Showing papers on "Steric effects published in 1991"
TL;DR: The conformations of calix [4] arenes can be deduced from the 13 C NMR chemical shift of the methylene groups connecting each pair of aromatic rings.
Abstract: The conformations of calix [4] arenes can be deduced from the 13 C NMR chemical shift of the methylene groups connecting each pair of aromatic rings. Inspection of 24 cases revealed that when the phenol rings beside each methylene are in a syn orientation (i.e., in cone conformations), the methylene signals appear aroung δ31, whereas they appear around δ37 when both phenol rings are anti oriented (i.e., in 1,3-alternate conformations). Steric effects are believed to be the main cause of such large differences
532 citations
TL;DR: The extent to which local strain is present in the polypeptide backbone of folded protein molecules has been examined and it is revealed that there are relatively few sterically strained features.
Abstract: The extent to which local strain is present in the polypeptide backbone of folded protein molecules has been examined. The occurrence of steric strain associated with nonproline cis peptide bonds and energetically unfavorable main chain dihedral angles can be identified reliably from the well ordered parts of high resolution, refined crystal structures. The analysis reveals that there are relatively few sterically strained features. Those that do occur are located overwhelmingly in regions concerned with function. We attribute this to the greater precision necessary for ligand binding and catalysis, compared with the requirements of satisfactory folding.
208 citations
207 citations
TL;DR: In this paper, thermodynamic and kinetic studies for reactions of (tetramesitylporphyrinato) rhodium(II) porphyrins with toluene exclusively at the benzylic C-H bond are presented.
Abstract: Thermodynamic and kinetic-mechanistic studies are reported for reactions of (tetramesitylporphyrinato) rhodium(II) monomer, (TMP)Rh . , and (tetraxylylporphyrinato) rhodium (II) dimer, [(TXP)Rh] 2 , with methane that produce hybride and methyl derivatives. Comparative studies demonstrate that rhodium(II) porphyrins react with toluene exclusively at the benzylic C-H bond, and kinetic studies suggest that this reaction proceeds through a transition state related to that for the methane reactions. Aromatic C-H bond reactions are kinetically excluded for rhodium(II) prophyrins due to steric effects in the transition state
188 citations
TL;DR: The first unsymmetrically substituted dialkylgermylene, Me 5 C 5 GeCH(SiMe 3 ) 2 with LiC-(SiME 3 ) 3, was shown to be stable at ambient temperature in this article.
86 citations
77 citations
TL;DR: In this paper, surface properties of clean and hydrogenated diamond (100) have been calculated using the atom superposition and electron delocalization molecular orbital (ASED-MO) and ASED-band methods.
74 citations
74 citations
73 citations
TL;DR: The results pave the road to the development of a new asymmetric synthesis of enantiomerically pure α-substituted β-amino acids.
Abstract: β-Alanine, an inexpensive α-amino acid, was converted into the 2-tert-butylperhydropyrimidin-4-one derivative 2, which can be alkylated with high diastereoselectivity via the corresponding enolate. The high stereoselectivity observed for the reaction of 2-Li with electrophiles seems to be due to steric hindrance generated by an axial disposition of the tert-butyl group at C(2), which directs addition from the enolate face opposite to this group. The hydrolysis of the resulting adducts proceeds with 6N hydrochloric acid to afford α-substituted β-amino acids in good yields. These results pave the road to the development of a new asymmetric synthesis of enantiomerically pure α-substituted β-amino acids
71 citations
TL;DR: In this article, the steric interaction between R and the OH group was found to be more important than that between the CH3 group and the O-methyl-substituted series, RCON(CH3)OH (R = CH3, C2H5 and C6H5), showing that the series exhibits cis-trans isomerism about the CN bond.
Abstract: 1H and 13C NMR studies of N-methyl-substituted hydroxamic acids, RCON(CH3)OH (R = CH3, C2H5 and C6H5), show that the series exhibits cis–trans isomerism about the CN bond. The Z/E ratio increases in the series CH3 < C2H5 < n-C5H11 < n-C6H13 for a given solvent, indicating that steric interaction between R and the OH group is more important than that between R and the CH3 group. In DMSO-d6 the Z isomer is preferentially stabilized by solvation, whereas in C6D6 and toluene-d8 the unsolvated E form is thermodynamically more stable, as predicted by ab initio molecular orbital calculations. (12) At high concentrations (in CDCl3, CD2Cl2, C6D6 and toluene-d8) the E isomer may also be stabilized by intermolecular association. In the O-methyl-substituted series, RCONHOCH3 (R = CH3, C6H5), although the Z isomer predominates in all solvents, at low temperatures there is evidence for enolic forms.
TL;DR: In this article, simple diastereoselectivity of several important categories of aldol reactions of persubstituted enolates has been investigated for sterically least biased cyclic and acyclic ketone and aldehyde enolate reactions.
Abstract: Simple diastereoselectivity of several important categories of aldol reactions of persubstituted enolates has been investigated for sterically least biased cyclic and acyclic ketone and aldehyde enolates, and has been found to be useful for the stereoselective construction of quaternary carbon centers. The types of reactions examined involve the reaction of lithium, borinate, borate, trialkoxytitanium, trichlorotitanium, and zirconium (Cp 2 ZrCl) enolates, and the reactions of enol silyl ethers under high pressure, fluoride catalysis, and Lewis acid catalysis. In contrast to the less substituted metal enolates, uncatalyzed reactions of persubstituted metal enolates proceeded in a sense anticipated from the conventional Zimmerman-Traxler chair transition state (TS) model
TL;DR: In this article, the authors investigated the rate effect of ring substituenta in the addition of 1,3-dicarbonylalkyl radicals to ring-substituted styrenes in MeOH and/or MeCN.
Abstract: # The addition reactions of 1,3-dicarbonylalkyl radicals to ring-substituted styrenes have been kinetically investigated in MeOH and/or MeCN It ha^ been observed that the rate effect of ring substituenta ie nearly identical in the reactions of MeCOCHCOMe (11, MeOCOCHCOMe (2) and MeOCOCHCOOMe (4, the p value, in MeOH being 496, -101 and -106, respectively Since the three radicals are relatively strong oxidants and have similar reduction potentials, an important contribution of the charge transfer structure RCOCHCORCH&HAf+ to the addition transition state is suggested It has also be found that in the reactions of 1 and 4 with a-alkyl-substituted styrenes the rate of addition is strongly influenced by the nature of the alkyl group, decreasing in the order: Me > Et > 'Pr > tBu The observed effecta are much larger than those reported for the corresponding reactions of the nucleophilic cyclohexyl radical It is suggested that the a-alkyl substituents exert an effect of steric inhibition of resonance thereby ring delocalization of the charge and/or unpaired electron in the transition state is significantly reduced Delocalization may be more important in the reactions of 1 and 4 than in those of the cyclohexyl radical since it is possible that the former utilizes a transition state occurring later along the reaction coordinate and/or characterized by a larger extent of charge transfer
TL;DR: In this article, the steric effect in the scattered density distributions is determined by a quadrupole mass spectrometer, and it is found that the sterics effect in peak in the distribution of direct inelastically scattered molecules depends linearly on the reflection angle.
Abstract: A pulsed supersonic and cold oriented beam of NO molecules is incident upon the (111) face of clean Ag and Pt single crystal surfaces. The steric effect in the scattered density distributions is determined by a quadrupole mass spectrometer. It is found that the steric effect in the peak in the distribution of direct inelastically scattered molecules depends linearly on the reflection angle. In all circumstances O‐end collisions lead to scattering angles more inclined towards the surface than N‐end collisions. For the Pt(111) surface a much stronger steric effect is measured than for the Ag(111) surface. The steric effect seems to scale with the incident normal velocity. These strong steric effects can be explained by the larger trapping probability for the N‐end orientation and a leverage effect due to the high trapping probability.
TL;DR: 15N NMR chemical shifts and coupling constants indicate that the modified base exists as an 8-keto tautomers of adenine, and a subtle change in the structure of the NH2 group results in a better understanding into N7-N9 tautomerism ofAdenine.
Abstract: Natural abundance 15N NMR spectroscopy and ancillary spectroscopic techniques have been employed to study the solution structure of 8-hydroxyadenosine. 8-Hydroxyadenosine is a naturally occurring oxidized nucleic acid adduct that is generally implied to have an 8-hydroxy tautomeric structure. 15N NMR chemical shifts and coupling constants, however, indicate that the modified base exists as an 8-keto tautomer. The pH dependence of 15N NMR and UV spectra showed the presence of two pKa's, at 2.9 and 8.7, corresponding to protonation at N1 and ionization at N7, respectively. The latter results in the formation of an 8-enolate structure. Unusual upfield shifts of the 1H and 15N resonances of the NH2 group, and a reduction in the one-bond coupling constant 1JN6-H6, is indicative of an unfavorable steric or electronic interaction between the NH2 group and the adjacent N7-H proton. This interaction results in a subtle change in the structure of the NH2 group. In addition to being a possible mechanism for alteration of hydrogen bonding in oxidized DNA, this type of interaction gives a better understanding into N7-N9 tautomerism of adenine. Furthermore, the structure of 8-hydroxyadenosine has been related to possible mechanisms for mutations.
TL;DR: In this article, two kinds of N-salicylideneaniline derivatives, N-tetrachlorosalicylidsene-1-pyrenylamine (CL4SA) and CL4SPY, have been prepared and subjected to structural and optical studies in the crystalline state.
Abstract: Two kinds of N-salicylideneaniline derivatives, N-tetrachlorosalicylideneaniline (CL4SA) and N-tetrachlorosalicylidene-1-pyrenylamine (CL4SPY), have been prepared and subjected to structural and optical studies in the crystalline state. A thermochromic-type behavior of the former crystal has been observed as a shift of the absorption edge, while that of the latter crystal is much less pronounced. Fairly short O–H···N hydrogen bonds have been found in both compounds through crystallographic studies, which may be mainly due to a steric effect of chlorine substituents. The difference in the proton transfer behavior of CL4SA and CL4SPY should presumably be caused by an intermolecular charge-transfer interaction in the latter crystal.
TL;DR: In this article, simple allenes have been converted cleanly to the corresponding 1,4-dioxaspiro [2.2] pentanes in good yields by oxidation with dimethyldioxirane.
Abstract: A number of simple allenes have been converted cleanly to the corresponding 1,4-dioxaspiro [2.2] pentanes in good yields by oxidation with dimethyldioxirane. Mono- and trisubstituted allenes give the anti diastereomers with good stereoselectivity, whereas 1,3-disubstituted allenes show only a slight preference for the anti,anti isomers, unless steric effects are extreme. Stereochemical assignments are based on steric considerations and a consistent set of NMR characteristics that permit structural attributions
TL;DR: The rate at which different metal hydrides carry out this reaction is mainly dependent on the bond dissociation energy of the M-H bond, with thermodynamics playing a less important role.
Abstract: Cleavage of the M-H bond of transition metal hydrides can result in delivery of a hydride (H−), hydrogen atom (H), or proton (H+) to an organic or organo-metallic substrate. Hydrogen atom transfer from a metal hydride to an olefin is an endothermic reaction; the rate at which different metal hydrides carry out this reaction is mainly dependent on the bond dissociation energy of the M-H bond. Hydrogen atom transfer from a metal hydride to a carbon-centered radical is an exothermic reaction in which the rate is largely determined by steric demands of the ligands of the metal hydride, with thermodynamics playing a less important role. Some neutral metal hydrides can function as hydride donors to carbocations. Anionic metal hydrides also exhibit hydridic reactivity, but in some cases the actual M-H bond cleavage step involves hydrogen atom transfer as a part of a radical chain reaction. Neutral metal hydrides can function as nucleophiles and as weakly coordinating ligands to coordinatively unsaturate...
TL;DR: In this article, the acidity and homolytic bond dissociation energies of diphenylmethanes, including 9-G-fluorenes and Ph 2 CHG, were investigated.
Abstract: The acidities and homolytic bond dissociation energies (BDEs) of the acidic H-C or H-N bonds in six α-substituted diphenylmethanes, Ph 2 CHG, nine remotely substituted diphenylamines, four bridged diphenylamines (iminodibenzyl, iminostilbene, phenothiazine, phenoxazine), three carbazoles, indole, and pyrrole are reported. The near constancy of the increased acidity for 9-G-fluorenes compared to α-G-diphenylmethanes, which represents the approximate relative aromatic stabilization energies of the fluorenide anions, indicates that electronic effects on 9-GFl − and Ph 2 CG − ions do not differ greatly. On the other hand, steric effects in Ph 2 CG • radicals cause destabilizing effects when G is SPh, CO 2 Et, or SO 2 Ph, whereas, except for SO 2 Ph, these functions are stabilizing in 9-GFl • radicals
TL;DR: In this paper, a Supersonic jet mass resolved excitation spectroscopy was employed to determine the minimum energy conformations of benzyl alcohol and a series of nine methyl-, ethyl-, fluoro-, and aminobenzyl alcohols.
Abstract: : Supersonic jet mass resolved excitation spectroscopy is employed to determine the minimum energy conformations of benzyl alcohol and a series of nine methyl-, ethyl-, fluoro-, and aminobenzyl alcohols. The interpretation of the time-of-flight mass spectra (TOFMS) of these molecules leads to the assignment of specific molecular geometries for each system. The minimum energy conformation of the -CH2O moiety is determined to be perpendicular relative to the plane of the aromatic ring. The hydroxy proton in the sterically unemcumbered benzyl alcohol points toward the benzene ring. The potential energy barrier for the low frequency torsional motion of the hydroxymethyl group arises mainly from an internal hydrogen bonding interaction between the OH group and pi-system of the ring. (jg)
TL;DR: In this article, the stereoselectivity of amides of 2,5-dimethylpyrrolidine was investigated for radical addition to acyclic chiral alkenes.
Abstract: The two alkenes studied were both amides of 2,5-dimethylpyrrolidine, available as either the R,R or S,S enantiomer from D- or L-alanine. Our alkene studied was the unsymmetrical monoamide derived from 4-oxo-2-pentenoic acid (1), while the other substrate examined was the diamide of fumaric acid, 2. Hexyl, cyclohexyl, and tert-butyl radical addition to the amide of 4-oxo-2-pentenoic acid (1) gave approximately equal amounts of addition at the carbonyl and amide ends of the alkene. This approach provides the highest reported stereoselectivity for radical addition to an acyclic chiral alkene. Furthermore, a rationale for the diastereoselectivity is presented that suggests that the amide selectivity in steric in origin and will be general
TL;DR: Steric influences on the insertion of CO 2 into metal-oxygen bonds have been investigated in this paper, where the steric bulk of the cis phosphine ligand has been attributed to the stericity of the ligand.
Abstract: Steric influences on the insertion of CO 2 into metal-oxygen bonds have been investigated The compounds [Et 4 N][W(CO) 5 O-2,6-Ph 2 -C 6 H 3 ] and [Et 4 N][W(CO) 4 (PPh 2 Me)OPh] have been synthesized and their reactivites toward CO 2 studied The anion demonstrated a retarded rate of reaction with CO 2 as compared to the parent complex, [Et 4 N][W(CO) 5 OPh], a result ascribed to the steric bulk of the cis phosphine ligand
TL;DR: In the case of Bilirubin IXα as discussed by the authors, the insertion of a methyl group at the pro-R site on the α-carbon of the propionic acid destabilizes the M-chirality intramolecularly hydrogen-bonded conformational enantiomer by introducing a severe nonbunded CH 3 |CH 3 steric interaction with a pyrrole methyl substituent.
Abstract: (4Z,15Z)-Bilirubin IXα, the cytotoxic yellow tetrapyrrole pigment of jaundice, readily adopts either of two interconverting, enantiomeric conformations, which are stabilized through complementary intramolecular hydrogen bonding between the pyrrole and lactam N-H and C=O residues of one dipyrrinone moiety and the CO 2 H group of propionic acid side chains on the second dipyrrinone. One conformational enantiomer can be destabilized relative to the other through allosteric action by judicious placement of methyl groups in the propionic acid side chains. Thus, insertion of a methyl group at the pro-R site on the α-carbon of the propionic acid destabilizes the M-chirality intramolecularly hydrogen-bonded conformational enantiomer by introducing a severe nonbonded CH 3 |CH 3 steric interaction with a pyrrole methyl substituent
TL;DR: In this paper, the steric interactions between the substituents at opposite bridgehead positions have been revealed by NMR and DNMR spectrometry, showing remarkable size-dependent hindrance of the degenerate interconversion of the two S4 symmetric conformers of the fenestrindans.
Abstract: All-cis-Tetrabenzo[5.5.5.5]fenestranes (fenestrindans) with four bridgehead substituents (4-10) and two doubly bridged, centrohexacyclic analogues, the bis-endo-peroxide 11 and the bis-disulfide 12, have been synthesized from the tetrabromofenestrindan 3. Pronounced steric interactions between the substituents at opposite bridgehead positions have been revealed by NMR and DNMR spectrometry, showing remarkable size-dependent hindrance of the degenerate interconversion of the two S4 symmetric conformers of the fenestrindans.
TL;DR: The mononuclear, four-coordinate aluminum compounds AlR 2 (BHT)L and AlR(BHT 2 L as discussed by the authors were obtained when treated with the correct molar equivalent of BHT-H and the appropriate Lewis base.
TL;DR: The surprisingly small steric shielding of the protein redox sites in the covalent complex, as deduced from the reactions at high ionic strength, may indicate that the proteins have multiple reaction domains on their surfaces or that the complex is dynamical or both.
Abstract: Cytochrome c and plastocyanin form an electrostatic complex, which can be reinforced by amide bonds in the presence of a carbodiimide. Besides this cross-linking, carbodiimide also converts carboxylate side chains into neutral N-acylurea groups. Four derivatives of the covalent diprotein complex, which differ in the degree of this charge neutralization, are separated by cation-exchange chromatography. Electron-transfer reactions at different ionic strengths involving the electrostatic complex and the four derivatives of the covalent complex are studied by laser flash photolysis with flavin semiquinones as reducing agents. The reactivity of the associated proteins toward external reductants cannot be predicted simply on the basis of this reactivity of the separate proteins. Qualitative analysis of the dependence on ionic strength of the reactions between FMN semiquinone and the covalent derivatives indicates sites at which this reductant interacts with the cross-linked proteins. The surprisingly small steric shielding of the protein redox sites in the covalent complex, as deduced from the reactions at high ionic strength, may indicate that the proteins have multiple reaction domains on their surfaces or that the complex is dynamical or both. The intracomplex (unimolecular) electron-transfer reaction is fast in the electrostatic complex (ket = 1300 +/- 200 s-1) but undetectably slow in each of the four derivatives of the covalent complex (ket less than 0.2 s-1).(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: In this article, the Fourier transform-ion cyclotron resonance (FT-ICR) spectrometry was used to measure deprotonation and protonation rates for alkyl benzenes.