Direct C─H Lactonization of Carboxylic Acids Enabled by LMCT Photoactivation

Area of Science:

• Organic Chemistry
• Photochemistry
• Catalysis

Background:

• Photoinduced ligand-to-metal charge-transfer (LMCT) activation is a versatile method for carboxylic acid functionalization.
• Decarboxylative coupling reactions using LMCT offer powerful synthetic strategies.
• Engaging acyloxy radical intermediates in reactions like hydrogen atom transfer (HAT) is desirable but challenging due to rapid decarboxylation.

Purpose of the Study:

• To develop a novel LMCT-promoted C─H lactonization reaction for benzoic and aliphatic carboxylic acids.
• To overcome the inherent challenge of rapid decarboxylation of acyloxy radical intermediates.
• To explore the potential of metal-centered stabilization for enabling alternative radical pathways.

Main Methods:

• Utilized photoinduced ligand-to-metal charge-transfer (LMCT) activation.
• Investigated C─H lactonization of various carboxylic acids.
• Conducted mechanistic studies to understand intermediate stabilization and reaction pathways.

Main Results:

• Successfully achieved LMCT-promoted C─H lactonization of benzoic and aliphatic carboxylic acids.
• Demonstrated that metal-centered stabilization of the acyloxy radical intermediate is crucial.
• Showed that 1,5-hydrogen atom transfer (1,5-zHAT) can outcompete decarboxylation under optimized conditions.

Conclusions:

• LMCT activation provides a viable route for C─H lactonization of carboxylic acids.
• Metal-centered stabilization of acyloxy radicals is key to controlling reactivity and enabling new transformations.
• This work broadens the synthetic utility of LMCT photochemistry by engaging acyloxy radicals in non-decarboxylative pathways.