Rotational isomerism: XVII—solvent effects on the NMR spectra and rotamer energies of some tetrahaloethanes

The NMR spectra of 1,1-dichloro-2,2-difluoroethane ( 1 ), 1,1-dibromo-2,2-difluoroethane ( 2 ), meso and dl 1,2-dichloro-1,2-difluoroethane ( 3 ) and 1,1,2,2-tetrachloroethane ( 4 ) have been analysed in a number of solvents. The ¹⁹F spectrum of 3 in L-bornyl acetate at 56·4 and 94·1 MHz allows an unambiguous identification of the meso and dl isomers. The spectra of the d and l isomers consist of two AA′XX′ spectra with a small chemical shift difference between the d and l forms, whilst that of the meso form is an apparent AA′XX′ spectrum at 56·4 MHz but an ABXX′ spectrum at 94·1 MHz, the ¹⁹F nuclei in this isomer being anisochronous in this solvent. The observed solvent and temperature dependence of the couplings of 1, 2 , meso 3 and 4 when combined with the calculated solvation energies, allow the determination of the rotamer energies and couplings in these molecules. The rotamer energy differences (E_g – E_t) in the liquid and vapour states are 0·6 and ?0·2 kcal/mol ( 1 ); 0·4 and ?0·5 kcal/mol ( 2 ); 0·9 and 0·2 kcal/mol meso ( 3 ) and ?0·1 and ?0·8 kcal/mol ( 4 ). The ³J(HH), ³J(HF) and ³J(FF) couplings for the distinct rotamers are considered together with those of similarly constituted molecules. The general agreement demonstrates that the solvation theory may be applied to multisubstituted ethanes without any basic modifications. The trans oriented HH couplings show a linear substituent electronegativity dependence, which differs appreciably from that obtained for disubstituted ethanes, however. The gauche couplings show the influence of dihedral angle variations as well as substituent electronegativity. The rotamer ³J(FF) couplings in meso 3 are ?38·2 Hz (J_t) and ?17·4 Hz (J_g).     

Dimers from α-alkoxybenzyl radicals. An NMR study

meso and dl Dimers (ArCHOR)₂ where R is Me, Et, ⁱPr, ^tBu, cyclohexyl and 1-adamantyl may readily be differentiated by their NMR spectra; the benzylic protons of the meso isomer always absorb at a slightly higher field than those of the dl isomer in each of the solvents used. Differences in chemical shift are discussed in terms of preferences in conformer distribution. The formation of equal amounts of both dimers from the corresponding radical ArCHOR shows that steric and polar factors are not important in influencing the dimerization. Magnetic non-equivalence due to the presence of asymmetric centres was found in some of the compounds discussed above.     

The 19F NMR spectrum of cis-1,2-dichlorohexafluorocyclobutane

The ¹⁹F spectrum of cis-1,2-dichlorohexafluorocyclobutane has been fully analysed on the basis of an [ABX]₂ spin system, with initial treatment at an [AMX]₂ level. The chemical shift and coupling constants are presented, and the latter are discussed. Some comments are made on the spectrum of the trans isomer.     

Fate of the Oxygen Atoms in the Diol‐Dehydratase‐Catalyzed Dehydration of meso‐Butane‐2,3‐diol

¹⁸O‐Substituted propane‐1,2‐diols and meso‐butane‐1,2‐diols were synthesized and fed to growing cells of Lactobacillus brevis. Propan‐1‐ol and butan‐2‐ol, prepared from such diols through diol‐dehydratase‐catalyzed dehydration followed by intracellular reduction, were analyzed for their ¹⁸O‐content. For each propane‐1,2‐diol enantiomer, partial retention or complete loss of the isotope appeared to be related to the mode of substrate binding. Specific retention of the O‐atom linked to the (R)‐configured C‐atom of meso‐butane‐1,2‐diol indicates that the diol dehydratase handles this substrate like (R)‐propane‐1,2‐diol.     

Carbon-13 NMR and conformational analysis of meso- and dl-α,α′-disubstituted succinic acids

meso-and dl-Diastereomers of a number of α,α′-disubstituted succinic acids have been shown to give different ¹³C NMR chemical shifts. The results can be satisfactorily explained on the basis of their conformational analyses. A discussion of the observed chemical shifts is presented, and the preferred conformation for each of several compounds is predicted on the basis of these chemical shifts.     

High-resolution proton magnetic resonance and infrared spectra of poly(vinyl formal) and its model compounds

High-resolution proton magnetic resonance and infrared spectra of poly(vinyl formal) were studied in comparison with those of the model formals obtained from stereoisomers of pentane-2,4-diol and heptane-2,4,6-triol in order to learn spectral changes due to differences of the steric structures of the polymer. In the NMR spectrum of transformal obtained from dl diol or dl,dl (syndiotactic) triol, all proton signals were well interpreted by assuming a rapid chair-chair inversion of the formal ring. On the other hand, no such inversion was observed spectroscopically in cis-formal obtained from the meso diol or meso,meso (isotactic) triol, and the cis-formal ring was supposed to take a diequatorial form preferentially. Consequently, dioxymethylene protons gave a single peak (equivalent) in trans-formal and an AB quartet (nonequivalent) in cis-formal. In the spectra of poly(vinyl formal), the dioxymethylene signal was an overlap of the singlet and quartet in dimethylsulfoxide solution. Observations of the spectra of various poly(vinyl formals) obtained from poly(vinyl alcohols) of different tacticities and study of temperature dependence of the signal have shown that the singlet and quartet are attributed to trans- and cis- formals, respectively, in the polymer spectrum also. In the infrared spectra of poly(vinyl formals), the 800 and 785 cm^-1 bands were found to be related to cis- and trans-formal rings respectively. A linear relationship was confirmed between D₇₈₅/D₈₀₀ and trans/cis ratios determined from the peak intensities of the dioxymethylene proton signals.     

Photo-oxidation of the 2,4-dichloropentanes and PVC

The photo-oxidation of PVC and its model compounds, the dl- and meso-2,4-dichloropentanes, were studied by high-resolution NMR spectroscopy. The structure, ? CHCl? CH₂? CO? CH₂? was found to be present in PVC after photo-oxidation. Two peaks, which corresponded to the CHCl and CH₂ groups next to the carbonyl group, were observed in carbon disulfide–acetone-d₅ mixture, at 6.15 and 7.25τ, respectively. From the NMR spectra, the ratio of the CH₂ to CHCl protons in PVC and the model compounds were found to increase linearly with the time of photo-oxidation after induction periods. The initial rates of the model compounds were comparable with that of PVC; i.e., 1.6 × 10^?2 hr^?1, at 30°C. Since similar gaseous products were also detected by mass spectrometry in the initial stage, their photo-oxidation probably assumed the same rate-determining step. According to the results that the dl-2,4-dichloropentane was photo-oxidized 1.5 times faster than the meso-compound, it would be likely that the syndiotactic sequences in PVC might be more easily photo-oxidized than the isotactic sequences.     

Chromium-catalyzed pinacol coupling of benzaldehyde in water

The pinacol coupling of benzaldehyde (0.25 M or 1.25 M) in water was catalyzed by 5-25 mol % CrCl₂ in the presence of Zn-dust or Al-dust at 20 °C or 60 °C. In all cases at most 50% of the pinacol coupling product, 1,2-diphenyl-1,2-ethanediol, was obtained with the major product, benzyl alcohol, being formed by a competitive 2e⁻ reduction of the carbonyl. The dl- to meso-diastereoselectivity of the pinacol products ranged from 0.6:1 to nearly 1:1.     

Asymmetric oxidation of 1,2-diols using N-bromosuccinimide in the presence of chiral copper catalyst

Asymmetric oxidation of 1,2-diols using N-bromosuccinimide (NBS) in the presence of copper(II) triflate and (R,R)-Ph-BOX has been exploited. This oxidation was applicable to asymmetric desymmetrization of meso-hydrobenzoin and kinetic resolution of dl-hydrobenzoin and racemic-cycloalkane-cis-1,2-diols to afford optically active α-ketoalcohols with good to high enantiomeric excess.     

13C n.m.r. spectra of rotameric dimethyl 1-isopropyldibenzo[b,e]bicyclo[2.2.2]octatriene-7,8-dicarboxylate: Non-equivalency of carbons along the rotational axis

¹³C n.m.r. spectra of dimethyl 1-isopropyldibenzo[b,e]bicyclo[2.2.2]octatriene-7,8-dicarboxylate disclose the existence of both dl and meso isomers, whereas ¹H n.m.r. spectra were interpreted by the sole existence of the former. Characteristic chemical shifts for the rotamers were found, among others, for carbons along the rotational axis. The non-equivalence is most probably attributed to the difference in geometry about the carbons in question. An unusual fact was also that the more compressed methyl carbon of the isopropyl group gives a signal at higher field.     

Syntheses,crystal structures and characterizations of tetraazamacrocyclic complexes meso -[Ni(1,7-CT)]I2 and dl -[Ni(1,7-CT)][Fe(CN)5NO] · H2O

A centrosymmetric meso tetraazamacrocyclic nickel(II) complex meso-[Ni(1,7-CT)]I₂ (1) and a novel racemic ion-pair double complex dl-[Ni(1,7-CT)][Fe(CN)₅NO]?·?H₂O (2) (1,7-CT?=?5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,7-diene) have been synthesized and characterized by elemental analyses, single crystal X-ray diffraction, ¹H NMR, IR, UV-vis spectra and TG-DTA analyses. Results reveal that the structure of 1 consists of the overlapping meso-[Ni(1,7-CT)]²⁺ cation with the perfect square-planar NiN₄ chromophore and outer iodides. Compound 2 contains four kinds of structurally distinct ion-pairs a, a′, b and b′ that are made up of distorted square dl-[Ni(1,7-CT)]²⁺ cations and distorted octahedral [Fe(CN)₅NO]^2? anions, which are divided into two groups with an enantiotropic relationship between them in each group. They are linked by van der Waals forces and hydrogen bonds involving H₂O and arranged alternatively in adjacent layers, forming an orderly three-dimensional network structure.     

The meso Helix: Symmetry and Symmetry‐Breaking in Dynamic Oligourea Foldamers with Reversible Hydrogen‐Bond Polarity

Oligoureas (up to n=6) of meso cyclohexane‐1,2‐diamine were synthesized by chain extension with an enzymatically desymmetrized monomer 2 . Despite being achiral, the meso oligomers adopt chiral canonical 2.5‐helical conformations, the equally populated enantiomeric screw‐sense conformers of which are in slow exchange on the NMR timescale, with a barrier to screw‐sense inversion of about 70 kJ mol^?1. Screw‐sense inversion in these helical foldamers is coupled with cyclohexane ring‐flipping, and results in a reversal of the directionality of the hydrogen bonding in the helix. The termini of the meso oligomers are enantiotopic, and desymmetrized analogues of the oligoureas with differentially and enantioselectively protected termini display moderate screw‐sense preferences. A screw‐sense preference may furthermore be induced in the achiral, meso oligoureas by formation of a 1:1 hydrogen‐bonded complex with the carboxylate anion of Boc‐d ‐proline. The meso oligoureas are the first examples of hydrogen‐bonded foldamers with reversible hydrogen‐bond directionality.     

Polyaryleneimides of meso- and dl- 1,2,3,4-butanetetracarboxylic acid dianhydrides

High molecular weight polyarylene amic acids have been prepared by reacting aromatic diamines with equimolar amounts of either meso-or dl-1,2,3,4-butanetetracarboxylic acid dianhydride (TCBA) in organic polar solvents. Heat converted these soluble precursors to insoluble polyimides having good high-temperature physical properties.     

Novel cage-like cavity accommodating a pair of guest molecules in the structure ofcatena-[dl-1,2-diaminopropanecadmium(II) tetra-μ-cyanocadmate(II)]-(1,2-dichloroethane)(1/1)

The title clathrate Cd(dl-pn)Cd(CN)₄·CH₂CICH₂CI (pn=dl-1,2-diaminopropane=dl-propy-lenediamine) crystallizes in the monoclinic space groupP2 ,a = 8.278(2),b = 14.904(2),c = 14.477(1) Å, = 91.24(1)°,V = 1785.7(1) ų,Z = 4,R = 0.068 for 4993 independent reflections. A novel three-dimensional host is built of the alternate array of the pn-chelated octahedral Cd and the tetrahedral Cd(CN)₄ linked at every N atom to the former Cd to provide a cage-like cavity for a pair of 1,2-dichloroethane guest molecules.     

Perfluoralkyljod-verbindungen: Die fluorierung von 1,2-dijod-tetrafluoräthan

Perfluoroalkyliodine compounds: the fluorination of 1,2-diiodo-tetrafluoroethane. F₂ICF₂CF₂IF₂ is formed as a white solid from the reaction of 1,2-diiodo-tetrafluoroethane with elemental fluorine in CCl₃F at -78°C. The preparation, the properties and the ¹⁹F-nmr spectrum are described.     

Ultrasonic effects on electroorganic processes: Part 16. Stereoselectivity control of hydrodimers formed in cathodic reduction of acetophenones

Stereoselectivity (meso/dl ratio) of the hydrodimers (HDs) formed in cathodic reduction of acetophenones was greatly affected by ultrasonic irradiation with power higher than an ultrasonic cavitation threshold in weakly acidic solutions, while such an ultrasonic effect on stereocontrol was not observed in either strongly acidic or alkaline solutions.     

1H NMR spectra of ethane‐1,2‐diol and other vicinal diols in benzene: GIAO/DFT shift calculations

The proton NMR spectra of several 1,2‐diols in benzene have been analysed so as to associate each magnetically nonequivalent proton with its chemical shift. The shifts and coupling constants of the OH and methylene protons of ethane‐1,2‐diol have been determined in a wide range of solvents. The conformer distribution and the proton NMR shifts of these 1,2‐diols in benzene have been computed on the basis of density functional theory. The solvent is included using the integral–equation–formalism polarizable continuum model implemented in Gaussian 09. Relative Gibbs energies for all stable conformers are calculated at the Perdew, Burke and Enzerhof (PBE)0/6‐311 + G(d,p) level, and shifts are calculated using the gauge‐including atomic orbital method with the PBE0/6‐311 + G(d,p) geometry but using the cc‐pVTZ basis set. Previous calculations on ethane‐1,2‐diol and propane‐1,2‐diol have been corrected and extended. New calculations on tert‐butylethane‐1,2‐diol, phenylethane‐1,2‐diol, butane‐2,3‐diols (dl and meso) and cyclohexane‐1,2‐diols (cis and trans) are presented. Overall, the computed NMR shifts are in good agreement with experimental values for the OH protons but remain systematically high for CH protons. Some results based on the Gaussian 03 solvation model are included for comparison. Copyright © 2012 John Wiley & Sons, Ltd.     

Reduction and coupling reaction of carbonyl compounds by KI/Fe(OH)3 in aqueous media

ABSTRACT

A small amount of ferric hydroxide was found to catalyze KI-activated coupling of aromatic aldehydes and acetophenone to give the corresponding pinacols in high yields. The ratio of meso to dl isomer was approximately 1:1. Most aliphatic aldehydes and ketones were unreactive under the same conditions.     

Caged phosphorane with P-C bond based on chloral and 4,5-dimethyl-2-(2-oxo-1,2-diphenylethoxy)-1,3,2-dioxaphospholane

The key methods of caged phosphoranes synthesis are analyzed. Reaction of 4,5-dimethyl-2-(2-oxo-1,2-diphenylethoxy)-1,3,2-dioxaphospholane (prepared from the meso-form of 2,3-butanediol) with chloral has yielded the caged phosphorane containing a phosphorus-carbon bond: 1,1-(1,2-dimethylethylenedioxy)-3,4-diphenyl-6-trichloromethyl-2,5,7,1-trioxaphosphabicyclo[2.2.1^1,4]heptane; spatial structure of the product has been elucidated with X-ray diffraction analysis.

     

Eindeutiger Nachweis der Konfiguration der diastereomerendl-3-Amino-2,3-diphenylpropansäuren

Zusammenfassung Zur Bestätigung der relativen Konfigurationen der diastereomerendl-3-Amino-2,3-diphenylpropansäuren hat man die Azide der N-Phthalyl-Derivate der beiden Verbindungen dem Abbau nachCurtius mit anschließender Hydrazinolyse unterworfen. Erwartungsgemäß ergab hierbei dieerythro-Aminosäure dasmeso-Stilbendiamin, ihrthreo-Isomeres hinggen dasdl-Stilbendiamin.