Photoredox activation of carbon dioxide and unactivated aliphatic carbonyl compounds

Abstract

Chapter 1: Photoredox Activation of Carbon Dioxide for Amino Acid Synthesis in Continuous Flow. Although carbon dioxide (CO₂ ) is highly abundant, its low reactivity has limited its use in chemical synthesis. In particular, methods for carbon-carbon bond formation generally rely on two-electron mechanisms for CO₂ activation and require highly activated reaction partners. Alternatively, radical pathways accessed via photoredox catalysis could provide new reactivity under milder conditions. Here we demonstrate the direct coupling of CO₂ and amines via the single-electron reduction of CO₂ for the photoredox-catalyzed continuous flow synthesis of [alpha]amino acids. By leveraging the advantages of utilizing gases and photochemistry in flow, a commercially available organic photoredox catalyst effects the selective [alpha]-carboxylation of amines that bear various functional groups and heterocycles.

The preliminary mechanistic studies support CO₂ activation and carbon-carbon bond formation via single-electron pathways, and we expect that this strategy will inspire new perspectives on using this feedstock chemical in organic synthesis. [color illustrations] Chapter 2: Direct [beta]-Selective Hydrocarboxylation of Styrenes with CO₂ Enabled by Continuous Flow Photoredox Catalysis. The direct [beta]-selective hydrocarboxylation of styrenes under atmospheric pressure of CO₂ has been developed using photoredox catalysis in continuous flow. The scope of this methodology was demonstrated with a range of functionalized terminal styrenes, as well as [alpha]-substituted and [beta]-substituted styrenes. [color illustrations] Chapter 3: Metal-free Reductive Coupling of Aliphatic Carbonyl Compounds and Styrenes by Photoredox Catalysis. Metal-free reductive coupling of aliphatic carbonyl compounds and styrenes by photoredox catalysis in continuous flow is described.

The method is applicable to both unactivated aliphatic ketones and aldehydes to afford the corresponding tertiary and secondary alcohols. Preliminary mechanistic investigations suggest the catalytic formation of a ketyl radical intermediate. [color illustrations]