Selective Hydrogenation of Amides to Amines and Alcohols Catalyzed by Improved Iron Pincer Complexes

A comparative study
on the synthesis, stability, and catalytic
activity of various iron pincer complexes with the general formula
[(R-PNHP)­Fe­(H) (CO) (BH4)]
is reported, where R denotes the substituent of the terminal PR2-groups (R = tBu, Cy, iPr, Ph, Et). By the example of the synthesized precatalysts,
it is shown that the nature of the ligands has a surprising influence
on the catalytic properties of the complexes. Bulky ligands and less
electron donating ligands affect the stability of the complexes, which
preferably react under the loss of CO or H2 to deactivated
products. In return, the reduced steric demand and the strong σ-donating
properties of the Et-substituted precatalyst (2a) lead
to an improved activity in the hydrogenation of esters to alcohols,
compared to that of the previously reported iPr-substituted complexes. The improved activity of complex 2a is clearly demonstrated in the direct hydrogenation of
amides to alcohols and amines under mild conditions.