Conformational analysis of p-tert-butylcalix[4]arene derivatives with trans-alkyl substituents on opposite methylene bridges: destabilization of the cone form by axial alkyl substituents

The stereochemistry of calix[4]arenes substituted by a pair of identical alkyl substituents in a trans fashion at two distal bridges is analyzed. MM3 calculations suggest that increasing the bulk of the alkyl group at the bridges destabilizes those conformations possessing an axial disposition of the substituent. In contrast to the 1,3-dimethyl ether of p-tert-butylcalix[4]arene, which adopts a cone conformation, solution NMR data indicate that the 1,2-alternate conformation is preferred in the dimethyl ether derivatives 5b (alkyl = i-Pr) and 5c (alkyl = t-Bu). In the derivative substituted by the less bulky methyl substituent (5a), both the cone and 1,2-alternate forms coexist in CDCl3. Increasing the polarity of the solvent increases the relative population of the cone form of 5a and 5b. The steric destabilization ensuing from the presence of the axial substituent is so large in the cone conformation of 5c that the 1,2-alternate conformer is the major form even in polar solvents. The cone --> 1,2-alternate interconversion barrier of 5a is 18.2 kcal mol(-1), indicating that the presence of an axial methyl group both destabilizes the cone conformation and decreases its rigidity.     

Theoretical study on the polymerization mechanism of substituted maleimides by using a chiral catalyst with Zn

It is possible to synthesize poly(N-substituted maleimide) by using a chiral complex consisting of a zinc and N-diphenylmethyl-1-benzyl-2-pyrrolidinoethanamine (DPhBP). The optical specific rotations [α]₄₃₅²⁵ in obtained polymers depend on the chirality of ligands in the catalysts. In the present study, density functional theory (DFT) calculations were adopted to investigate the polymerization mechanism in detail. The bulky diphenylmethyl group in the chiral ligand is effective to enhance the formation of the product in the initiation reaction. The geometry related to the pyrrolidine ring of the chiral ligand in the Zn catalyst is responsible for determining the configuration of polymers. It was also confirmed that the bulky substituent on the N atom of the N-substituted maleimide is another factor for obtaining polymers with high [α]₄₃₅²⁵.     

Catalytic Effect of Cesium Cation Adduct Formation on the Decarboxylation of Carboxylate Ions in the Gas Phase

The effect of Cs⁺ ligation on the decarboxylation of malonic acids (unsubstituted and methyl‐, dimethyl‐, ethyl‐, and phenyl‐substituted) in their carboxylate form was studied in the gas phase using tandem mass spectrometry. The study is based on the comparison of the decarboxylation of the bare monoanion (hydrogen malonates) and of the cesium adduct of the cesium salt (Cs⁺[cesium hydrogen malonates]) under collisional activation. Energy‐resolved dissociation curves of the negative and positive ions exhibit major differences. Decarboxylation of the cationic adducts of substituted malonic acid salts occurs at significantly lower collisional activation than for the corresponding bare hydrogen malonate anions. The conclusions from these experiments are supported by DFT calculations. The calculated activation parameters (enthalpy and Gibbs energy) confirm that the cesium cation coordination assists the decarboxylation of the carboxylate form.     

Synthesis of a novel ferrocene derivative having flame-retardant and smoke-suppressant properties

Reaction of ferrocene with chlorendic anhydride (1,4,5,6,7,7-hexachloro-5-norbornene-2,3-dicarboxylic acid anhydride) under Friedel–Crafts reaction conditions affords a new monosubstituted dervative of ferrocene which has significant flameretardant and smoke-suppressant properties when incorporated into poly(vinyl chloride) (PVC). The monocarboxylic acid from the above reaction undergoes smooth methylation with diazomethane to give the corresponding methyl ester. ¹H and ¹³C NMR spectra of these compounds have been compared with those obtained from similar compounds, namely β-ferrocenoylpropanoic acid and its methyl ester. Distant asymmetric centres in the chlorendic anhydride substituent markedly affect the proton spectra of the ferrocene derivative.     

Spectroscopic study of (−)-trans-1,2—epoxypropylphosphonic acid derivatives

Disodium, calcium and (+)-α-phenethylammonium salts of (−)-trans-1,2-epoxypropylphosphonic acid, the sodium salt of the monomethyl ester, and the dimethyl ester of this acid have been studied by ¹H, and ¹³C NMR, IR and Raman spectroscopy. From the NMR study it can be deduced that the steric factors are mainly responsible for the molecular arrangement of these compounds in solution. IR and Raman data suggest the existence of rotational isomerism in the dimethyl ester derivative.     

Malonic to Acetic Ester Transformation by Boric Acid

One of our synthetic projects involves a step of removing a carbalkoxy group from a substituted malonic ester. For accomplishing that goal, Krapcho's method¹ was deemed unsuitable, because the malonic ester contains an allylic halogen which is liable to attack by dimethyl sulfoxide (Kornblum reaction). Our recent work on using iodotrimethylsilane² to achieve decarbal-koxylation of such esters was partly impelled by that need; however, the prohibitive economics associated with even a medium-scale preparation forced us to develop a less expensive alternative.     

Electronic structure and optical properties of poly[3‐(4‐octylphenoxy)thiophene]: Experimental and theoretical studies

This article reports the synthesis and characterization of a new polythiophene derivative phenoxy‐substituted, the poly[3‐(4‐octylphenoxy)thiophene] (POPOT). The oxidative polymerization was found to yield low molecular weight material, whereas a modified Grignard metathesis (GRIM) yielded polymers of high molecular weights. One‐ and two‐dimensional NMR indicated the latter to be highly regioregular. POPOTs exhibited higher thermal stabilities than equivalent alkoxy‐substituted polythiophenes and exhibited red shifts in the absorption spectra with respect to equivalent. The absorption spectra showed a red shifted λ_max at 540 nm in tetrahydrofuran solutions and 580 nm in spin‐coated films, with respect to poly(3‐alkylthiophene)s. A further red shift of 40 nm in going from solution (540 nm) to solid states (580 nm) is correlated with results from density functional theory electronic structure calculations. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7505–7516, 2008     

Comparative studies of chemically synthesized polyaniline and poly(o-toluidine) doped with p-toluene sulphonic acid

Polyaniline and poly(o-toluidine) doped with p-toluene sulphonic acid (p-TSA) were synthesized by in situ chemical polymerization method using ammonium per sulphate as an oxidizing agent. This is a novel polymerization process for the direct synthesis of emeraldine salt phase of the polymer. The polymers were characterized by using UV-Vis and FT-IR spectroscopy, SEM, elemental analyzer, TGA/DSC and conductivity measurements. Thermal analysis shows that poly(o-toluidine) is less thermally stable compared to polyaniline. The less conductivity in poly(o-toluidine) is due to the cumulative steric as well as electronic effect of the bulky methyl substituent present on the benzene ring. High temperature conductivity measurements show ‘thermal activated behavior’.     

Configuration and conformational equilibrium of (±)-trans-1-oxo-3-thiophen-2-yl-isochroman-4-carboxylic acid methyl ester

The X-ray analysis of 1-oxo-3-thiophen-2-yl-isochroman-4-carboxylic acid methyl ester 1 confirmed its trans-configuration and a conformation with diaxial H-3 and H-4 atoms in solid state. NMR experiments indicated that trans-1 exists in solution in both expected conformers. In CDCl₃ and especially in CD₃OD or DMSO, the conformational equilibrium is shifted towards the conformer with diequatorial H-3 and H-4, which was also determined by 2D NOESY experiments. The shift is due to the greater polarity of that conformer deduced by ab initio calculations.     

Phosphates d'énols: XI—Etude des constantes de couplage 3J(P?O?C?H) et 4J(P?O?CC?H)

NMR studies on E and Z enolic phosphates with no substituent on carbon 1 provided evidence for the existence of planar and gauche conformations. When no bulky substituents are present in the cisoid position of the phosphates two planar and two gauche forms are possible. The most stable conformation is the planar form having the phosphate group trans to the double bond. With a bulky substituent in the cisoïd position of the phosphates the planar form with the group cis to the double bond could not be observed. A close agreement is obtained between the values of dihedral angles determined from observed coupling constants and those calculated with the CNDO/2 method. Changes observed in cisoïd and transoïd ⁴J(POCCH) coupling constants with temperature variation and the reversal of magnitude of these constants when a bulky substituent is on carbon 1⁵⁸ are explained.     

Kinetics of the metathesis polymerization of 5,6-Di(methoxycarbonyl)bicyclo[2.2.1]hept-2-enes on an original Hoveyda-Grubbs II type catalyst

The kinetics of the metathesis polymerization of three isomeric 2,3-dicarbomethoxy-5-norbornenes on an original Hoveyda-Grubbs II type catalyst Has been investigated by ¹H NMR spectroscopy. The activation energies of metathesis polymerization and the homopolymerization rate constants have been calculated for all esters from experimental data. The polymerization rate constant of endic acid dimethyl ester (endo,endo-2,3-dicarbomethoxy-5-norbornene) is almost two orders of magnitude lower than that of the reaction of exo,exo-2,3-dicarbomethoxy-5-norbornene. Selected parameters of the microstructure of the resulting polymers are presented.     

3,7-disubstituted bicyclo[3.3.1]nonanes—III: Synthesis and conformation of bicyclo[3.3.1]nonane-3α,7α-dicarboxylic acid,its dimethyl ester and some other 3,7-disubstituted bicyclo[3.3.1]nonanes; adamantane as an integrated holding system

The conformation of bicyclo[3.3.1]nonane-3α,7α-dicarboxylic acid and its dimethyl ester has been studied by comparing ¹H NMR and ¹³C NMR spectra of these compounds with those of some model 3,7-disubstituted bicyclo[3.3.1]nonanes, fixed in a single conformation by the use of adamantane as an integrated holding group or by means of suitable substitution. It is shown that the dicarboxylic acid and its dimethyl ester exist predominantly as two rapidly interconverting (identical) chair-boat conformations with distinctly flattened rings; the population of the double-boat conformation appears to be very small.     

One‐pot synthesis of polymeric nanoparticle by ring‐opening metathesis polymerization

Shell‐functionalized polymeric nanoparticle was prepared through the method of polymerization‐induced self‐assembly of block copolymers [poly(2,3‐bis(2‐bromoisobutyryloxymethyl)‐5‐norbornene)‐block‐poly(7‐oxanorborn‐5‐ene‐exo‐exo‐2,3‐dicarboxylic acid dimethyl ester), PBNBE‐b‐PONBDM] via one‐pot ring‐opening metathesis polymerization of 2,3‐bis(2‐bromoisobutyryloxymethyl)‐5‐norbornene (BNBE) and 7‐oxanorborn‐5‐ene‐exo‐exo‐2,3‐dicarboxylic acid dimethyl ester (ONBDM) in a selective solvent. The compositions and the molecular weights of the copolymers were estimated by ¹H‐NMR and gel permeation chromatography. The micelles were characterized by dynamic light scattering, transmission electron micrograph, and atomic force microscopy. The results indicated that the spherical micelles constructed with bromine‐bearing PBNBE shell and PONBDM core were stable and reproducible in toluene. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Helicity Induction and Its Static Memory of Poly(biphenylylacetylene)s Bearing Pyridine N‐Oxide Groups and Their Use as Asymmetric Organocatalysts

Novel poly(biphenylylacetylene) derivatives carrying different types of pyridine N‐oxide units with a bulky or less‐bulky substituent at a different position as the functional pendant groups (poly‐ 2a and poly‐ 2b ) were synthesized by the rhodium‐catalyzed polymerization of the corresponding monomers. The influence of the steric environment around the catalytically active pyridine N‐oxide sites on the helicity induction and its static memory as well as the asymmetric catalytic activities of the resulting helical polymers with a macromolecular helicity memory was investigated. The polyacetylenes formed an excess one‐handed helical conformation upon noncovalent interactions with optically active alcohols and the induced macromolecular helicities of the polyacetylenes were efficiently memorized after the removal of the chiral inducers. Poly‐ 2b with the macromolecular helicity memory showed an enantioselectivity for the catalytic asymmetric allylation of benzaldehydes, producing optically active allyl alcohols, although their enantioselectivities were low. On the other hand, poly‐ 2a exhibited a negligible catalytic activity probably due to the bulky substituent at the o‐position of the pyridine N‐oxide residues, while poly‐ 2a underwent a unique helix‐inversion with the increasing concentration of chiral alcohols and the opposite helicity of poly‐ 2a was further successfully memorized. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2481–2490     

Chiral ethylene-bridged flavinium salts: the stereoselectivity of flavin-10a-hydroperoxide formation and the effect of substitution on the photochemical properties

A series of chiral non-racemic N¹,N¹⁰-ethylene bridged flavinium salts 4 was prepared using enantiomerically pure 2-substituted 2-aminoethanols (R?=?isopropyl, phenyl, benzyl, 4-methoxybenzyl, 4-benzyloxybenzyl) derived from amino acids as the sole source of chirality. The flavinium salts were shown to form 10a-hydroperoxy- and 10a-methoxy-adducts with moderate to high diastereoselectivity depending on the ethylene bridge substituent originating from the starting amino acid. High diastereoselectivities (dr values from 80:20 to >95:5) were observed for flavinium salts bearing benzyl substituents attached to the ethylene bridge. The benzyl group preferred the face-to-face (syn) orientation relative to the flavinium unit; thereby effectively preventing nucleophilic attack from one side. This conformation was found to be the most stable according to the DFT calculations. Consequently, the presence of benzyl groups causes intermolecular fluorescence quenching resulting in a significant decrease in the fluorescence quantum yield from 11% for 4a bearing an isopropyl substituent to 0.3% for 4c containing a benzyl group and to a value lower than 0.1% for the benzyloxybenzyl derivative 4e.     

Beiträge zur Chemie der Pyrrolpigmente, 11. Mitt.: Die Lichtabsorption geometrisch isomerer Arylmethyliden-pyrrolinone —Konformationsanalytische Studien

By investigation of the geometrical isomers of several substituted arylmethylidene-pyrrolinones using lanthanide induced shift experiments in connection with a computer program, the ring current model,PPP—SCF—LCAO—MO—CI calculations and UV spectroscopy the following conclusions could be drawn: The light absorptions ofZ- andE-isomers vary only in the case of different dihedral angles at the arylmethine bond. The dihedral angles in theE-isomers are determined by the substituent in 4-position of the lactam ring. Hydrogen makes no difference whereas methyl leads to a more twistedE-structure which therefore shows a marked hypsochromic shift of the intense long wavelength absorption band, as compared to the correspondingZ-isomer.     

Acyl chloride‐facilitated condensation polymerization for the synthesis of heat‐sensitive poly(anhydride‐ester)s

We succeeded in developing the acyl chloride‐facilitated condensation polymerization method for the synthesis of new poly(anhydride‐ester)s with aromatic side groups, which cannot be polymerized by the classic melt condensation polymerization method. Using chlorinated and acylated carboxylic acids as the intermediates, the polymerization was carried out at low temperatures of 120 or 135 °C to yield pure poly(anhydride‐ester)s of molecular weights as high as 1.55 × 10⁵ with minimal side‐reactions. A homogeneous route of preparation was developed and optimized, using butyric anhydride as the acylating reagent and oxalyl chloride as the chlorinating reagent. A comparison of the mechanisms of the classic method and the new method indicates that the effects of transacylation—cyclization and oligomer formation—were greatly reduced due to the high reactivity of carboxylic acid chloride and the steric effect of bulky acyl groups. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5899–5915, 2007     

Synthesis of telechelic methacrylic siloxanes with cycloaliphatic substituents groups for UV-curable applications

Methacrylic functionalized siloxanes were prepared in a two-step process. In the first step, a series of telechelic glycidyl epoxy siloxanes were prepared, substituted with either methyl, cyclopentyl, or cyclohexyl groups. In the second step, the telechelic glycidyl epoxy groups were reacted with methacrylic acid. The reaction was monitored via acid value, and when the acid value was ?10, the reaction was terminated. Characterization of the methacrylic telechelic siloxane polymers were performed using ¹H NMR, ¹³C NMR and FT-IR. The methacrylated siloxanes were formulated with a free radical photo-initiator, UV-cured, and the rate of polymerization was monitored via photo-differential calorimetry. After curing, viscoelastic properties, and oxygen-permeability were evaluated. In addition, X-ray was used to evaluate the structure of the cured films. The rate of polymerization was dependent on substituent and increased with increasing substituent size. The oxygen permeability was dependent on crosslink density, and increased with increasing substituent size. The increase in permeability, and thus free volume was supported by X-ray studies which showed an increase in d-spacing with increasing alkyl size.     

The electronic structure of 6a-thiothiophthen and related molecules: non-empirical calculations and reassignment of the photoelectron spectra

Non-empirical calculations of the electronic structure of the 2,5-dimethyl derivative of the title compound, and the isosteric molecules (1b–4b; 5a, b) formed by replacement of the 1- and/or 6-sulphur atoms by O or NMe are reported. A reassignment of the photoelectron spectra of the molecules in the light of these calculations, and using intensity changes between the unsubstituted and dimethyl derivatives, and substituent effects, shows substantial differences from those recently reported.     

300 Mhz 1H NMR spectra and conformations of biotin and related hexahydrothienoimidazolone derivatives

¹H NMR spectra of biotin and four related hexahydrothienoimidazolones in which the endo pentanoate side chain of biotin is replaced by another endo or exo substituent, and the urea nitrogen atoms are substituted with benzyl groups, have been obtained at 300 MHz. Vicinal coupling constants differentiate cis and trans proton pairs. The generalized Karplus equation was utilized to calculate dihedral angles from vicinal proton-proton coupling constants. The conformation of biotin in solution, calculated from coupling constants, is in good agreement with solid state X-ray crystallographic data.