Reductive Arylation of Nitroarenes with Chloroarenes: Re-ducing Conditions Enable New Reactivity from Palladium Catalysts

Abstract

Palladium-catalyzed C–N bond forming reactions are a key tool in modern synthetic organic chemistry. Despite advances in catalyst design enabling the use of a variety of aryl (pseudo)halides, the neces-sary aniline coupling partner is often synthesized in a discrete reduc-tion step from a nitroarene. An ideal synthetic sequence would avoid the necessity of this step while maintaining the reliable reactivity of palladium catalysis. Herein we describe how reducing conditions enable new chemical steps and reactivity from well-studied palladium catalysts, resulting in a new, useful transformation: the reductive arylation of nitroarenes with chloroarenes to form diarylamines. Mechanistic experiments suggest that under reducing conditions, BrettPhos-palladium complexes catalyze the dual N-arylation of typi-cally inert azoarenes—generated via the in situ reduction of ni-troarenes—via two distinct mechanisms. Initial N-arylation proceeds via a novel association-reductive palladation sequence followed by reductive elimination to yield an intermediate 1,1,2-triarylhydrazine. Arylation of this intermediate by the same catalyst via a traditional amine arylation sequence forms a transient tetraarylhydrazine, un-locking reductive N–N bond cleavage to liberate the desired product. The resulting reaction allows for the synthesis of diarylamines bearing a variety of synthetically valuable functionalities and heteroaryl cores in high yield.