Mechanistic Studies on Garratt-Braverman Cyclization: The Diradical-Cycloaddition Puzzle

Area of Science:

• Organic Chemistry
• Reaction Mechanisms

Background:

• The Garratt-Braverman (GB) cyclization is a key reaction in organic synthesis.
• Controversies exist regarding the precise reaction mechanisms for different substrate classes.

Purpose of the Study:

• To elucidate the mechanistic pathways of Garratt-Braverman cyclization in bis-propargyl sulfones and ethers.
• To resolve mechanistic ambiguities through comprehensive experimental investigations.

Main Methods:

• Deuterium-labeled substrate studies to track atom fate.
• Electron Paramagnetic Resonance (EPR) spectroscopy to detect radical intermediates.
• Trapping experiments to identify transient species.
• Laser Ablation-Laser Desorption (LA-LDI) mass spectrometry for sensitive detection.

Main Results:

• Evidence supports a diradical mechanism for Garratt-Braverman cyclization in bis-propargyl sulfones.
• An anionic [4 + 2] pathway is identified as the preferred mechanism for bis-propargyl ethers.
• The nature of the heteroatom (sulfur vs. oxygen) dictates the reaction pathway.

Conclusions:

• The mechanistic outcome of Garratt-Braverman cyclization is highly sensitive to the heteroatom bridging the propargyl groups.
• This study clarifies the distinct reaction pathways for sulfone and ether systems, advancing the understanding of this cyclization reaction.