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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.


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Journal ArticleDOI
TL;DR: A family of 4-substituted 1-cubanecarboxylic acids have been synthesized and their X-ray crystal structures analyzed as discussed by the authors, and the rare syn-anti O−H−O catemer 6 is a recurring pattern in this series of compounds.
Abstract: A family of 4-substituted-1-cubanecarboxylic acids have been synthesized and their X-ray crystal structures analyzed. The rare syn-anti O−H···O catemer 6 is a recurring pattern in this series of compounds. Catemer 6 is observed in the crystal structures of 4-chloro-1-cubanecarboxylic acid (10), 4-bromo-1-cubanecarboxylic acid (11), 4-iodo-1-cubanecarboxylic acid (12), and 4-(methoxycarbonyl)-1-cubanecarboxylic acid (13). The ready occurrence of catemer 6 in this family is ascribed to its stabilization by auxiliary C−H···O hydrogen bonds formed by the relatively acidic cubyl C−H groups. The frequency of occurrence of 6 also facilitates its definition as a useful supramolecular synthon. As is true in many catemers, the formation of 6 is sensitive to steric factors. Therefore, the robustness of this synthon may be assessed by analyzing the crystal structures of molecules wherein the 4-substituent is too small (R = H, 14), too large (R = Ph, 15), or has a specific hydrogen bonding preference of its own (R = C...

239 citations

Journal ArticleDOI
TL;DR: Several lines of evidence suggest that associative substitution pathways operate to get the hydrocarbon substrate into, and out of, the coordination sphere; that is, the mechanism of benzene substitution proceeds by a solvent (TFE)-assisted associative pathway.
Abstract: A series of bis(aryl)diimine-ligated methyl complexes of Pt(II) with various substituted aryl groups has been prepared. The cationic complexes [(ArN CR−CR NAr)PtMe(L)]^+[BF_4]- (Ar = aryl; R = H, CH_3; L = water, trifluoroethanol) react smoothly with benzene at approximately room temperature in trifluoroethanol solvent to yield methane and the corresponding phenyl Pt(II) cations, via Pt(IV)-methyl-phenyl-hydrido intermediates. The reaction products of methyl-substituted benzenes suggest an inherent reactivity preference for aromatic over benzylic C−H bond activation, which can however be overridden by steric effects. For the reaction of benzene with cationic Pt(II) complexes bearing 3,5-disubstituted aryl diimine ligands, the rate-determining step is C−H bond activation, whereas for the more sterically crowded analogues with 2,6-dimethyl-substituted aryl groups, benzene coordination becomes rate-determining. This switch is manifested in distinctly different isotope scrambling and kinetic deuterium isotope effect patterns. The more electron-rich the ligand is, as assayed by the CO stretching frequency of the corresponding carbonyl cationic complex, the faster the rate of C−H bond activation. Although at first sight this trend appears to be at odds with the common description of this class of reaction as electrophilic, the fact that the same trend is observed for the two different series of complexes, which have different rate-determining steps, suggests that this finding does not reflect the actual C−H bond activation process, but rather reflects only the relative ease of benzene displacing a ligand to initiate the reaction; that is, the change in rates is mostly due to a ground-state effect. The stability of the aquo complex ground state in equilibrium with the solvento complex increases as the diimine ligand is made more electron-withdrawing. Several lines of evidence, including the mechanism of degenerate acetonitrile exchange for the methyl-acetonitrile Pt(II) cations in alcohol solvents, suggest that associative substitution pathways operate to get the hydrocarbon substrate into, and out of, the coordination sphere; that is, the mechanism of benzene substitution proceeds by a solvent (TFE)-assisted associative pathway.

237 citations

Journal ArticleDOI
TL;DR: The results of crystal structure determinations of eighteen nucleosides and nucleotides have been analyzed to determine the nature of the steric barriers to rotation about the glycosidic (C-N) bond.

237 citations

Journal ArticleDOI
TL;DR: The results illustrate that the electronic properties of an NHC ligand can be tuned over a wide range without the need to change its constitution or steric attributes.
Abstract: A high yielding approach to planar chiral carbene ligands is described, in which an imidazo[1,5-a]pyridine-3-ylidene unit is embedded into a cyclophane scaffold. As evident from the IR data of the corresponding rhodium complex 18 as the parent member of this family, these new ligands turned out to be exceptionally strong electron donors, rivaling or even outperforming the other diamino-stabilized five-membered N-heterocyclic carbenes (NHC) known to date. If the remote ring of the cyclophane is substituted by four fluorine atoms, however, the donor capacity is significantly reduced by the through-space interaction of the tetrafluorobenzene moiety with the underneath carbene entity. Since X-ray data suggest that the steric demand of the fluorinated and the nonfluorinated cyclophane-2-ylidenes are virtually identical, these results illustrate that the electronic properties of an NHC ligand can be tuned over a wide range without the need to change its constitution or steric attributes. The synthesis route lea...

237 citations

Journal ArticleDOI
TL;DR: The first structurally characterized examples of the 1,2,3-triazole motif employed in a terdentate ligand display enhanced steric freedom and a facile receptivity towards a reversible aquation in the case of an electrogenerated Fe(III) state.

237 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023942
20221,917
2021346
2020292
2019296
2018307