Indium-Catalyzed Reductive Coupling Enabled Efficient Synthesis of Acylphosphine Oxides and Diphosphines

The unique low level of the ionization potential of indium(0) to indium(I) makes it an appealing metal for organic synthesis. Here, we present an indium-catalyzed reductive cross-coupling between chlorophosphines (R2PCl) or dichlorophosphines (RPCl2) with acyl chlorides (RCOCl). This one-pot approach, using indium-catalysis followed by oxidation, generates a variety of mono- or bis-acylphosphine oxides in good yields. Additionally, this protocol offers a convenient pathway to obtain the photoinitiators, Luricin TPO and IRGACURE 819, that are widely used in the industry. Experimental and computational studies indicate the intermediacy of phosphorus-based radical species that dimerize to diphosphines, which then couple with acyl chlorides through a four-membered transition state. Furthermore, we also explore indium-catalyzed reductive homocoupling of R2PCl or RPCl2, which provides facile access to various P–P bond formations.