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.

Acceleration Effects of Phosphine Ligands on the Rhodium-Catalyzed Dehydrogenative Silylation and Germylation of Unactivated C(sp3)–H Bonds

Abstract: The current work describes the marked rate of acceleration caused by phosphine ligands on the rhodium-catalyzed dehydrogenative silylation and germylation of unactivated C(sp3)–H bonds. The reactivity was affected by the steric and electronic nature of the phosphine ligands. The use of the bulky and electron-rich diphosphine ligand (R)-DTBM-SEGPHOS was highly effective to yield the dehydrogenative silylation products selectively in the presence of a hydrogen acceptor. An appropriate choice of C2-symmetric chiral diphosphine ligand enables the asymmetric dehydrogenative silylation via the enantioselective desymmetrization of the C(sp3)–H bond. The unprecedented catalytic germylation of C(sp3)–H bonds with dehydrogenation was also examined with the combination of the rhodium complex and a wide bite angle diphosphine ligand to provide the corresponding 2,3-dihydrobenzo[b]germoles in good yield.

Mechanism of selective oxidation of organic sulfides with Oxo(salen)chromium(V) complexes

Abstract: The selective oxidation of organic sulfides to sulfoxides by oxo(salen)chromium(V) complexes in acetonitrile is overall second-order, first-order each in the oxidant and the substrate. The rate constant, k2, values of several para-substituted phenyl methyl sulfides correlate linearly with Hammett σ constants and the ρ values are in the range of −1.3 to −2.7 with different substituted oxo(salen)chromium(V) complexes. The reactivity of different alkyl sulfides is in accordance with Taft's steric substituent constant, ES. A mechanism involving direct oxygen atom transfer from the oxidant to the substrate rather than electron transfer is envisaged. Correlation analyses show the presence of an inverse relationship between reactivity and selectivity in the reaction of various sulfides with a given oxo(salen)chromium(V) complex and vice versa. Mathematical treatment of the results shows that this redox system falls under strong reactivity−selectivity principle (RSP).

Structural analysis of specificity: alpha-lytic protease complexes with analogues of reaction intermediates.

Abstract: To better understand the structural basis of enzyme specificity, the structures of complexes formed between a-lytic protease, an extracellular serine protease of Lysobacter enzymogenes, and five inhibitory peptide boronic acids ( R2-boroX, where R2 is methoxysuccinyl-Ala-Ala-Pro- and boroX is the a-aminoboronic acid analogue of Ala, Val, Ile, Norleu, or Phe) have been studied at high resolution by X-ray crystallography. The enzyme has primary specificity for Ala in the PI position of peptide substrates with catalytic efficiency decreasing with increasing side-chain volume. Enzyme affinity for inhibitors with boroVal, borolle, and boroPhe residues is much higher than expected on the basis of the catalytic efficiencies of homologous substrates. Covalent tetrahedral adducts are formed between the active-site serine and the boronic acid moieties of R2-boroAla, R,-boroVal R2-boroIle, and R2-boroNorleu. Though R2-boroVal is a slowly bound inhibitor and R2-boroAla is rapidly bound (Kettner, C. A., Bone, R., Agard, D. A., & Bachovchin, W. W. (1988) Biochemistry 27, 7682-76881, there appear to be no structural differences that could account for slow binding. The removal from solution of 20% more hydrophobic surface on binding accounts for the improved affinity of a-lytic protease for R2-boroVal relative to R2-boroAla. The high affinity of the enzyme for R2-boroIle derives from the selective binding of the L-all0 stereoisomer of the boroIle residue, which can avoid bad steric interactions in the binding pocket. While R2-boroNorleu buries as much hydrophobic surface as R2-boroVal, its larger side chain causes alterations in enzyme conformation and inhibitor position, leading to a distortion of hydrogen bonds between the enzyme and inhibitor. A trigonal adduct is formed between the active-site serine and the boronic acid moiety of R2-boroPhe in which the catalytic histidine occupies a position axial to the plane of the trigonal adduct. The histidine Nc2 is 2.2 A from the boron,