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Crossed Aldol Reactions: Overview

Crossed Aldol Reactions: Overview

Crossed aldol addition is the reaction between two different carbonyl compounds under acidic or basic conditions. Here, both the carbonyl compounds function as nucleophiles and electrophiles. As shown in Figure 1, such a reaction yields a mixture of products, two of which are formed via self-condensation, while the remaining two are formed via crossed-condensation. Without adjustment, the reaction's usefulness in organic chemistry is decreased.

Preparation of Amines: Reductive Amination of Aldehydes and Ketones

Preparation of Amines: Reductive Amination of Aldehydes and Ketones

Carbonyl compounds and primary amines undergo reductive amination first to produce imines, followed by secondary amines in the same reaction mixture, using selective reducing agents like sodium cyanoborohydride or sodium triacetoxyborohydride. Reductive amination produces different degrees of substitution of amines depending on the starting amine substrate.

Amides to Amines: LiAlH4 Reduction

Amides to Amines: LiAlH₄ Reduction

Amide reduction with strong reducing agents like lithium aluminum hydride proceeds through a nucleophilic acyl substitution to form amines. Primary, secondary, and tertiary amides yield primary, secondary, and tertiary amines, respectively.
Amide reduction requires two equivalents of the reducing agent, acting as a source of hydride ions. As shown in the figure, the reaction is initiated with a nucleophilic attack by the hydride ion at the carbonyl carbon to form a tetrahedral intermediate.

Alkenes via Reductive Coupling of Aldehydes or Ketones: McMurry Reaction

Alkenes via Reductive Coupling of Aldehydes or Ketones: McMurry Reaction

The radical dimerization of ketones or aldehydes gives vicinal diols through a pinacol coupling reaction. However, the behavior of titanium metals used for the reaction as a source of electrons is unusual. When the reaction is carried out in the presence of titanium, diols can be isolated at low temperatures. Else titanium further reacts with diols, forming alkenes through the McMurry reaction.

Crossed Aldol Reaction Using Weak Bases

Crossed Aldol Reaction Using Weak Bases

This lesson deals with the crossed aldol reaction using weak bases. The self-condensation of an aldehyde having α hydrogen is prevented by adding it slowly to a mixture of formaldehyde and weak bases like hydroxide and alkoxide. Upon slow addition of the aldehyde, the base deprotonates the α carbon of the aldehyde to form the corresponding enolate. The enolate subsequently attacks the formaldehyde to form a single crossed product. Figure 1 depicts the aforementioned reaction.

Crossed Aldol Reaction Using Strong Bases: Directed Aldol Reaction

Crossed Aldol Reaction Using Strong Bases: Directed Aldol Reaction

The reaction between two different carbonyl compounds comprising α hydrogen in the presence of a strong base like lithium diisopropylamide (LDA) to form a crossed aldol product is known as a directed aldol reaction. The directed aldol reaction is depicted in Figure 1.

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Related Experiment Video

Updated: Apr 28, 2026

Facile Preparation of 2Z,4E-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate

Facile Preparation of 2Z,4E-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate

Published on: June 21, 2017

Asymmetric Reductive Cross-Coupling of Aldimines.

Yuhao Dai¹, Fang Fang², Guangqing Xu³

¹School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China.

Organic Letters

|April 27, 2026

Summary

A new method efficiently synthesizes unsymmetrical chiral 1,2-diamines using chiral diboron-templated reductive cross-coupling. This approach offers good selectivity and functional group tolerance for valuable chemical compounds.

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Retropinacol/Cross-pinacol Coupling Reactions - A Catalytic Access to 1,2-Unsymmetrical Diols

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Versatile CO2 Transformations into Complex Products: A One-pot Two-step Strategy

Versatile CO2 Transformations into Complex Products: A One-pot Two-step Strategy

Published on: November 9, 2019

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Facile Preparation of 2Z,4E-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate

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Published on: June 21, 2017

Retropinacol/Cross-pinacol Coupling Reactions - A Catalytic Access to 1,2-Unsymmetrical Diols

Retropinacol/Cross-pinacol Coupling Reactions - A Catalytic Access to 1,2-Unsymmetrical Diols

Published on: April 4, 2014

Versatile CO2 Transformations into Complex Products: A One-pot Two-step Strategy

Versatile CO2 Transformations into Complex Products: A One-pot Two-step Strategy

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Area of Science:

Organic Chemistry
Asymmetric Synthesis
Catalysis

Background:

Unsymmetrical chiral 1,2-diamines are crucial building blocks in pharmaceuticals, chiral auxiliaries, and ligands.
Existing synthetic routes may lack efficiency, selectivity, or versatility.

Purpose of the Study:

To develop a novel and efficient modular strategy for synthesizing unsymmetrical chiral 1,2-diaryl ethylenediamines.
To achieve high chemoselectivity and enantioselectivity in the synthesis.

Main Methods:

Chiral diboron-templated reductive cross-coupling of aldimines.
Utilizing steric and concentration effects to control reactivity.

Main Results:

Successful modular synthesis of unsymmetrical chiral 1,2-diaryl ethylenediamines.
Achieved good chemoselectivity and enantioselectivity.
Effectively inhibited homocoupling side-reactions.
Demonstrated good functional group tolerance and scalability.

Conclusions:

The established method provides a practical and versatile route to unsymmetrical chiral 1,2-diamines.
This strategy enhances the accessibility of important chiral diamine motifs.