Enhancing the Orienting Properties of Poly(γ‐benzyl‐L‐glutamate) by means of Additives

Residual dipolar couplings (RDCs) have recently become increasingly important in organic structure determination due to their unique information content. One main limitation for the use of RDCs in organic compounds is the orientation that needs to be induced to be able to measure RDCs. So far, there are very few possibilities to modulate the orientational properties of organic solutes and even less when chiral media are considered. Based on our recent findings that the critical concentration of the liquid‐crystalline phase of homopolypeptides depends on their molecular weight, we sought for further ways to modulate the orienting properties. We were especially interested in seeing whether we could not only influence the induced degree of orientation, but whether we could also change the solute′s preferred orientation and even enhance enantiodifferentiation. We thus tried different aprotic and protic additives and were successful in all of the above‐mentioned aspects by using CCl₄ as the additive. Furthermore, we consider DMSO to be a very useful additive. The LC phase of low MW poly(γ‐benzyl‐L ‐glutamate) (PBLG) is usually unstable when DMSO is added. The high MW PBLG used in this study, however, remained stable up to a DMSO/CDCl₃ ratio of 1:2. By using this combination of solvents, the alignment of the two enantiomers of a compound, which is insoluble in CDCl₃, namely, the HCl salt of a tryptophane ester, was possible leading to high‐quality spectra. The two enantiomers of the tryptophane ester showed different couplings, thus indicating that enantiodifferentiation is taking place. Thus we were able to modulate the orienting properties (degree of orientation, preferred orientation and enantiodifferentiation) of PBLG by using additives and to increase the accessible solvent and solute range significantly.     

RDC‐enhanced structure calculation of a β‐heptapeptide in methanol

Residual dipolar couplings (RDCs) are a rich source of structural information that goes beyond the range covered by the nuclear Overhauser effect or scalar coupling constants. They can only be measured in partially oriented samples. RDC studies of peptides in organic solvents have so far been focused on samples in chloroform or DMSO. Here, we show that stretched poly(vinyl acetate) can be used for the partial alignment of a linear β‐peptide with proteinogenic side chains in methanol. ¹D_CH, ¹D_NH, and ²D_HH RDCs were collected with this sample and included as restraints in a simulated annealing calculation. Incorporation of RDCs in the structure calculation process improves the long‐range definition in the backbone of the resulting 3₁₄‐helix and uncovers side‐chain mobility. Experimental side‐chain RDCs of the central leucine and valine residues are in good agreement with predicted values from a local three‐state model. Copyright © 2016 John Wiley & Sons, Ltd.     

Computer‐Assisted 3D Structure Elucidation of Natural Products using Residual Dipolar Couplings

An enhanced computer‐assisted procedure for the determination of the relative configuration of natural products, which starts from the molecular formula and uses a combination of conventional 1D and 2D NMR spectra, and residual dipolar couplings (RDCs), is reported. Having already the data acquired (1D/2D NMR and RDCs), the procedure begins with the determination of the molecular constitution using standard computer‐assisted structure elucidation (CASE) and is followed by fully automated determination of relative configuration through RDC analysis. In the case of moderately flexible molecules the simplest data‐explaining conformational model is selected by the use of the Akaike information criterion.     

Application of residual dipolar couplings in organic compounds

Residual dipolar couplings (RDCs) induced by anisotropic media are a powerful tool for the structure determination of biomolecules through NMR spectroscopy. Recent advances have proven it to be a valuable tool for determination of the stereochemistry of organic molecules. By simple inspection or order matrix calculations, RDCs provide unambiguous information about the relative configurations or complete stereochemistry of organic compounds.     

A Self‐Assembled Oligopeptide as a Versatile NMR Alignment Medium for the Measurement of Residual Dipolar Couplings in Methanol

Residual dipolar coupling (RDC) is a powerful structural parameter for the determination of the constitution, conformation, and configuration of organic molecules. Herein, we report the first liquid crystal‐based orienting medium that is compatible with MeOH, thus enabling RDC acquisitions of a wide range of intermediate to polar organic molecules. The liquid crystals were produced from self‐assembled oligopeptide nanotubes (AAKLVFF), which are stable at very low concentrations. The presented alignment medium is highly homogeneous, and the size of RDCs can be scaled with the concentration of the peptide. To assess the accuracy of the RDC measurement by employing this new medium, seven bioactive natural products from different classes were chosen and analyzed. The straightforward preparation of the anisotropic alignment sample will offer a versatile and robust protocol for the routine RDC measurement of natural products.     

Parameters influencing the molecular weight of 3,6‐carbazole‐based D‐π‐A‐type copolymers

Condensation copolymerization reactions of carbazole 3,6‐diboronate with 4,7‐bis(5‐bromo‐2‐thienyl)‐2,1,3‐benzothiadiazole (DTBT) only produce low‐molecular‐weight donor (D)‐π‐acceptor (A) copolymers. High‐molecular‐weight copolymers for use in optoelectronic devices are necessary for achieving extended π‐conjugation and for controlling the copolymer processibility. To elucidate the cause of the persistently low molecular weight, we synthesized three 3,6‐carbazole‐based D‐A copolymers using copolymerizations of N‐9′‐heptadecanyl‐3,6‐carbazole with DTBT, N‐9′{2‐[2‐(2‐methoxy‐ethoxy)‐ethoxy]‐ethyl}‐3,‐6‐carbazole with DTBT, and N‐9′‐heptadecanyl‐3,6‐carbazole with alkyl‐substituted DTBT. We investigated several parameters for their influence on molecular copolymer weight, including the conformation of the chain during growth, the solubility of the monomers, and the dihedral angles between the donor and acceptor units. Size exclusion chromatography, UV–vis absorption spectroscopy, and computational studies revealed that the low molecular weights of 3,6‐carbazole‐based D‐A copolymers resulted from conjugation breaks and the resulting high coplanarity, which led to strong interactions between polymer chains. These interactions limited formation of high‐molecular‐weight‐copolymers during copolymerization. The strong intermolecular interactions of the 3,6‐carbazole moiety were exploited by incorporating 3,6‐carbazole units into poly[9′,9′‐dioctyl‐2,7‐flourene‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] prepared from 9′,9′‐dioctyl‐2,7‐flourene and DTBT. Interestingly, the number average molecular weight increased gradually with increasing 2,7‐fluorene monomer content but the number of conjugation breaks was a range of 6–7. The hole mobilities of the copolymers were studied for comparison purposes. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Asymmetric Catalytic Formal 1,4‐Allylation of β,γ‐Unsaturated α‐Ketoesters: Allylboration/Oxy‐Cope Rearrangement

A highly enantioselective formal conjugate allyl addition of allylboronic acids to β,γ‐unsaturated α‐ketoesters has been realized by employing a chiral Ni^II/N,N′‐dioxide complex as the catalyst. This transformation proceeds by an allylboration/oxy‐Cope rearrangement sequence, providing a facile and rapid route to γ‐allyl‐α‐ketoesters with moderate to good yields (65–92 %) and excellent ee values (90–99 % ee). The isolation of 1,2‐allylboration products provided insight into the mechanism of the subsequent oxy‐Cope rearrangement reaction: substrate‐induced chiral transfer and a chiral Lewis acid accelerated process. Based on the experimental investigations and DFT calculations, a rare boatlike transition‐state model is proposed as the origin of high chirality transfer during the oxy‐Cope rearrangement.     

Modulation of a Molecular π‐Electron System in a Purely Organic Conductor that Shows Hydrogen‐Bond‐Dynamics‐Based Switching of Conductivity and Magnetism

New important aspects of the hydrogen‐bond (H‐bond)‐dynamics‐based switching of electrical conductivity and magnetism in an H‐bonded, purely organic conductor crystal have been discovered by modulating its tetrathiafulvalene (TTF)‐based molecular π‐electron system by means of partial sulfur/selenium substitution. The prepared selenium analogue also showed a similar type of phase transition, induced by H‐bonded deuterium transfer followed by electron transfer between the H‐bonded TTF skeletons, and the resulting switching of the physical properties; however, subtle but critical differences due to sulfur/selenium substitution were detected in the electronic structure, phase transition nature, and switching function. A molecular‐level discussion based on the crystal structures shows that this chemical modification of the TTF skeleton influences not only its own π‐electronic structure and π–π interactions within the conducting layer, but also the H‐bond dynamics between the TTF π skeletons in the neighboring layers, which enables modulation of the interplay between the H‐bond and π electrons to cause such differences.     

Chiral Phosphoric Acid Dual‐Function Catalysis: Asymmetric Allylation with α‐Vinyl Allylboron Reagents

We report a dual function asymmetric catalysis by a chiral phosphoric acid catalyst that controls both enantioselective addition of an achiral α‐vinyl allylboronate to aldehydes and pseudo‐axial orientation of the α‐vinyl group in the transition state. The reaction produces dienyl homoallylic alcohols with high Z‐selectivities and enantioselectivities. Computational studies revealed that minimization of steric interactions between the alkyl groups of the diol on boron and the chiral phosphoric acid catalyst influence the orientation of α‐vinyl substituent of the allylboronate reagent to occupy a pseudo‐axial position in the transition state.     

High‐performance liquid chromatography enantioseparation of polyhalogenated 4,4′‐bipyridines on polysaccharide‐based chiral stationary phases under multimodal elution

An investigation on the high‐performance liquid chromatography enantioseparation of 12 polyhalogenated 4,4′‐bipyridines on polysaccharide‐based chiral stationary phases is described. The overall study was directed toward the generation of efficient separations in order to obtain pure atropisomers that will serve as ligands for building homochiral metal organic frameworks. Four coated columns—namely, Lux Cellulose‐1, Lux Cellulose‐2, Lux Cellulose‐4, and Lux Amylose‐2—and two immobilized columns—namely, Chiralpak IC and IA—were used under normal, polar organic, and reversed‐phase elution modes. Moreover, Chiralcel OJ was considered under normal‐phase and polar organic conditions. The effect of the chiral selector and mobile phase composition on the enantioseparation, the enantiomer elution order and the beneficial effect of nonstandard solvents were studied. The effect of water in the mobile phase on the enantioselectivity and retention was investigated and retention profiles typical of hydrophilic interaction liquid chromatography were observed. Interesting phenomena of solvent‐induced enantiomer elution order reversal occurred under normal‐phase mode. All the considered 4,4′‐bipyridines were enantioseparated at the multimilligram level.     

Synthesis,crystallization, and morphology of star‐shaped poly(ε‐caprolactone)

Six‐arm star‐shaped poly(ε‐caprolactone) (sPCL) was successfully synthesized via the ring‐opening polymerization of ε‐caprolactone with a commercial dipentaerythritol as the initiator and stannous octoate (SnOct₂) as the catalyst in bulk at 120 °C. The effects of the molar ratios of both the monomer to the initiator and the monomer to the catalyst on the molecular weight of the polymer were investigated in detail. The molecular weight of the polymer linearly increased with the molar ratio of the monomer to the initiator, and the molecular weight distribution was very low (weight‐average molecular weight/number‐average molecular weight = 1.05–1.24). However, the molar ratio of the monomer to the catalyst had no apparent influence on the molecular weight of the polymer. Differential scanning calorimetry analysis indicated that the maximal melting point, cold crystallization temperature, and degree of crystallinity of the sPCL polymers increased with increasing molecular weight, and crystallinities of different sizes and imperfect crystallization possibly did not exist in the sPCL polymers. Furthermore, polarized optical microscopy analysis indicated that the crystallization rate of the polymers was in the order of linear poly(ε‐caprolactone) (LPCL) > sPCL5 > sPCL1 (sPCL5 had a higher molecular weight than both sPCL1 and LPCL, which had similar molecular weights). Both LPCL and sPCL5 exhibited a good spherulitic morphology with apparent Maltese cross patterns, whereas sPCL1 showed a poor spherulitic morphology. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5449–5457, 2005     

9H‐Quinolino[3,2,1‐k]phenothiazine: A New Electron‐Rich Fragment for Organic Electronics

A new electron‐rich fragment, namely the quinolinophenothiazine (QPTZ) is reported. The QPTZ fragment incorporated in spiroconfigured materials leads to higher performance in blue Phosphorescent OLEDs than structurally related phenylacridine and indoloacridine based materials (increasing the HOMO energy level, modulating the spin‐orbit coupling, etc.) and leads to highly efficient blue phosphorescent organic light emitting diodes, indicating the strong potential of this new molecular fragment in organic electronics.     

Self‐Assembly of π‐Conjugated Gelators into Emissive Chiral Nanotubes: Emission Enhancement and Chiral Detection

A series of new π‐conjugated gelators that contain various aromatic rings (phenyl, naphthyl, 9‐anthryl) and amphiphilic L ‐glutamide was designed, and their gel formation in organic solvents and self‐assembled nanostructures was investigated. The gelators showed good gelation ability in various organic solvents that ranged from polar to nonpolar. Those gelator molecules with small rings such as phenyl and naphthyl self‐assembled into nanotube structures in most organic solvents and showed strong blue emission. However, the 9‐anthryl derivative formed only a nanofiber structure in any organic solvent, probably owing to the larger steric hindrance. All of these gels showed enhanced fluorescence in organogels. Furthermore, during the gel formation, the chirality at the L ‐glutamide moiety was transferred to the nanostructures, thus leading to the formation of chiral nanotubes. One of the nanotubes showed chiral recognition toward the chiral amines.     

Optimization of the LC enantioseparation of chiral pharmaceuticals using cellulose tris(4‐chloro‐3‐methylphenylcarbamate) as chiral selector and polar non‐aqueous mobile phases

The resolving power of a new commercial polysaccharide‐based chiral stationary phase, Sepapak‐4, with cellulose tris(4‐chloro‐3‐methylphenylcarbamate) coated on silica microparticles as chiral selector, was evaluated toward the enantioseparation of ten basic drugs with widely different structures and hydrophobic properties, using ACN as the main component of the mobile phase. A multivariate approach (experimental design) was used to screen the factors (temperature, n‐hexane content, acidic and basic additives) likely to influence enantioresolution. Then, the optimization was performed using a face‐centered central composite design. Complete enantioseparation could be obtained for almost all tested chiral compounds, demonstrating the high chiral discrimination ability of this chiral stationary phase using polar organic mobile phases made up of ACN and containing an acidic additive (TFA or formic acid), 0.1% diethylamine and n‐hexane. These results clearly illustrate the key role of the nature of the acidic additive in the mobile phase.     

Chiral stationary phases based on chitosan bis(4‐methylphenylcarbamate)‐(alkoxyformamide)

Highly N‐deacetylated chitosan was chosen as a natural chiral origin for the synthesis of the selectors of chiral stationary phases. Therefore, chitosan was firstly acylated by various alkyl chloroformates yielding chitosan alkoxyformamides, and then these resulting products were further derivatized with 4‐methylphenyl isocyanate to afford chitosan bis(4‐methylphenylcarbamate)‐(alkoxyformamide). A series of chiral stationary phases was prepared by coating these derivatives on 3‐aminopropyl silica gel. The content of the derivatives on the chiral stationary phases was nearly 20% by weight. The chiral stationary phases prepared from chitosan bis(4‐methylphenylcarbamate)‐(ethoxyformamide) and chitosan bis(4‐methylphenylcarbamate)‐(isopropoxyformamide) comparatively showed better enantioseparation capability than those prepared from chitosan bis(4‐methylphenylcarbamate)‐(n‐pentoxyformamide) and chitosan bis(4‐methylphenylcarbamate)‐(benzoxyformamide). The tolerance against organic solvents of the chiral stationary phase of chitosan bis(4‐methylphenylcarbamate)‐(ethoxyformamide) was investigated, and the results revealed that this phase can work in 100% ethyl acetate and 100% chloroform mobile phases. Because as‐synthesized chiral selectors did not dissolve in many common organic solvents, the corresponding chiral stationary phases can be utilized in a wider range of mobile phases in comparison with conventional coating type chiral stationary phases of cellulose and amylose derivatives.     

Random poly(3‐hexylthiophene‐co‐3‐cyanothiophene‐co‐3‐(2‐ethylhexyl)thiophene) copolymers with high open‐circuit voltage in polymer solar cells

For the purpose of developing poly(3‐hexylthiophene) (P3HT) based copolymers with deep‐lying highest occupied molecular orbital (HOMO) levels for polymer solar cells with high open‐circuit voltage (V_oc), we report a combined approach of random incorporation of 3‐cyanothiophene (CNT) and 3‐(2‐ethylhexyl)thiophene (EHT) units into the P3HT backbone. This strategy is designed to overcome CNT content limitations in recently reported P3HT‐CNT copolymers, where incorporation of more than 15% of CNT into the polymer backbone leads to impaired polymer solubility and raises the HOMO level. This new approach allows incorporation of a larger CNT content, reaching even lower‐lying HOMO levels. Importantly, a very low HOMO level of ?5.78 eV was obtained, representing one of the lowest HOMO values for exclusively thiophene‐based polymers. Lower HOMO levels result in higher V_oc and higher power conversion efficiencies (PCE) compared to the previously reported P3HT‐CNT copolymers containing only 3‐hexylthiophene and CNT units. As a result, solar cells based on P3HT‐CNT‐EHT(15:15) , which contains 70% of P3HT, 15% of CNT and 15% of EHT, yield a V_oc of 0.83 V in blends with PC₆₁BM while preserving high fill factor (FF) and high short‐circuit current density (J_sc), resulting in 3.6% PCE. Additionally, we explored the effect of polymer number‐average molecular weight (M_n) on the optoelectronic properties and solar cell performance for the example of P3HT‐CNT‐EHT(15:15). The organic photovoltaic (OPV) performance improves with polymer M_n increasing from 3.4 to 6.7 to 9.6 kDa and then it declines as M_n further increases to 9.9 and to 16.2 kDa. The molecular weight study highlights the importance of not only the solar cell optimization, but also the significance of individual polymer properties optimization, in order to fully explore the potential of any given polymer in OPVs. The broader ramification of this study lies in potential application of these high band gap copolymers with low‐lying HOMO level in the development of ternary blend photovoltaics as well as tandem OPV. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1526–1536     

Enantioselective separation of chiral vicinal diols in capillary electrophoresis using a mono‐6A‐aminoethylamino‐β‐cyclodextrin as a chiral selector

This paper describes an improved access to mono‐6^A‐aminoethylamino‐β‐CD (β‐CDen), a very efficient cationic chiral selector for CZE in the separation of eight chiral aromatic vicinal diols. The β‐CDen concentration has a strong influence on the efficiency of enantioseparation. The effects of the pH and concentration of the BGE, the capillary temperature, and the applied voltage on the resolution and separation selectivity have been studied. Excellent chiral resolution was achieved under the optimal conditions of β‐CDen 10 mM, pH 10, 200 mM borate buffer at 15 kV and 20°C within 20 min. Moreover, the developed method was successfully applied to the determination of the enantiomeric purity of the catalytic asymmetric dihydroxylation (AD) reaction products.     

Lipase‐Catalyzed Ring‐Opening Polymerization of 3(S)‐sec‐Butylmorpholine‐2,5‐dione

Lipase‐catalyzed ring‐opening bulk polymerizations of 3(S)‐sec‐butylmorpholine‐2,5‐dione (BMD) were investigated. Selected commercial lipases were screened as catalysts for BMD polymerization at 110°C. Polymerizations catalyzed with 10 wt.‐% of lipase PPL and PC result in BMD conversions of about 70% and in molecular weights of the products ranging from 5 500 to 10 700. Lipases MJ, CR and ES showed lower catalytic activities for the polymerization of BMD. Poly(3‐sec‐butylmorpholine‐2,5‐dione) has a carboxylic acid group at one end and a hydroxy group at the other end. During the polymerization racemization of the isoleucine residue takes place. Lipase PPL was selected for a more detailed study. The apparent rate of polymerization increases with increasing PPL concentration when the polymerization temperature is 110°C. When the PPL concentration is 5 and 10 wt.‐% with respect to the monomer, a conversion of about 70% is reached after 5 d and 3 d, respectively, while for a PPL concentration of 1 wt.‐% the conversion is less than 7% even after 6  d. High concentrations of PPL (10 wt.‐%) result in high M_n values (< 4  d). The highest molecular weight poly(BMD), M_n = 19 900, resulted from a polymerization conducted at 120°C with 5 wt.‐% PPL for 6 d. The general trend observed by varying the polymerization temperature is as follows: (i) monomer conversion and M_n increase with increasing reaction temperature from 110 to 125°C, (ii) monomer conversion and M_n decrease with an increase in reaction temperature from 125 to 130°C. Water content was found to be an important factor that controls both the conversion and the molecular weight. With increasing water content, enhanced polymerization rates are achieved while the molecular weight of poly(BMD) decreases.     

NMR characterization of 1,1′‐binaphthyl‐2,2′‐diyl hydrogen phosphate binding to chiral molecular micelles

NMR spectroscopy was used to characterize the binding of the chiral compound 1,1′‐binaphthyl‐2,2′‐diyl hydrogen phosphate (BNP) to five molecular micelles with chiral dipeptide headgroups. Molecular micelles have covalent linkages between the surfactant monomers and are used as chiral mobile phase modifiers in electrokinetic chromatography. Nuclear overhauser enhancement spectroscopy (NOESY) analyses of (S)‐BNP:molecular micelle mixtures showed that in each solution the (S)‐BNP interacted predominately with the N‐terminal amino acid of the molecular micelle's dipeptide headgroup. NOESY spectra were also used to generate group binding maps for (S)‐BNP:molecular micelle mixtures. In these maps, percentages are assigned to the (S)‐BNP protons to represent the relative strengths of their interactions with a specified molecular micelle proton. All maps showed that (S)‐BNP inserted into a previously reported chiral groove formed between the molecular micelle's dipeptide headgroup and hydrocarbon chain. In the resulting intermolecular complexes, the (S)‐BNP protons nearest to the analyte phosphate group were found to point toward the N‐terminal Hα proton of the molecular micelle headgroup. Finally, pulsed field gradient NMR diffusion experiments were used to measure association constants for (R) and (S)‐BNP binding to each molecular micelle. These K values were then used to calculate the differences in the enantiomers' free energies of binding, Δ(ΔG). The NMR‐derived Δ(ΔG) values were found to scale linearly with electrokinetic chromatography (EKC) chiral selectivities from the literature. Copyright © 2010 John Wiley & Sons, Ltd.