Unusual inversion phenomenon of β-cyclodextrin dimers in water

A conformational analysis of three triazole-containing bridged bis-β- cyclodextrins (CD) has been carried out to evaluate their recognition ability. NMR spectroscopy and ITC measurements clearly demonstrate that one of the CD glucopyranose units undergoes a 360° rotation in water so that the spacer linking the two CDs is deeply included into one of the CD cavities. The amplitude of this inversion phenomenon depends on the nature of the spacer and results in a limited accessibility to the CD cavities in line with previous catalytic results.     

Systematic construction of a monotetrahydrofuran-ring library in annonaceous acetogenins by asymmetric alkynylation and stereodivergent tetrahydrofuran-ring formation

All eight diastereoisomers of the monotetrahydrofuran-ring cores of annonaceous acetogenins have been synthesized through utilization of asymmetric alkynylation and stereodivergent one-pot tetrahydrofuran-ring formation. In all cases, the asymmetric alkynylation proceeded with very high diastereoselectivity to give eight kinds of optically pure tetrahydrofuran core from a common alpha-oxyaldehyde. We also describe a comparison of the (1)H NMR, (13)C NMR, and CD spectral data of the eight isomers and give full details of the tetrahydrofuran-ring construction including a model study of asymmetric alkynylation.     

Molecular hydrogen tweezers: structure and mechanisms by neutron diffraction, NMR, and deuterium labeling studies in solid and solution

The mechanism of reversible hydrogen activation by ansa-aminoboranes, 1-N-TMPH-CH(2)-2-[HB(C(6)F(5))(2)]C(6)H(4) (NHHB), was studied by neutron diffraction and thermogravimetric mass-spectroscopic experiments in the solid state as well as with NMR and FT-IR spectroscopy in solution. The structure of the ansa-ammonium borate NHHB was determined by neutron scattering, revealing a short N-H···H-B dihydrogen bond of 1.67 ?. Moreover, this intramolecular H-H distance was determined in solution to be also 1.6-1.8 ? by (1)H NMR spectroscopic T(1) relaxation and 1D NOE measurements. The X-ray B-H and N-H distances deviated from the neutron and the calculated values. The dynamic nature of the molecular tweezers in solution was additionally studied by multinuclear and variable-temperature NMR spectroscopy. We synthesized stable, individual isotopic isomers NDDB, NHDB, and NDHB. NMR measurements revealed a primary isotope effect in the chemical shift difference (p)Δ(1)H(D) = δ(NH) - δ(ND) (0.56 ppm), and hence supported dihydrogen bonding. The NMR studies gave strong evidence that the structure of NHHB in solution is similar to that in the solid state. This is corroborated by IR studies providing clear evidence for the dynamic nature of the intramolecular dihydrogen bonding at room temperature. Interestingly, no kinetic isotope effect was detected for the activation of deuterium hydride by the ansa-aminoborane NB. Theoretical calculations attribute this to an "early transition state". Moreover, 2D NOESY NMR measurements support fast intermolecular proton exchange in aprotic CD(2)Cl(2) and C(6)D(6).     

Hydrogen-bonding interactions in cinchonidine-2-methyl-2-hexenoic acid complexes: a combined spectroscopic and theoretical study

Molecular interactions between cinchonidine (CD) and 2-methyl-2-hexenoic acid (MHA) have been studied by means of NMR, ATR-IR MES, DFT, and ab initio molecular dynamics. These interactions are of particular interest due to their pivotal role in the chiral induction occurring in the heterogeneous catalytic asymmetric hydrogenation of alpha,beta-unsaturated acids. The population density of the Open(3) conformer of CD, the most populated one at room temperature in apolar solvents, considerably increased to a maximum by addition of MHA to CD in toluene. The CD-MHA complex showed prominent symmetric and asymmetric carboxylate stretching vibrations in the regions of 1350-1410 and 1520-1580 cm(-1), respectively. DFT calculations revealed that these vibrational frequencies are expected to significantly shift depending on the chemical surrounding of MHA, that is, the hydrogen bond network. Earlier postulated 1:1 binding between CD and MHA was considered unlikely; instead, a dynamic equilibrium involving the MHA monomer and dimer, the 1:3 and possibly 1:2 CD-MHA complexes, were rationalized. Stable CD-MHA structures suggested by DFT calculations are the "1:3, halfN, cyclic" and the "1:3, halfN, cyclic tilted" complexes, where three MHA molecules are connected in wire by hydrogen bonding, two having direct interaction with CD. The confinement of CD's torsional motions in the complexes, leading to a slightly distorted Open(3) conformer via specific hydrogen-bonding interactions, was clearly reproduced by ab initio molecular dynamics, and the stable and flexible nature of the interaction was verified. Theoretical IR spectra of the complexes reproduced the characteristic vibrational frequencies of the complexes observed experimentally, supporting the stability of the 1:3 and implying the possibility of even higher molecular weight CD-MHA complexes.     

BPOB的绝对不对称合成机理及固体CD光谱

以苯乙酮与间苯二甲酸甲酯为原料经由Claisen缩合制备了1,3-二(3-苯基-3-氧代丙烯醇)苯(BPOB). 用元素分析、MS、1H NMR、UV吸收光谱、X射线单晶衍射和固体CD光谱等对其进行了表征. 晶体结构数据显示, BPOB属于Sohncke空间群P212121, UV和1H NMR谱表明, BPOB在溶液中主要以烯醇式存在, 固体CD光谱和重结晶实验证明, BPOB手性晶体的形成是结晶诱导的绝对不对称合成.     

Fine tuning of sulfoalkylated cyclodextrin structures to improve their mass-transfer properties in an aqueous biphasic hydroformylation reaction

The structure of sulfoalkylated cyclodextrins (CDs) have been varied and optimised to improve their performances as mass-transfer promoters in an aqueous biphasic hydroformylation reaction. Their surface tensions have been measured using the Wilhelmy plate technique and compared. Their behaviour towards two hydrosoluble derivatives of triphenylphosphine has been evaluated by ³¹P{¹H} and ¹H NMR measurements and their catalytic activity has been assessed in a rhodium-catalyzed hydroformylation reaction of 1-decene. The best result was obtained using a β-CD sulfobutylated on the primary face and methylated on the secondary face. Indeed, this CD increased the reaction rate by a factor of 250 without inducing selectivity decrease. The accessibility to the secondary face of the CD appears to be determining in the catalytic process as it governs the approach between the CD-included substrate and the water-soluble catalyst. The impact of the nature of the CD substituents on the chemo- and regioselectivity of the reaction is also discussed.     

Structure of brefeldin A

On the basis of X-ray crystal structure analysis, CD. measurements, and asymmetric synthesis the absolute configuration of brefeldin A has been determined.     

NMR investigation on the conformational properties of N-[2-pyridyl-N-oxide]-amino derivatives

The proton NMR spectra of N-[2-pyridyl-N-oxide]-derivatives of primary and secondary ethylamines, containing a substituent R on the C atom bearing the amino function, have been completely analysed in terms of the fundamental NMR parameters. The preferred conformations of the compounds investigated were established by the indications from NOE experiments as well as: (1) the long range coupling across the five bond between the aminic hydrogen and the proton in 4-position of the pyridine-N-oxide ring (⁵J_m^H,NH ～ 0·5 c/s), (2) the value of the vicinal coupling constant in the fragment CHNH (³J_NHCH ～ 7–9 c/s), (3) the large deshielding (Δτ ～ 1–1·5 ppm) observed for the resonance position of the proton on the asymmetric C atom in secondary amine derivatives with respect to the corresponding primary ones, and (4) the diamagnetic shielding produced on protons in position 3 and 4 of the pyridine-N-oxide ring by different aromatic groups introduced in the R substituent.The NMR data confirmed the preferred rotamers previously suggested on the basis of ORD and CD measurements.     

A foldamer-based chiroptical molecular switch that displays complete inversion of the helical sense upon anion binding

Chiral indolocarbazole dimers fold into a helical conformation by virtue of intramolecular hydrogen bonds, as demonstrated by (1)H NMR and CD spectra and optical rotations. In particular, the optical properties of the dimers were found to be extremely sensitive to the nature of the solvent, depending on whether they are folded or not. The helical sense of the dimers can be reversibly switched by binding sulfate ion, which gives rise to complete inversion of the CD spectra. The binding mode and absolute stereochemistry of the sulfate complexes was unequivocally determined by single-crystal X-ray structures, which are all consistent with the CD and (1)H NMR spectra in solution.     

Neutral high-potential nickel triad bisdithiolenes: structure and solid-state NMR properties of Pt[S2C2(CF3)2]2

X-ray crystallography and solid-state NMR techniques were used to determine the structure and 195Pt NMR chemical shift (CS) tensor of Pt[S2C2(CF3)2]2. This is the first reported crystal structure of a highly oxidizing (CN- or CF3-substituted) neutral bis(dithiolene) complex of a Ni triad metal in its pure form. The 195Pt NMR CS tensor is highly anisotropic and asymmetric; the latter property is attributed to the noninnocent nature of the ligand. The tensor components and orientation are determined with density functional theory calculations.     

Frequency-switching inversion-recovery for severely hyperfine-shifted NMR: evidence of asymmetric electron relaxation in high-spin Co(II)

A new method for reliably measuring longitudinal relaxation rates for severely hyperfine-shifted NMR signals in aqueous solutions is presented. The method is illustrated for a well-defined cobalt tetracysteinate, with relevance to cobalt-substituted metalloproteins. The relaxation measurements are indicative of asymmetric electronic relaxation of the high-spin Co(II) ion.     

Interaction between β‐Cyclodextrin and Mixed Cationic‐Anionic Surfactants (1): Thermodynamic Study

The interactions between β‐cyclodextrin (β‐CD) and the mixtures of cationic‐anionic surfactants in aqueous solution were investigated by surface tension and ¹H NMR measurements. It was shown that the critical micelle concentration (cmc) increased linearly with the increase of β‐CD concentration. Furthermore, β‐CD formed 1∶1 inclusion complex with both cationic and anionic surfactants in the mixed surfactant systems, and no significant selective inclusion was observed. The thermodynamic parameters of the inclusion process of β‐CD to mixed cationic‐anionic surfactants were calculated by a numerical method based on the surface tension measurements, and it was found that the inclusion process was both enthalpy and entropy favorable.     

Synthesis and optical properties of (<Emphasis Type="Italic">S</Emphasis>)-Nobin

A preparative method of synthesis of optically pure S-2-amino-2′-hydroxy-1,1′-binaphthyl (S-Nobin), the precoursor in the synthesis of other asymmetric 1,1-binaphthyls applied as the ligands at the creation of the catalysts of asymmetric synthesis, is modified. The UV and CD spectra of this compound were studied in detail and essential dependence of spectral information on the solvent nature was shown.     

Binary and ternary complexes containing alpha-cyclodextrin and bromonaphthalene derivatives: a note of caution in interpreting UV absorption spectral data

Ultraviolet absorption spectra, NMR spectra, and phosphorescence measurements were used to confirm that alpha-cyclodextrin (CD) and 2-bromo-6-beta-D-glucopyranosidylnaphthalene (BGN) form only a binary complex and to characterize its properties. The binding constant for the CD.BGN complex was found to be 886 +/- 24 M(-1) and 770 +/- 110 M(-1) from NMR and UV absorbance measurements, respectively. Comparison of spectral properties revealed the CD.BGN complex to be binary and complexes containing CD and n-alkoxy (n-alkanoloxy) derivatives of 2-bromonaphthalene (N) to be of higher order, notably ternary. A red shift was observed in the UV absorption spectra of the CD(2).N complexes. The absence of a hydroxyl hydrogen atom on the naphthalene ring of N molecules made it impossible for hydrogen bond formation to a glucosidic oxygen in the CD cavity to be the cause of the red shift. The similar red shifts reported herein and for the ternary complexes of CD with 2-naphthol and 2-bromo-6-hydroxynaphthalene (BOHN) indicated that hydrogen bond formation between the hydroxyl hydrogen and glucosidic oxygen atom might not be the cause of the red shift for the latter guest molecules, as has been proposed previously. This result emphasizes the caution necessary in using UV absorption spectral data as evidence for hydrogen bond formation in molecular complexes containing CD.     

Broadband dielectric spectroscopy on a discotic liquid crystalline polymer

Broadband dielectric spectroscopy is employed for the first time to study the molecular dynamics in discotic liquid crystalline polymers. One dielectric relaxation process is found which is strongly broadened and asymmetric. It is assigned a local hindered rotation of the ester groups attaching the spacer to the discotic mesogen. This assignment is supported by NMR measurements on the identical substance.Dedicated to Prof. E. W. Fischer on the occasion of his 60th birthday.     

Optically active alternating copolymers of maleic acid and alkylvinylethers

Alternating copolymers of maleic acid (MAc) with optically active alkylvinylethers (RVE)[R = (S)-1-methylpropyl, (R)-1-methylbutyl, (S)-2-methylbutyl, (S)-3-methylpentyl, (S)-4-methylhexyl, (S)-1-methylheptyl and (S)-3,7-dimethyloctyl] were obtained by hydrolysis in aqueous alkaline medium of the corresponding copolymers with maleic anhydride (MAn). i.r. Absorption spectra, viscosity and potentiometric titration curves are described as well as u.v., ORD and CD spectra recorded in ethanol and water. Conductometric titrations in non-aqueous medium confirm the alternance of dicarboxylic and ethereal structural units. It is shown that the nature of alkyl substituents strongly affects the ionization process of the first carboxylic group. The u.v. bands connected with the $\text{n → π}{}^1$ electronic transitions of carboxylic chromophores are optically active and the ellipticities of the dichroic bands around 215 nm depend on the position of the asymmetric carbon atoms present in the side chain. Moreover for copolymers with ethereal structural unit containing an asymmetric carbon atom α with respect to the oxygen atom, the rotatory strength of such bands is dependent on the length of the alkyl substituents for measurements carried out in water, whilst no significant dependence is evident in measurements performed in ethanol. No ordered secondary structure is considered to account for these features which are probably due to asymmetric induction phenomena involving optically active side chains.     

Conformational consequences of the dynamic processes in the stereolabile atropisomers of acyl-substituted m-terphenyl derivatives

By means of low-temperature NMR spectroscopy, conformers (stereolabile atropisomers) due to the restricted rotation about the Ar-Ar and Ar-C(O)R bonds were detected in a number of acylphenyl derivatives, substituted in positions 2 and 6 by the 3-isopropylphenyl moiety (compounds 1-3, R=H, Me, and t-Bu, respectively). The conformational assignment was accomplished on the basis of the symmetry of the low-temperature 13C NMR spectra with the added support of ab initio calculations. The interconversion barriers were also determined by complete line shape simulation of the NMR spectra, and the experimental values were satisfactorily reproduced by ab initio calculations. In the case of the asymmetric derivative 4, two enantiomers, generated by the restricted t-BuC(O)-Ar rotation, were found sufficiently stable to allow their separation by means of the enantioselective HPLC technique at ambient temperature and to obtain the corresponding CD spectra.     

Evidence of a self-inclusion phenomenon for a new class of mono-substituted alkylammonium-beta-cyclodextrins

A new class of mono-substituted N-alkyl-N,N-dimethylammonium-beta-cyclodextrins has been synthesized in a three step procedure from the native beta-cyclodextrin. The structural analysis of these compounds undertaken by combined use of 1D and 2D NMR spectra indicate that the two methyl groups bound on the nitrogen are magnetically inequivalent due to a self-inclusion phenomenon of the alkyl chain inside the CD cavity. A variable-temperature 1H NMR study showed that these mono-substituted CD derivatives formed temperature-independent intramolecular complexes with their own alkylammonium substituent. The strength of the interaction between the alkyl moiety and the cyclodextrin cavity has been evaluated by a competitive method using an adamantane derivative. Finally, surface tension measurements demonstrated the surface active character of these compounds and confirmed their self-inclusion ability.     

New class of pyridine catalyst having a conformation switch system: asymmetric acylation of various sec-alcohols

We have developed a new class of pyridine catalyst for asymmetric acylation of sec-alcohols having a conformation switch system in which interconversion between self-complexation and uncomplexation is induced by acylation and deacylation steps, respectively. Kinetic resolution of various sec-alcohols is performed by the asymmetric acylation with isobutyric anhydride using 0.05 to 0.5 mol % catalyst 1a with s values of up to 30. In addition, dl-diols are also resolved in a similar manner in good selectivity. Moreover, asymmetric desymmetrization of meso-1,X-diols (X = 2-6) are achieved in the presence of 0.5-5 mol % catalyst 1a. A working model for the reaction mechanism is proposed on the basis of the (1)H NMR measurements, X-ray structural analyses, and AM1 and DFT calculations, where the conformation switch system governed by an intramolecular cation-pi interaction between a pyridinium ring and a thiocarbonyl group would play a key role to attain both good selectivity and high catalytic activity.     

Asymmetric Hydrogenation with Highly Active IndolPhos–Rh Catalysts: Kinetics and Reaction Mechanism

The mechanism of the IndolPhos–Rh‐catalyzed asymmetric hydrogenation of prochiral olefins has been investigated by means of X‐ray crystal structure determination, kinetic measurements, high‐pressure NMR spectroscopy, and DFT calculations. The mechanistic study indicates that the reaction follows an unsaturate/dihydride mechanism according to Michaelis–Menten kinetics. A large value of K_M (K_M=5.01±0.16 M ) is obtained, which indicates that the Rh–solvate complex is the catalyst resting state, which has been observed by high‐pressure NMR spectroscopy. DFT calculations on the substrate–catalyst complexes, which are undetectable by experimental means, suggest that the major substrate–catalyst complex leads to the product. Such a mechanism is in accordance with previous studies on the mechanism of asymmetric hydrogenation reactions with C₁‐symmetric heteroditopic and monodentate ligands.