Steric effects

About: Steric effects is a research topic. Over the lifetime, 16112 publications have been published within this topic receiving 319615 citations. The topic is also known as: steric hindrance.

Comparative study of the catalytic oxidation of catechols by copper(II) complexes of tripodal ligands

Abstract: Copper(II) complexes of the ligands tris(2-pyridylmethyl)amine (tpyma), tris(2-pyridylethyl)amine (tpyea), tris(3,5-dimethylpyrazol-1-ylmethyl)amine (tpzma) and tris(3,5-dimethylpyrazol-1-ylethyl)-amine (tpzea) were prepared. The complexes, [Cu(ligand)Cl]Cl or [Cu(ligand)(H2O)][BF4]2, were characterized by a combination of absorption and EPR spectroscopies and chemical analysis. The ability of the complexes to oxidize 3,5-di-tert-butylcatechol to 3,5-di-tert-butyl-o-benzoquinone has been studied and the results show that the rate of reaction is dependent on the nature of the heterocyclic donor, its basicity, steric considerations, the chelate ring size and the type of exogenous donor present. Large variations in the rate were observed with the most effective catalysts being those with pyridine donors which formed six-membered chelate rings; the complex [Cu(tpyea)(H2O)][BF4]2 was the most active while [Cu(tpzea)(H2O)][BF4]2 and [Cu(tpzea)Cl]Cl were inactive. Electrochemical data for the series of compounds show that there is a non-linear relationship between their ability to oxidize catechols and their reduction potentials. The most effective catalysts were those complexes which exhibited reduction potentials close to 0.00 V, while those that deviated from that potential by 200–300 mV in either direction were largely inactive. Within the range of complexes which were active, a steric match between the substrate and the complex also largely defined their reactivity. Comparisons to the biological system tyrosinase are drawn.

Hydrogen bond strength and β‐sheet propensities: The role of a side chain blocking effect

Abstract: Amino acid side chains can enhance peptide group hydrogen bond strength in protein structures by obstructing the competing hydrogen bond to solvent in the unfolded state. Available data indicate that the steric blocking effect contributes an average of 0.5 kJ per residue to protein hydrogen bond strength and accounts for the intrinsic beta-sheet propensities of the amino acids. In available data for helical models, the contribution to alpha-helix propensities is obscured especially by large context-dependent effects. These issues are all related by a common side chain-dependent steric clash which disfavors peptide to water H-bond formation, peptide to catalyst complexation in hydrogen exchange reactions (Bai et al., Proteins 17:75-86, 1993), and peptide to peptide H-bonding in the helical main chain conformation (Creamer and Rose, Proc. Natl. Acad. Sci. U.S.A. 89:5937-5941, 1992) but not in beta-strands.

Vinyl-vinyl coupling on late transition metals through C-C reductive elimination mechanism. A computational study.

Abstract: A detailed density functional study was performed for the vinyl−vinyl reductive elimination reaction from bis-σ-vinyl complexes [M(CHCH2)2Xn]. It was shown that the activity of these complexes decreases in the following order: PdIV, PdII > PtIV, PtII, RhIII > IrIII, RuII, OsII. The effects of different ligands X were studied for both platinum and palladium complexes, which showed that activation barriers for C−C bond formation reaction decrease in the following order: X = Cl > Br, NH3 > I > PH3. Steric effects induced either by the ligands X or by substituents on the vinyl group were also examined. In addition, the major factors responsible for stereoselectivity control on the final product formation stage and possible involvement of asymmetric coupling pathways are reported. In all cases ΔE, ΔH, ΔG, and ΔGaq energy surfaces were calculated and analyzed. The solvent effect calculation shows that in a polar medium halogen complexes may undergo a reductive elimination reaction almost as easily as compound...

Conformational studies on pyranoid sugar derivatives by NMR spectroscopy. Correlations of observed proton—proton coupling constants with the generalized Karplus equation

Abstract: Proton spin-spin coupling values measured for a wide range of tetrasubstituted tetrahydropyran ring-systems obtained from aldopentopyranose sugars have been correlated with values calculated from a generalized version of the Karplus equation which takes into account the electronegativities of the various atoms in the molecule and the configurations of these substituents. Minor discrepancies between observed and calculated values can be attributed to slight flattening of the tetrahydropyran ring as a result of steric interactions of the substituents. The magnitude of the geminal coupling J5e,5a of these aldopentopyranose derivatives provides a rough measure of the conformational population; when H-4 is axial [Cl(D) or 1C(L) conformation] J5e,5a is ∼10.9 Hz, and is ∼13.4 Hz when H-4 is equatorial [1C(D) or Cl(L) conformation].