Ti-Catalyzed Oxidative Amination Using Anilines

In this report, Ti-catalyzed transfer hydrogenation from anilines to alkynes is leveraged to generate nitrenes from anilines in the Ti-catalyzed [2 + 2+1] synthesis of N-aryl pyrroles. While there are myriad methods for accessing nitrene reactive intermediates in chemical synthesis, methods for the direct transformation of amines into nitrene equivalents are uncommon. This transfer hydrogenation-coupled [2 + 2 + 1] pyrrole synthesis is catalyzed by a recently reported bis(phenoxide) Ti complex, (TPO)Ti(NMe2)(2) (TPO = (3,3 ''-ditert-butyl-5,5 ''-dimethyl-[1,1 ':2 ',1 ''-terphenyl]-2,2 ''-bis(olate)). Preliminary mechanistic insight indicates that a balance of the competition between alkyne insertion into titanacycle Ti-C bonds versus Ti-C aminolysis at several points along the catalytic cycle is critical for productive catalysis over unwanted side reactions, such as alkyne hydroamination or alkyne cyclotrimerization.