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.