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Reduction of Alkenes: Asymmetric Catalytic Hydrogenation02:17

Reduction of Alkenes: Asymmetric Catalytic Hydrogenation

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Catalytic hydrogenation of alkenes is a transition-metal catalyzed reduction of the double bond using molecular hydrogen to give alkanes. The mode of hydrogen addition follows syn stereochemistry.
The metal catalyst used can be either heterogeneous or homogeneous. When hydrogenation of an alkene generates a chiral center, a pair of enantiomeric products is expected to form. However, an enantiomeric excess of one of the products can be facilitated using an enantioselective reaction or an...
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Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical 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
1.8K
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

2.3K
Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
2.3K
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

2.0K
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.
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Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule02:17

Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule

14.1K
If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others.
The hydrohalogenation of an unsymmetrical alkene can yield two haloalkane products, depending on which vinylic carbon takes up the halogen. However, one product usually predominates, where hydrogen adds to the vinylic carbon bearing the...
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Regioselectivity of Electrophilic Additions-Peroxide Effect02:35

Regioselectivity of Electrophilic Additions-Peroxide Effect

8.5K
In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination.
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Updated: Jun 23, 2025

Synthesis and Performance Characterizations of Transition Metal Single Atom Catalyst for Electrochemical CO2 Reduction
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Synthesis and Performance Characterizations of Transition Metal Single Atom Catalyst for Electrochemical CO2 Reduction

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The Selectivity Origins in Ag-Catalyzed CO2 Electroreduction.

Shenyu Shen1, Wenshan Zhao1, Mei Xiang2

  • 1School of Chemistry, Engineering Research Center of Energy Storage Materials and Devices of Ministry of Education, National Innovation Platform (Center) for Industry-Education Integration of Energy Storage Technology, Xi'an Jiaotong University, Xi'an, 710049, China.

The Journal of Physical Chemistry Letters
|June 18, 2024
PubMed
Summary

Silver catalysts selectively convert CO2 to C1 products. Applied potential and pH determine if formate or CO is produced via concerted proton-electron transfer (CPET) or sequential electron-proton transfer (SEPT) mechanisms.

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Area of Science:

  • Electrochemistry
  • Surface Science
  • Catalysis

Background:

  • Electrochemical CO2 reduction (CO2R) is crucial for C1 product synthesis.
  • Understanding the reaction mechanism (CPET vs. SEPT) on silver is debated.
  • Ag(111) shows high selectivity for C1 products in CO2R.

Purpose of the Study:

  • Investigate Ag-catalyzed electrochemical CO2R selectivity.
  • Elucidate the influence of applied potential and pH on reaction mechanisms.
  • Determine C1 product distribution based on reaction conditions.

Main Methods:

  • Microkinetic modeling based on Marcus electron transfer theory.
  • Computational investigation of CO2 reduction pathways on Ag(111).
  • Analysis of selectivity under varying applied potentials and pH.

Main Results:

  • At pH 1.94, formate is favored below -0.94 V (CPET), while CO dominates above -0.94 V (SEPT).
  • At pH 13.95, formate is the main product via the SEPT mechanism.
  • Applied potential and pH significantly alter product selectivity and mechanism.

Conclusions:

  • Provides mechanistic insights into Ag-catalyzed CO2R.
  • Demonstrates control over C1 product selectivity by tuning potential and pH.
  • Offers a theoretical framework for designing efficient CO2 reduction catalysts.