Synthetic and Mechanistic Studies into the Reductive Functionalization of Nitro Compounds Catalyzed by an Iron(salen) Complex

We report on the
use of a simple, bench-stable [Fe(salen)2]-μ-oxo
precatalyst in the reduction of nitro compounds. The
reaction proceeds at room temperature across a range of substrates,
including nitro aromatics and aliphatics. By changing the reducing
agent from pinacol borane (HBpin) to phenyl silane (H3SiPh),
we can chemoselectively reduce nitro compounds while retaining carbonyl
functionality. Our mechanistic studies, which include kinetics, electron
paramagnetic resonance (EPR), mass spectrometry, and quantum chemistry,
indicate the presence of a nitroso intermediate and the generation
of an on-cycle iron hydride as a key catalytic intermediate. Based
on this mechanistic insight, we were able to extend the chemistry
to hydroamination and identified a simple substrate feature (alkene
lowest unoccupied molecular orbital (LUMO) energy) that could be used
to predict which alkenes would result in productive catalysis.