Visible Light-Induced [2+2+1] Cycloaddition of 1,6-Enones with α-Bromocarbonyls

Area of Science:

• Organic Chemistry
• Synthetic Chemistry
• Photochemistry

Background:

• Cycloaddition reactions are fundamental in organic synthesis.
• Developing efficient methods for synthesizing fused heterocyclic compounds remains a key challenge.
• Visible light photocatalysis offers a sustainable and mild approach to chemical transformations.

Purpose of the Study:

• To develop a novel visible light-induced cycloaddition reaction.
• To synthesize cis-hexahydro-2H-cyclopenta[b]furans from readily available starting materials.
• To elucidate the reaction mechanism and explore its synthetic utility.

Main Methods:

• Visible light irradiation of 1,6-enones with alpha-carbonyl alkyl bromides.
• Radical cascade reaction pathway.
• Mechanistic studies including radical addition, hydrogen atom transfer, bromination, and SN2 cyclization.

Main Results:

• Successful synthesis of cis-hexahydro-2H-cyclopenta[b]furans with high yields and excellent diastereoselectivity.
• Identification of a radical cascade mechanism involving key steps like C=O bond radical addition and SN2 cyclization at a quaternary center.
• Demonstration of an extended scope for [2+2+1] cycloaddition methodology.

Conclusions:

• A novel and efficient visible light-driven dehydrobrominative [2+2+1] cycloaddition has been developed.
• The reaction provides direct access to valuable cis-hexahydro-2H-cyclopenta[b]furan scaffolds.
• The mechanistic insights reveal an uncommon SN2 cyclization, expanding the repertoire of radical chemistry.