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Related Concept Videos

Crossed Aldol Reaction Using Strong Bases: Directed Aldol Reaction00:56

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.
Conjugate Addition to α,β-Unsaturated Carbonyl Compounds01:09

Conjugate Addition to α,β-Unsaturated Carbonyl Compounds

α,β-Unsaturated carbonyl compounds are molecules bearing a carbonyl and alkene functionality in conjugation with each other. The conjugation in the molecule leads to three resonance structures. The hybrid form exhibits two probable electrophilic sites: the carbonyl carbon and the β carbon.
Conjugate Addition (1,4-Addition) vs Direct Addition (1,2-Addition)01:27

Conjugate Addition (1,4-Addition) vs Direct Addition (1,2-Addition)

α,β-Unsaturated carbonyl compounds with two electrophilic sites, the carbonyl carbon, and the β carbon, are susceptible to nucleophilic attack via two modes: conjugate or 1,4-addition and direct or 1,2-addition.
Conjugate addition results in a thermodynamically stable product. The reaction retains the stronger C=O bond at the expense of the weaker C=C π bond. The process is slow as the β carbon is less electrophilic than the carbonyl carbon.
Direct addition products are formed faster owing to...
Alkenes via Reductive Coupling of Aldehydes or Ketones: McMurry Reaction01:22

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.
Base-Catalyzed Aldol Addition Reaction01:08

Base-Catalyzed Aldol Addition Reaction

As depicted in Figure 1, base-catalyzed aldol addition involves adding two carbonyl compounds in aqueous sodium hydroxide to form a β-hydroxy carbonyl compound.
[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction01:16

[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction

The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.

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Retropinacol/Cross-pinacol Coupling Reactions - A Catalytic Access to 1,2-Unsymmetrical Diols
10:12

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

Published on: April 4, 2014

Tetranuclear BINOL-titanium complex in selective direct aldol additions.

Bernd Schetter1, Burkhard Ziemer, Gregor Schnakenburg

  • 1Institut für Chemie der Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany.

The Journal of Organic Chemistry
|January 9, 2008
PubMed
Summary

A robust titanium complex catalyzes direct aldol additions, forming quaternary stereocenters with high regioselectivity. This catalyst is effective even at low loadings (0.2 mol %) and works with various substrates.

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Area of Science:

  • Organometallic Chemistry
  • Asymmetric Catalysis
  • Organic Synthesis

Background:

  • Direct aldol addition reactions are fundamental in organic synthesis for carbon-carbon bond formation.
  • Developing highly regioselective and stereoselective catalysts remains a key challenge, particularly for creating quaternary stereocenters.
  • Unsymmetrical ketones and oxygen-containing ene components present unique challenges due to steric and electronic factors.

Purpose of the Study:

  • To introduce a novel, robust, and water-stable tetranuclear titanium complex as a catalyst for direct aldol addition.
  • To demonstrate the catalyst's ability to achieve high regioselectivity, especially at sterically hindered positions.
  • To explore the formation of quaternary stereocenters and the use of diverse substrates, including oxygen-containing enes and aliphatic aldehydes.

Main Methods:

  • Synthesis and characterization of the tetranuclear titanium complex Ti(4)(micro-BINOLato)(6)(micro(3)-OH)(4) (1).
  • Catalytic testing of complex 1 in direct aldol addition reactions with various unsymmetrical ketones and ene components.
  • Analysis of reaction products to determine regioselectivity, stereoselectivity, and identify any side products (e.g., acetals).

Main Results:

  • Complex 1 efficiently catalyzes direct aldol additions with high regioselectivity, favoring the sterically encumbered alpha-side of unsymmetrical ketones.
  • The formation of quaternary stereocenters was successfully achieved.
  • The catalyst demonstrated broad substrate scope, accommodating oxygen-containing ene components and aliphatic aldehydes, with effective catalysis at loadings as low as 0.2 mol %.

Conclusions:

  • The tetranuclear titanium complex (1) is a highly effective catalyst for direct aldol additions, offering excellent regioselectivity and enabling the synthesis of complex structures.
  • The catalyst's robustness, water stability, and low loading requirements make it a practical tool for organic synthesis.
  • Mechanistic insights into the catalytic process were discussed, providing a foundation for further catalyst development.