Search results for " Bases"
showing 10 items of 272 documents
Über die wirksamkeit von lewis-basen als initiatoren der anionischen polymerisation von formaldehyd. 27. Mitt. über polyoxymethylene
1966
Die anionische Polymerisation des monomeren Formaldehydes mit LEWIS-Basen wurde bei −78°C in Ather, Aceton und Toluol untersucht. Die Basizitat der LEWIS-Basen ist ein wichtiger Faktor for ihre Initiatoraktivitat, ist jedoch nicht allein entscheidend; es spielen auch strukturelle und steren rische Faktoren eine Rolle. Primare Amine sind wahrscheinlich deshalb schlechte Initiatoren, weil sie mit Formaldehyd Methylol- und Dimethylolamine bilden. Verzweigungen aliphatischer Substituenten wirken sich um so starker retardierend aus, je naher sich die Verzweigungsstelle am Stickstoffatom befindet. Die Polymerisationsgeschwindigkeit steigt bei den meisten Aminen mit zunehmender Polaritat des Losun…
Why Do Five-Membered Heterocyclic Compounds Sometimes Not Participate in Polar Diels–Alder Reactions?
2013
The reactions of bicyclic enone (BCE, 1) with cyclopentadiene (Cp, 2) and the five-membered heterocyclic compounds (FHCs) furan 3 and N-methyl pyrrole 4 for the construction of polycyclic heterocyclic compounds have been studied at the B3LYP/6-31G* level. No reaction takes place in the absence of Lewis acid (LA) catalysts as a consequence of the high activation energy associated with these reactions. Electrophilic activation of BCE 1 by formation of a complex with the BF3 LA, 1-BF3, and solvent effects favor the reactions. However, a different reactivity is manifested by Cp 2 and FHCs 3 and 4. Thus, while the reaction of 1-BF3 with Cp 2 yields the expected exo [4 + 2] cycloadduct, the react…
ChemInform Abstract: Low-Loading Asymmetric Organocatalysis
2012
Asymmetric organocatalysis is now recognized as the third pillar of asymmetric synthesis. Recent years have witnessed increasing interest towards the use of highly active and stereoselective organocatalysts. This critical review documents the advances in the development of chiral organocatalysts which are systematically used in ≤3 mol% loading in all the sub-areas of the field, namely aminocatalysis, Bronsted acids and bases, Lewis acids and bases, hydrogen bond-mediated catalysis, phase transfer and N-heterocyclic carbene catalyses (194 references).
Kronenether mit einem Lewis-sauren Zentrum, eine neue Klasse heterotoper Wirtmoleküle
1991
Aminosäureester als chirale Hilfsgruppen in Lewis-Säure-katalysierten Umsetzungen elektronenreicher Siloxydiene mit Iminen
1990
ChemInform Abstract: Reversal of Asymmetric Induction in Stereoselective Strecker Synthesis on Galactosyl Amine as the Chiral Matrix.
1989
Abstract The reversal of the direction of asymmetric induction in Lewis acid catalyzed Strecker synthesis using the 2,3,4,6-tetra-O-pivaloyl-β-D-galactopyranosyl amine 1 is described. In isopropanol or tetrahydrofuran 1 had given ( R )-diastereomers of the corresponding α-amino nitriles preferably. However, in chloroform in the presence of heterogeneous zinc chloride the same auxiliary alternatively leads to an excess of the ( S )-diastereomers.
Nickel as a Lewis Base in a T‐Shaped Nickel(0) Germylene Complex Incorporating a Flexible Bis(NHC) Ligand
2018
Flexible, chelating bis(NHC) ligand 2, able to accommodate both cis- and trans-coordination modes, was used to synthesize (2)Ni(η 2 -cod), 3. In reaction with GeCl2, this produced (2)NiGeCl2, 4, featuring a T-shaped Ni(0) and a pyramidal Ge center. Complex 4 could also be prepared from [(2)GeCl]Cl, 5, and Ni(cod)2, in a reaction that formally involved Ni-Ge transmetalation, followed by coordination of the extruded GeCl2 moiety to Ni. A computational analysis showed that 4 possesses considerable multiconfigurational character and the Ni→Ge bond is formed through σ-donation from the Ni 4s, 4p, and 3d orbitals to Ge. (NHC)2Ni(cod) complexes 9 and 10, as well as (NHC)2GeCl2 derivative 11, incor…
The Aluminyl Anion : A New Generation of Aluminium Nucleophile
2020
Trivalent aluminium compounds are well known for their reactivity as Lewis acids/electrophiles, a feature that is exploited in many pharmaceutical, industrial and laboratory-based reactions. Recently, a series of isolable aluminium(I) anions ('aluminyls') have been reported, which offer an alternative to this textbook description: these reagents behave as aluminium nucleophiles. This minireview covers the synthesis, structure and reactivity of aluminyl species reported to date, together with their associated metal complexes. The frontier orbitals of each of these species have been investigated using a common methodology to allow for a like-for-like comparison of their electronic structure a…
Crystal structure ofS-octyl (E)-3-(4-methoxybenzylidene)dithiocarbazate
2015
As already observed in similar molecules, the dithiocarbazate group in the title compound, C17H26N2OS2, adopts anEEconfiguration with respect to the C=N bond of the benzylidene moiety. In the crystal, molecules are connected into inversion dimers by pairs of N—H...S hydrogen bonds. The dimers are linked by weak π–π interactions, with centroid-to-centroid distances of 3.723 (11) Å, forming chains parallel to [110].
Crystal structure of (E)-N-(3,4-dimethoxybenzylidene)morpholin-4-amine
2014
In the title compound, C13H18N2O3, the benzene ring makes a dihedral angle of 17.19 (11)° with the least-squares plane formed by the four C atoms of the morpholine ring, which adopts a chair conformation. In the crystal, C—H...N hydrogen bonds link the molecules into supramolecular chains running along a 21screw axis parallel to theb-axis direction. Weak C—H...π interactions are also observed.