Isomerism of copper(II) coordination compounds with hydroxamic acids

The structure of Cu^II complexes with hydroxamic acids Cu[R¹N(O)−(O)CR²]₂, where R¹=Ph, R²=Me; R¹=Me, R²=Ph, was studied by ESR spectroscopy. In toluene solutions and low-temperature glasses, the complexes exist as two forms, which were identified ascis-andtrans-isomers. The proportions of the isomers were determined. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 726–729, April, 1999.     

A scheme for rapid prediction of cooperativity in hydrogen bond chains of formamides,acetamides, and N‐methylformamides

A scheme is proposed in this article to predict the cooperativity in hydrogen bond chains of formamides, acetamides, and N‐methylformamides. The parameters needed in the scheme are derived from fitting to the hydrogen bonding energies of MP2/6‐31+G** with basis set superposition error (BSSE) correction of the hydrogen bond chains of formamides containing from two to eight monomeric units. The scheme is then used to calculate the individual hydrogen bonding energies in the chains of formamides containing 9 and 12 monomeric units, in the chains of acetamides containing from two to seven monomeric units, in the chains of N‐methylformamides containing from two to seven monomeric units. The calculation results show that the cooperativity predicted by the scheme proposed in this paper is in good agreement with those obtained from MP2/6‐31+G** calculations by including the BSSE correction, demonstrating that the scheme proposed in this article is reasonable. Based on our scheme, a cooperativity effect of almost 240% of the dimer hydrogen bonding energy in long hydrogen bond formamide chains, a cooperativity effect of almost 190% of the dimer hydrogen bonding energy in long hydrogen bond acetamide chains, and a cooperativity effect of almost 210% of the dimer hydrogen bonding energy in long hydrogen bond N‐methylformamide chains are predicted. The scheme is further applied to some heterogeneous chains containing formamide, acetamide, and N‐methylformamide. The individual hydrogen bonding energies in these heterogeneous chains predicted by our scheme are also in good agreement with those obtained from Møller‐Plesset calculations including BSSE correction. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

A new approach to cyclic hydroxamic acids: Intramolecular cyclization of N-benzyloxy carbamates with carbon nucleophiles

N-Alkyl-N-benzyloxy carbamates, 2, undergo facile intramolecular cyclization with a variety of carbon nucleophiles to give functionalized five- and six-membered protected cyclic hydroxamic acids, 3, in good to excellent yields. This method can be extended to prepare seven-membered cyclic hydroxamic acids in moderate yields. The sulfone intermediates 3 from this study can be alkylated while the corresponding phosphonates have been shown to undergo HWE reaction. The α,β-unsaturated synthon, 8, prepared by thermal elimination of sulfoxide 3m, undergoes Michael addition with secondary amines. The usefulness of this approach to prepare polydentate chelators has been demonstrated by the synthesis of bis cyclic hydroxamic acids 12, 14, and 15.     

Comparison of Bifurcated Halogen with Hydrogen Bonds

Bifurcated halogen bonds are constructed with FBr and FI as Lewis acids, paired with NH₃ and NCH bases. The first type considered places two bases together with a single acid, while the reverse case of two acids sharing a single base constitutes the second type. These bifurcated systems are compared with the analogous H-bonds wherein FH serves as the acid. In most cases, a bifurcated system is energetically inferior to a single linear bond. There is a larger energetic cost to forcing the single σ-hole of an acid to interact with a pair of bases, than the other way around where two acids engage with the lone pair of a single base. In comparison to FBr and FI, the H-bonding FH acid is better able to participate in a bifurcated sharing with two bases. This behavior is traced to the properties of the monomers, in particular the specific shape of the molecular electrostatic potential, the anisotropy of the orbitals of the acid and base that interact directly with one another, and the angular extent of the total electron density of the two molecules.     

Solubilities of normal alkanoic acids by the UNIFAC group contribution method

The UNIFAC group contribution method was applied to predict solubilities of C₁₀–C₂₂ normal alkanoic acids in hydrocarbons, esters, ketones, and alcohols. The results were compared with the literature experimental data. It was found that taking into account dimerization of the acids improves considerably the prediction accuracy.     

Hydrogen bond and the resonance effect on the formamide–water complexes

The interaction of formamide and the two transition states of its amide group rotation with one, two, or three water molecules was studied in vacuum. Great differences between the electronic structure of formamide in its most stable form and the electronic structure of the transition states were noticed. Intermolecular interactions were intense, especially in the cases where the solvent interacted with the amide and the carbonyl groups simultaneously. In the transition states, the interaction between the lone pair of nitrogen and the water molecule becomes important. With the aid of the natural bond orbitals, natural resonance theory, and electron localization function (ELF) analyses an increase in the resonance of planar formamide with the addition of successive water molecules was observed. Such observation suggests that the hydrogen bonds in the formamide–water complexes may have some covalent character. These results are also supported by the quantitative ELF analyses. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Thermodynamic analysis of receptors based on guanidinium/boronic acid groups for the complexation of carboxylates, alpha-hydroxycarboxylates, and diols: driving force for binding and cooperativity

The thermodynamics of guanidinium and boronic acid interactions with carboxylates, alpha-hydroxycarboxylates, and diols were studied by determination of the binding constants of a variety of different guests to four different hosts (7-10). Each host contains a different combination of guanidinium groups and boronic acids. The guests included molecules with carboxylate and/or diol moieties, such as citrate, tartrate, and fructose, among others. The Gibbs free energies of binding were determined by UV/Vis absorption spectroscopy, by use of indicator displacement assays. The receptor based on three guanidinium groups (7) was selective for the tricarboxylate guest. The receptors that incorporated boronic acids (8-10) had higher affinities for guests that included alpha-hydroxycarboxylate and catechol moieties over guests containing only carboxylates or alkanediols. Isothermal titration calorimetry revealed the enthalpic and entropic contributions to the Gibbs free energies of binding. The binding of citrate and tartrate was investigated with hosts 7-10, for which all the binding events were exothermic, with positive entropy. Because of the selectivity of hosts 8-10, a simple boronic acid (14) was also investigated and determined to be selective for alpha-hydroxycarboxylates and catechols over amino acids and alkanediols. Further, the cooperativity of 8 and 9 in binding tartrate was also investigated, revealing little or no cooperativity with 8, but negative cooperativity with 9. A linear entropy/enthalpy compensation relationship for all the hosts 7-10, 14, and the carboxylate-/diol-containing guests was also obtained. This relationship indicates that increasing enthalpy of binding is offset by similar losses in entropy for molecular recognition involving guanidinium and boronic acid groups.     

Conformational analysis of a secondary hydroxamic acid in aqueous solution by NOE spectroscopy

Hydroxamic acids are metal‐binding compounds used by micro‐organisms and possess applications in medicine and industry. Hydroxamic acids favor two conformations, E and Z; metal binding is limited to the Z conformation. The Z conformation may be identifiable by NOE spectroscopy, but analysis is complicated by the potential for long‐range coupling as well as for relayed NOEs due to conformational switching. In this report, we re‐examine the reported conformational preference of N‐methyl acetohydroxamic acid (NMHA) in D₂O using NOE spectroscopy. We find that the favored conformation of NMHA in aqueous solution is the E conformation, contrary to an earlier report. NOE build‐up curves are proposed as a valuable tool to probe conformational behavior in similar systems. Copyright © 2013 John Wiley & Sons, Ltd.     

On the importance of intramolecular hydrogen bond cooperativity in d‐glucose – an NMR and QTAIM approach

The idea that hydrogen bond cooperativity is responsible for the structure and reactivity of carbohydrates is examined. Density functional theory and gauge‐including atomic orbital calculations on the known conformers of the α and β anomers of d ‐glucopyranose in the gas phase are used to compute proton NMR chemical shifts and interatomic distances, which are taken as criteria for probing intramolecular interactions. Atom–atom interaction energies are calculated by the interacting quantum atoms approach in the framework of the quantum theory of atoms in molecules. Association of OH1 in the counterclockwise conformers with a strong acceptor, pyridine, is accompanied by cooperative participation from OH2, but there is no significant change in the bonding of the two following 1,2‐diol motifs. The OH6 ^... O5 (G?g+/cc/t and G+g?/cc/t conformers) or OH6 ^... O4 (Tg+/cc/t conformer) distance is reduced, and the OH6 proton is slightly deshielded. In the latter case, this shortening and the associated increase in the OH6–O4 interaction energy may be interpreted as a small cooperative effect, but intermolecular interaction energies are practically the same for all three conformers. In most of the pyridine complexes, one ortho proton interacts with the endocyclic oxygen O5. Analogous results are obtained when the clockwise conformer, G?g+/cl/g?, detected for the α anomer, and a hypothetical conformer, Tt/cl/g?, are complexed with pyridine through OH6. Generally, the cooperative effect does not go beyond the first two OH groups of a chain. Copyright © 2017 John Wiley & Sons, Ltd.     

Quantum-chemical calculations of the methylation of hydroxamic and thiohydroxamic acids with diazomethane

We made calculations about the methylation of both hydroxamic and thiohydroxamic acids with CH₂N₂. The potential-energy surfaces of several proposed pathways leading to possible site-selective products (N-methylated and O-methylated hydroximates) are presented. Our results agree satisfactorily with an experimental observation by Liguori et al. who found site selectivity in the formation of dimethylated products. Simultaneous deprotonation and methylation occurs in both forms (E and Z) of hydroxamic acid and thiohydroxamic acid, and the net energy barrier via this pathway is the smallest. In most corresponding processes the energy barriers are smaller for thiohydroxamic acid, and the Z-form has an energy barrier smaller than that of the E-form in both hydroxamic and thiohydroxamic acids.     

两亲性分子聚集体的相变及其协同性

两亲性分子聚集体是一类重要的软物质,它们有着丰富而复杂的相行为.本文主要从两个方面综述了作者所在的研究组在两亲性分子聚集体相变研究方面的工作进展.(1)磷脂相关体系相变热力学:归纳了多种小分子(二甲基亚砜、甘油、海藻糖、尿素等)对于磷脂体系相行为的调控,比较并讨论了固醇类分子和葡萄糖神经酰胺分子诱导磷脂分子形成液态有序相的能力,还介绍了计算机模拟磷脂相行为的工作进展.(2)两亲性分子聚集体相变的协同性:先介绍了相变协同性(即分子头部、尾部、界面等基团在相变过程中的一致性)问题的提出,然后通过双十八烷基二甲基溴化铵分子和硬脂酰溶血卵磷脂两个体系的研究实例,说明两亲性分子聚集体相变过程中存在着头尾不一致的现象.对这个问题的研究,将为我们打开挑战相态转变的一系列重大问题(如相变动力学、相态多型性、相态稳定性以及相变可逆性等)的新窗口.     

A convenient procedure for the solid-phase synthesis of hydroxamic acids on PEGA resins

An efficient method for the solid-phase synthesis of hydroxamic acids is described. The method comprises the nucleophilic displacement of esters immobilized on PEGA resins with hydroxylamine/sodium hydroxide in isopropanol. The hydroxyaminolysis protocol is compatible with a broad range of PEGA-supported peptide and peptidomimetic esters. The methodology was found to be compatible with two new strategies for the synthesis of solid-supported lactams and diketopiperazines, respectively, both relying on the high inter- and intramolecular reactivity of cyclic N-acyliminium ions with electron-rich aromatics and heteroaromatics, ultimately affording hydroxamic acid derivatives in high purities.     

DFT study on cooperativity in the interactions of hydrazoic acid clusters

Density functional theory at the B3LYP level with the 6‐311G** basis set is performed to calculate the systems consisting of up to four hydrazoic acid molecules. The dimers are found to exhibit cyclic and chain structures with N … H contacts at ca. 2.1–2.7 Å. However, there are only cyclic structures with N … H contacts at ca. 2.0–2.3 Å and 2.0–2.1 Å in the trimer and tetramer, respectively. Hydrogen bond distances in the trimer and tetramer are shorter than those in the cyclic dimer as a result of the stronger interaction between molecules. The contribution of cooperative effect to the interaction energy is significant. After the correction of the basis set superposition error and zero‐point energy, the binding energies are ?10.69, ?29.34, and ?54.26 kJ·mol^?1 for the most stable dimer, trimer, and tetramer, respectively. The calculated IR shifts for N? H stretching mode increase with the size of the cluster growths, reaching more than 200 cm^?1 in the tetramer. For the most stable clusters, the transition from the monomer to dimer, dimer to trimer, and trimer to tetramer involve changes of ?14.40, ?25.68, and ?31.88 kJ·mol^?1 for the enthalpies at 298.15 K and 1atm, respectively. We also perform Mulliken populations analysis and find the Mulliken populations on intermolecular N … H increasing in the sequence of the dimer, trimer, and tetramer. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 279–286, 2003     

Hydrogen bond and proton transfer in complexes of trioctylamine with halogenoacetic acids

The infrared spectra of solutions containing trioctylamine and monohalogenoacetic acids have been investigated in the region 1500–1800 cm^?1. Two forms of complexes 1: 1 acid-amine, the molecular complex with hydrogen bond OH N and the ionic pair, resulting from proton transfer from acid to amine, have been found to exist in equilibrium with each other.     

Modulation of hydrogen bonding upon ion binding: Insights into cooperativity

The impact due to the of presence of ions, such as Mg²⁺, Na⁺, H⁺, Cl^?, and OH^? on hydrogen bonded clusters of increasing size (water, formamide, and acetamide [n = 1–10]) in the context of associated cooperativity has been explored using density functinal theory (DFT) calculations. Sequential binding energies (SBE) rise on addition of monomer in case of parent clusters. SBE for ionic clusters are several times higher than that of parent clusters initially. This behavior is more dramatic on addition of either Mg²⁺ or H⁺ compared to other ions. Interestingly, SBE of both parent and ionic clusters approach nearly uniform values beyond n = 6 irrespective of kind of ion present in the cluster with the exception of magnesium. © 2013 Wiley Periodicals, Inc.     

The dimerization of radical anions of aromatic carboxylic acids competing with the self-protonation reaction

Quantum-chemical calculations (CNDO/2) of the theoretical relationship between the rate constants for the dimerization and self-protonation of radical anions show that dimer formation in the one-electron electroreduction of aromatic carboxylic acids (benzoic (1), 1-naphthoic (2), and 9-anthroic (3) is most probable for1. It is established that during the constant potential electrolysis (CPE) of1 a mixture of “head-to-tail” dimers is formed in the presence of 0.1M Bu₄NClO₄ (DMF). Their ratio depends on the amount of electricity passed through the solution. The CPE of 2 in the presence of 20 % H₂O affords 1,4-dihydro-1-naphthoic acid in up to 70 % yield. The high yield (∼70 %) of 9,10-dihydro-9-anthroic acid during the CPE of 3 can be accounted for by the decomposition of the dimeric product followed by protonation of the anionic species. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1735–1738, October, 1993.     

Intra and intermolecular hydrogen bonding in formohydroxamic acid

The presence of hydrogen bonding interactions in several tautomeric forms of formohydroxamic acid (FHA) and 1:1 association among the tautomeric forms and water‐coordinated tautomeric forms of FHA is explored theoretically. Out of the seven equilibrium structures, four tautomeric forms have been selected for aggregation with single water molecule and dimer formation. Fifteen aggregates of FHA with H₂O have been optimized at MP2/AUG‐cc‐PVDZ level and analyzed for intramolecular and intermolecular H‐bond interactions. Twenty‐seven dimers of the four tautomeric forms have been obtained at MP2/6‐31+G* level. The stabilization energies associated with dimerization and adduct formation with water are the result of H‐bond interactions and range from very weak to medium. The atomic charges and NBO analysis indicate that the electrostatic and the charge transfer are the important components favoring H‐bond formation. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Copper-catalyzed amide bond formation from formamides and carboxylic acids

A highly efficient copper-catalyzed approach to form amide bonds from formamides and carboxylic acids was developed.This protocol shows broad substrate scopes and high yields in the presence of 1 mol% catalyst and 4.0 equiv.formamides.