Catalytic Appel reactions

Organophosphorus chemistry plays a key role in the modern synthesis of organic compounds. The formation of phosphine oxides from phosphines often drives reactions to completion. However, these oxides also result in waste and purification issues. Organophosphorus catalysis aims to address these problems, and in this paper we present our progress in developing and utilizing new organophosphorus catalysts. More specifically, the in situ reduction of a range of cyclic phosphine oxides was explored, leading to the development of dibenzophospholes as new organophosphorus catalysts and the application thereof in the phosphorus-mediated substitution of alcohols by halides, which is also known as the Appel reaction. We show that the electronic fine-tuning of the catalyst is crucial for the success of the reaction and that the development of reactions using the in situ reduction protocol all depends on finding the right balance in the reactivities of the reaction components. This balance has been successfully found for the bromination of alcohols, and significant progress has been made for the chlorination reactions.