scispace - formally typeset
Search or ask a question
Topic

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.


Papers
More filters
Journal ArticleDOI
TL;DR: L-(+)-Histidine (C6NaO2H9) as discussed by the authors is an open, extended form of Histidine which is stabilized by an imramolecular hydrogen bond between the amino nitrogen atom and the adjacent imidazole nitrogen atom.
Abstract: L-(+)-Histidine (C6NaO2H9) crystallizes in the orthorhombic space group P212121, with a = 5-177, b = 7.322, c= 18.87 A, and Z=4. Data were collected with Mo Kct radiation, using balanced filters. The structure was solved by direct phasing methods and refined to a final agreement index of 0.034 for all reflections. The conformation of the molecule is that of the open, extended form, and is stabilized principally by an imramolecular hydrogen bond between the amino nitrogen atom and the adjacent imidazole nitrogen atom. Where this conformation is found in proteins, it is likely to reduce the chemical reactivity of tha+ ;midazole group, because one of the imidazole nitrogen atoms is sterically hindered by the peptide ba,.~, one.

101 citations

Journal ArticleDOI
TL;DR: In this article, the molecular structure of tetra-tert-butyldiphosphine has been determined in the gas phase by electron diffraction using the new DYNAMITE method and in the crystalline phase by X-ray diffraction.
Abstract: The molecular structure of tetra-tert-butyldiphosphine has been determined in the gas phase by electron diffraction using the new DYNAMITE method and in the crystalline phase by X-ray diffraction. Ab initio methods were employed to gain a greater understanding of the structural preferences of this molecule in the gas phase, and to determine the intrinsic P–P bond energy, using recently described methods. Although the P–P bond is relatively long [GED 226.4(8) pm; X-ray 223.4(1) pm] and the dissociation energy is computed to be correspondingly small (150.6 kJ mol−1), the intrinsic energy of this bond (258.2 kJ mol−1) is normal for a diphosphine. The gaseous data were refined using the new Edinburgh structure refinement program ed@ed, which is described in detail. The molecular structure of gaseous P2But4 is compared to that of the isoelectronic 1,1,2,2-tetra-tert-butyldisilane. The molecules adopt a conformation with C2 symmetry. The P–P–C angles returned from the gas electron diffraction refinement are 118.8(6) and 98.9(6)°, a difference of 20°, whilst the C–P–C angle is 110.3(8)°. The corresponding parameters in the crystal are 120.9(1), 99.5(1) and 109.5(1)°. There are also large deformations within the tert-butyl groups, making the DYNAMITE analysis for this molecule extremely important.

101 citations

Journal ArticleDOI
TL;DR: In this article, the electron deficiency at the dicoordinate boron atom is reduced through π-π delocalization of a neighboring nitrogen atom's formally nonbonding electron pair, that is, through formation of a classical two-center, two-electron (2c-2e) bond.
Abstract: Methyleneboranes are stable compounds when their reactive CB bonds are shielded by sterically demanding substituents and the electron deficiency at the dicoordinate boron atom is reduced through delocalization of adjacent electron pairs. In the classical amino(methylene)boranes of Noth and Paetzold, the electron deficiency at the dicoordinate boron atom is relieved through π-π delocalization of a neighboring nitrogen atom's formally nonbonding electron pair, that is, through formation of a classical two-center, two-electron (2c–2e) bond. In nonclassical methyleneboranes, which are the focus of this review, the electrondeficient center at the dicoordinate boron atom forms nonclassical three-center, two-electron (3c2e) bonds with neighboring σ bonds. These methyleneboranes exhibit structural and spectroscopic properties similar to carbenium ions having strong σ–π delocalization (hyperconjugation, bridging) and to transition metal complexes having strong agostic interactions.

101 citations

Journal ArticleDOI
TL;DR: The chemical mechanism of hydroxylation, catalyzed by dopamine beta-monooxygenase, has been explored with a combination of secondary kinetic isotope effects and structure-reactivity correlations, and an alpha-deuterium isotope effect is obtained, indicating the occurrence of a stepwise process for C-H bond cleavage and C-O bond formation and hence the presence of a substrate-derived intermediate.
Abstract: The chemical mechanism of hydroxylation, catalyzed by dopamine beta-monooxygenase, has been explored with a combination of secondary kinetic isotope effects and structure-reactivity correlations. Measurement of primary and secondary isotope effects on Vmax/Km under conditions where the intrinsic primary hydrogen isotope effect is known allows calculation of the corresponding intrinsic secondary isotope effect. By this method we have obtained an alpha-deuterium isotope effect, Dk alpha = 1.19 +/- 0.06, with dopamine as substrate. The beta-deuterium isotope effect is indistinguishable from one. The large magnitude of Dk alpha, together with our previous determination of a near maximal primary deuterium isotope effect of 9.4-11, clearly indicates the occurrence of a stepwise process for C-H bond cleavage and C-O bond formation and hence the presence of a substrate-derived intermediate. To probe the nature of this intermediate, a structure-reactivity study was performed by using a series of para-substituted phenylethylamines. Deuterium isotope effects on Vmax and Vmax/Km parameters were determined for all of the substrates, allowing calculation of the rate constants for C-H bond cleavage and product dissociation and dissociation constants for amine and O2 loss from the enzyme-substrate ternary complex. Multiple regression analysis yielded an electronic effect of p = -1.5 for the C-H bond cleavage step, eliminating the possibility of a carbanion intermediate. A negative p value is consistent with formation of either a radical or a carbocation; however, a significantly better correlation is obtained with sigma p rather than sigma p+, implying formation of a radical intermediate via a polarized transition state. Additional effects determined from the regression analyses include steric effects on rate constants for substrate hydroxylation and product release and on KDamine, consistent with a sterically restricted binding site, and a positive electronic effect of p = 1.4 on product dissociation, ascribed to a loss of product from an enzyme-bound Cu(II)-alkoxide complex. These results lead us to propose a mechanism in which O-O homolysis [from a putative Cu(II)-OOH species] and C-H homolysis (from substrate) occur in a concerted fashion, circumventing the formation of a discrete, high energy oxygen species such as hydroxyl radical. The substrate and peroxide-derived radical intermediates thus formed undergo a recombination, kinetically limited by displacement of an intervening water molecule, to give the postulated Cu(II)-alkoxide product complex.

100 citations

Journal ArticleDOI
TL;DR: This simple and mild 4-pyridination proceeds in a highly chemoselective manner especially at benzylic C(sp3)–H bonds without affecting polar functional groups, and enables intermolecular formation of sterically hindered bonds between alkylaromatics and 4- pyridine.
Abstract: Direct substitution of hydrogen in C(sp3)–H bonds by 4-pyridine was achieved by employing benzophenone and 4-cyanopyridine in aqueous acetonitrile under photo-irradiating conditions. This simple and mild 4-pyridination proceeds in a highly chemoselective manner especially at benzylic C(sp3)–H bonds without affecting polar functional groups, and enables intermolecular formation of sterically hindered bonds between alkylaromatics and 4-pyridine. The present methodology thus serves as a powerful tool for construction of biologically active and functional molecules with 4-pyridine substructures.

100 citations


Network Information
Related Topics (5)
Alkyl
223.5K papers, 2M citations
97% related
Aryl
95.6K papers, 1.3M citations
95% related
Ligand
67.7K papers, 1.3M citations
94% related
Palladium
64.7K papers, 1.3M citations
94% related
Nuclear magnetic resonance spectroscopy
42.6K papers, 1M citations
93% related
Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023942
20221,917
2021346
2020292
2019296
2018307