Steric Hinderance Research Articles

Cobalt- and Nickel-Catalyzed Carboxylation of Alkenyl and Sterically Hindered Aryl Triflates Utilizing CO2.

A highly efficient cobalt-catalyzed reductive carboxylation reaction of alkenyl trifluoromethanesulfonates (triflates) has been developed. By employing Mn powder as a reducing reagent under 1 atm pressure of CO2 at room temperature, diverse alkenyl triflates can be converted to the corresponding α,β-unsaturated carboxylic acids. Moreover, the carboxylation of sterically hindered aryl triflates proceeds smoothly in the presence of a nickel or cobalt catalyst.

Photo-catalytic degradation of organic dyes: ultrasonic-assisted synthesis of PdO nanoparticles

PdO nanoparticles were prepared by a new precursor without using any surfactants. The novel Pd(II) complex, [Pd(8Q)(pn)] NO3 (8Q = 8-hydroxyquinolin, pn = 1,3-propylenediamine) has been synthesized and characterized. Ligands as a capping agent with steric hinderance lead to formation of nanoparticles. Images confirm that by ultrasonic-assisted reactions nanostructures were synthesized. Interaction between the Pd(II) complex and DNA has been investigated by UV–visible absorption and fluorescence spectroscopic techniques. The photo-catalytic behaviour of PdO nanoparticles was evaluated using the degradation of eight organic dyes under ultraviolet light irradiation. The results show that PdO nanoparticles have applicable photo-catalytic performance.

The crystal structure of 1-(2-hydroxy-5-meth-oxy-phen-yl)ethanone 4,4-di-methyl-thio-semicarbazone.

The asymmetric unit of the title compound, C12H17N3O2S, contains two independent mol­ecules, A and B. Both mol­ecules are nearly planar with the dihedral angle between the mean planes of the thio­amide group and benzene ring being 7.5 (1)° in A and 4.3 (2)° in B. In each mol­ecule, the hy­droxy group participates in intra­molecular O—H⋯N hydrogen bonding, while the amino H atom is not involved in hydrogen bonding because of the steric hinderence caused by two neighboring methyl groups. In the crystal, the individual molecules are linked by weak C—H⋯O hydrogen bonds, forming A–A and B–B inversion dimers. The dimers are linked via C—H⋯π inter­actions which help stabilize the packing.

Hydrogenolysis of 1-Alkoxybenzocyclobutenes with Site-selective Cleavage of the Sterically Hindered C(sp2)–C(sp3) Bond

Abstract 1-Alkoxybenzocyclobutenes undergo ring-opening hydrogenolysis with site-selective cleavage of the sterically hindered C(sp2)–C(sp3) bond on Pd/C.

P‐124: Excimer Formation in Organic Emitter Films Associated with a Molecular Orientation Promoted by Steric Hinderance

White emission with two sharp strong peaks — a molecular emission peak at 455nm an excimer emission peak at 591nm — was obtained by introducing a terphenyl group into a highly twisted core chromophore, which promoted a molecular orientation in the film state suitable for excimer formation.

Enhanced production of fructose ester by biocatalyzed continuous flow process

Abstract Background Fatty acid sugar esters are non-toxic, odorless, non-irritanting surfactants. They can be synthesized by renewable resources and are completely biodegradable in aerobic and anaerobic conditions. Their application has been expanded in innumerous areas including pharmaceuticals, cosmetics, detergents and food industry. Lipase-catalyzed esterification have been investigated as a potential substitute to the traditional chemical, demanding milder reaction conditions, allowing better reaction control and providing higher-quality products. So, the lipase catalyzed sugar ester synthesis becomes an interesting strategy for producing biodegradable, non- ionic surfactants. The main disadvantage of this protocol is the poor solubility of substrates and long reaction time required for performed the esterification reaction with moderated to good yields. Results Here in, we report the enzymatic esterification of steric hindered fructose derivative with free fatty acids derived from palm oil refining process (RePO) under continuous flow conditions at concentrations up to 0.5 M, increasing the productivity up to 100 mg. min −1.g immob. enzyme −1. Conclusions The immobilized commercial enzyme from Rhizomucor miehei generated the best performance between the catalysts tested.

Conjugated polymers based on poly(fluorenylene ethynylene)s: Syntheses and sensing performance for nitroaromatics

Four conjugated polymers (P1, P2, P3 and P4) based on poly(fluorenylene ethynylene)s (PFEs) have been synthesized by introducing 9,9-diphenylfluorene units and/or thiophene units into the backbone of PFEs. These exotic units remarkably change the behaviors of polymer, such as the solubility, the peak wavelengths of the optical spectra, and the spectral red-shift of solid polymers, etc. As sensing materials for detecting nitroaromatics, all the four polymers show observable fluorescence quenching when DNT exists. For example, in DNT/THF solution, the quenching efficiencies (ηEP) of P2 and P4 are 95% and 90% at quenching equilibrium respectively, which are much higher than those of P1 (77%) and P3 (72%). The high ηEP of P2 and P4 may be ascribed to the electron-rich character of thiophenylene units which can facilitate the electron-transfer between polymer and DNT. However, in another case of polymer films in DNT vapor, P3 shows higher quenching efficiency even than P2 at quenching equilibrium, which may be attributed to the fact that the steric hinderance of 9,9-diphenylfluorene units in P3 can improve the permeability of the film sensor. P4, which contains both thiophenylene and 9,9-diphenylfluorene units, shows the highest quenching efficiency of 91% in DNT vapor at quenching equilibrium.

Rigid and steric hindering bisphosphate flame retardants for polycarbonate

Rigid steric hindering spiro-bisphosphates (PDMPDP and PDBPDP) were synthesized from spiro-pentaerythritol chlorophosphate and 2,6-dimethylphenol and o-tert-butylphenol respectively in order to investigate the effect of steric hindering groups on the flame retardant mechanism of bisphosphates and the effect of rigid spiro-pentaerythritol structure on the heat distortion temperature (HDT) of polycarbonate. A comparison of the steric hindering spiro-bisphosphates with the simplest phenyl spiro-bisphosphate (PDPDP) and commercial resorcinol bis(diphenyl phosphate) (RDP) on the flame retardancy of polycarbonate reveals that the presence of a thermally stable steric hindering group not only changes the efficiency but also shifts the mode of flame retardancy. In spite of low phosphorus content, PDMPDP and RDP show the highest efficiency of flame retardancy toward polycarbonate as evidenced by UL-94 combustion results with RDP exhibiting the strongest vapor phase action, PDMPDP the second, and PDPDP the least as judged from the phosphorus distribution in the vapor and the condensed phase. This is parallel to the tendency of chemical interactions between polycarbonate and flame retardants with PDPDP demonstrating the greatest interaction as proven by TGA study in air. It is suggested that the charring process of PDPDP interferes with that of polycarbonate and leads to microporous char that is seen in SEM and partially responsible for the poor flame retardancy of PDPDP. Compared to PC/RDP, PC/PDMPDP has a high HDT value.

Reversibly Compressible, Highly Elastic, and Durable Graphene Aerogels for Energy Storage Devices under Limiting Conditions

High porosity combined with mechanical durability in conductive materials is in high demand for special applications in energy storage under limiting conditions, and it is fundamentally important for establishing a relationship between the structure/chemistry of these materials and their properties. Herein, polymer‐assisted self‐assembly and cross‐linking are combined for reduced graphene oxide (rGO)‐based aerogels with reversible compressibility, high elasticity, and extreme durability. The strong interplay of cross‐linked rGO (x‐rGO) aerogels results in high porosity and low density due to the re‐stacking inhibition and steric hinderance of the polymer chains, yet it makes mechanical durability and structural bicontinuity possible even under compressive strains because of the coupling of directional x‐rGO networks with polymer viscoelasticity. The x‐rGO aerogels retain >140% and >1400% increases in the gravimetric and volumetric capacitances, respectively, at 90% compressive strain, showing reversible change and stability of the volumetric capacitance under both static and dynamic compressions; this makes them applicable to energy storage devices whose volume and mass must be limited.

Structural Conformers of (1,3-Dithiol-2-ylidene)ethanethioamides: The Balance between Thioamide Rotation and Preservation of Classical Sulfur-Sulfur Hypervalent Bonds.

The reaction of N-(2-phthalimidoethyl)-N-alkylisopropylamines and S2Cl2 gave 4-N-(2-phthalimidoethyl)-N-alkylamino-5-chloro-1,2-dithiol-3-thiones that quantitatively cycloadded to dimethyl or diethyl acetylenedicarboxylate to give stable thioacid chlorides, which in turn reacted with one equivalent of aniline or a thiole to give thioanilides or a dithioester. Several compounds of this series showed atropisomers that were studied by a combination of dynamic NMR, simulation of the signals, conformational analysis by DFT methods, and single crystal X-ray diffraction, showing a good correlation between the theoretical calculations, the experimental values of energies, and the preferred conformations in the solid state. The steric hindering of the crowded substitution at the central amine group was found to be the reason for the presence of permanent atropisomers in this series of compounds and the cause of a unique disposition of the thioxo group at close-to-right angles with respect to the plane defined by the 1,3-dithiole ring in the dithiafulvene derivatives, thus breaking the sulfur-sulfur hypervalent bond that is always found in this kind of compounds.

ChemInform Abstract: Iridium‐Catalyzed Borylation of Sterically Hindered C(sp3)—H Bonds: Remarkable Rate Acceleration by a Catalytic Amount of Potassium tert‐Butoxide.

ChemInform Abstract: Iridium‐Catalyzed Borylation of Sterically Hindered C(sp³)—H Bonds: Remarkable Rate Acceleration by a Catalytic Amount of Potassium tert‐Butoxide.

Structural Study on Hen-egg Yolk High Density Lipoprotein (HDL) Granules

Hen egg yolk’s main fractions, granules and plasma contribute to the formation and the stability of disperse systems like emulsions by constituting an interfacial film. However, yolk is still used empirically and physical properties of yolk fractions are not entirely understood. Therefore, one of the main approaches is to fractionate egg yolk in its single components and to close the lack of understanding of the molecular and structural characteristics of each fraction. As our sedimentation experiments show, separation efficiency of granules from hen egg yolk varies with changing solution pH. Under isoelectric conditions at pH 4.0, granules bind more low density lipoprotein (LDL; plasma-fraction), as atomic force microscopy reveals. We show that besides reduced electrostatic repulsion forces also structural changes of the granules support a higher incorporation of LDL-vesicle at pH 4.0. Varying amounts of lipid with pH make a structural analysis difficult by imaging techniques alone. We report on X-ray scattering experiments in reflection geometry performed on films of hen-egg granules. The observed scattering patterns provide prominent lateral distances in the film and a repeating distance from the phospholipids of the LDL vesicles. At pH 4.0 the repeating distance is larger and less diffuse compared to pH 6.5. We extracted two prominent lateral particle sizes in the HDL-granule film from two significant shoulders in the scattering functions. At pH 4.0 the obtained radii of gyration are Rg1 = 83 nm and Rg2 = 2.4 nm. At pH 6.5 we observed a significant expansion of both sizes. This indicates rougher contact areas and a stronger steric hindering for the incorporation of the LDL vesicle in the HDL-granules under native milieu conditions.

ChemInform Abstract: Palladium Nanoparticles Stabilized by Sterically Hindered Phosphonium Salts as Suzuki Cross‐Coupling Catalysts.

AbstractIn situ generated palladium nanoparticles stabilized by a sterically hindered phosphonium salt are shown to act as an efficient catalyst for the Suzuki cross‐coupling reaction of haloarenes with arylboronic acids.

Menshutkin reaction between DABCO and benzyfluoride/fluorodiphenylmethane: a mechanistic study.

The microcosmic mechanism of the Menshutkin reaction between DABCO and benzyl fluoride/fluorodiphenylmethane has been investigated in both the gas and solvent phase by performing DFT calculations at B3LYP/6-31G(d,p) level of theory. The Gibbs free energy profiles to reach the possible transition states, i.e., five-membered ring transition state and SN2 transition state show that the reaction between DABCO and benzyl fluoride proceeds through SN2 transition state in accordance with previously reported studies, while the reaction between DABCO and fluorodiphenylmethane proceeds through five-membered ring transition state contrary to earlier literature. The role of solvent has been elucidated by reoptimizing the structures using SMD model of solvation. Hydrogen bonding and steric hinderance have been identified as the key factors in guiding the reaction pathway of commercially important Menshutkin reaction.

Aza-Diels-Alder reaction between N-aryl-1-oxo-1H-isoindolium ions and tert-enamides: Steric effects on reaction outcome.

The synthesis of 5-substituted 6,6a-dihydroisoindolo[2,1-a]quinolin-11(5H)-ones via [4 + 2] imino-Diels–Alder cyclization from N-aryl-3-hydroxyisoindolinones and N-vinyl lactams under Lewis acid-catalysed anhydrous conditions is reported. Reactions of N-(2-substituted-aryl)-3-hydroxyisoindolinones with N-vinylpyrrolidone under identical conditions resulted in the formation of 2-(2-substitued-aryl)-3-(2-(2-oxopyrrolidin-1-yl)vinyl)isoindolin-1-one analogues indicating steric hinderance as the cause of deviation. The probable mechanism of the reaction based on the results from X-ray crystallography and molecular modelling is discussed.

Synthesis, Toxicity, and Antituberculosis Activity of Isoniazid Derivatives Containing Sterically Hindered Phenols

The toxicity and tuberculostatic activity of new isoniazid derivatives containing sterically hindered phenols were investigated. The investigated isoniazid derivatives exhibited lower activity and toxicity than isoniazid.

Structure and properties research on montmorillonite modified by flame-retardant dendrimer

A novel approach to highly branched polymer chains for making advanced flame-retardant organoclays (OC) was presented. In this effort, a series of dendrimer based montmorillonite (Mt) were synthesized by tetrahydroxymethyl phosphonium chloride (THPC), N, N-dihydroxyl-3-aminomethyl propionate, and boric acid.Mt was first modified by THPC, and organic Mt (OMt) was obtained. Three generations of dendrimer modified organic clay minerals, DOMt-1, DOMt-2, and DOMt-3, were successfully prepared by OMt and the monomer prepared. Their chemical structures were characterized by FTIR, 1H NMR, elemental analysis, XRD, and TGA. These DOMts had expanded interlayer space and possessed good thermal stabilities. With an increase of generations, the thermal stability was not further enhanced for highly branched polymers due to the increasing content of CH bonds. These DOMts were terminated by boric acid, and a flame-retardant and dendrimer modified OMt (FR-DOMt) was successfully prepared. Experiments demonstrated that this FR-DOMt had better thermal stability and larger d-value due to steric hindering and thermal stability of the surfactant. The chemical structure and properties of FR-DOMt were valuable for its further applications.

Structural Dynamics and Relaxation Processes with Participation of Excited Singlet and Triplet States in Sterically Hindered Porphyrins and Their Chemical Dimers

steric interactions and porphyrin non-planarity remained to be not studied yet. We found for the first time that mono- and di-meso-phenyl substitution in octaethylporphyrins led to a drastic shortening of T 1 decays (∼1.5 ms → 2–5 µs) in deaerated solutions at 295 K without considerable influence on spectral-kinetic parameters of singlet states. These effects have been systematically studied for porphyrins and their chemical dimers with a controllable structure of meso-phenyl substituents as well as for meso-phenyl-substituted octaarylporphyrins with increasing number (n=1÷4) of meso-phenyl rings. In some cases, quantum-chemical calculations have been used to explain experimental findings. Paper cont ains also some recent results showing what circumstances should be taken into account when using these compounds in various areas (excited state deactivation in multiporphyrin complexes, photoinduced electron transfer, singlet oxygen generation).

Carbamate Stabilities of Sterically Hindered Amines from Quantum Chemical Methods: Relevance for CO2 Capture

The influence of electronic and steric effects on the stabilities of carbamates formed from the reaction of CO2 with a wide range of alkanolamines was investigated by quantum chemical methods. For the calculations, B3LYP, M11-L, MP2, and spin-component-scaled MP2 (SCS-MP2) methods were used, coupled with SMD and SM8 solvation models. A reduction in carbamate stability leads to an increased CO2 absorption capacity of the amine and a reduction of the energy required for solvent regeneration. Important factors for the reduction of the carbamate stability were an increase in steric hindrance around the nitrogen atom, charge on the N atom and intramolecular hydrogen bond strength. The present study indicates that secondary ethanolamines with sterically hindering groups near the N atom show significant potential as candidates for industrial CO2-capture solvents.

ChemInform Abstract: (IPr)Pd(pydc) (pydc = Pyridine‐2,6‐dicarboxylate) — A Highly Active Precatalyst for the Sterically Hindered C—N Coupling Reactions.

AbstractNovel N‐heterocyclic carbene complexes incorporating a pyridine‐2‐carboxylate or pyridine‐2,6‐dicarboxylate ligand are synthesized and successfully applied as catalysts in the Buchwald—Hartwig amination reaction of sterically hindered aryl chlorides.