A charge-remote allylic cleavage reaction: Mechanistic possibilities

Area of Science:

• Organic Chemistry
• Mass Spectrometry
• Physical Chemistry

Background:

• Monounsaturated fatty acids are vital biomolecules.
• Understanding their fragmentation mechanisms is crucial for analytical applications.
• Collision-induced dissociation (CID) is a key technique in mass spectrometry.

Purpose of the Study:

• To elucidate the mechanisms of collision-induced allylic cleavage reactions in fatty acids.
• To investigate the role of deuterium labeling in understanding reaction pathways.
• To differentiate between concerted and radical mechanisms.

Main Methods:

• Utilized deuterium-labeled ions of monounsaturated fatty acids.
• Performed collision-induced dissociation (CID) experiments.
• Analyzed fragmentation patterns and isotopic labeling results.

Main Results:

• Identified potential concerted mechanisms: 1,4-elimination, retro-ene, and 1,4-conjugate elimination.
• Considered two-step radical pathways for retro-ene and 1,4-conjugate elimination.
• Isotopic labeling supported radical reactions but suggested high activation energies.
• Concerted retro-ene and 1,4-conjugate elimination pathways are energetically favorable and consistent with data.

Conclusions:

• Both radical and concerted mechanisms are plausible for allylic cleavage.
• Lower-energy concerted pathways (retro-ene and 1,4-conjugate elimination) are favored.
• The charge-remote concept is challenged by the possibility of distinct mechanisms for proximal and distal C-C bond cleavage.