Related Concept Videos
01:11ortho–para-Directing Activators: –CH3, –OH, –NH2, –OCH3
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All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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02:36Regioselectivity and Stereochemistry of Hydroboration
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A significant aspect of hydroboration–oxidation is the regio- and stereochemical outcome of the reaction.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
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01:26Photochemical Electrocyclic Reactions: Stereochemistry
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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Selection Rules: Photochemical Activation
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01:17Thermal Electrocyclic Reactions: Stereochemistry
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The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
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01:24ortho–para-Directing Deactivators: Halogens
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Halogens are ortho–para directors. They are more electronegative than carbon. Therefore, as ring substituents, they can withdraw electrons through the inductive effect and deactivate the aromatic ring towards electrophilic substitution. Halogens also have an electron-donating resonance effect on the ring, which influences the orientation of the incoming electrophile. If an electrophile attacks at the ortho or the para position, the halogen donates electrons and stabilizes the intermediate...
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02:34Regioselectivity and Stereochemistry of Acid-Catalyzed Hydration
8.8K
The rate of acid-catalyzed hydration of alkenes depends on the alkene's structure, as the presence of alkyl substituents at the double bond can significantly influence the rate.
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The ortho effect in directed C-H activation.
Balázs L Tóth1, Anna Monory1, Orsolya Egyed2
1ELTE "Lendület" Catalysis and Organic Synthesis Research Group, Faculty of Science, Institute of Chemistry, Eötvös Loránd University Pázmány Péter Sétány. 1/A H-1117 Budapest Hungary tothb@zng.elte.hu novakz@elte.hu.
Chemical Science
|June 24, 2021
Summary
A new substrate-based model explains the "Ortho Effect" in transition metal-catalyzed C-H activation, predicting reactivity using steric hindrance near directing groups. This aids chemists in optimizing reactions and selecting substrates for efficient synthesis.
Area of Science:
- Organic Chemistry
- Catalysis
- Reaction Mechanism
Background:
- Transition metal-catalyzed *ortho*-directed C-H activation is crucial in organic synthesis.
- Reactions are often hindered by steric and electronic interactions between directing groups (DG) and other substituents on the aromatic ring.
- Neighboring group interactions significantly impact the efficiency of C-H activation.
Purpose of the Study:
- To introduce a substrate-only-based model to interpret the influence of steric hindrance in *ortho*-directed C-H activation.
- To define and explain the 'Ortho Effect' (OE) arising from steric interactions.
- To provide a predictive tool for evaluating substrate reactivity and optimizing reaction conditions.
Main Methods:
- Development of a model using descriptors like torsion angle and torsional energy.
- Analysis of over 250 diverse substrate examples to validate the model.
- Establishment of structural and energetic criteria for predicting metalation efficiency.
Main Results:
- The 'Ortho Effect' model successfully predicts and explains reactivity in directed *ortho*-C-H activation.
- Demonstrated the nature of the OE across a wide variety of chemical structures.
- Provided criteria to *a priori* assess the efficiency of the rate-determining metalation step.
Conclusions:
- The developed model offers a simple, general protocol for estimating substrate reactivity in C-H activation.
- The criteria help determine minimum reaction temperatures for achieving reasonable reaction rates.
- Synthetic validation via palladium-catalyzed trifluoroethylation and predictions for various DGs showcase practical applicability.