Search results for "Base"
showing 10 items of 8362 documents
Stereoselective Synthesis of γ-Fluorinated α-Amino Acids Using 2-Hydroxy-3-pinanone as an Auxiliary
2000
(+)-(S)-2-Amino-4-fluorobutanoic acid (5a) (> 96% ee), its α-methylated derivative (+)-(S)-5b (85% ee), and (−)-(S)-2-amino-4-fluoro-4-pentenoic acid (10) (81% ee) were synthesized by diastereoselective alkylation in the presence of LDA at low temperatures. Alkylation of (+)-(R,R,R)-2-hydroxy-3-pinanone based imines of glycine tert-butyl ester 1a or alanine isopropyl ester 1b with 1-bromo-2-fluoroethane (2) or 3-bromo-2-fluoropropene (7), respectively, followed by stepwise deprotection was used. The selectivity of the alkylation increased by lithium/magnesium exchange or for 1b also by addition of DMPU. Differences in the reactivity of enolate alkylations of enantiomerically pure or racemic…
Molecular “Floppyness” and the Lewis Acidity of Silanes: A Density Functional Theory Study
2001
A comprehensive set of Lewis acid-base adducts of silanes was investigated by means of the density functional theory geometry optimization [B3LYP/6−31G(d)], and thermochemical calculations, [B3LYP/6−311+G(2d,p)//B3LYP/6−31G(d)]. Complex formation was found to weaken Si−Cl and Si−Br bonds more than Si−F or Si−H bonds. Comparable distances between Si and a Lewis base L (L = NH3, OH2, F−) are shorter in hexa- than in pentacoordinated complexes. The molecular structures of the pentacoordinated Si complexes allowed for a mapping of an SN2 reaction pathway by correlating the lengths of the Si−X and Si−L bonds. Complex formation was found to be exothermic for most of the coordination compounds, an…
Mesoporous Materials Incorporating Metal Triflates
2016
Metal triflates attracted a large interest as catalysts in the past three decades especially due to the Lewis acid properties exhibited by these compounds and the variety of the functional group transformations. This chapter proposes an overview presenting the catalytic performances of triflates both as molecular compounds and as triflates dispersed in different solid matrices. In the first part the performances of molecular triflates in organic reactions carried out in the presence of water or water as solvent considering both water-sensitive and -insensitive metals are discussed. Then, the importance of materials incorporating triflates is demonstrated for catalysts obtained via immobiliz…
B-DNA Structure and Stability as Function of Nucleic Acid Composition. Dispersion-Corrected DFT Study of Dinucleoside-Monophosphate Single and Double…
2013
actions of the sugar-phosphate skeleton with water; (6) hydrophobic interactions of the DNA cylindrical core, made up by the hydrogen-bonded and stacked nitrogen bases, with the water solvent. Recently, there has been increasing effort in developing and applying quantum chemical methods able to reproduce the structure of native B-DNA and to correctly describe the energy involved in the intrastrand and interstrand noncovalent interactions between the nucleotide monomers. This topic has been approached by both wave function methods and density functional theory. [2] Water solvent and sodium counterions also play an important role in the formation and relative stabilization of the double-helic…
Receptors for Nucleotides
2012
Recognition and activation of nucleotides by polyammonium receptors constitute an important target in supramolecular chemistry since the very beginning of this field. Nucleotides have three components: (i) the polyphosphate chain, (ii) the sugar moiety, and (iii) the nucleobase, which permit their multipoint binding through attractions between opposite charges, hydrogen bonding, π-stacking, CH–π interactions, and so on. In this chapter, different receptors for nucleotides, most but not all of them consisting polyamines, are examined, focusing on their molecular structure that enables different binding modes to be operated. Also, a number of examples of nucleotide binding through metal compl…
Mannich Reactions with Amino Alcohols
2000
The condensation of resorcarenes 1 with various amino alcohols and an excess of formaldehyde was studied. The tetrabenzoxazines 2a−e were found as the only products in the reaction with 6-aminohexan-1-ol, 4-aminobutan-1-ol, and 2-aminoethanol, while 3-aminopropan-1-ol forms the tetraoxazine 3 as the main product. In the case of aminoethanols substituted at the 2-position with alkyl groups, the tetraoxazolidines 4 are the preferred reaction products, while 1-methyl aminoethanol (1-amino-propan-2-ol) yields predominantly the tetrabenzoxazine 2f. The structures of all these compounds have been confirmed by NMR spectroscopy and additionally by single-crystal X-ray analysis in the case of 2a and…
Effect of ionic liquid organizing ability and amine structure on the rate and mechanism of base induced elimination of 1,1,1-tribromo-2,2-bis(phenyl-…
2006
Abstract The kinetics of the elimination reaction of 1,1,1-tribromo-2,2-bis(phenyl-substituted)ethanes into the corresponding 1,1-dibromo-2,2-bis(phenyl-substituted)ethenes induced by amines were studied in three room temperature ionic liquids ([BMIM][BF 4 ], [BMIM][PF 6 ], [BdMIM][BF 4 ]). In order to have information about reagent–ionic liquid interactions, the reaction was carried out over the temperature range (293.1–313.1 K). To study the effect of the amine on the rate and occurrence of the elimination reaction, several primary, secondary and tertiary amines with different structure (cyclic and acyclic), basicity and steric requirements were used. The data collected show that the reac…
ChemInform Abstract: In situ DRIFT Spectroscopic Investigation on the Chemical Evolution of Zinc Phosphate Acid-Base Cement.
2010
Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy has been used to follow the chemical evolution of zinc phosphate acid−base cement (ZPC), which has been prepared from unmodified, as well as from aluminum- and zinc-modified orthophosphoric acid. For the first time, amorphous dizinc cyclotetraphosphate octahydrate, Zn2P4O12·8H2O, although hydrolytically instable, has been observed as precursor phase prior to crystallization of α-hopeite, α-Zn3(PO4)2·4H2O, in both systems. Within minutes from onset of mixing the infrared spectrum of unmodified ZPC is dominated by the characteristic vibrations of α-hopeite, due to matrix crystallization. Setting of modified ZPC is strongly af…
Aluminum complexes of sterically hindered tetradentate Schiff bases: Synthesis, structure, and reactivity toward ɛ-caprolactone
1997
Abstract The sterically hindered Schiff bases tbmSalenH2 [tbmSalen = N,N′-1,2-ethylenebis(3-tert-butyl-5-methylsalicylideneimine)] and tbmSalcenH2 [tbmSalcen = N,N′-trans-1,2-cyclohexanediyl-bis(3-tert-butyl-5-methylsalicylideneimine)] afforded a series of aluminum complexes of the general formulae [Al(tbmSalen)X] and [Al(tbmSalcen)X] (X = Cl, Me, Et). The molecular structure of [Al(tbmSalcen)Cl] was determined by single-crystal X-ray structural analysis which revealed a five-coordinate aluminum center with a distorted square pyramidal geometry. The alkyl complexes were found to oligomerize ɛ-caprolactone.
SN1 Reactions in Supercritical Carbon Dioxide in the Presence of Alcohols: the Role of Preferential Solvation
2016
Ethanol (3b) inhibits SN1 reactions of alkyl halides 1 in supercritical carbon dioxide (scCO2) and gives no ethers as products. The unexpected behaviour of alcohols 3 in the reaction of alkyl halides 1 with 1,3-dimethoxybenzene (2) in scCO2 under different conditions is rationalised in terms of Bronsted and Lewis acid–base equilibria of reagents, intermediates, additives and products in a singular solvent characterised by: (i) the strong quadrupole and Lewis acid character of carbon dioxide, which hinders SN2 paths by strongly solvating basic solutes; (ii) the weak Lewis base character of carbon dioxide, which prevents it from behaving as a proton sink; (iii) the compressible nature of scCO…