16 mol%, which corresponds to the Ag2O/B2O3 ratio of 0.25 and the alkali oxide content of 20 mol% in the binary borate glasses. These results are ascribed to the formation of nonbridging oxygen atoms in the tetrahedral BO4 and FeO4 units. A glass transition temperature (Tg) of the glasses shows a distinct composition dependency, having a maximum around 16 mol% Ag2O. These lead to the conclusions that planar BO3 units are changed into the tetrahedral BO4 units in the lower Ag2O content region and that the nonbridging oxygen atoms are formed in the BO4 and FeO4 units when the Ag2O content is higher than 16 mol%. These conclusions are well consistent with the composition dependency of the electric conductivity which shows a minimum when the Ag2O content is 10–15 mol%.

Mössbauer and DTA Studies of Superionic Conducting AgCl–Ag2O–B2O3 Glasses

Abstract A structural study of silver borate glasses containing 20 or 30 mol% AgCl and 1 mol% Fe2O3 has been performed by means of Mössbauer spectroscopy. The distinct decreases in the Mössbauer parameters observed in the borate glasses with Ag2O contents higher than about 10 mol% suggest that chloride ions are covalently bonded with tetrahedral iron or boron atoms as nonbridging chlorine. On the other hand, chloride ions are considered to be ionically bonded with Ag+ ions at the interstitial sites of the three-dimensional network composed of BO3, BO4, and FeO4 units when the Ag2O content is lower than 10 mol%. Measurements of the glass transition temperatures and the electric conductivity show a good agreement with the Mössbauer results, suggesting that mobile and electric conducting Ag+ ions change their configurational sites from the interstitial sites which are close to Cl− ions to different interstitial sites which are close to the nonbridging chlorine atom.

Structural studies of organoboron compounds. XIV. Preparation and crystal and molecular structure of 4-isopropylidene-2,5,5-triphenyl-1,3-dioxa-4-azonia-2-bora-5-boratacyclopentane

Details of the synthesis, physical properties, and molecular structure of 4-isopropylidene-2,5,5-triphenyl-1,3-dioxa-4-azonia-2-bora-5-boratacyclopentane are reported. Crystals of this compound are monoclinic, a = 14.126(3), b = 16.008(1), c = 8.642(2) Å, β = 99.052(8)°, Z = 4, space group P21/n. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.032 and Rw = 0.033 for 1117 reflections with I ≥ 3σ(I). The molecule contains a nearly planar five-membered B—O—B—O—N ring which incorporates both trigonal planar boron (B—O = 1.333(4) and 418(4), B—C = 1.548(5) Å) and tetrahedral boron atoms (B—O = 1.495(4), B—N = 1.676(4), B—C = 2 at 1.603(5) Å). Other important libration-corrected bond distances include: C=N = 1.284(3) and N—O = 1.418(3) Å.

Crystal and molecular structure of 8,8-dimethyl-3,5-diphenyl-2,4,6-trioxa-1-azonia-3-bora-5-boranatabicyclo[3.3.0]octane

Crystals of 8,8-dimethyl-3,5-diphenyl-2,4,6-trioxa-1-azonia-3-bora-5-boranatabicyclo[3.3.0]-octane are monoclinic, a = 22.323(1), b = 9.555(1), c = 16.0027(6) Å, β = 109.055(4)°, Z = 8, space group C2/c. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to a final R of 0.048 and Rw of 0.066 for 2629 reflections with I ≥ 3σ(I). The central part of the molecule consists of two fused five-membered rings. One of them is a familiar BOCCN ring while the other is a novel and nearly planar BOBON ring containing one tetrahedral boron atom and one trigonal planar boron atom. Bond lengths are: O—B, 1.337(2)–1.467(2), N—B, 1.692(2), O—N, 1.440(2), O—C, 1.426(2), N—C, 1.513(2), B—C, 1.557(2) and 1.599(2), C(sp3)—C(sp3), 1.520(2)–1.530(2), C—C(ar), 1.372(4)–1.407(2), N—H, 0.89(2), mean C(ar)—H, 0.97(1), and mean C(sp3)—H, 1.01(1) Å. The crystal structure consists of discrete molecules each linked to two others by N—H … O hydrogen bonds (N … O = 2.853(2) Å).