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

Abstract

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.