Influence Of Steric Factors Research Articles

Influence of steric factors on reversible reactions of hydrogenation-dehydrogenation of polycyclic aromatic hydrocarbons on a Pt/C catalyst in hydrogen storage systems

The reversible reactions of hydrogenation-dehydrogenation of polycyclic aromatic hydrocarbons with a different extent of condensation (naphthalene, anthracene, biphenyl, meta- and para-terphenyl isomers) were studied in comparison with benzene on a 3%Pt/C catalyst. Hydrogenation of naphthalene and anthracene was carried out at T = 280 °C, P = 90 atm, whereas the reaction of benzene, biphenyl and terphenyl isomers was performed at T = 180 °C, P = 70 atm in an autoclave. Dehydrogenation of the corresponding polycyclic naphthenes (cyclohexane, decalin, perhydroanthracene, bicyclohexyl, ortho, meta, and para-isomers of perhydroterphenyl) was carried out in the temperature range of 280 to 340 °C in a flow setup at the volume hourly space velocity of 1 h−1 and atmospheric pressure. Correlation dependences of the reaction rates of hydrogenation-dehydrogenation of the studied substrates on their structure and extent of condensation were found.

Study of gas-phase reactions within the modified Marcus model. III. CH4 + CH3 → CH3 + CH4. 250–2000 K

Previous work (Romanskii, 2018) was devoted to research in the framework of the modified Marcus model (calculation level UCCSD(T) 6-311++G**//B3LYP/6-31+G**) on the reaction kinetics of methane with a methyl radical in the temperature range of 10–800 K. In the present work, these studies are extended to the region of high temperatures, up to 2000 K. Analysis of data on the kinetics and KIE in the temperature range of 250–2000 K is carried out in comparison with the data of studies performed using the quantum scattering method (Remmert et al., 2009). The abnormally high values of kH/kD observed in our work are considered a consequence of a violation of the generalized Frank Condon principle (GFCP) for the deuterium atom transfer reaction. The influence of steric factor on the value of kH/kD is quantitatively analysed; the similarity of the obtained picture to similar observations for the reaction of enzymes is noted.

Influence of Steric Factors on Chiral Lithium Amide Aggregates

The solution structures of three mixed aggregates dissolved in toluene-d8 consisting of the lithiated amides derived from (S)-N-isopropyl-1-((triisopropylsilyl)oxy)propan-2-amine, (R)-N-(1-phenyl-2-((triisopropylsilyl)oxy)ethyl)propan-2-amine, or (S)-N-isobutyl-3-methyl-1-((triisopropylsilyl)oxy)butan-2-amine and n-butyllithium are characterized by various NMR experiments including diffusion-ordered NMR spectroscopy with diffusion coefficient-formula weight correlation analyses (D-FW) and other one- and two-dimensional NMR techniques. We report that steric hindrance of R1 and R2 groups of the chiral lithium amide controls the aggregation state of the mixed aggregates. With a less hindered R2 group, lithium (S)-N-isopropyl-1-((triisopropylsilyl)oxy)propan-2-amide forms mostly a 2:2 ladder-type mixed aggregate with n-butyllithium. Increase of steric hindrance of the R1 and R2 groups suppresses the formation of the 2:2 mixed aggregate and promotes formation of a 2:1 mixed aggregate. We observe that lithium (S)-N-isobutyl-3-methyl-1-((triisopropylsilyl)oxy)butan-2-amide forms both a 2:2 mixed aggregate and a 2:1 mixed trimer with n-butyllithium. Further increase in the steric hindrance of R1 and R2 groups results in the formation of only 2:1 mixed aggregate as observed with lithium (R)-N-(1-phenyl-2-((triisopropylsilyl)oxy)ethyl)propan-2-amide.

Dual Role of Acetate as a Nucleophile and as an Internal Base in Cycloplatination Reaction of sym-N,N′,N″-Triarylguanidines

Reaction of cis-[Cl(2)Pt(S(O)Me(2))(2)] with 1 equiv of sym-N,N',N″-triarylguanidines, ArN═C(NHAr)(2) (sym = symmetrical; Ar = 2-MeC(6)H(4) (LH(2)(2-tolyl)), 2-(MeO)C(6)H(4) (LH(2)(2-anisyl)), 4-MeC(6)H(4) (LH(2)(4-tolyl)), 2,5-Me(2)C(6)H(3) (LH(2)(2,5-xylyl)), and 2,6-Me(2)C(6)H(3) (LH(2)(2,6-xylyl))) in toluene under reflux condition for 3 h afforded cis- or trans-[Cl(2)Pt(S(O)Me(2))(ArN═C(NHAr)(2))] (Ar = 2-MeC(6)H(4) (1), 2-(MeO)C(6)H(4) (2), 4-MeC(6)H(4) (3), 2,5-Me(2)C(6)H(3) (4), and 2,6-Me(2)C(6)H(3) (5), respectively) in 83-96% yield. Reaction of cis-[Cl(2)Pt(S(O)Me(2))(2)] with 1 equiv of LH(2)(2-tolyl) and LH(2)(4-tolyl) in the presence of 1 equiv of NaOAc in methanol under reflux condition for 3 h afforded acetate-substituted products, cis-[(AcO)ClPt(S(O)Me(2))(ArN═C(NHAr)(2))] (Ar = 2-MeC(6)H(4) (6) and 4-MeC(6)H(4) (7)) in 83% and 84% yields, respectively. Reaction of cis-[Cl(2)Pt(S(O)Me(2))(2)] with 1 equiv of LH(2)(2-anisyl) and LH(2)(2-tolyl) in the presence of 1 equiv of NaOAc in methanol under reflux condition for 3 and 12 h afforded six-membered [C,N] platinacycles, [Pt{κ(2)(C,N)-C(6)H(3)R-3(NHC(NHAr)(═NAr))-2}Cl(S(O)Me(2))] (Ar = 2-RC(6)H(4); R = OMe (8) and Me (9)), in 92% and 79% yields, respectively. The new complexes have been characterized by analytical and spectroscopic techniques, and further the molecular structures of 1, 2, 4, 5, 6, and 8 have been determined by single-crystal X-ray diffraction. The platinum atom in 1, 4, and 5 exhibited the trans configuration, while that in 2, 6, and 8 exhibited the cis configuration. Complex 6 is shown to be the precursor for 9, and the former is suggested to transform to the latter possibly via an intramolecular C-H activation followed by elimination of AcOH. The solution behavior of new complexes has been studied by multinuclear NMR ((1)H, (195)Pt, and (13)C) spectroscopy. The new complexes exist exclusively as a single isomer (trans (1 and 5) and cis (6 and 7)), a mixture of cis and trans isomers with the former isomer being predominant in the case of 2 and the latter isomer being predominant in the case of 3. Complex 5 in the trans form revealed the presence of one isomer at 0.007 mM concentration and two isomers in about 1.00:0.12 ratio at 0.154 mM concentration as revealed by (1)H NMR spectroscopy, and this has been ascribed to the restricted Pt-S bond rotation at higher concentration. Platinacycle 8 exists as one isomer, while 9 exists as a mixture of seven isomers in solution. The influence of steric factor, π-acceptor property of the guanidine, subtle solid-state packing forces upon the configuration of the platinum atom, and the number of isomers in solution have been outlined. Factors that accelerate or slow down the cycloplatination reaction, the role of NaOAc, and a plausible mechanism of this reaction have been discussed.

Acylative kinetic resolution of racemic heterocyclic amines using N-phthaloyl-(S)-amino acyl chlorides with alkyl side chains

A comparative study of the acylative kinetic resolution of racemic 2-methyl-1,2,3,4 tetrahydroquinoline and 3,4-dihydro-3-methyl-2H-[1,4]benzoxazine using N-phthaloyl-(S)-amino acyl chlorides with alkyl side chains has been carried out. The influence of steric factors on the stereoselectivity of the acylation was demonstrated. The (S)-enantiomers of the heterocyclic amines (ee >99%) were obtained in good yields via a kinetic resolution protocol using N-phthaloyl-(S)-leucyl chloride.

Supramolecular architectures of novel chromium(iii) oxalate complexes: steric effects of the ligand size and building-blocks approach

Five new oxalate complexes of chromium(III), [Hphen][Cr(phen)(C(2)O(4))(2)]·2H(2)O (1), [Cr(phen)(2)(C(2)O(4))][Cr(phen)(C(2)O(4))(2)]·3H(2)O (2), [Cr(phen)(2)(C(2)O(4))]NO(3)·H(2)C(2)O(4)·H(2)O (3), [Cr(bpy)(2)(C(2)O(4))][Cr(bpy)(C(2)O(4))(2)]·3H(2)O (4) and [Cr(bpy)(2)(C(2)O(4))]NO(3)·1/2H(2)C(2)O(4)·4H(2)O (5) (phen = 1,10-phenanthroline, bpy = 2,2'-bipyridine), were prepared by using an (oxalato)tantalate(V) solution as a source of oxalate ligands. The compounds contain either the discrete mononuclear [Cr(L)(2)(C(2)O(4))](+) cation [L = phen (3); L = bpy (5)] or the discrete mononuclear [Cr(L)(C(2)O(4))(2)](-) anion [L = phen (1)], or both types of mononuclear ions [L = phen (2); L = bpy (4)]. The crystal structures are dominated by the hydrogen-bonding and π···π-stacking interactions that give rise to the overall two- (compounds 1, 2, 4, 5) or three-dimensional (compound 3) architectures. Compounds 2 and 4 represent a borderline case between isostructurality and non-isostructurality; they exhibit an analogous packing of the cation and the anion units, but the crystallization water molecules occupy different positions - due to a difference in size between the phen and bpy ligands. The influence of steric factors is evident also in the case of 3 and 5, which, despite very similar chemical formulae, exert a completely different packing of the constituents. By the self-assembling of 1 and 4, used as building blocks in the reaction with calcium(II) cations, the heterobimetallic polymeric compounds {[CaCr(2)(phen)(2)(C(2)O(4))(4)]·5H(2)O}(n) (6) and {[CaCr(2)(bpy)(2)(C(2)O(4))(4)]·0.83H(2)O}(n) (7) were obtained. The crystal structure of 7 is reported: the [Cr(bpy)(C(2)O(4))(2)](-) unit, through the two oxalate groups, acts as a chelating ligand towards Ca cations, resulting in heterometallic one-dimensional double zigzag chains, formed of diamond-shaped units. The characterization of the compounds obtained was accomplished by the spectroscopy and thermal analysis methods.

Influence of steric factors on the direction of reactions

Our attempts to convert it into hydroxycyclohexanone II by ozonolysis under the conditions of Criegee rearrangement [2, 3] resulted only in diketone III [1] inert with respect to m-chloroperbenzoic acid. We also failed to perform the oxidative cleavage of epoxyalcohol IV by treating with HIO4, we obtained only diol V. At the reduction of epoxyalcohol IV with LiAlH4 alongside the expected diol VI we isolated regioisomeric diols VII and O O O

Structural Concept of Nitroxide As a Lipid Peroxidation Inhibitor

Nitroxides have antioxidative activities toward lipid peroxidation, but the influence of steric factors is not known. We synthesized alkyl-substituted nitroxides at the α-position of the N-O moiety to enhance lipophilicity and the bulk effect. There was good correlation between the IC(50) and lipophilicity (log P(o/w)) of nitroxides with use of the thiobarbituric acid-reactive substances (TBARS) assay. Furthermore, an inhibitory effect on the TBARS assay was dependent upon the number and length of alkyl groups, though nitroxides had almost identical lipophilicity.

ChemInform Abstract: Formation and Structural Aspects of Four-, Five-, and Six-Membered Phosphorus Nitrogen Ring Compounds.

Abstract The donor-stabilized dithiomonometaphosphoryl chloride py-PS2Cl (py = pyridine) reacts with primary amines, RNH2, in defined molar ratios to four-ring heterocycles. Whether diaza - or azathiadiphosphetidines are formed depends on the rest R, the basicity of the amines and the reaction conditions. In a similar way the ring size of the P-N heterocycles obtained from py-PS2Cl and monosubstituted hydrasines is determined by the substituents at hydrazine, The influence of steric factors on the probable reaction mechanisms and some structural features of the obtained heterocycles are discussed.

Synthesis and Electrooptical Properties of Metal-Containing Azopolymers. The Influence of Steric Factors on the Electro-Optical Effect in Polycomplexes of Azobenzene Derivatives with Cobalt

We propose new structures of metal-containing azopolymers with changeable electro-optical properties in consequence of different coordination metal ions. The new monomers – 4-methacroyloxy-(4′-carboxy-3′-hydroxy)-azobenzene, 4-methacroyloxy-2′-carboxy-azobenzene and their complexes with Co and polymers based of these ones are synthesized. The influence of the electric field on the absorption spectra of new metal-containing azopolymers films are investigated. Electro-optical properties of polymeric films is determined by the re-orientation of dipole moments of azobenzene groups photo induced by polarized light in the electric field. Increase of structural inflexibility by the photo isomerization of azobenzene groups decrease the electro-optical effect value.

Co-crystallisation of 2,2′-dithiodibenzoic acid with the isomeric n-pyridinealdazines, n = 2, 3 and 4: supramolecular polymers and the influence of steric factors upon aggregation patterns

The structure of the 1:1 co-crystal formed between the dicarboxylic acid, 2,2′-dithiodibenzoic acid, and each of the isomeric 3- and 4-pyridinealdazines features a supramolecular zig-zag chain constructed about the O–Hacid⋯Npyridine synthon. By contrast, using the same experimental conditions, only a 2:3 co-crystal could be formed when the isomeric 2-pyridinealdazine was used. Here, a pentamer is found, stabilised by the O–Hacid⋯Npyridine synthon and these associate to form a supramolecular chain via C–H⋯π contacts. Theory shows no significant differences in the basicity of the pyridine-nitrogen atoms leading to the conclusion that it is the steric congestion related to the relative disposition of the 2-pyridine nitrogen that is responsible for the observed behaviour.

Near-infrared photosensitive composites with electron conductivity

Polymer composite films were prepared on the basis of poly(vinyl ethylal) doped with a low concentration of an anionic polymethine dye that absorbs light in the near IR region and a higher concentration of 2,4,5,7-tetranitro-9-fluorenone or tetrakis(benzylamino)octahydrofullerene organic acceptor. The photoconductivity of the polymer composite films increases on passing to fullerene derivatives. The main reason for this effect is a decrease in the influence of steric factors on intermolecular electronic transitions in the case of the near-spherical structure of acceptor molecules as compared with the planar structure.

Synthesis and Characterization of New Trimeric Rhenium(I) Complexes. The Influence of Steric Factors on the Size of Pyrazolonaterhenium(I) Metallomacrocycles

The reaction of [ReX(CO)5] with thiosemicarbazones H2L(R) derived from beta-keto esters (X = Cl, Br; R = Me, Ph) allowed the isolation of cyclic trimeric complexes [Re3(pyz(R))3(CO)9], where pyz(R) is the pyrazolonate corresponding to the thiosemicarbazone. Electron spray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) monitoring of the reactions of H2L(Ph) in toluene confirmed that the trimer was formed in the reaction mixture and no higher oligomer was detected. These studies, together with the X-ray structures of the trimeric complexes, afford new insight into the factors influencing the self-assembly of pyrazolonaterhenium(I) complexes.

疎水性化学物質の魚類における濃縮性に及ぼす分子の立体的かさ高さの影響について

疎水性化学物質の魚類における濃縮性に及ぼす分子の立体的かさ高さの影響について

Influence of steric factors on exciplex energy and magnetic field effect

With the advent of spin chemistry, magnetic field effect (MFE) on exciplex luminescence has emerged as an important domain of research. MFE is a diffusion controlled phenomenon and hence is solvent dielectric ( ε) dependent. It maximizes at a particular ε ( ε max) for a specific exciplex system. Various attempts have been made to explain the variation of this ε max from one exciplex to another. In our present work we have succeeded for the first time to enmark the energy of exciplex ( E ex) as the prime factor in determining the ε max. We have indicated a definite inverse correlation (1:1) between ε max and E ex. We have also tried to correlate some parameters that are important in exciplex formation, e.g. Charton's steric constant ( ν c), repulsive energy ( R e) and E ex.

Chalcogen-Containing π-Complexes of Lanthanides: Crystal-Chemical Analysis and a System of Descriptors for Intermolecular Contacts

A sample comprising 241 π-complexes LnC n H m Z k (Z = O, S, Se, Te) was used to analyze the influence of the nature, the oxidation state, and the coordination number of the central atom and the nature of the Z atom on the activity area of the lanthanide (Ln). A new system of criteria based on the concepts of the atomic and molecular Voronoi–Dirichlet polyhedra is proposed for the quantitative analysis of intermolecular contacts. This system was used to analyze the influence of steric factors on the stability of complex groups and the presence or absence of agostic interactions in the structures of 475 complexes LnC n H m , LnC n H m X k (X = F, Cl, Br), and LnC n H m Z k .

ChemInform Abstract: Palladium‐Catalyzed Allylic Alkylation Using Pyridino‐Oxazolines and Quinolino‐Oxazolines as Ligands — Influence of Steric Factors.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Investigation of selectively substituted monosaccharides by the method of vibrational spectroscopy and X-ray analysis

The influence of steric factors on the formation of vibrational spectra of carbohydrates was illustrated by the example of selectively substituted monosaccharides. We consider compounds in which the difference and similarity of vibrational spectra are largely determined by the difference and similarity of the geometry of their molecular base (the molecule's skeleton). At the same time, in the case of selectively substituted nitrates of methyl-β-d-glucopyranoside, the skeletal bases of which are identical, with differences in the localization site of volumetric substituents and their total number, or in rotational isomers (conformations) of the lateral nitrate groups, their Fourier Raman spectra are different over a wide spectral range. It is concluded that steric factors and the position of volumetric substituents of hydroxyls produce an exceptionally important effect on the formation of vibrational spectra of carbohydrates.

New Axially Chiral Bis(dihydrooxazoles) as Ligands in Stereoselective Transition‐Metal Catalysis

AbstractThe new, axially chiral bis(4,5‐dihydrooxazoles) 4 have been synthesized in a straightforward manner, starting from the substituted, racemic 1,1′‐biphenyl‐2,2′‐dicarboxylic acids 1 and optically active amino alcohols 2. The adducts were resolved by medium‐performance liquid chromatography (MPLC; see Scheme 1). Formation of Cu1 complexes of 4 was followed by 1H‐NMR spectroscopy. The catalytic behavior of these complexes has been investigated by asymmetric cyclopropanation of styrene with ethyl diazoacetate. Beside the influence of steric factors, a significant electronic effect on asymmetric induction could also be observed (see Scheme 2 and Table).

Stereochemical factors affecting second-sphere co-ordination of ferrioxamine B with cis-syn-cis and cis-anti-cis isomers of dicyclohexano-18-crown-6 in chloroform and a comparison with alkali-metal and ammonium cations

Second-sphere co-ordination of ferrioxamine B [Fe(HL)]+ by dicyclohexano-18-crown-6 (eicosahydrodibenzo[b,k][1,4,7,10,13,16]hexaoxacyclooctadecine) occurs through host–guest complex formation with the protonated amine side chain (CH2)5NH3+. The influence of steric factors on the association constants (Ka) for both nitrate and perchlorate salts of ferrioxamine B with cis-syn-cis and cis-anti-cis isomers of dicyclohexano-18-crown-6 (CE) in CHCl3 has been investigated. For comparison purposes, association of the perchlorate salt of the indium(III) analogue of ferrioxamine B [In(HL)]+ and the picrate salts of NH4+ and alkali-metal cations are included in this study (picrate = 2,4,6-trinitrophenolate). Although the absolute values of Ka for [M(HL)]X association with CE are strongly dependent on the anion identity, the selectivity ratio expressed for the syn crown ether isomer is 3 : 1 and is independent of anion variations. Comparison with alkali-metal and ammonium cations shows that the discrimination between syn and anti isomers of cis-dicyclohexano-18-crown-6 is dependent upon cation size and is the least for K+ where the ionic radius best matches the cavity size. While syn isomer discrimination in CHCl3 is closely related to the ionic radius, in water it is related to the hydrated radius. The syn over anti isomer preference exhibited by alkali-metal and ammonium cations is much more pronounced in water than in chloroform, which is likely due to the steric hindrance imposed by the large effective radius of the cations due to their hydration shell. Discrimination between the syn and anti isomers by [M(HL)]+ in CHCl3 is greater than expected based on comparison with the parent NH4+ ion. This is likely due to the significant steric requirements of the bulky metal complex molecule. The selectivity of the syn over the anti isomer is dominated by the preference of the bulky [M(HL)]+ complex for the less hindered face of the syn isomer.