Search results for "medicinal"

Linked Amido—Cyclopentadienyl Complexes of Group 3 and 4 Metals: The First “Post-Metallocenes”

1999

Linked amido—cyclopentadienyl ligands were introduced by Bercaw and Shapiro in the late eighties with the catalysts of the type Sc(η5:η1-C5Me4SiMe2N t Bu)X (X = H, alkyl) [1]. These electronically more unsaturated and sterically more accessible analogs of ansa-scandocene complexes were found to be capable of catalyzing the living oligomerization of the α-olefins propylene, 1-butene, and 1-pentene. In these complexes the steric constraint [2] of Brintzinger-type ansa-metallocenes is alleviated by the replacement of one cyclopentadienyl ligand by an amido ligand NR’ (Scheme 1). In order to explore sterically demanding derivatives of this dianionic ligand (C5R4ZNR’)2-, iron and titanium comple…

Mononuclear tungsten(VI) complexes with methylenebis(6-alkylphenol)s

2003

Abstract Reaction of monomeric tungsten(VI) complex [W(eg)3] (1) (eg=1,2-ethanediolate dianion) with either one or 2 equiv. of 2,2′-methylenebis(6-alkylphenol) [alkyl=Me (H2L1), iPr (H2L2), tBu (H2L3)] provides heteroleptic complexes [W(eg)2(Ln)] (2) and [W(eg)(Ln)2] (3), respectively. Sterically hindered ligand precursor H2L3, which bear bulky tert-butyl groups in ortho-positions, can form only a complex corresponding to formula 2.

Gas-phase and solution basicities of some alkyl 2,6-dialkylphenyl ketones: A comparative analysis

2000

Abstract The gas-phase basicity of a number of alkyl 2,6-dialkylphenyl ketones (2,6-R2C6H3COR′) has been found to be almost insensitive to structural variations, as a result of a compensation of steric and electronic effects associated with the bulkiness and to the polarizability, respectively, of R and/or R′. On the contrary, the basicity in concentrated sulfuric acid undergoes, along the same series of compounds, a variation of nearly 8 pK units, as a consequence of steric inhibition of solvation of the protonated carbonyl as the main effect played by R and/or R′. The results in the condensed phase agree very nicely with recent findings relevant to some 4-substituted 2,6-dimethylacetophen…

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…

Weak Intermolecular Anion–π Interactions in Pentafluorobenzyl-Substituted Ammonium Betaines

2012

A series of ammonium–carboxylate and ammonium–sulfonate betaines was synthesized and studied by single-crystal X-ray diffraction analysis to investigate the weak intermolecular interactions as well as the intramolecular interactions in the solid state. None of the expected intramolecular anion–π interactions could be observed, probably because of the steric demands and the reduced nucleophilicity of the anionic part of the betaines. Nevertheless, a weak intermolecular anion–π interaction between the anionic part of the betaine and the pentafluorophenyl unit is present in the structure of 5a.

ChemInform Abstract: Gas-Phase and Solution Basicities of Some Alkyl 2,6-Dialkylphenyl Ketones: A Comparative Analysis.

2000

Abstract The gas-phase basicity of a number of alkyl 2,6-dialkylphenyl ketones (2,6-R2C6H3COR′) has been found to be almost insensitive to structural variations, as a result of a compensation of steric and electronic effects associated with the bulkiness and to the polarizability, respectively, of R and/or R′. On the contrary, the basicity in concentrated sulfuric acid undergoes, along the same series of compounds, a variation of nearly 8 pK units, as a consequence of steric inhibition of solvation of the protonated carbonyl as the main effect played by R and/or R′. The results in the condensed phase agree very nicely with recent findings relevant to some 4-substituted 2,6-dimethylacetophen…

ChemInform Abstract: Ortho-Substituent Effects in N-Arylacetamides. NMR and Molecular Mechanics Investigation.

2010

1H, 13C, 15N and 17O NMR spectra of N-phenylacetamide (acetanilide) and 21 ortho-substituted acetanilides were measured and assigned. The observed NMR parameters are related to the Hammett substituent parameters and conformational characteristics of the acetamido moiety estimated by molecular mechanics calculations. Significant relationships were found for the 13C NMR chemical shifts of C-5 (para to substituent) and the direct spin–spin coupling constant, 1J(C, H), of C-3 (ortho to substituent) with Hammett substituent parameters. For 15N NMR chemical shifts of the amido nitrogen, no general correlation with the Hammett substituent parameters was found. The interactions between functionalit…

Durch sterische effekte stabilisierte ß-ketocarbonsäuren

1989

Abstract Increasing steric hindrance in β-keto carboxylic acids leads to an increasing kinetic stability towards decarboxylation, till systems are reached which are completely stable at room temperature. Simultaneously the tautomeric equilibrium is changed in favour of the (Z)-enol, and finally in favour of the (E)-configurated enol.

A sterically crowded triazene: 1,3-bis(3,5,3′′,5′′-tetramethyl-1,1′:3′;1′′-terphenyl-2′-yl)triazene

2007

PHOSPHORORGANISCHE VERBINDUNGEN 991VERSUCHE ZUR AUFKLÄRUNG DER O-SELEKTIVITÄT VON VERBINDUNGEN MIT DER P(O)F-GRUPPE

1982

Abstract The mechanism of the reaction of phosphoryl fluoride ( P(O)F) with alcohols in the presence of an amine is fundamentally different from the reaction of phosphoryl chlorides ( P(O)Cl) with primary or secondary amines. The following observations strongly support this proposal: 1. 1H-NMR-, 31P-NMR- and 19F-NMR-spectroscopic investigations show that methyl-phenyl-phosphinicacid-fluoride and n-butylamine form a thermolabile adduct, which yields the methyl-phenyl-phosphinicacid-amide only very slowly. 2. The rate of the reaction of methyl-phenyl-phosphinicacid-fluoride with ethanol is independent of the basicity of the amine but very sensitive to steric factors. Imidazole (which is only …