Studies in the pinacol rearrangement—V The rearrangements of cis- and trans-1,2-diphenyl-1,2-ditolylethylene oxides

The intermediate formation of the two epoxides was observed in the perchloric acid-catalysed rearrangement of meso-2,2′-dimethylbenzpinacol (P_meso) to 2-methylbenzoyl tolyl diphenyl methane (K) Two kinds of epoxides were separated with preparative TLC and the configuration (cis and trans) of these epoxides was inferred in the light of UV, NMR and IR spectra, dipole moment and kinetic data. The rearrangement rates of these epoxides were measured in HClO₄-anhydrous acetic acid. Linear correlations of logarithm of the rate constants (log k) with the Hammett acidity function of the medium (H₀) were obtained, which supports the A-1 mechanism, together with the large positive values of the entropy of activation. It is shown that the perchloric acid-catalysed rearrangement of meso-2,2′-dimethylbenzpinacol (P_meso) occurs via such many kinetically distinguishable routes as the following where C and T are cis- and trans-epoxide respectively. The mechanisms of the rearrangements were discussed from the data of kinetics.     

顺、反-1,2-环己二胺对有机-无机杂化锑碘异构体的影响

在浓氢碘酸水溶液中,顺式和反式-1,2-环己二胺(DAC)分别与三碘化锑反应得到2种有机-无机杂化异构体(cis-1,2-DACH_2)[SbI_5]·H_2O(1)和{(trans-1,2-DACH_2)[SbI_5]·H_2O}_n(2)。X射线单晶衍射表明化合物1中的无机成分是由2个[SbI_6]八面体通过共I-I边形成的二聚体[Sb_2I_(10)],而化合物2中的无机部分是[SbI_6]八面体通过共享顶点形成的一维锯齿链。此外,利用紫外-可见光谱、荧光光谱和密度泛函理论对化合物1和2进行了比较研究。     

The vibrational analysis and torsional study of trans-1,2-dimethylcyclopropane and cis-1,2-dimethylcyclopropane

The infrared spectra of cis-1,2-dimethylcyclopropane and trans-1,2-dimethylcyclopropane have been recorded between 4000 and 200 cm^?1 in the polycrystalline solid phase, and 4000 to 80 cm^?1 in the gas phase. The Raman spectra of these two compounds in the gaseous and liquid phases were also recorded between 3100 and 10 cm^?1. An assignment of the thirty-nine fundamental vibrations for both cis- and trans-1,2-dimethylcyclopropane is proposed, and comparisons are made with the vibrations of other similar molecules. Additionally, ten torsional transitions were observed in the far infrared and Raman spectra of cis-1,2-dimethylcyclopropane, and four transitions were observed in the spectra of the trans compound. From these spectral data, torsional barriers were determined. The effective barriers to methyl torsion are 2.92 kcal mol^?1 (12.20 kJ mol^?1) for cis-1,2-dimethylcyclopropane and 2.61 kcal mol^?1 (11.14 kJ mol^?1) for trans-1,2-dimethylcyclopronane.     

Trapping of unstable molecular conformations in argon matrices: Gauche- and trans-1,2-difluoroethane

Experiments are reported which describe the trapping of the unstable trans conformer of 1,2-difluoroethane in argon matrices. Assignments of the infrared spectra of the gauche and trans conformers are given. The latter is shown to have a 600 cal/mol higher free enthalpy.     

Thermal study of palladium complexes with trans-1,2-diaminecy clohexanetetraacetic acid

Thermal decomposition processes for cyclohexanediaminetetraacetic acid (CDTA-H₄) complexes of palladium, [Pd(CDTA-H₂)] and [Pd(CDTA-H₄)Cl₂]·2 HCl·2 H₂O have been studied using TG—DTA techniques. Infrared spectroscopy and X-ray diffraction have been also used for the characterization of intermediate and final products. In the decomposition of the dichloro complex, chloride ions are released simultaneously to a ring closure reaction in which CDTA becomes tetradentate. For both compounds, the final product in the decomposition is PdO, as confirmed by the X-ray difraction pattern of a sample heated at 600°C.     

Stereoselective alkynylation of trans-2,3-epoxy sulfides with double inversion of configuration by alkynylaluminums

A stereoselective alkynylation reaction of rans-1-phenylthio-2,3-epoxyalkanes with alkynylaluminums which proceeds via episulfonium ions, that is, with double inversion of configuration, has been developed.     

Vibrational spectra and structure of chlorine substituted cyclopropanes : Part II. cis-1,2-Dichlorocyclopropane and trans-1,2,3-trichlorocyclopropane

The infrared and Raman spectra of the newly synthesized title compounds have been recorded from 4000 to 50 cm⁻¹ in the pure liquid state. An assignment of all the fundamentals of the two molecules is proposed and compared to those of related molecules. Evidence is obtained for the way the symmetry of substitution determines frequency shifts of the ring modes.     

Photophysical behavior of trans-2-styrylquinoxaline conformers

The fluorescence and excitation spectra of trans-2-styrylquinoxaline changed markedly on varying the excitation and emission wavelengths, probably because of an equilibrium between conformers originating from the rotation of a quasi-single bond between the quinoxalyl group and the ethylenic carbon atom. The values of the fluorescence quantum yields depend on the solvent polarity at room temperature. However, the fluorescence of each conformer shows a different dependence on the solvent polarity and only one conformer is fluorescent in non-polar hydrocarbon solvents, while the composition of the two conformers does not change significantly on varying the solvent.     

Determination of compositions and isomer structures in mixtures of cis- and trans-1,2-dichloroethene (ClHC=CHCl) by gas-phase electron diffraction

Using gas-phase electron diffraction it has been demonstrated that a composition of known isomer mixtures can be determined with error limits of about 5%, all relevant structural parameters being refined simultaneously by the least-squares method. If, however, corresponding bond distances and valence angles have erroneously been assigned equal values in the two isomers, a large increase in the least-squares error limits from 5% to 12% is noticed. Apparently innocent assumptions about some of the parameters can lead to incorrect isomer composition and to too small error limits as estimated by the least-squares formulae.

From the reinvestigation of pure cis-1,2-dichloroethene the following bond distances (r_a) and valence angles () were determined: r(C---H) = 1.090(29) Å, r(C=C) = 1.345(6) Å, r(C---Cl) = 1.716(4) Å, C=C---Cl = 123.8(2)°, C=C---H = 119.4(26)°. Error limits are 2σ.     

Stereochemistry of the halolactonisation reaction The reaction of cis- and trans- stilbene-2-carboxylic acids with halogens

The stereochemistry of the reaction of cis- and trans-stilbene-2-carboxylic acids with chlorine and bromine, yielding the two diastereoisomeric 3-phenyl-4-halo-3:4-dihydroisocoumarins (III) and (IV) has been investigated. The reactions are entirely stereospecific and involve intramolecular attack by the carboxyl group on an intermediate carbonium or halonium ion. The lactones (III) and (IV) yield the two 3-(-hydroxybenzyl) phthalides (V) and (VI), with retention of configuration. Thionyl chloride transforms (V) and (VI) into the 3-(-chlorobenzyl) phthalides (IX) and (X), with inversion. Steric effects are mainly responsible for the alternative formation of halolactones or normal dihalogenated derivatives in the reactions between unsaturated acids and halogens.     

The use of trans-2,3-divinyl epoxides as precursors to carbonyl ylides

trans-2,3-Divinyl epoxides have been found to be good substrates for the generation of carbonyl ylides in CCl₄ and 145 °C. These ylides, to a limited extent, undergo isomerization to cis-2,3-divinyl epoxides, leading to the isolation of 4,5-dihydrooxepins. Of greater potential usefulness is the finding that these ylides can be efficiently trapped in an intermolecular sense by a dipolarophile, leading to dihydrofurans.     

The application of Pd-complexes of trans-2,5-dialkylpyrrolidinylbenzyldiphenylphosphines to enantioselective allylic alkylation

The preparation of cationic palladium (η³-C₃H₅) complexes 6 and 7, possessing an enantiopure trans-2,5-dialkylpyrrolidinyl unit, is described. These complexes were applied to the enantioselective alkylation of the test substrate, 1,3-diphenylpropenyl acetate 8, with dimethyl malonate in good to high conversions with enantioselectivities of up to 90% ee for the 2,5-diethylpyrrolidinyl substituted complex 7. Attempts to optimise reaction conditions included variation of solvent and temperature and the source of the malonate nucleophile.     

Studies in stereochemistry—I Stereospecific routes to trans- anti- and cis- syn-Δ-dodecahydrophenanthrenes

Reduction of trans-1-oxo-7-methoxy-1,2,3,4,9,10,11,12-octahydrophenanthrene (XI) by lithium tri-t-butoxyaluminohydride gave trans-1β-hydroxy-7-methoxy-1,2,3,4,9,10,11,12-octahydrophenanthrene (XII) which on lithium—liquid ammonia reduction gave trans-anti-1β-hydroxy-7-oxo-Δ⁸⁽¹⁴⁾-dodecahydrophenanthrene (XIII). Reduction of cis-1-oxo-7-methoxy-1,2,3,4,9,10,11,12-octahydrophenanthrene (XV) by sodium borohydride gave cis-1-hydroxy-7-methoxy-1,2,3,4,9,10,11,12-octahydrophenanthrene (XVI) which on lithium—liquid ammonia reduction gave cis-syn-1-hydroxy-7-oxo-Δ⁸⁽¹⁴⁾-dodecahydrophenanthrene (XVII).     

Proton magnetic resonance studies of cyclic compounds—III cis- and trans-2,6-dimethylmorpholines

Commercial 2,6-dimethylmorpholine has been separated into cis- and trans-isomers by vapour phase chromatography. The PMR spectra of the bases, analysed as ABXK₃ systems (J_AK = J_BK = 0), show that the most abundant isomer has the cis-configuration, consisting probably of a single conformation in which both methyl groups are equatorial. The trans-isomer is probably a mixture of rapidly inverting and energetically equivalent conformations, since the vicinal coupling constants deduced from the PMR spectrum are averages of those expected for the two conformations.     

cis- and trans-N-(Benzylsulfinyl)hexahydrobenzoxazolidin-2-ones as novel chiral sulfinyl transfer reagents

The synthesis of N-benzylsulfinyl derivatives 5a–d from both pairs of enantiomeric hexahydrobenzoxazolidin-2-ones 4a–d is reported. The use of 5a–d as effective chiral sulfinylating reagents in the preparation of enantiopure sulfoxides (e.e.>98%) is also reported.     

The absolute configuration of streptonigrin

A theoretical study of the molecular circular dichroism (CD) of the antitumor antibiotic natural product streptonigrin is described, aiming at the secure assignment of its absolute configuration by comparison of the CD spectra predicted for M and P, with the experimental one. The stereostructure of streptonigrin was previously investigated by two other groups, yet leading to two different attributions. Although streptonigrin possesses two biaryl axes, only the ‘southern’ one is configurationally stable and thus responsible for the chiroptical properties, since the ‘northern’ AB-ring system of the molecule is kept in plane with ring C by hydrogen bonding. All computational methods applied within this work to simulate the CD spectrum—semiempirical approaches and time-dependent density functional theory (TDDFT)—consistently attribute the M-configuration to streptonigrin.