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Thermal Electrocyclic Reactions: Stereochemistry

Thermal Electrocyclic Reactions: Stereochemistry

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.

Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation

Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation

Introduction
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.

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Reduction of Alkenes: Catalytic Hydrogenation

Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a transition-metal catalyst, the reaction is called catalytic hydrogenation.
Metals like palladium, platinum, and nickel are commonly used in their solid forms — fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
The hydrogenation process takes place on the...

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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.
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[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement

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The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.

Hydroboration-Oxidation of Alkenes

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In addition to the oxymercuration–demercuration method, which converts the alkenes to alcohols with Markovnikov orientation, a complementary hydroboration-oxidation method yields the anti-Markovnikov product. The hydroboration reaction, discovered in 1959 by H.C. Brown, involves the addition of a B–H bond of borane to an alkene giving an organoborane intermediate. The oxidation of this intermediate with basic hydrogen peroxide forms an alcohol.

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Updated: Oct 8, 2025

Catalytic Reactions at Amine-Stabilized and Ligand-Free Platinum Nanoparticles Supported on Titania During Hydrogenation of Alkenes and Aldehydes

Catalytic Reactions at Amine-Stabilized and Ligand-Free Platinum Nanoparticles Supported on Titania During Hydrogenation of Alkenes and Aldehydes

Published on: June 24, 2022

Catalytic, contra-Thermodynamic Positional Alkene Isomerization.

Gino Occhialini¹, Vignesh Palani¹, Alison E Wendlandt¹

¹Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.

Journal of the American Chemical Society

|December 30, 2021

Summary

This study introduces a novel dual catalyst system for contra-thermodynamic alkene isomerization, enabling direct synthesis of terminal alkenes from internal ones using photochemical irradiation.

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A Microwave-Assisted Direct Heteroarylation of Ketones Using Transition Metal Catalysis

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Catalytic Reactions at Amine-Stabilized and Ligand-Free Platinum Nanoparticles Supported on Titania During Hydrogenation of Alkenes and Aldehydes

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A Microwave-Assisted Direct Heteroarylation of Ketones Using Transition Metal Catalysis

A Microwave-Assisted Direct Heteroarylation of Ketones Using Transition Metal Catalysis

Published on: February 16, 2020

Area of Science:

Organic Chemistry
Photochemistry
Catalysis

Background:

Positional isomerization of carbon-carbon double bonds (C═C) is crucial for interconverting alkene regioisomers.
Current methods typically yield thermodynamically favored products, limiting synthetic flexibility.

Purpose of the Study:

To develop a novel method for contra-thermodynamic positional alkene isomerization.
To access terminal alkene isomers directly from conjugated internal alkene starting materials.

Main Methods:

Utilized a dual catalyst system under photochemical irradiation.
Investigated the deconjugation of electron-rich and electron-poor alkenes.
Applied the method to natural product synthesis.

Main Results:

Achieved contra-thermodynamic positional alkene isomerization.
Successfully synthesized terminal alkene isomers from internal alkenes.
Demonstrated broad utility with diverse alkene substrates and in natural product synthesis.

Conclusions:

The discovered dual catalyst system enables unprecedented access to less stable alkene isomers.
The reaction proceeds via a regiospecific bimolecular homolytic substitution (S_H2') mechanism involving an allyl-cobaloxime intermediate.