Search results for "steric"
showing 10 items of 476 documents
Die oligomerisierung endständiger hydroxyacetylene mit rhodiumkomplexkatalysatoren
1982
The substrate and ligand influence on oligomerisation of terminal hydroxyacetylenes catalysed by rhodium complexes has been studied. In this reaction α- and β-hydroxyacetylenes were compared with each other and with an unsubstituted terminal alkyne. The concentration dependant ligand control with Ph3As and Ph3P has been compared and different ligand association behaviour is found, because only with Ph3As as ligand at intermediate concentrations cyclic trimers are selectively formed. Some brief experiments with different p- and o-substituted triphenylarsines show electronic and marked steric influences on the oligomerisation of 3-methylbutyne-1-ol-3.
Bis- and Trisamides Derived From 1′-Aminoferrocene-1-carboxylic Acid and α-Amino Acids: Synthesis and Conformational Analysis
2009
Ferrocene derivatives with one or two achiral and chiral arms based on α-amino acids (Gly, l-Ala, l-Val) attached to the cyclopentadienyl rings were prepared by solution-phase peptide synthesis from N-acetyl- and N-Boc-protected 1′-aminoferrocene-1-carboxylic acids (Boc = tert-butoxycarbonyl). The conformational preference in the solid state of selected examples was elucidated by X-ray crystallography. The chiroptical properties of chiral bis- and trisamides were investigated by circular dichroism (CD) spectroscopy in solution. The conformational preference was studied by NMR and IR spectroscopy, as well as by molecular modeling (DFT). For the bisamides, a conformational library is observed…
Theoretical study of 1,3-dipolar cycloaddition reactions with inverse electron demand - A DFT study of the lewis acid catalyst and solvent effects in…
2000
The molecular mechanism for the inverse electron demand 1,3-dipolar cycloaddition of nitrones with vinyl ethers has been characterized using density functional theory methods with the B3LYP functional and the 6−31G* basis set. Relative rates, regioselectivity, endo/exo stereoselectivity, Lewis acid catalyst and solvent effects are analyzed and discussed. Four reactive channels associated with the formation of two pairs of diasteromeric regioisomers have been characterized. Analysis of the geometries of the corresponding transition structures shows that the reaction in the gas phase takes place by an asynchronous concerted mechanism. These 1,3-dipolar cycloadditions present a large ortho reg…
Asymmetric synthesis of bicyclic amino acid derivatives by Aza-Diels-Alder reactions in aqueous solution
1991
Derivatives 7–10 and 13–16 of methyl and ethyl 2-azabicyclo[2.2.1]heptane-3-carboxylates are synthesized by Aza-Diels-Alder reactions of chiral iminium ions, formed in situ from glyoxylic acid and chiral amines, with cyclopentadiene. Whereas the heterodienophiles derived from phenylglycinol and esters of sterically more demanding amino acids fail to undergo asymmetric cycloadditions, with alanine methyl ester and (R)-1-phenylethylamine hydrochoride the cycloadducts are formed in yields of 15 and 52%, respectively, reaching de values of up to 90:10 for the exo isomers.
Understanding the selectivity in the formation of δ-lactams vs. β-lactams in the Staudinger reactions of chloro-cyan-ketene with unsaturated imines. …
2014
The reactions of chloro-cyan-ketene with two phenyl substituted unsaturated imines yielding β- or δ-lactams have been investigated using DFT methods at the MPWB1K/6-311G(d,p) level in diethyl ether. The reactions are initialised by the nucleophilic attack of the unsaturated imines on the ketene with formation of zwitterionic intermediates. The subsequent C–C single bond formation at the imine carbon or at the β-conjugated position enables the formation of β- or δ-lactams. Analysis of the energies involved in the two competitive channels explains the selectivity experimentally observed; in the absence of any steric hindrance, formation of δ-lactams is favoured over the formation of β-lactams…
A theoretical study of carbon-carbon bond formation by a Michael-type addition.
2012
A theoretical study of the Michael-type addition of 1,3-dicarbonyl compounds to α,β-unsaturated carbonyl compounds has been performed in the gas phase by means of the AM1 semiempirical method and by density functional theory (DFT) calculations within the B3LYP and M06-2X hybrid functionals. A molecular model has been selected to mimic the role of a base, which is traditionally used as a catalyst in Michael reactions, an acetate moiety to modulate its basicity, and point charges to imitate the stabilization of the negative charge developed in the substrate during the reaction when taking place in enzymatic environments. Results of the study of six different reactions obtained at the three di…
Lanthanide Discrimination with Hydroxyl-Decorated Flexible Metal–Organic Frameworks
2018
We report two new highly crystalline metal-organic frameworks (MOFs), derived from the natural amino acids serine (1) and threonine (2), featuring hexagonal channels densely decorated with hydroxyl groups belonging to the amino acid residues. Both 1 and 2 are capable of discriminating, via solid-phase extraction, a mixture of selected chloride salts of lanthanides on the basis of their size, chemical affinity, and/or the flexibility of the network. In addition, this discrimination follows a completely different trend for 1 and 2 because of the different locations of the hydroxyl groups in each compound, which is evocative of steric complementarity between the substrate and receptor. Last bu…
Syntheses and Crystal Structures of two Non-isostructural Copper(I) Chloride and Bromide π-Complexes with (2-Allylthio)benzimidazole of 1:1 Compositi…
2002
By alternating-current electrochemical synthesis crystals of C10H10N2S · CuCl (I) and C10H10N2S · CuBr (II) π-complexes have been obtained and structurally investigated. The symmetry of I is orthorhombic, space group Pbcn, a = 16.799(2) A, b = 7.302(2) A, c = 18.214(2) A, V = 2234.1(9) A3, Z = 8 for C10H10N2S · CuCl composition, R = 0.0635, Rw = 0.0683 for 970 reflections. Crystals of II are monoclinic, space group P21/n, a = 7.4439(5) A, b = 17.6952(3) A, c = 17.0420(7) A, s = 94.662(2)°, V = 2237.4(2) A<3, Z = 4 for (Cu · C10H10N2S)2Br2 composition, R = 0.0463 for 4123 reflections. (2-Allylthio)benzimidazole moiety acts in both structures discussed as a bidentate ligand, chelating the cop…
Comparison of linear and hyperbranched polyether lipids for liposome shielding by 18F-radiolabeling and positron emission tomography
2018
Multifunctional and highly biocompatible polyether structures play a key role in shielding liposomes from degradation in the bloodstream, providing also multiple functional groups for further attachment of targeting moieties. In this work hyperbranched polyglycerol (hbPG) bearing lipids with long alkyl chain anchor are evaluated with respect to steric stabilization of liposomes. The branched polyether lipids possess a hydrophobic bis(hexadecyl)glycerol membrane anchor for the liposomal membrane. hbPG was chosen as a multifunctional alternative to PEG, enabling the eventual linkage of multiple targeting vectors. Different hbPG lipids (Mn = 2900 and 5200 g mol-1) were examined. A linear bis(h…
Spectral and structural studies on Ni(II) dithiocarbamates: Nickel sulfide nanoparticles from a dithiocarbamate precursor
2015
Abstract Three new planar Ni(II) dithiocarbamate complexes; [Ni(4-dpmpzdtc)2] (1), [Ni(4-dpmpzdtc)(PPh3)(NCS)] (2) and [Ni(bupcbzdtc)(PPh3)(NCS)] (3) (where, 4-dpmpzdtc = 4-(diphenylmethyl)piperazinecarbodithioato anion, bupcbzdtc = N-butyl-N-(4-chlorobenzyl))dithiocarbamato anion and PPh3 = triphenylphosphine) with “NiS4” and “NiS2PN” chromophore units were synthesized and characterized by single crystal X-ray structural analysis as well as UV–Vis, IR and NMR (1H, 13C and 31P) spectroscopy. For 2, 1H–1H COSY spectrum was also recorded. Single crystal X-ray structural analysis of 1–3, reveals a slightly distorted square planar geometry in all three complexes wherein the steric and electroni…