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: A number of alkoxypyrrolophenanthridinium salts and their analogues related to the antileukemic alkaloid ungeremine were prepared by a practical photochemical cyclization.
Abstract: A number of alkoxypyrrolophenanthridinium salts and their analogues related to the antileukemic alkaloid ungeremine were prepared by a practical photochemical cyclization. The importance of the quaternary nitrogen atom and of alkoxy groups, the planarity of a molecule, and steric considerations relative to antileukemic activity are discussed.

74 citations

Journal ArticleDOI
TL;DR: In this article, the nature of the hydrosilane activation mediated by ruthenium(II) thiolate complexes of type [(R3P)Ru(SDmp)]+[BArF4]− is elucidated by an in-depth experimental and theoretical study.
Abstract: The nature of the hydrosilane activation mediated by ruthenium(II) thiolate complexes of type [(R3P)Ru(SDmp)]+[BArF4]− is elucidated by an in-depth experimental and theoretical study. The combination of various ruthenium(II) thiolate complexes and tertiary hydrosilanes under variation of the phosphine ligand and the substitution pattern at the silicon atom is investigated, providing detailed insight into the activation mode. The mechanism of action involves reversible heterolytic splitting of the Si–H bond across the polar Ru–S bond without changing the oxidation state of the metal, generating a ruthenium(II) hydride and sulfur-stabilized silicon cations, i.e. metallasilylsulfonium ions. These stable yet highly reactive adducts, which serve as potent silicon electrophiles in various catalytic transformations, are fully characterized by systematic multinuclear NMR spectroscopy. The structural assignment is further verified by successful isolation and crystallographic characterization of these key intermediates. Quantum-chemical analyses of diverse bonding scenarios are in excellent agreement with the experimental findings. Moreover, the calculations reveal that formation of the hydrosilane adducts proceeds via barrierless electrophilic activation of the hydrosilane by sterically controlled η1 (end-on) or η2 (side-on) coordination of the Si–H bond to the Lewis acidic metal center, followed by heterolytic cleavage of the Si–H bond through a concerted four-membered transition state. The Ru–S bond remains virtually intact during the Si–H bond activation event and also preserves appreciable bonding character in the hydrosilane adducts. The overall Si–H bond activation process is exergonic with ΔG0r ranging from −20 to −40 kJ mol−1, proceeding instantly already at low temperatures.

74 citations

Journal ArticleDOI
TL;DR: The resonance Raman spectra and CD spectra of 13-demethylrhodopsin indicate that its chromophore, an analog in which the nonbonded interaction between the 10-H and the 13-CH3 groups is removed, is less distorted in the C10...C13 region than the native Chromophore.
Abstract: The role of intramolecular steric interactions in the isomerization of the 11-cis-retinal chromophore in the photoreceptor protein rhodopsin is examined with resonance Raman and CD spectroscopy combined with quantum yield experiments. The resonance Raman spectra and CD spectra of 13-demethylrhodopsin indicate that its chromophore, an analog in which the nonbonded interaction between the 10-H and the 13-CH3 groups is removed, is less distorted in the C10···C13 region than the native chromophore. The reduced torsional and hydrogen-out-of-plane resonance Raman intensities further indicate that the excited state potential energy surface has a much shallower slope along the isomerization coordinate. This is consistent with the decrease in quantum yield from 0.67 in rhodopsin to 0.47 in 13-demethylrhodopsin. The resonance Raman intensities show that the steric twist is reintroduced by addition of a methyl group at the C10 position. However, the quantum yield of 10-methyl-13-demethylrhodopsin is found to be only...

74 citations

Journal ArticleDOI
TL;DR: In this article, substituent effects on azide reactivity during CU I -catalyzed click reactions were investigated by reacting a variety of model azides with propargyl alcohol in DMF-d 7 using CuBr as catalyst.
Abstract: An investigation was conducted into substituent effects on azide reactivity during CU I -catalyzed click reactions by reacting a variety of model azides with propargyl alcohol in DMF-d 7 using CuBr as catalyst, and monitoring conversions via 1 H NMR. The model azides utilized for this investigation were selected due to structural similarity with the end groups of polymers typically employed in azide-alkyne cycloadditions including polystyrene, poly(methyl acrylate), poly(methyl methacrylate), and polyacrylonitrile. Reactivities of alkyl azides depend on both the electronic properties of the substituent and steric congestion around the end group. The fastest reactions were observed for azides with electron-withdrawing substituents and less sterically congested end groups.

74 citations

Journal ArticleDOI
TL;DR: In this article, the mutarotation of D-fructopyranose has been studied and it has been shown that the furanose-pyranoses transformation is very rapid.
Abstract: Studies on the mutarotation of D-fructose show that whereas the furanose-pyranose transformation is rapid, the furanose-furanose transformation is very rapid. As fructose mutarotation is found to be multiphasic at ambient temperatures, estimates of the tautomer distribution and the specific rotation of the furanose forms, by application of the unimolecular equation to optical data, are in error. Calculations from fructose tautomer shifts with temperature, and also determinations made from the invertase hydrolysis of methyl -D-fructofuranoside indicate that the specific rotation of -D-fructofuranose is +78° and that for a-D-fructofuranose is -120°. The "rules' of rotation and of anomeric specification do not therefore apply to the fructofuranoses. The saporous unit responsible for the sweet taste of -D-fructopyranose has been indentified as a tripartite grouping involving the anomeric hydroxyl group, the oxygen atom of the primary alcohol group and the ring methylene carbon atom. These represent, respectively, an appropriate proton donor (AH), and a proton acceptor (B) for an intermolecular hydrogen bonding system and also a hydrophobic bonding site (s). In the -D-fructopyranose structure they possess the proper steric disposition to elicit sweet taste. The tasteless furanose forms of fructose do not have the required steric relation between these functional groups.

74 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