Izabela D. Madura

Reactivity of Various Four‐Coordinate Aluminum Alkyls towards Dioxygen: Evidence for Spatial Requirements in the Insertion of an Oxygen Molecule into the Al−C Bond

The interaction of dioxygen with various tetrahedral aluminum alkyls, (tBu)3Al.OEt2 (1), tBu2Al(mu-OtBu)2AltBu2 (6), (tBu)2Al(mesal) (2) [mesal=methyl salicylate anion], R2Al(mu-pz)2AlR2 [pz=deprotonated pyrazole, R= Me (3a), Et (3b), and tBu (3c)], R2Al(mu-3,5-Me2pz)2AIR2[3,5-Me2pz = deprotonated 3,5-dimethylpyrazole, R= Me (4a), and Et (4b)], and Et2B(mu-pz)2AlEt2 (5), has been investigated. We were particularly interested in the effect of steric hindrances both caused by the metal-bonded substituents and those that result from the nature of the bifunctional ligand used in the oxygenation reaction. In the reaction of 1 with O2, only the formation of the monoalkoxide compound6 was observed. The latter di-tert-butyl compound as well as all planar aluminapyrazoles, that is, the tert-butyl derivative 3c and lower alkylaluminum derivatives with the more demanding 3,5-dimethylpyrazoyl ligands 4a and 4b, are stable under an atmosphere of dry oxygen and ambient conditions. Inspection of the space-filling representation of these compounds has undoubtedly shown that the bulky tert-butyl groups or pyrazolyles ligands, respectively, provide steric protection for the metal center from the dioxygen attack. In contrast, the dialkylaluminum derivatives of pyrazole, 3a and 3b, and the diethylaluminum bis(1-pyrazolyl)borate complex 5, all with the metal center eclipsed with respect to the plane defined by the four nitrogen atoms, react smoothly with O2 to form the alkyl(alkoxy)aluminum complexes. In the reaction of 5 with O2 for example, the Et-B bonds remained intact, and the dimeric five-coordinate compound [Et2B(mu-pz)2 Al(mu-OEt)Et]2 (9) was isolated in good yield. The interaction of mononuclear di-tert-butyl chelate complex 2 with O2 at -15 degrees C gives (tBuOO)(tBuO)Al(mu-OtBu)2Al(mesal)2 (7) in high yield, and the presence of the alkylperoxo moiety is a particularly significant point in the resulting product. All the compounds have been characterized spectroscopically, and the structures of 3c, 4a, 6, 7, and 9 have been confirmed by X-ray crystallography. Structural features of 1-6 are discussed and are considered in relation to the possible approach pathways of the O2 molecule to the four-coordinate metal center. This analysis and the observed apparent dissimilarity in the reactions of model four-coordinate aluminum alkyls with O2 clearly show that the stereoelectronic prerequisites are responsible for the fundamentally different reactivity.

Structural aspects of the aromaticity of cyclic π-electron systems with BN bonds

An extension of the HOMA model and Birds I6 model for systems with BN bonds is given. Analysis of the aromatic character of 11 pairs of borazine derivatives and their benzene analogues is done based on Cambridge Structural Database (CSD) release using HOMA and I6 indices. Borazine derivatives exhibit less aromatic character than their benzene analogues and are much less sensitive to the chemical and topological effects from the environment of the rings in question. In contrast to cyclooctatetraene, 1,3,5,7-tetra-t-butyl-2,4,6,8-tetramethyl-1,3,5,7,2,4,6,8-tetraazatetraborocine does not exhibit antiaromatic character.

A tautomeric equilibrium between functionalized 2-formylphenylboronic acids and corresponding 1,3-dihydro-1,3-dihydroxybenzo[c][2,1]oxaboroles

Functionalized 2-formylphenylboronic acids undergo an unprecedented tautomeric rearrangement in solution to form corresponding 1,3-dihydro-1,3-dihydroxybenzo[c][2,1]oxaboroles. X-Ray analyses of selected examples revealed diverse solid-state molecular structures from a planar open form with a hydrogen-bonded carbonyl group (X = 3-F) through a twisted conformer showing a weak carbonyl–boron interaction (X = 3,5-Br2) to a cyclic oxaborole derivative (X = 3-Br). Variable-temperature 1H NMR spectroscopy was used to determine equilibrium constants as well as enthalpies and entropies of tautomerization in a mixed solvent [D6]acetone–D2O (95 : 5). A computational approach to the process by DFT (B3LYP) and MP2 methods has also been performed.

Lewis acidity and sugar receptor activity of 3-amino-substituted benzoxaboroles and their ortho-aminomethylphenylboronic acid analogues

The pKas as well as apparent binding constants of several 3-amino-substituted benzoxaboroles with alizarin red S (ARS) and some biologically important sugars have been evaluated and compared with that of the parent aminomethylphenylboronic acids. The investigated benzoxaboroles reveal lower acidity than the corresponding boronic acids. All the studied boronic compounds display the usual order of apparent binding constant values with glucose, galactose and fructose. The X-ray structure of the model catechol ester of the morpholinylmethylphenylboronic acid has been determined. The molecules are monomeric zwitterions with a tetracoordinated boron atom. The intermolecular interactions have been described with the aid of the Hirshfeld surface analysis.

Amination-reduction reaction as simple protocol for potential boronic molecular receptors. Insight in supramolecular structure directed by weak interactions

The synthesis of the potential molecular receptors in the amination-reduction reaction has been investigated within the model system comprising (2-formylphenyl)boronic acid and morpholine. The 3-amine substituted benzoxaborole was identified to be the intermediate of the synthesis and the unsubstituted benzoxaborole as the by-product resulting from reduction of the starting material. The insight into the reactivity of the starting materials as well as the intermediate benzoxaborole enabled significant rise in the yield of 2-(aminomethyl) phenylboronic acids synthesis. The solid state structure of 2-(piperidylmethyl)phenylboronic acid has been re-determined, and the description of the molecule and the crystal is given. The supramolecular layer structure directed by the weak C-H…O and C-H…π interactions was identified and scrutinized based on the geometry and Hirshfeld surface analyses

On the mechanism of four-coordinate aluminum alkyls interaction with dioxygen: evidence for spatial demands in the autoxidation reaction

Abstract The reactivity of four-coordinate dialkylaluminum derivatives of pyrazole towards dioxygen, particularly with regard to the effect of steric hindrances caused by the metal center bonded substituents and those resulting from the ligand, has been investigated. Our preliminary results show that spatial demands play a vital role in the autoxidation of four-coordinate aluminum alkyls.

5-Fluoro-1,3-dihydro-2,1-benzoxaborol-1-ol.

In the crystal structure of the title compound, C7H6BFO2, a broad-spectrum antifungal drug (AN2690), the planar [maximum deviation 0.035 (1) Å] molecules form centrosymmetric R 2 2(8) dimers via strong O—H⋯O hydrogen bonds. The dimers are arranged into layers by weak intermolecular C—H⋯O and C—H⋯F hydrogen bonds. The symmetry of this two-dimensional supramolecular assembly can be described by the layer group p and topologically classified as a simple uninodal four-connected two-dimensional network of a (4.4.4.4.6.6) topology. Further weak C—H⋯O interactions build up the three-dimensional structure.

The products formed in the reaction of MeAlCl2 with alcohols (1:1)

Abstract The products formed in the reaction (1:1) of MeAlCl2 with alcohols ROH, where R=Et, iPr, nBu, iBu, sBu, tBu, tBuCH2 were investigated. Depends on the alcohol used the formation of [Cl2Al(μ-OR)2]2AlCl or/and [Cl2Al(μ-OR)2]3Al except dimeric or/and trimeric ROAlCl2 were found. The products have been characterized by NMR and cryoscopically in benzene. The crystal structures of [Cl2Al(μ-OnBu)2]3Al and [Cl2Al(μ-OCH2tBu)]2 were determined by X-ray diffraction studies.

Influence of alkoxy ligands on the Cp-Al bonding mode in [Cp2Al-μ-OR]2 from X-ray crystallographic and 27Al-NMR spectroscopic solution studies

Abstract The dicyclopentadienylaluminium alkoxides of general formula [Cp2Al-μ-OR]2 where R=Me, Et, n-Bu, i-Bu, CH2t-Bu, s-Bu, CH2Ph, C6H4-4-t-Bu (1–8) were prepared by reacting CpNa with ROAlCl2 at the molar ratio 2:1, respectively (2–8) or from the reaction of Cp3Al with alcohol (1). The compounds were characterised by multinuclear NMR (1H, 13C, 27Al) method. The molecular structures of compounds 2, 5 and 6 were determined by X-ray crystallography. The Cp–Al bonding mode in dicyclopentadienylaluminium alkoxides regarding the steric demands of alkoxy groups is discussed based on 27Al-NMR chemical shifts and structural data obtained. The meaningful correlation between Cp ring-slippage and 27Al-NMR chemical shifts was observed.

Hydrogen-bonding patterns in two structural isomers of 3,6-bis(2-chlorophenyl)-1,4-dihydro-1,2,4,5-tetrazine.

Two structural isomers, 3,6-bis(2-chlorophenyl)-1,4-dihydro-1,2,4,5-tetrazine, (I), and 3,5-bis(2-chlorophenyl)-4-amino-1H-1,2,4-triazole, (II), both C(14)H(10)Cl(2)N(4), form chain-like structures in the solid state, stabilized by N-H...N and N-H...Cl hydrogen bonds. A contribution from weak interactions to the strong hydrogen-bond network is observed in both structures. The secondary graph sets for intermolecular hydrogen bonds [R(2)(2)(11) for (I) and R(2)(2)(12) for (II)] indicate the similarity between the networks.