PubMed Health⌕ Search

Biomedical subjects

J H Reibenspies

Publications and source records attributed to J H Reibenspies.

12 recordsLinked to original sources
PubMed2001

Subtle bite-angle influences on N(2)S(2)Ni complexes.

A new N(2)S(2)Ni complex based on the 1,4-diazacycloheptane (dach) framework allows the study of the effects of ring size, in fused diamines, on the structural and chemical properties of nickel(II) dithiolate and dithioether complexes. Compared to its 1,5-diazacyclooctane (daco) derivatives, the dithiolate complex (bmedach)Ni and the S-templated, macrocyclic dithioether complex (propyl-bmedach)NiBr(2) show decreased cavity sizes with narrower angleN-Ni-N angles (by ca. 6 degrees ) and wider angleS-Ni-S angles (also by ca. 6 degrees ). The electrochemical properties of the dithiolate complexes based on dach and daco are nearly identical, while the (propyl-bmedach)NiBr(2) complex shows a 140 mV destabilization of the Ni(I) oxidation state relative to its daco analogue. Molecular structures for the (bmedach)Ni and (propyl-bmedach)NiBr(2) complexes and their respective electrochemical and spectroscopic properties are reported.

Journal Article↗
PubMed2001

Coordination sphere flexibility of active-site models for Fe-only hydrogenase: studies in intra- and intermolecular diatomic ligand exchange.

A series of dinuclear complexes, (mu-SRS)Fe(2)(CO)(6) (R = -CH(2)CH(2)-, -CH(2)CH(2)CH(2)-, -CH(2)-C(6)H(4)-CH(2)-; edt, pdt, and o-xyldt, respectively) has been examined for specific characteristics that might relate to structural similarity with the active site of Fe-only hydrogenases. Variable-temperature proton NMR studies display the fluxionality of the iron-dithiocyclohexane unit in (mu-pdt)Fe(2)(CO)(6) while in the (mu-o-xyldt)Fe(2)(CO)(6) compound, the bridge is fixed. Temperature-dependent (13)C NMR spectral studies establish intramolecular CO site exchange localized on discrete Fe(CO)(3) units in all complexes, which is influenced by steric effects of the mu-SRS unit. Kinetic studies of intermolecular CO/CN(-) ligand-exchange reactions establish associative or I(a) mechanisms in sequential steps to form the dicyano dianion, (mu-SRS)[Fe(CO)(2)(CN)](2)(=) with 100% selectivity. Theoretical calculations (DFT) of transition states in the intramolecular site-exchange processes lead to a rationale for the interesting cooperativity in the CN(-)/CO intermolecular ligand-exchange process. The hinge motion of the three light atom S-to-S bridge is related to a possible heterolytic H(2) activation/production process in the enzyme.

Binding Sites↗
PubMed2001

New, optically active phosphine oxazoline (JM-Phos) ligands: syntheses and applications in allylation reactions.

Three different syntheses of the phosphine oxazoline systems 1 are presented. Two of these approaches are divergent routes designed to involve an advanced intermediate that can be transformed into several different end-products. The third is a shorter route specifically designed to facilitate preparations of these systems on a larger scale using minimal functional group protection. Overall, eight different phosphine oxazolines were prepared. These were screened in several palladium-mediated allylation reactions. They proved to be most useful for asymmetric alkylation of 3-acetoxy-1,3-diphenylpropene and less suitable/effective for the more challenging substrates (a pentenyl derivative and a cyclohexenyl system). X-ray crystallographic analysis of the complex [(eta 3-PhCHCHCHPh)Pd(1a)][PF6] led to the conclusion that the origins of asymmetric induction in these systems might be indirectly attributed to interaction of the oxazoline-phenyl substituent with the palladium and with an allyl-phenyl substituent. Finally, data is presented for allylation of a silylenolate of an N-acyl oxazolidinone; excellent enantioselectivities and yields were obtained.

Journal Article↗
PubMed2000

On the behavior of alpha,beta-unsaturated thioaldehydes and thioketones in the Diels-Alder reaction.

Alpha,beta-unsaturated thioaldehydes and thioketones, R1CH=CH-C(=S)R2, are prepared in situ by the reaction between the corresponding carbonyl compounds and bis(dimethylaluminum) sulfide. These compounds undergo [4 + 2] self-dimerization reactions in which one molecule serves as the heterodiene component and the other as the dienophile to afford different types of dimeric products depending on the R1 and R2: 1,2-dithiin and 1,3-dithiin (R1 = R2 = H), 1,2-dithiin (R1 = Ph, R2 = H, CH3), or dihydrothiopyran (R1 = R2 = Ph). These differences in selectivity are explained on the basis of the relative energies evaluated by molecular orbital (MO) calculations at the DFT (density functional theory) level. The calculations show that in the dimerization reaction of thioacrolein (I), the head-to-tail (S-C-S bonded) dimers are kinetically more stable by about 5 kcal/mol but slightly thermodynamically unstable by about 2 kcal/mol than the head-to-head (S-S bonded) dimers. The calculations on thiocinnamaldehyde (IV) indicate that the dimerization reactions of phenyl-substituted alpha,beta-unsaturated thioaldehydes and thioketones are almost equally controlled by thermodynamic and kinetic factors. These unsaturated thiocarbonyl compounds also function as heterodienes (C=C-C=S) in the cycloaddition reaction with norbornadiene and as dienophiles (C=S) in the reaction with cyclopentadiene.

Aldehydes↗
PubMed2000

Crystal structures of heptakis(2,6-di-O-tert-butyldimethylsilyl)cyclomaltoheptaose, heptakis(2-O-methyl-3,6-di-O-tert-butyldimethylsilyl)cyclomaltoheptaose and heptakis(2-O-methyl)cyclomaltoheptaose.

Crystal structures of heptakis(2,6-di-O-tert-butyldimethylsilyl)cyclomaltoheptaose, heptakis(2-O-methyl-3,6-di-O-tert-butyldimethylsilyl)cyclomaltohep taose and heptakis(2-O-methyl)cyclomaltoheptaose were determined from X-ray diffraction patterns obtained for single crystals of the title compounds grown from ethyl acetate and ethanol, respectively, as solvent. The crystal structures prove conclusively that quantitative migration of the tert-butyldimethylsilyl group from the 2-O- to the 3-O-position [D. Icheln, B. Gehrcke, Y. Piprek, P. Mischnick, W.A. Konig, M.A. Dessoy, A.F. Morel, Carbohydr. Res., 280 (1996) 237-250] was achieved during methylation of heptakis(2,6-di-O-tert-butyldimethylsilyl)cyclomaltoheptaose by iodomethane-sodium hydride.

Crystallography↗
PubMed2000

Interactions between divalent metal ions and an octacoordinate macrocyclic ligand.

A dinucleating hexaazadiphenol macrocyclic ligand, 15,31-dimethyl-3,11,19,27,33,35-hexaazapentacyclo[27.3.1.1.(5,9)1.(13,17)1. (21,25)]hexatriaconta-5,7,9(33),13,15,17(34),21,23,25(35),29,31,1(36)- dodecaene-34,36-diol (H2L), forms a number of protonated, neutral, and/or hydroxo mononuclear, homodinuclear, and heterodinuclear complexes with the divalent metal ions Cu2+, Cd2+, Mn2+, and Zn2+, controlled by the stoichiometry of the metal ion and ligand as well as the pH values of the solution. Their stability constants and species distribution as a function of p[H] are determined. The pH potentiometric studies show that the dinuclear complexes are formed via the mononuclear chelates in which two kinds of coordination patterns are observed. One is that the metal ions are complexed by exactly half of coordination sites of the dinucleating macrocycle (N3O-), and the other is that the metal ions occupy salen-like sites of the macrocycle (N3O(2)2-). In the 2:1 systems (2:1 molar ratio of metal ion to ligand), the mononuclear species predominate in acidic solutions while the dinuclear species predominate in basic solutions, except for the case of copper. The protonated mononuclear complex [H2LZn](NO3)(2).5H2O forms triclinic crystals, of space group P1, with a = 10.7797(12) A, b = 10.9047(12) A, c = 17.0176(15) A, alpha = 106.857(9) degrees, beta = 95.822(8) degrees, gamma = 100.191(9) degrees, and Z = 2; the neutral heterodinuclear complex [LZnCdCl2].6H2O forms monoclinic crystals, of space group C2/c, with a = 16.234(5) A, b = 15.976(9) A, c = 29.829(11) A, alpha = 90 degrees, beta = 90.28(2) degrees, gamma = 90 degrees, and Z = 8.

Journal Article↗
PubMed2000

Synthesis and characterization of racemic mixture and meso isomers of bis(trans-2-aminocyclohexyl)aminepentaacetic acid and the stabilities of their Gd(III) complexes.

The synthesis and characterization of two multidentate ligands, the racemic mixture and meso isomers of bis-(trans-2-aminocyclohexyl)aminepentaacetic acid, are described. Equilibrium constants for their Gd(III) complexes were determined by direct potentiometry. The formation constants (KML = [ML]/[M][L]) of Gd(III)-Cycyracemic in 0.10 M KCl at 25.0 degrees C is 10(20.71; that for the meso isomer is 10(20.42). The crystal structure of (1S,2S,1'S,2'S)-bis(trans-2-aminocyclohexyl)amine-N,N,N',N",N"-pentaacetic acid, penta-tert-butyl ester (C42H75N3O10), is reported. This compound crystallizes in the triclinic system with space group P1 with cell parameters a = 10.805(2) A, b = 11.382(2) A, c = 20.999(4) A, alpha = 91.41(3) degrees, beta = 98.23(3) degrees, gamma = 113.88(3) degrees, V = 2327.8(8) A3, Z = 2, Dx = 1.116 g mL-1. The results are compared to the crystal structures of the gadolinium complexes and the predictions derived from molecular mechanics.

Algorithms↗
PubMed1993

New sesquiterpene alpha-methylene lactones from the Egyptian plant Jasonia candicans.

Extracts of the plant Jasonia candicans possess antimicrobial activity. ET2O/MeOH extracts were subjected to liquid chromatography, and several sesquiterpenes were isolated and identified including the known compounds confertin [1], 4,11(13)-eudesmadien-12-oic acid [2], and 11-eudesmen-4-ol [3]. Two new diol alpha-methylene lactone antimicrobial agents were identified from nmr and mass spectral data and X-ray crystallography as (4 alpha, 5 alpha, 8 beta, 10 beta)-4,10-dihydroxy-1,11(13)-guaidien-12,8-olide [4] and (4 alpha, 5 alpha, 8 beta, 10 alpha)-4,10-dihydroxyl-1,11 (13)-guaidien-12,8-olide[5], which differ in stereochemistry at the C-10 tertiary alcohol center.

Lactones↗
PubMed1992

Isolation and identification of xochitloldione and isoxochitlolone from Cnidosculus urens.

Major components of MeOH extracts from the plant roots of Cnidosculus urens purified by cc and tlc and crystallization were lupeol acetate and the previously unreported compounds isoxochitlolone [1] and xochitloldione [2], which were identified through mass, ir, nmr, and uv spectroscopy and X-ray crystallography. In preliminary testing, isoxochitlolone has been found to be active against Escherichia coli and Staphylococcus aureus.

Anti-Bacterial Agents↗
PubMedDate not supplied

Kukulkanins A and B, new chalcones from Mimosa tenuefolia.

Two new methoxychalcones, kukulkanins A (2',4'-dihydroxy-3',4-dimethoxychalcone) and B (2',4',4-trihydroxy-3'-methoxychalcone), were isolated by flash chromatography of extracts of small branches of Mimosa tenuefolia. The compounds were identified using ir, nmr, uv, and mass spectroscopy, and their structures were confirmed by X-ray crystallography.

Chalcone↗