Steric Hindrance Of Methyl Groups Research Articles

Computational Exploration of 1,2-Carboamine Carbonylation Catalyzed by Nickel.

Nickel-catalyzed carbonylation of alkenes is a stereoselective and regioselective method for the synthesis of amide compounds. Theoretical predictions with density functional theory calculations revealed the mechanism and origin of stereoselectivity and regioselectivity for the nickel-catalyzed carbonylation of norbornene. The carbonylation reaction proceeds through oxidative addition, migration insertion of alkenes, and subsequent reduction elimination to afford cis-carbonylation product. The C-N bond activation of amides is unfavorable because the oxidative addition ability of the C-C bond is stronger than that of the C-N bond. The determining step of stereoselectivity is the migratory insertion of the strained olefin. The structural analysis shows that steroselectivity is controlled by the steric hindrance of methyl groups to olefins and substituents to IMes in ligands.

Steric Hindrance of Methyl Group on the Reaction Pathway of Hydrodesulfurization in the Presence of Quinoline

Steric hindrance of methyl on reaction pathway of hydrodesulfurization in presence of quinoline was investigated over a NiMo/Al2O3 catalyst at 4.0 MPa and 320 °C. The results show that steric hindrance of methyl changed the main pathway of HDS while presence of quinoline further exacerbated this transformation. Changes in inhibitory factor curve show that quinoline poisoning on DBT HDS is a relatively mild. When adjacent methyl is present, even at low concentration, the inhibitory factor of quinoline on 4,6-DMDBT HDS is still greater than 85%. At high levels, the inhibitory factor of quinoline on DDS pathway even exceeds 99%. Quinoline poisons HYD pathway of DBT HDS to a stable level in a short time (30 to 60 min). However, after 6 h of reaction, HYD pathway yield of 4,6-DMDBT HDS is still increasing. Quinoline simultaneously poisons HYD and DDS pathways of 4,6-DMDBT HDS to a stable level within 120 min, and then DDS pathway will continue to be poisoned, thereby increasing HYD pathway yield. Regardless of presence of adjacent methyl in sulfur-containing molecules, an increase in quinoline concentration will lead to an increase in HYD/DDS pathway yield ratio. Addition of quinoline inhibits conversion of hydrogenated intermediates to DDS products by poisoning C–S bond dissociation center (such as CUS active site) and leads to formation of deep hydrogenated products.

Complexation of U(VI) with BiPDA, DmBiPDA, and PhenDA: Comparison on Structures and Binding Strengths in Aqueous and DMSO/20%(v)H2O Solutions.

The stability constants (log β) of 1:1 uranyl complexes with three N,O-mixed donor ligands (L = 2,2'-dipyridyl-6,6'-dicarboxylate, 3,3'-dimethyl-2,2'-bipyridine-6,6'-dicarboxylate, and 1,10-phenanthroline-2,9-dicarboxylate, denoted as BiPDA, DmBiPDA, and PhenDA, respectively) in aqueous and DMSO/20%(v)H2O solutions were determined by spectrophotometry in 0.1 M tetraethylammonium perchlorate. The effects of ligand preorganization, steric hindrance, and solvation on the binding strength of U(VI) with the three ligands were discussed. In aqueous solution, PhenDA forms stronger complexes with U(VI) than BiPDA due to its well-preorganized structure. In DMSO/20%(v)H2O solution, in contrast, the strong solvation effect of DMSO on the ligands reduces the energy gap between the trans- and cis-conformations of BiPDA, resulting in log β(UO2(BiPDA)) > log β(UO2(PhenDA)). The steric hindrance of methyl groups on DmBiPDA makes the complex UO2(DmBiPDA) of the lowest stability in both aqueous and DMSO/20%(v)H2O solutions. Single-crystal structural data of U(VI) complexes with the three ligands indicate that the ligand coordinates with UO22+ via aromatic nitrogen atoms and carboxylate oxygen atoms. There is no clear correlation between the trend of the stability constants in solutions and the U-N/O bond lengths of the three crystal complexes. Nevertheless, DmBiPDA coordinates to UO22+ in a high-strain fashion as a result of the steric hindrance of methyl groups while BiPDA in a low-strain fashion, which is in accordance with the relative complexation strength of the two respective complexes. The results from this work help us understand the effect of ligand preorganization and solvation on the binding strength of actinides with multidentate N,O-mixed ligands in solid and solutions, which is of importance in designing ligands for the partitioning of actinides from nuclear wastes.

Highly Efficient Thermally Activated Delayed Fluorescence Emitter Developed by Replacing Carbazole With 1,3,6,8-Tetramethyl-Carbazole.

Carbazole (Cz) is the one of the most popular electron donors to develop thermally activated delayed fluorescence (TADF) emitters, but additional groups are generally required in the molecules to enhance the steric hindrance between Cz and electron acceptor segments. To address this issue, we replaced Cz with its derivative 1,3,6,8-tetramethyl-carbazole (tMCz) to develop TADF emitters. Two novel compounds, 6-(4-(carbazol-9-yl)phenyl)-2,4-diphenylnicotinonitrile (CzPN) and 2,4-diphenyl-6-(4- (1,3,6,8-tetramethyl-carbazol-9-yl)phenyl) nicotinonitrile (tMCzPN) were designed and synthesized accordingly. With the same and simple molecular framework, tMCzPN successfully exhibits TADF behavior, while CzPN is a non-TADF fluorophor, as the additional steric hindrance of methyl groups leads to a more twisted structure of tMCzPN. In the organic light-emitting diodes (OLEDs), tMCzPN exhibits extremely high forward-viewing maximum external quantum efficiency of 26.0%, without any light out-coupling enhancement, which is significantly higher than that of 5.3% for CzPN. These results indicate that tMCzPN is an excellent TADF emitter and proves that tMCz is a more appropriate candidate than Cz to develop TADF emitters.

Direct ring C[sbnd]H bond activation to produce cresols from toluene and hydrogen peroxide catalyzed by framework titanium in TS-1

A series of TS-1 samples with varying Ti content was prepared and applied in the partial oxidation of toluene with H2O2 as oxidant. The TS-1 samples and relevant systems were characterized by XRD, FT-IR, in situ FT-IR, nitrogen adsorption/desorption, DR UV–vis, XPS, ICP-AES, EPR, SEM, TEM and TG. The results showed that the framework Ti species was responsible for the selective ring CH bond activation, resulting in toluene hydroxylation; while the extra-framework Ti species was advantageous to the formation of side chain oxidation products and to the deep oxidation of both kinds of the products formed. Under the optimized reaction conditions, the toluene conversion reached 38.9% with 84.3% selectivity to cresols (TOF, 2.66 h−1), while, the selectivity of para-cresol was 66.8%, which might be caused by the pore structure and steric hindrance of methyl group. The catalyst regenerated by calcination exhibited stable activity and selectivity after the second run.

Side-chain crystallization in alkyl-substituted cellulose esters and hydroxypropyl cellulose esters

The differences in side chain crystallization behavior between cellulose esters (CEs) and hydroxypropyl cellulose esters (HPCEs) were systematically investigated by a combination of differential scanning calorimetry (DSC) and small and wide-angle X-ray scattering techniques. DSC investigation indicated that under the same side chain length, the fusion enthalpy and the number of crystallized CH2 of CEs were smaller than HPCEs. At the same time, their d-spacing and molecular arrangements were also different from each other. For the CEs, the side chains are perpendicular to the main chain, while the side chains most probably tend to tilt to main chain in the HPCEs as was evidenced by X-ray scattering results. The phenomenon can be understood as a consequence of different flexibility of attachment bridges in both kinds of side chain polymers and the steric hindrance of methyl group in the hydroxypropyl group in HPCEs. In addition, the added hydroxypropyl substituents make the side chain length increasing.

Separation of Vitamin E on a 100-Å Phenogel Column

The effect of π-electrons and hydroxyl group on the separations of vitamin E on a swelling-controlled polystyrene-divinylbenzene (Phenogel) column using toluene/isooctane as the mobile phase was investigated. The effect of the π-electrons was demonstrated in the baseline separation of α-tocopherol and α-tocotrienol on a 100-Å Phenogel column. In addition, baseline separation of α-, (β-+γ-)- and δ-tocopherol could be achieved on this column. The separation mechanism of these isomers are due to the difference in the interactions between the hydroxyl group on the chromanol ring of each tocopherol and the gel matrix caused by the steric hindrance of methyl group(s). It was concluded that solutes of the same molecular size but different in the polar groups could be separated on a high performance size-exclusion chromatography by controlling the swelling of the gel matrix via modification of the mobile phase.

Hydrodeoxygenation of Cresols Over Mo/Al2O3 and CoMo/Al2O3 Sulfided Catalysts

The hydrodeoxygenation (HDO) of cresol isomers was investigated at 340 °C under 4 MPa over Mo/Al2O3 and CoMo/Al2O3 sulfided catalysts. Over both catalysts, the reactivity of cresols followed the order: m-cresol > p-cresol > o-cresol. These phenolic compounds were converted by two deoxygenation routes. The desired direct deoxygenation (DDO) route, leading to toluene, was strongly promoted by cobalt. On the contrary, the HYDrogenation route (yielding methylcyclohexenes and methylcyclohexane) was favored over Mo/Al2O3 and practically not affected by the presence of cobalt. To explain the formation of these deoxygenated products, the participation of different kinds of active sites such metal-decorated edge and brim-model was suggested. Thus, we proposed a reaction mechanism involving a schematic CoMoS active site which allowed to explain (i) the highest reactivity of m-cresol by the involvement of a stabilized cationic intermediate and (ii) the lowest reactivity of o-cresol, ascribed to a steric hindrance of methyl group during the initial tautomerization step.

Rational Design of Photoresponsive Supramolecular Assemblies Based on Diarylethene

Photochromism: Functionalization of a diarylethene derivative with π-conjugated oligomers resulted in a higher aggregation capability in the open rather than the closed form due to tunable steric hindrance of methyl groups of the diarylethene core (see scheme). Distinct aggregation abilities of the open and the closed isomers in nonpolar solvent and phototunable energy transfer between the two functional units enabled visible-light-triggered formation of fluorescent organogels. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Silylation of graphite oxide by dimethyloctylchlorosilane

Graphite oxide was silylated by dimethyloctylchlorosilane under various conditions. Silylated graphite oxide was successfully obtained and the maximum silicon content reached 0.17 mol/graphite oxide unit when the butylamine/dimethy-loctylchlorosilane/ graphite oxide unit ratio was about 19/16/1. The above maximum silicon content was much smaller than that obtained for graphite oxide silylated by octyltrichlorosilane, probably because of the steric hindrance of methyl groups against oxygen containing groups on the surface of graphite oxide. At the same time, the smaller amount of butylamine was needed to scavenge the HC1 molecules formed as a byproduct, reflecting the smaller number of chlorine atom in the dimethy-loctylchlorosilane molecule.

Spectroscopic study on binding behaviors of different structural nonionic surfactants to cyclodextrins

The binding of polyethylene glycol (10) n-octylphenyl ether (OPE) and polyethylene glycol (10) tert-octylphenyl ether (Triton X-100, TX) to β-cyclodextrin ( β-CD) and heptakis(2,3- O-dimethyl)- β-CD (DM- β-CD) was described in detail by surface tension, steady-state fluorescence of OPE and TX, and phosphorescence of 1-bromonaphthalene (BN) probe. Surface tension and fluorescence measurements show that β-CD entraps the hydrophobic moieties of OPE and TX to form inclusion complexes with the stoichiometry of 1:1. Unlike the n-octyl group of OPE, however, the tert-octyl group of TX fails to be encapsulated into the cavity of DM- β-CD because of the steric hindrance of methyl groups at the rim of the cavity. The inclusion of the phenyl group of OPE and TX was demonstrated by dynamic quenching effect of iodide ion on fluorescence of OPE and TX in the presence of β-CD. Static fluorescence quenching of OPE and TX by BN, phosphorescence of BN, and energy transfer between TX and BN provide additional evidence for the inclusion of their phenyl groups into the CD cavity. Analyses of molecular size suggest that the longer n-octyl group of OPE is situated in curled manner in the cavity and the tert-octyl group of TX undergo a slight distortion for fit of β-CD. Further introduction of the third guest component into the CD cavity occupied by OPE and TX will force the flexible octyl groups of OPE and TX to deform to a greater extent.

Hydrogen-bonding interaction of methyl-substituted pyridines with thioacetamide: steric hindrance of methyl group

The hydrogen-bonding interaction between a series of methyl-substituted pyridines as proton acceptors and thioacetamide as a proton donor in CCl 4 has been investigated using near-infrared absorption spectroscopy. The stability of the 1:1 hydrogen-bonded complex increases with the number of methyl groups and depends on the position of methyl groups. The steric hindrance of ortho-methyl groups particularly reduces the stability of complex. The relative stability agrees with the ease of miscibility of pyridines with water for methyl and dimethyl homologs. The calculated proton affinities and the DFT association energies using 6-31+G(d, p) and 6-311++G(2d, 2p) basis sets reveal the steric hindrance of ortho-methyl groups.

The steric hindrance of methyl groups in collisional quenching of highly vibrationally excited methyl-substituted pyrazines by He, Ar, and Kr

The collisional quenching of vibrationally excited methyl-substituted pyrazine series with an initial energy of 38 000–39 000 cm −1 by He, Ar, and Kr has been investigated using time-resolved infrared fluorescence of C–H stretching vibrations. The methyl group facilitates the quenching process, but its enhancement effect decreases as the number of methyl groups increases. The quenching efficiency of dimethylpyrazines depends on the position of methyl groups, which becomes significant when going from He to Kr. The steric hindrance of methyl groups plays an important role in the quenching of dimethylpyrazines and trimethylpyrazine by inert gas.

Dimethylpyridine-temperature programmed desorption (DMP-TPD) for measurement of strength of Brønsted and Lewis acid sites on metal oxide catalysts

Temperature-programmed desorption of dimethylpyridine (DMP-TPD) was performed for the measurement of strength of Brønsted acid and Lewis acid sites on metal oxide catalysts. Alumina, alumina–boria, silica–magnesia and silica–alumina were employed as examples of solid acid catalysts. Infrared (IR) spectra of adsorbed pyridine (Py) and ammonia showed that Lewis acid sites were abundant on alumina, Brønsted acid sites were abundant on alumina–boria, and both were present on silica–magnesia and silica–alumina surfaces. 2,6-dimethylpyridine-TPD (2,6-DMP-TPD) spectra of alumina depended strongly on purging temperatures. The amount of desorbed 2,6-DMP decreased with the increase in the purging temperature, and only a negligible amount of 2,6-DMP was detected in the TPD run from 523 K. On the other hand, 3,5-dimethylpyridine (3,5-DMP) adsorbed on Lewis acid sites on alumina. Selective adsorption of 2,6-DMP on Lewis acid sites was also confirmed from in-situ IR spectra. In the case of alumina–boria on which Brønsted acid sites are abundant, both 2,6-DMP-TPD and 3,5-DMP-TPD spectra gave similar desorption profiles. Profiles of acid strength determined from DMP-TPD agreed well with those determined from the IR spectra of adsorbed 3,5-DMP. These results demonstrated the selective adsorption of 2,6-DMP on Brønsted acid sites because of steric hindrance of methyl groups, and the possibility of measurement of respective strength of Brønsted acid and Lewis acid sites. Through a comparison of 2,6-DMP-TPD and 3,5-DMP-TPD spectra of silica–magnesia and silica–alumina, the scope of DMP-TPD for the measurement of strength of Brønsted and Lewis acid sites on metal oxide catalysts was discussed.

Hydrodesulfurization Reactivities of Narrow-Cut Fractions in a Gas Oil

A gas oil (bp 227--377 C) was fractionated by the distillation at intervals of 20 C into five fractions. Each fraction was desulfurized over the NiMo and CoMo catalyst, respectively, in an autoclave reactor at 360 C under a total pressure of 2.9 MPa to examine its hydrodesulfurization (HDS) reactivities. The fractions exhibited the different reactivities which were expressed by the pseudo-second-order kinetics, giving the decreasing rate constants of 0.636, 0.270, 0.118, 0.039, and 0.023 [min{times}wt %]{sup {minus}1} with an increase of their boiling point ranges. Composition analyses of the fractions indicated that alkyldibenzothiophenes, polyaromatic hydrocarbons, and nitrogen compounds in the heavier fractions restricted their reactivities. Steric hindrance of methyl groups as well as the competitive retardation of latter compounds are suggested. The NiMo catalyst showed higher activity for HDS of the heavier fractions than the CoMo through its higher hydrogenation activity, while the latter was slightly superior to the former in HDS of the lighter fractions. The polyaromatic species existing dominantly in the heaviest fraction (> 340 C) are responsible for the fluorescent color in the oils. On the basis of such understanding, a deep HDS process which combined a single-stage HDS for lighter fractions and a multistage HDSmore » for the heavier fractions was proposed and examined. Such a process achieved the sulfur level below 0.05 wt % with the color removal and its superiority over the conventional HDS process was discussed.« less

Solvent Effect on Radical Polymerization of Phenyl Acrylate

Abstract The radical polymerization of phenyl acrylate (PhA) in various aromatic solvents was studied under the same conditions as that of phenyl methacrylate (PMA) reported previously. The propagation rate constants (kp ) and termination rate constants (kt ) were determined by intermittent illumination methods. The variation in the kp values for PhA with aromatic solvents had the same trend as that for PMA. The solvent effect on PhA was somewhat larger than that on PMA, indicating that the steric hindrance of methyl group of the latter is important. Variation in kp values with solvents can be explained by the formation of an electron donor-acceptor complex and is correlated with the derealization stabilization of complex formation.

Synthesis and radical polymerization of spiro orthocarbonates bearing exomethylene groups

AbstractSynthesis and radical polymerization of spiro orthocarbonates (SOCs) bearing exomethylene groups at the α to the ether oxygen (1a–1e) were studied. SOCs 1a–1e were prepared by the successive reactions of dichlorodiphenoxymethane with two different diols followed by dehydrochlorination. Radical polymerization of the SOCs was carried out in the presence of an appropriate initiator (3 mol % versus monomer) at 130 and 180°C. The obtained polymer insoluble in n‐hexane contained both vinyl polymerization unit 13 and double ring‐opening polymerization unit 14 (ketone‐carbonates). The degree of ring‐opening followed the order: 1a <1b < 1d, 1e. n‐Hexane‐insoluble polymer was not obtained in the polymerization of 1c. Both steric hindrance of methyl group and ring size affected the degree of ring‐opening. The introduction of methyl group into SOC increased the degree of ring‐opening (1a <1b), whereas the degree of ring‐opening of either 1d, consisting of six‐ and seven‐membered rings, and 1e, consisting of two seven‐membered rings, was higher than those of 1a and 1b, consisting of five and seven‐membered rings. From the molecular orbital calculation (PM3, UHF method), it was concluded that if the first single ring‐opening occurs, then the successive second ring‐opening takes place more smoothly. The first ring‐opening requires more energy than the vinyl polymerization does. © 1994 John Wiley & Sons, Inc.

Role of steric hindrance of methyl groups on the rates of reduction of N-methyl substituted ethylenediamine complexes of platinum(IV)

The reduction of platinum(IV) complexes of the type [PtCl 4(N-N)] by FeCp 2 I −,[Pt(NH 2Et) 4] 2+ or [Pt(en)(NH 2Et) 2] 2+ in the presence of chloride ions in methanol have been investigated (N-N = en, meen, s-dmen, a-dmen, trimen, tmen). The reactions obey second- or third-order rate laws: rate = k FeCp 2 [PtCl 4(N-N)][FeCp 2, k I-[PtCl 4(N-N)][I −] or k Pt II [PtCl 4(N-N)][PtL 2+ 4][Cl −]. The rate constants increase by increasing the steric hindrance at the amine ends of the N-N ligand, even if in a measure which is quantitatively dependent on the reducing agent I − > FeCp 2 > [Pt(NH 2Et) 4] 2+ ≈ [Pt(en)(NH 2Et) 2] 2+. The steric hindrance of the terminal amine groups appears to behave as an additive parameter and linear relationships are found which correlate the reactivities (log k Reductant) of platinum(IV) complexes to a proposed parameter of relative overall steric hindrance of the N-N ligand. The observed reactivity trend of [PtCl 4(N-N)] complexes is thought to have basically a thermodynamic origin. Driving forces should also be responsible for the higher reactivity ( ca 20 times) of [Pt(en)(NH 2Et) 2] 2+ with respect to that of [Pt(NH 2Et) 4] 2+.

Stereospecificity of two plant diamine oxidases on 2-methyl-1,4-diaminobutane

Abstract The enzymatic deamination of pure enantiomers of 2-methyl-1-4-diaminobutane catalyzed by diamine oxidases from two plants (Euphorbia characias latex and Lens esculenta seedlings) was studied to investigate the conformations of the active sites. The results suggest that, in 2-methyl-1-4-diaminobutane, the steric hindrance of methyl group is primarily involved in decreasing activity compared with 1-4-diaminobutane (Pu), the natural substrate of diamine oxidases. This approach suggests that by replacement of hydrogen with a methyl group in the physiological substrate, useful informations on the active site of diamine oxidases from different sources can be obtained.