Showing papers on "Steric effects published in 2006"

Structural conformation and vibrational spectroscopic studies of 2,6-bis(p-N,N-dimethyl benzylidene)cyclohexanone using density functional theory

Abstract: NIR-FT Raman and FT-IR spectra of the crystallized 2,6-bis (p-N,N-dimethyl benzylidene)cyclohexanone (C24H28N2O) have been recorded in the region 3200–500 and 4000–400 cm−1, respectively. The spectral interpretation has been done following full structure optimization and vibrational wavenumber calculations based on the density functional theory (DFT) using the standard B3LYP/6-31G* basis set. The predicted vibrational spectra are in excellent agreement with the experiment, permitting an unambiguous assignment for the unusual downshifting of νCO caused by expanded conjugation effects in the dienone system. The optimized geometry clearly demonstrates the ‘half-chair’ conformation of the central nonheterocyclic ring. The ring CH stretching vibrational modes involving aromatic hydrogen atoms participating in steric interaction have been observed with low intensities. There are valid structural and spectral bases for the enhancement of its bioactivity due to full charge transfer reaction by the substitution of the electron-donating 4-dimethylamino group into the aryl rings. The existence of intramolecular, CH…O, improper, blue-shifted hydrogen bond was investigated by means of the natural bonding orbitals (NBO) analysis. There are also slight dissimilarities in the bond lengths and endocyclic bond angles of both phenyl rings due to the effect of the heavy substitution. The characteristic ring modes have also been assigned in detail. Copyright © 2006 John Wiley & Sons, Ltd.

Steric effects in the dynamics of electrolytes at large applied voltages: II. Modified Poisson-Nernst-Planck equations

Abstract: In situations involving large potentials or surface charges, the Poisson Boltzman(PB) equation has shortcomings because it neglects ion-ion interactions and steric effects. This has been widely recognized by the electrochemistry community, leading to the development of various alternative models resulting in different sets "modified PB equations", which have had at least qualitative success in predicting equilibrium ion distributions. On the other hand, the literature is scarce in terms of descriptions of concentration dynamics in these regimes. Here, adapting strategies developed to modify the PB equation, we propose a simple modification of the widely used Poisson-Nernst-Planck (PNP) equations for ionic transport, which at least qualitatively accounts for steric effects. We analyze numerical solutions of these MPNP equations on the model problem of the charging of a simple electrolyte cell, and compare the outcome to that of the standard PNP equations. Finally, we repeat the asymptotic analysis of Bazant, Thornton, and Ajdari(2004} for this new system of equations to further document the interest and limits of validity of the simpler equivalent electrical circuit models introduced in Part I for such problems.

Study of ligand substituent effects on the rate and stereoselectivity of lactide polymerization using aluminum salen-type initiators

Abstract: A series of aluminum salen-type complexes [where salen is N,N′-bis(salicylaldimine)-1,2-ethylenediamine] bearing ligands that differ in their steric and electronic properties have been synthesized and investigated for the polymerization of rac-lactide. X-ray crystal structures on key precatalysts reveal metal coordination geometries intermediate between trigonal bipyramidal and square-based pyramidal. Both the phenoxy substituents and the backbone linker have a significant influence over the polymerization. Electron-withdrawing groups attached to the phenoxy donor generally gave an increased polymerization rate, whereas large ortho substituents generally slowed down the polymerization. The vast majority of the initiators afforded polylactide with an isotactic bias; only one exhibited a bias toward heteroselectivity. Isoselectivity generally increases with increased flexibility of the backbone linker, which is presumed to be better able to accommodate any potential steric clashes between the propagating polymer chain, the inserting monomer unit, and the substituents on the phenoxy donor.

An improved method for the computation of ligand steric effects based on solid angles

Abstract: An improved algorithm has been designed to characterize ligand interactions in organometallic and coordination complexes in terms of the percentage of the metal coordination sphere shielded by a given ligand. The computations for ligand solid angles are performed numerically and employ introduced atomic radii that are larger than covalent but smaller than van der Waals radii. This approach enables facile evaluation of steric congestion in the metal coordination sphere, quantification of unfavorable interligand contacts, and in some cases prediction of the complex composition or ligand coordination on purely geometrical grounds.

Catalytic hydrodeoxygenation of methyl-substituted phenols: correlations of kinetic parameters with molecular properties.

Abstract: The hydrodeoxygenation of methyl-substituted phenols was carried out in a flow microreactor at 300 degrees C and 2.85 MPa hydrogen pressure over a sulfided CoMo/Al(2)O(3) catalyst. The primary reaction products were methyl-substituted benzene, cyclohexene, cyclohexane, and H(2)O. Analysis of the results suggests that two independent reaction paths are operative, one leading to aromatics and the other to partially or completely hydrogenated cyclohexanes. The reaction data were analyzed using Langmuir-Hinshelwood kinetics to extract the values of the reactant-to-catalyst adsorption constant and of the rate constants characterizing the two reaction paths. The adsorption constant was found to be the same for both reactions, suggesting that a single catalytic site center is operative in both reactions. Ab initio electronic structure calculations were used to evaluate the electrostatic potentials and valence orbital ionization potentials for all of the substituted phenol reactants. Correlations were observed between (a) the adsorption constant and the two reaction rate constants measured for various methyl-substitutions and (b) certain moments of the electrostatic potentials and certain orbitals' ionization potentials of the isolated phenol molecules. On the basis of these correlations to intrinsic reactant-molecule properties, a reaction mechanism is proposed for each pathway, and it is suggested that the dependencies of adsorption and reaction rates upon methyl-group substitution are a result of the substituents' effects on the electrostatic potential and orbitals rather than geometric (steric) effects.

Synthetic, structural, mechanistic, and computational studies on single-site beta-diketiminate tin(II) initiators for the polymerization of rac-lactide.

Abstract: A family of tin(II) complexes supported by beta-diketiminate ligands has been investigated as initiators for the polymerization of rac-lactide. Kinetic studies reveal a first-order dependence on [lactide], but with a significant induction period. Linear plots of M(n) versus conversion and [M](o)/[I](o) versus conversion, along with narrow molecular weight distributions (typically 1.07-1.10), are indicative of well-controlled, "living" polymerizations. Less sterically hindered derivatives promote faster propagation than their bulky analogues, in accord with a more accessible active site. Enhanced rates of polymerization are observed for ligands bearing halogenated N-aryl substituents, a consequence of the more Lewis acidic nature of the Sn(II) centers. All of the initiators exhibit a similar bias toward heterotactic polylactide, which is attributed to a chain-end control mechanism influenced predominantly by the presence of the Sn 5s(2) lone pair of electrons rather than the steric or electronic properties of the beta-diketiminate ligand. The tin(II) isopropyl-(S)-lactate complex, ((Me)BDI(DIPP))SnOCH(Me)COO(i)Pr (14), has been synthesized as a model compound for the propagating species by treatment of ((Me)BDI(DIPP))Sn(NMe(2)) with isopropyl-(S)-lactate. An X-ray structure determination showed that the lactate ligand forms a five-membered chelate ring with a weak donor bond from the carbonyl oxygen atom to the tin center. A B3LYP density functional computational study indicates that insertion of the first lactide monomer into the tin(II) alkoxide bond is facile, with the induction period arising from a slower insertion of the second (and possibly third and fourth) monomer units.

Ruthenium-Catalyzed Regiospecific Borylation of Methyl C−H Bonds

Abstract: We report the regiospecific, ruthenium-catalyzed borylation of saturated terminal C−H bonds. Alkylboronates were obtained in 78−98% yields. The borylations of alkanes, trialkylamines, protected alcohols, and fluoroalkanes occurred regiospecifically at the methyl group that is least sterically hindered. In contrast to most organometallic C−H activation, the reactions of alkanes occurred in higher yields than the reactions of arenes. Reactions were conducted that probed steric and electronic effects on the alkyl borylation. These reactions showed that the borylation occurred preferentially at the methyl group that is least sterically hindered and most electron-deficient. Ruthenium compounds containing boryl ligands were synthesized, and one was characterized by X-ray crystallography. One of these compounds contained a rare bridging boryl ligand and served as a catalyst precursor for the borylation of octane.

A Direct Catalytic Asymmetric Mannich-type Reaction via a Dinuclear Zinc Catalyst: Synthesis of Either anti- or syn-α-Hydroxy-β-Amino Ketones

Abstract: The use of imines bearing a hydrolyzable nitrogen substituent in direct asymmetric Mannich reactions with α-hydroxyketones is developed. Previous work focused on the use of N-arylimines or nonenolizable imines, and the latter with only methoxy-substituted α-hydroxyacetophenones. Using a dinuclear catalyst devised from 2,6-di-(S)-2‘-diphenylhydroxymethylpyrrolidino-N-methyl)-4-methylphenol and diethylzinc, a broad array of hydroxyacetylated aromatics, including phenyl, 2-furyl, 1-naphthyl, and 2-naphthyl, react well. In addition, the reactions focused on the use of enolizable imines. With the N-diphenylphosphinoyl, the reactions are anti selective with enantiomeric excesses ranging from 83 to 99%, except for the reaction of the 2-methoxy-2‘-hydroxyacetylbenzene. With the N-Boc-imines, the reactions were syn selective with enantiomeric excesses from 90 to 94%. The dependence of the diastereoselectivity on the nature of the N-substituent presumably arises from the steric demands of the diphenylphosphinoyl group.

Steric communication of chiral information observed in dendronized polyacetylenes.

Abstract: Structural and retrostructural analysis of helical dendronized polyacetylenes (i.e., self-organizable polyacetylenes containing first generation dendrons or minidendrons as side groups) synthesized by the polymerization of minidendritic acetylenes with [Rh(nbd)Cl]2 (nbd = 2,5-norbornadiene) reveals an ∼10% change in the average column stratum thickness (l) of the cylindrical macromolecules with a chiral periphery, through which a strong preference for a single-handed screw-sense is communicated. The cylindrical macromolecules reversibly interconvert between a three-dimensional (3D) centered rectangular lattice (Φr-c,k) exhibiting long-range intracolumnar helical order at lower temperatures and a two-dimensional (2D) hexagonal columnar lattice (Φh) with short-range helical order at higher temperatures. A polymer containing chiral, nonracemic peripheral alkyl tails is found to have a larger l as compared to the achiral polymers. In methyl cyclohexane solution, the same polymer exhibits an intense signal in ...

Triphenylene analogues with B2N2C2 cores: synthesis, structure, redox behavior, and photophysical properties.

Abstract: A series of alkyl (1−3), aryl (6), and benzo-annulated (4, 5) heteroaromatic triphenylene analogues with B2N2C2 cores have been synthesized via chelation of pyridazine derivatives using difunctional Lewis acidic diborabiphenyl precursors. In contrast to triphenylene, NICS(1) calculations on 1 suggested high aromaticities for the central (−11.3 ppm) and outer borabenzene rings (−7.7 ppm), along with nonaromatic behavior for the pyridazine ring (−0.7 ppm). Crystal structure analyses supported this analysis. When the a- and c-faces of the pyridazine moiety were free of substitution (1, 3), planar structures resulted, but upon substitution, a twisted B2N2C2 core was observed due to steric repulsion of neighboring hydrogen atoms (e.g., 5). The increase of steric bulk from H (1) to iPr (3) in the planar species was found to result in a dimeric, head-to-tail herringbone packing motif, held together by close intermolecular B···N interactions of 3.39 A. One-electron reduction by Cp*2Co was found to afford the radi...

Nucleophilic substitution at phosphorus (S(N)2@P): disappearance and reappearance of reaction barriers.

Abstract: Pentacoordinate phosphorus species play a key role in organic and biological processes. Yet, their nature is still not fully understood, in particular, whether they are stable, intermediate transition complexes (TC) or labile transition states (TS). Through systematic, theoretical analyses of elementary SN2@C, SN2@Si, and SN2@P reactions, we show how increasing the coordination number of the central atom as well as the substituents' steric demand shifts the SN2@P mechanism stepwise from a single-well potential (with a stable central TC) that is common for substitution at third-period atoms, via a triple-well potential (featuring a pre- and post-TS before and after the central TC), back to the double-well potential (in which pre- and postbarrier merge into one central TS) that is well-known for substitution reactions at carbon. Our results highlight the steric nature of the SN2 barrier, but they also show how electronic effects modulate the barrier height.

Reciprocity of steric and stereoelectronic effects in the collagen triple helix.

Abstract: In previous work, we demonstrated that 4-fluoroproline residues can contribute greatly to the conformational stability of the collagen triple helix, and that this stability arises from stereoelectronic effects that fix the pucker of the pyrrolidine ring and thereby preorganize the backbone properly for triple-helix formation. Here, we take a reciprocal approach, demonstrating that the steric effect of a 4-methyl group confers stability similar to that from a 4-fluoro group in the opposite configuration. Such fundamental interplay between steric and stereoelectronic effects is heretofore unknown in proteinsnatural or syntheticand provides a new means to modulate conformational stability.

The Elusive Mechanism of Olefin Metathesis Promoted by (NHC)Ru-Based Catalysts: A Trade between Steric, Electronic, and Solvent Effects

Abstract: The reaction mechanism operative in olefin metathesis has been and still is a challenging area of research. Here we contribute to the discussion showing that the actual mechanism is a balance of the title effects. In particular, we show that the electronic and solvent effects evidenced by experimental studies can be easily counterbalanced by steric effects.

What Factors Influence the Ratio of C¿H Hydroxylation versus C¿C Epoxidation by a Nonheme Cytochrome P450 Biomimetic?

Abstract: Density functional calculations on a nonheme biomimetic (Fe=O(TMCS+) have been performed and its catalytic properties versus propene investigated Our studies show that this catalyst is able to chemoselectively hydroxylate C=H bonds even in the presence of C=C double bonds This phenomenon has been analyzed and found to occur due to Pauli repusions between protons on the TMCS ligand with protons attached to the approaching substrate The geometries of the rate determining transition states indicate that the steric hindrance is larger in the epoxidation transition states than in the hydroxylation ones with much shorter distances; hence the hydroxylation pathway is favored over the epoxidation Although, the reactant experiences close lying triplet and quintet spin states, the dominant reaction mechanism takes place on the quintet spin state surface; ie, Fe=O(TMCS)+ reacts via single-state reactivity Our calculations show that this spin state selectivity is the result of geometric orientation of the transition state structures, whereby the triplet ones are destabilized by electrostatic repulsions between the substrate and the ligand while the quintet spin transition states are aligned along the ideal axis The reactivity patterns and geometries are compared with oxoiron species of dioxygenase and monoxygenase enzymes Thus, Fe=O(TMCS)+ shows some similarities with P450 enzyme reactivity: it chemoselectively hydroxylates C=H bonds even in the presence of a C=C double bond and therefore is an acceptable P450 biomimetic However, the absolute barriers of substrate oxidation by Fe=O(TMCS)+ are higher than the ones obtained with heme enzymes, but the chemoselectivity is lesser affected by external perturbations such as hydrogen bonding of a methanol molecule toward the thiolate sulfur or a dielectric constant This is the first oxoiron complex whereby we calculated a chemoselective hydroxylation over epoxidation in the gas phase

Iridium(III)-induced isomerization of 2-substituted pyridines to N-heterocyclic carbenes.

Abstract: Steric strains (F-strain or front-strain) in the acid−base interaction between the Lewis acid [TpMe2Ir(C6H5)2] and 2-substituted pyridines Lewis bases, are responsible for the stabilization of the C-adducts (2) of the respective pyridine carbene tautomers over the expected N-adducts.

Mechanistic insights into triterpene synthesis from quantum mechanical calculations. Detection of systematic errors in B3LYP cyclization energies

Abstract: Most quantum mechanical studies of triterpene synthesis have been done on small models. We calculated mPW1PW91/6-311+G(2d,p)//B3LYP/6-31G* energies for many C30H51O+ intermediates to establish the first comprehensive energy profiles for the cationic cyclization of oxidosqualene to lanosterol, lupeol, and hopen-3β-ol. Differences among these 3 profiles were attributed to ring strain, steric effects, and proton affinity. Modest activation energy barriers and the ample exothermicity of most annulations indicated that the cationic intermediates rarely need enzymatic stabilization. The course of reaction is guided by hyperconjugation of the carbocationic 2p orbital with parallel C–C and C–H bonds. Hyperconjugation for cations with a horizontal 2p orbital (in the plane of the ABCD ring system) leads to annulation and ring expansion. If the 2p orbital becomes vertical, hyperconjugation fosters 1,2-methyl and hydride shifts. Transition states leading to rings D and E were bridged cyclopropane/carbonium ions, which allow ring expansion/annulation to bypass formation of undesirable anti-Markovnikov cations. Similar bridged species are also involved in many cation rearrangements. Our calculations revealed systematic errors in DFT cyclization energies. A spectacular example was the B3LYP/6-311+G(2d,p)//B3LYP/6-31G* prediction of endothermicity for the strongly exothermic cyclization of squalene to hopene. DFT cyclization energies for the 6-311+G(2d,p) basis set ranged from reasonable accuracy (mPW1PW91, TPSSh with 25% HF exchange) to underestimation (B3LYP, HCTH, TPSS, O3LYP) or overestimation (MP2, MPW1K, PBE1PBE). Despite minor inaccuracies, B3LYP/6-31G* geometries usually gave credible mPW1PW91 single-point energies. Nevertheless, DFT energies should be used cautiously until broadly reliable methods are established.

Synthesis of bulky and electron-rich MOP-type ligands and their applications in palladium-catalyzed C-N bond formation.

Abstract: A series of 2-dialkylphosphino-2'-alkoxy-1,1'-binaphthyl ligands (6a-c and 8a-c) have been prepared conveniently by a lithium-initiated ring-opening reaction of dinaphthofuran, followed by selective phosphorylation. These compounds displayed a remarkable air and moisture stability, both in solid form and in solution. Application of these phosphine ligands in palladium-catalyzed C-N bond forming reactions revealed the crucial roles of the steric bulk of the substituents on the phosphorus atom governing the catalytic activity. Specifically, 2-di-tert-butylphosphino-2'-isopropoxy-1,1'-binaphthyl (8b) proved to be the most effective for the aminations of aryl halides with primary amines, while the less bulky 2-dicyclohexyl-2'-methoxy-1,1'-binaphthyl (6a) was more effective for the aminations with secondary amines. The steric and electronic effects of the ligands were analyzed to account for these observations.

The effect of steric hindrance on the association of telluradiazoles through Te-N secondary bonding interactions

Abstract: DFT calculations were used to compare the magnitude of steric repulsion to the strength of secondary bonding interactions (SBIs) in the (Te−N)2 supramolecular synthon to explain or predict the supramolecular structures assembled by two derivatives of the 1,2,5-telluradiazole ring: benzo-2,1,3-telluradiazole (4c) and 3,6-dibromobenzo-2,1,3-telluradiazole (5). The crystallographically determined structures were consistent with the computational predictions. In sharp contrast with the previously known structures of its sulfur and selenium analogues, 4c forms infinite ribbon chains in the solid state with 2.682(7)−2.720(7) A Te−N SBIs. Steric hindrance in 5 restricted the supramolecular association to form discrete dimers with 2.697(8) A Te−N SBIs. In addition to discrete dimers, the dibromo derivative crystallizes as solvated dimers in 5·DMSO with 2.834(5) A Te−O SBIs. Other weaker SBIs were identified in the crystal lattices and were assessed by the computational method.

Cyanophenyloximes: Reliable and Versatile Tools for Hydrogen-Bond Directed Supramolecular Synthesis of Cocrystals

Abstract: A systematic structural and spectroscopic examination of the products resulting from cocrystallization reactions between three types of phenyloximes R−CN−OH (where R = H, Me, or CN) and a series of N-heterocyclic hydrogen-bond acceptors demonstrates that the acidity of the oxime -OH hydrogen-bond donor is crucial to the efficacy of the supramolecular assembly process. Cyanophenyloximes are comparable to carboxylic acids, in terms of success rate, whereas the significantly less acidic CH3- and H-substituted analogues are not effective at generating cocrystals despite close similarities in steric and geometric parameters. The importance and validity of using experimental pKa values and calculated electrostatic potential surfaces as a basis for predicting the supramolecular yield of an O−H···N interaction for driving the formation of cocrystals (within a functional group class) is unambiguously established, and six new crystal structures of cocrystals assembled using oxime···heterocycle-based hydrogen bonds ...

Molecular electrostatic potential approach to determining the steric effect of phosphine ligands in organometallic chemistry.

Abstract: A two-layer ONIOM(B3LYP/6-31G(d,p):UFF) quantum mechanics (QM)−molecular mechanics (MM) optimization of PR3 ligands, where the QM layer is always constructed as PH3, followed by molecular electrostatic potential (MESP) analysis of the QM layer is suggested as a simple and effective method for evaluating the steric effect of PR3 ligands. The subtle variations in the electron distribution that arise as a result of the steric bulkiness as well as the conformational changes in the substituent groups is well reflected in the value of the MESP minimum (Vmin) located in the QM region. In general, a sterically bulky group has always shown a more negative Vmin than a sterically less bulky group. The difference between the Vmin value of free PH3 and the Vmin value at the QM region of PR3 is used as a measure of the steric effect of the PR3 ligand. This value, designated as MESPsteric, showed a good linear correlation with the cone angle values as well as the average of the intervalence HPH angles found in the QM la...

Gold Catalysis: No Steric Limitations in the Phenol Synthesis

Abstract: Different dihydroisoindol-4-ols and 8-hydroxytetrahydroisoquinolines were prepared by the gold-catalyzed phenol synthesis The reaction was investigated with several sterically demanding groups in the 5-position of the furan starting material The influence of the reaction time and the catalyst on the yield was investigated

C–H bond activation through steric crowding of normally inert ligands in the sterically crowded gadolinium and yttrium (C5Me5)3M complexes

Abstract: Synthesis of the sterically crowded Tris(pentamethylcyclopentadienyl) lanthanide complexes, (C5Me5)3Ln, has demonstrated that organometallic complexes with unconventionally long metal ligand bond lengths can be isolated that provide options to develop new types of ligand reactivity based on steric crowding. Previously, the (C5Me5)3M complexes were known only with the larger lanthanides, La–Sm. The synthesis of even more crowded complexes of the smaller metals Gd and Y is reported here. These complexes allow an evaluation of the size/reactivity correlations previously limited to the larger metals and demonstrate a previously undescribed type of C5Me5-based reaction, namely C–H bond activation. (C5Me5)3Gd, was prepared from GdCl3 through (C5Me5)2GdCl2K(THF)2, (C5Me5)2Gd(C3H5), and [(C5Me5)2Gd][BPh4] and structurally characterized by x-ray crystallography. Although Gd3+ is redox-inactive, (C5Me5)3Gd functions as a reducing agent in reactions with 1,3,5,7-cyclooctatetraene (COT) and triphenylphosphine selenide to make (C5Me5)Gd(C8H8), [(C5Me5)2Gd]2Se2, and [(C5Me5)2Gd]2Se depending on the stoichiometry used. When the analogous synthetic method was attempted with yttrium in arene solvents, the previously characterized (C5Me5)2YR complexes (R=C6H5, CH2C6H5) were isolated instead, i.e., C–H bond activation of solvent occurred. To avoid this problem, (C5Me5)3Y was synthesized in high yield from [(C5Me5)2YH]2 and tetramethylfulvene in aliphatic solvents. Isolated (C5Me5)3Y was found to metalate benzene and toluene with concomitant formation of C5Me5H, a reaction contrary to the normal pKa values of these hydrocarbons. In this case, the normally inert (C5Me5)1− ligand engages in C–H bond activation due to the extreme steric crowding.

Pyridyl-supported pyrazolyl-N-heterocyclic carbene ligands and the catalytic activity of their palladium complexes in Suzuki-Miyaura reactions.

Abstract: Palladium complexes of two new types of unsymmetrical pyridyl-supported pyrazolyl-N-heterocyclic carbene ligands were synthesized and structurally characterized. A strategy to release the steric strain of the ligand was realized by the introduction of methylene linkers to the ligand molecule. All the palladium complexes exhibited good to excellent catalytic activity in Suzuki-Miyaura reactions of phenyl or p-tolylboronic acid with aryl halides including iodobenzene, aryl bromides, and activated aryl chlorides under mild conditions, revealing that the new ligands are promising for the construction of highly active transition-metal catalysts.

Assembly of Lipophilic Tetranuclear (Cu4 and Zn4) Molecular Metallophosphonates from 2,4,6-Triisopropylphenylphosponic Acid and Pyrazole Ligands

Abstract: A sterically hindered aryl phosphonic acid ArP(O)(OH)2 (2) (Ar = 2,4,6-isopropylphenyl) was synthesized and structurally characterized. ArP(O)(OH)2 forms an interesting hydrogen-bonded corrugated sheet-type supramolecular structure in the solid-state. A three-component reaction involving ArP(O)(OH)2, 3,5-dimethylpyrazole(DMPZH), and Cu(CH3COO)2·H2O produces the tetranuclear Cu(II) compound [Cu4(μ3-OH)2{ArPO2(OH)}2(CH3CO2)2(DMPZH)4][CH3COO]2·CH2Cl2 (3). A similar three-component reaction involving ArP(O)(OH)2, 3,5-dimethylpyrazole, and Zn(CH3COO)2·2H2O yields the tetranuclear Zn(II) compound [Zn4{ArPO3}2{ArPO2(OH)}2{DMPZH}4(DMPZ)2]·5MeOH (4). While 3 has been found to have an asymmetric cage structure where two dinuclear copper cores are bridged by bidentate [ArPO2(OH)]- ligands, 4 possesses an open-book tricyclic structure composed of three fused metallophosphonate rings. Magnetic studies on 3 revealed antiferromagnetic behavior.

Theoretical Calculation of Hydrogen-Bonding Strength for Drug Molecules

Abstract: Hydrogen bond is an important type of interaction between drug molecules and their receptors. We present here a computational method for accurately predicting the hydrogen-bonding strength for different acceptors with respect to a given donor or vice versa. The method is based on quantum chemistry DFT calculation of the interaction energy between hydrogen bond donors and acceptors. An excellent linear correlation is observed between the calculated hydrogen-bonding energies and the experimentally measured hydrogen-bonding constants log Kβ on a variety of known hydrogen bond acceptors and donors. These results not only indicate the predictive power of this method but also shed light on factors that determine the magnitude of experimentally measured hydrogen-bonding constants for different acceptors with respect to a given donor, suggesting a primarily enthalpic contribution from hydrogen-bonding energy. The method can be used for evaluating the effects of steric interference and inhibitor binding geometry on hydrogen-bonding strength in drug design.

Solvent free esterification reactions using Lewis acids in solid phase catalysis

Abstract: A clean, efficient and fast method for esterification reactions for sterically (biodiesels) or otherwise inactive (aromatic) precursors was developed, using catalysts supported in a solid phase under “solvent free” conditions, and whose reactions can be promoted by MW irradiation.