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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: The chemical reactivity of metal alkoxides toward hydrolysis, condensation and complexation mainly depends on the electronegativity of the metal atom, its coordination number and the steric hindrance of alkoxide groups.
Abstract: Sol-gel chemistry involves nucleophilic reactions The chemical reactivity of metal alkoxides toward hydrolysis, condensation and complexation mainly depends on the electronegativity of the metal atom, its coordination number and the steric hindrance of alkoxide groups Silicon alkoxides are poorly reactive Hydrolysis and condensation rates have to be enhanced by acid and base catalysis or nucleophilic activation Transition metal alkoxides are usually too reactive They have to be stabilized by complexation in order to avoid fast condensation The molecular design of alkoxide precursors opens the way to tailor-made materials

355 citations

Journal ArticleDOI
21 Feb 2014-Science
TL;DR: A catalytic intermolecular C–H silylation of unactivated arenes that manifests very high regioselectivity through steric effects of substituents meta to a potential site of reactivity is reported.
Abstract: Regioselective C–H functionalization of arenes has widespread applications in synthetic chemistry. The regioselectivity of these reactions is often controlled by directing groups or steric hindrance ortho to a potential reaction site. Here, we report a catalytic intermolecular C–H silylation of unactivated arenes that manifests very high regioselectivity through steric effects of substituents meta to a potential site of reactivity. The silyl moiety can be further functionalized under mild conditions but is also inert toward many common organic transformations, rendering the silylarene products useful building blocks. The remote steric effect that we observe results from the steric properties of both the rhodium catalyst and the silane.

350 citations

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate quantitatively the contributions of each of two mechanisms for molecular shape selectivity for catalytic cracking of C6 to C9 hydrocarbons on ZSM-5.
Abstract: For the catalytic cracking of C6 to C9 hydrocarbons on ZSM-5, we demonstrate quantitatively the contributions of each of two mechanisms for molecular shape selectivity. Using crystallites of different sizes and activities, and classical methods for evaluating diffusion inhibition of the reaction rate, we separate the effects of mass-transport-induced selectivity from that created by steric inhibition by the size of a reaction complex. The selective cracking of n-paraffins compared to monomethyl paraffins (from C6 to C9) is due to a higher intrinsic rate constant of the n-paraffin, with diffusional mass transport playing no appreciable role. In contrast, dimethyl paraffin cracking is strongly diffusion-inhibited. The methyl paraffin/n-paraffin discrimination is a result of steric constraint on the sizeable methyl paraffin/carbonium ion reaction complex. This structural selectivity is shown to be absent for the corresponding olefins where such complexes do not arise. The diffusivities at reaction conditions have been determined. For the linear hydrocarbon, diffusivity notably exceeds that expected from the Knudsen model. This reminds us to review assumptions of conventional concepts of mass transport. The availability of zeolites now allows us to probe many basic phenomena in catalysis, molecular configuration and dynamics, including mass transport.

347 citations

Journal ArticleDOI
TL;DR: Simple two-coordinate acyclic silylene species, SiR(2), can be synthesized which is stable in the solid state up to 130 °C and undergoes facile oxidative addition reactions with dihydrogen and with alkyl C-H bonds, thus demonstrating fundamental modes of reactivity more characteristic of transition metal systems.
Abstract: Simple two-coordinate acyclic silylenes, SiR2, have hitherto been identified only as transient intermediates or thermally labile species. By making use of the strong σ-donor properties and high steric loading of the B(NDippCH)2 substituent (Dipp = 2,6-iPr2C6H3), an isolable monomeric species, Si{B(NDippCH)2}{N(SiMe3)Dipp}, can be synthesized which is stable in the solid state up to 130 °C. This silylene species undergoes facile oxidative addition reactions with dihydrogen (at sub-ambient temperatures) and with alkyl C–H bonds, consistent with a low singlet–triplet gap (103.9 kJ mol–1), thus demonstrating fundamental modes of reactivity more characteristic of transition metal systems.

334 citations


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