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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...

Thermal and Photochemical Electrocyclic Reactions: Overview

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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.

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.

Metal-Ligand Bonds

Metal-Ligand Bonds

The hemoglobin in the blood, the chlorophyll in green plants, vitamin B-12, and the catalyst used in the manufacture of polyethylene all contain coordination compounds. Ions of the metals, especially the transition metals, are likely to form complexes.
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Thermal 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.

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

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Updated: Jan 17, 2026

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Published on: July 30, 2017

Ring-Expanded N-Heterocyclic Carbene (reNHC) Complexes: Applications in Transition Metal Catalysis.

Tongliang Zhou¹, Greta Utecht-Jarzyńska^1,2, Michal Szostak¹

¹Department of Chemistry, Rutgers University, 73 Warren Street, Newark, NJ 07102, United States.

Coordination Chemistry Reviews

|September 18, 2025

Summary

This summary is machine-generated.

Ring-expanded N-heterocyclic carbenes (reNHCs) offer significant advantages in transition metal catalysis. This review surveys their diverse applications, organometallic complexes, and pivotal role in modern chemistry.

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Accessing Valuable Ligand Supports for Transition Metals: A Modified, Intermediate Scale Preparation of 1,2,3,4,5-Pentamethylcyclopentadiene

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Published on: March 20, 2017

Area of Science:

Organometallic Chemistry
Catalysis
Materials Science

Background:

N-heterocyclic carbenes (NHCs) are crucial ligands in transition metal catalysis.
Traditionally, 5-membered NHCs dominated the field.
Recent advancements include ring-expanded NHCs (reNHCs) with 6-10 membered rings.

Purpose of the Study:

To comprehensively review the applications of reNHCs in transition metal catalysis.
To highlight the types of reNHCs and their corresponding organometallic complexes.
To elucidate the impact and significance of reNHCs in catalysis and organometallic chemistry.

Main Methods:

Literature review spanning from initial reports to June 2023.
Analysis of various catalytic reactions employing reNHCs with different metals.
Examination of diverse reaction mechanisms facilitated by reNHCs.

Main Results:

reNHCs provide significant advantages over traditional NHCs in numerous catalytic reactions.
A wide array of reNHCs, their metal complexes, and catalytic applications have been identified.
The review details the structural diversity and reactivity of reNHCs.

Conclusions:

reNHCs represent a vital and expanding class of ligands in transition metal catalysis.
Their unique properties enable enhanced performance in various catalytic transformations.
reNHCs are pivotal in advancing organometallic chemistry and catalytic science.