Stereoelectronics of Carboxylate-Imidazolium Hydrogen Bonds in Models of the Aspartate-Histidine Couple in Serine Proteases.

The geometry of the carboxylate-imidazolium hydrogen bond in the crystalline state and the effect of microenvironment are investigated in models of the aspartate-histidine (Asp-His) couple. Synthetic methods leading to the preparation of intramolecular models, possessing syn and anti-oriented hydrogen bonds, also are described. A single-pot procedure has been developed for converting phenoxyacetonitriles into their respective benzimidazole derivative in higher yields than the traditional two step procedure. An efficient single-pot procedure also has been developed for converting benzofuran to its acetylene derivative, 2-acetoxyphenylacetylene. This method involves in situ acetylation of the unstable phenol intermediate. Nine intermolecular imidazolium-benzoate couples have been prepared and their structures analyzed by single crystal X-ray crystallography. The structure of an intramolecular model also has been determined by single crystal X-ray crystallography. The orientation of all the imidazolium hydrogen bonds is syn relative to the carboxylate. Furthermore, syn hydrogen bonds are shorter than anti hydrogen bonds. The strength of the hydrogen bond increases as $\Delta$pK\sb{\rm a} between the donor and acceptor decreases. The basicity of the carboxylate is governed by the number of hydrogen bonds accepted by it: successive hydrogen bonding to carboxylate decreases its basicity. Consequently, correlations of hydrogen-bond strength and basicity (pK\sb{a} and PA) of carboxylates are more reliable when carboxylates accept equal number of hydrogen bonds. Stereoelectronics of syn hydrogen bonding to carboxylates are affected by the presence of strong and geometrically constrained anti hydrogen bonding. With respect to the plane of the carboxylate, syn hydrogen bonding lies (1) within 10\sp\circ in the absence of anti hydrogen bonding, (2) out-of-plane (30-35\sp\circ) in the presence of one anti hydrogen bond, and (3) within 10\sp\circ in the presence of two anti hydrogen bonds. Out-of-plane distortion in the presence of an anti-oriented hydrogen bond suggests a catalytic role for the anti-oriented Ser-214 side-chain in serine proteases

Zn(II) and Cd(II) Coordination Dimers Based on Mixed Benzodioxole-Carboxylate and N-Donor Ligands: Synthesis, Characterization, Crystal Structures and Photoluminescence Properties

Four new compounds, formulated as [Zn(m-Pip)2(3-Phpy)]2 (1), [Zn(m-Pip)2(4-Phpy)]2 (2), [Cd(m-Pip)(Pip)(3-Phpy)2]2 (3) and [Cd(m-Pip)(Pip)(4-Phpy)2]2 (4) (HPip=1,3-benzodioxole-5-carboxylic acid; Phpy= phenylpyridine), have been successfully assembled based on rigid carboxylate/pyridine ligands. These four compounds have been fully characterized by analytical and spectroscopic methods. The aim of the present study is to investigate the structural effect and the influence of the size of metal on the class, geometry and type of coordination of the carboxylate ligands (syn-syn, syn-anti) in the final 3D-arrangements of the structures. Finally, luminescence properties of these new four coordination dimers have been investigated

Entanglement and Bell's inequality violation above room temperature in metal carboxylates

In the present work we show that a special family of materials, the metal carboxylates, may have entangled states up to very high temperatures. From magnetic susceptibility measurements, we have estimated the critical temperature below which entanglement exists in the cooper carboxylate \{Cu$_2$(O$_2$CH)$_4$\}\{Cu(O$_2$CH)$_2$(2-methylpyridine)$_2$\}, and we have found this to be above room temperature ($T_e \sim 630$ K). Furthermore, the results show that the system remains maximally entangled until close to $\sim 100$ K and the Bell's inequality is violated up to nearly room temperature ($\sim 290$ K)

Stereoelectronic Effects in General Base Catalysis by Carboxylate. Theoretical Calculations and Studies in the Synthesis of New Intramolecular Models of Enzymatic Catalysis.

The hypothesis of the existence of stereoelectronic effects in the reactions in which carboxylate acts as a general base catalyst is further studied through the use of theoretical methods. Ab initio calculations on the structures and relative energies of the syn and anti conformations of formic acid at different degrees of solvation are described. These calculations indicate that the syn conformation is more stable in aqueous phase than the anti by 11.9 (at the STO-3G//STO-3G level), 6.9 (4-31G//4-31G), and 2.5 kcal/mol (6-31G(,**)//4-31G). This last number, though, comes from single point calculations at the 6-31G(,**) level on the 4-31G optimized structure and is not consistent with the trends observed with other basis sets. The greater stability of the syn conformation of carboxylic acids in aqueous solution should cause the higher basicity of carboxylate ion when acting as a base in the syn orientation. In order to experimentally test this hypothesis, an intramolecular model of ester hydrolysis reactions with a syn-oriented carboxylate is designed and progress towards its synthesis is described. To test the more critical reactions in the synthetic scheme, a series of stilbene-containing cyclic ethers is synthesized through the use of a novel etherification procedure in mixed solvents in very high yields, followed by a low-valent titanium induced coupling of bis(carbonyl) compounds. Overall yields of 61 to 74% for the two-step conversions are obtained. The crystal structures by X-ray diffraction of some of these new compounds are determined and described. To extend the synthetic sequence described above to functionalized stilbenes, a large number of 2,6-disubstituted benzaldehydes with various protecting groups are prepared and their selective deprotections investigated. Several bis(aryl) ethers are prepared using the etherification reaction used for the cyclic stilbenes and transformed into bis(benzaldehyde) compounds containing protected functional groups by dilithiation, followed by reaction with N,N-dimethylformamide. All attempts to form stilbenes from these compounds by reaction with low-valent titanium result in decomposition. Alternative preparations, both short and highly probable, of the desired models from the intermediates already synthesized are suggested

Pseudopeptidic ligands: exploring the self-assembly of isopthaloylbisglycine (H2IBG) and divalent metal ions

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Design and Synthesis of Oxazoline-Based Scaffolds for Hybrid Lewis Acid/Lewis Base Catalysis of Carbon–Carbon Bond Formation

A new class of hybrid Lewis acid/Lewis base catalysts has been designed and prepared with an initial objective of promoting stereoselective direct aldol reactions. Several scaffolds were synthesized that contain amine moieties capable of enamine catalysis, connected to heterocyclic metal-chelating sections composed of an oxazole–oxazoline or thiazole–oxazoline. Early screening results have identified oxa­zole–oxazoline-based systems capable of promoting a highly diastereo- and enantioselective direct aldol reaction of propionaldehyde with 4-nitrobenzaldehyde, when combined with Lewis acids such as zinc triflate

Formal [3+2] Cycloaddition Reactions of Electron-Rich Aryl Epoxides with Alkenes under Lewis Acid Catalysis Affording Tetrasubstituted Tetrahydrofurans

We report on the regio- and stereoselective synthesis of tetrahydrofurans by reaction between epoxides and alkenes in the presence of a Lewis acid. This is an unprecedented formal [3+2] cycloaddition reaction between an epoxide and an alkene. The chemical reaction represents a very concise synthesis of tetrahydrofurans from accessible starting compounds