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Electron-withdrawing Effect (electron-withdrawing + effect)
Selected AbstractsN -Acetylation as a Means to Activate Polyfluoroarylamines for Selective ortho -Hydrodefluorination by Zinc in Aqueous Ammonia: A Concise Route to Polyfluorobenzo Azaheterocycles,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 2 2007Sergey S. Laev Abstract N -Acetylation of polyfluoroarylamines is proposed as a meansto remove the amino group blocking effect of their hydrodefluorination by zinc in aqueous ammonia. With pentafluoroacetanilide, the Zn ion specific effect has been demonstrated to be responsible for ortho hydrodefluorination. This regiochemistry is accompanied by the removal of a fluorine atom from the para position, which occurs predominantly in the initial phase of the process in the absence of deliberately added zinc salt. The CuCl2 additive has been found to accelerate the reaction and to propel it to double defluorination. Quantum chemical calculations suggest a diminished electron affinity of pentafluoroaniline, which is responsible for its inertness in relation to the hydrodefluorination reaction. The pentafluoroaniline radical anion, which essentially has a nonplanar structure, is prone to easy fragmentation to give an aminotetrafluorophenyl radical. For pentafluoroacetanilide, CVA experiments and quantum chemical calculations predict that the pentafluorophenyl moiety serves as the electron receptor and that the acetamido group is twisted out of coplanarity with the benzene ring; thus, together with the electron-withdrawing effect of the acetyl group, the amino group blocking effect is suppressed. On this ground, the selective ortho hydrodefluorination of polyfluoroacetanilides is developed as an important protocol for the expeditious and general synthesis of polyfluorobenzo azaheterocycles via readily accessible polyfluoroarylamines from base polyfluoroarenes. Its applicability has been illustrated by preparing quinolines that possess a polyfluorinated benzene moiety by the Skraup synthesis utilizing crude polyfluoroacetanilide hydrodefluorination products as starting materials. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Competition between Non-Classical Single and Double Epimerizations in Cyclitol ChemistryEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 9 2004Ralf Miethchen Abstract Two competitive regio- and stereoselective epimerization reactions were investigated in four cyclitols characterized by four contiguous OH groups with a cis - trans - trans sequence and by varied substituents (OMe, OBz, F, H) adjacent to this tetrol unit. The starting materials were synthesized from L -quebrachitol (compounds 5,7) and myo -inositol (compound 8). Their acetalization with the chloral/DCC reagent system gave cyclic acetals with one epimerized chiral ring atom and also with two epimerized chiral centres. The single epimerization takes place exclusively at the middle C-atom of the cis - trans triol unit in the tetrol sequence (products 15, 17, 19/20 and 24,27), whereas the double epimerization occurs at both of the "centrally located" C-atoms in the cis - trans - trans tetrol unit (products 16, 18, 21 and 28). The product ratios of singly to doubly inverted compounds change as follows: the lower the electron-withdrawing effect of the substituents adjacent to the tetrol unit, the higher the percentage of the corresponding doubly inverted product. However, the singly inverted products remain the major products in all cases. X-ray analyses are given for the starting material 1-fluoro-2- O -(methyl)cyclohexane-2,3,4,5,6-pentol (5) and for the products 1- O -cyclohexylcarbamoyl-2,3- O -(2,2,2-trichloroethylidene)5- O -(methyl)cyclohexane-1,2,3,4,5-pentol (17), 3- O -acetyl-1- O -benzoyl-6- O -cyclohexylcarbamoyl-2- O -methyl-4,5- O -(2,2,2-trichloroethylidene)- muco -inositol (22) and 2,3-di- O -ethylidene)-(+/-)- chiro -inositol (24). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Nine Times Fluoride can be Good for your Syntheses.ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 17 2009Not just Cheaper: Nonafluorobutanesulfonates as Intermediates for Transition Metal-Catalyzed Reactions Abstract How much fluoride is good for a strong electron-withdrawing effect? In this review we summarize recent results on the use of perfluoroalkanesulfonates, in particular of the cost effective nonafluorobutanesulfonates (nonaflates), in transition metal-catalyzed reactions and a few other typical transformations. During the last decade many advantages over the commonly used triflates have been discovered. The generation of alkenyl and (het)aryl nonaflates and their applications in metal-catalyzed processes such as Heck, Suzuki, Sonogashira, Stille, and Negishi couplings or amination reactions are described. Although far from a systematic investigation, all the presented results clearly demonstrate the many advantages of nonaflates and of similar higher fluorinated sulfonates in laboratory and industrial scale organic synthesis. [source] Comparative study of electronic structure and optical properties of a series of Pt(II) complexes containing different electron-donating and -withdrawing groups: a DFT studyJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 3 2010Xiao-Na Li Abstract We report a quantum-chemistry study of electronic structures and spectral properties of a series of Pt(II) complexes containing different substituents (CH3 (1), OCH3 (2), NO2 (3), CF3 (4), and COOH (5)). 1 and 2 have been previously synthesized in experiment, while 3,5 are artificial complexes that we suggest can be used to investigate the electron-withdrawing effect on charge injection, transport, absorption, and phosphorescence properties. The results reveal that the stronger electron-donating and -withdrawing groups show stronger absorption intensity, while the phosphorescence efficiency is generally higher for complexes containing electron-donating substituents. 1 and 2 are easier for hole injection, while 3,5 are easier for electron injection. The enhanced electron injection abilities of 3,5 will confine more excitons in the light-emitting layer (EML) and may not result in lower electroluminescence (EL) efficiency than 1 and 2. These results suggest that the three artificial complexes may be new emitters in organic light-emitting diodes (OLEDs). Copyright © 2009 John Wiley & Sons, Ltd. [source] Experimental and theoretical NMR study of selected oxocarboxylic acid oximesMAGNETIC RESONANCE IN CHEMISTRY, Issue 1 2004Kamilla Malek Abstract 1H and 13C NMR spectra of the oxocarboxylic acid oximes 2-hydroxyiminopropanoic acid (1), 2-(4-methylthiazol-2-yl)-2-(hydroxyimino)acetic acid (2) and 2-cyano-2-(hydroxyimino)acetic acid (3) were measured in DMSO- d6, D2O and acetone- d6 solutions. The data indicate the presence of hydrogen bonding in 1 and 2 and a strong electron-withdrawing effect due to the cyano group in 3. The effect of intra- and intermolecular hydrogen bonding on the hydrogen and carbon chemical shifts in these molecules was studied theoretically. Total energy calculations of the stability of various hydrogen-bonded species, in addition to equilibrium parameters and chemical shifts, were calculated using ab initio methods (RHF, MP2) and density functional theory (B3LYP), implemented in the Gaussian 98 software package. The gauge-including atomic orbital (GIAO) method was used to predict magnetic shielding constants. Chemical shift calculations for the most stable species agree fairly well with the observed data, especially for the hydroxyl protons. Substituents adjacent to the ,-carbon show some influence of the oximic and carboxyl groups on the 13C chemical shifts, as expected for groups with different polar and anisotropic character. Copyright © 2003 John Wiley & Sons, Ltd. [source] Unusual Cycloadducts from the Dipolar Cycloaddition of Allenyl Perfluoroalkyl Sulfones to NitronesCHINESE JOURNAL OF CHEMISTRY, Issue 5 2007Xiao-Jin Wang Abstract The dipolar cycloaddition reaction of allenyl perfluoroalkyl sulfones (1) to nitrones (2) was described. Unlike nonfluorine-containing allenyl sulfones, 1 reacted readily with 2 in ether at room temperature and unusual zwitterionic cycloadducts (3) were obtained in good yields due to the strong electron-withdrawing effect of perfluoroalkyl groups. The structure of 3 was characterized by spectral analyses and X-ray crystallography. [source] | ||||||||||