posted on 2021-04-27, 12:36authored byJuan C. Babón, Miguel A. Esteruelas, Ana M. López, Enrique Oñate
The hexahydride OsH6(PiPr3)2 competently catalyzes the hydration
of aliphatic nitriles
to amides. The main metal species under the catalytic conditions are
the trihydride osmium(IV) amidate derivatives OsH3{κ2-N,O-[HNC(O)R]}(PiPr3)2, which have been isolated and fully characterized
for R = iPr and tBu. The rate of hydration is
proportional to the concentrations of the catalyst precursor, nitrile,
and water. When these experimental findings and density functional
theory calculations are combined, the mechanism of catalysis has been
established. Complexes OsH3{κ2-N,O-[HNC(O)R]}(PiPr3)2 dissociate the carbonyl group of the chelate to afford
κ1-N-amidate derivatives, which
coordinate the nitrile. The subsequent attack of an external water
molecule to both the C(sp) atom of the nitrile and the N atom of the
amidate affords the amide and regenerates the κ1-N-amidate catalysts. The attack is concerted and takes place
through a cyclic six-membered transition state, which involves Cnitrile···O–H···Namidate interactions. Before the attack, the free carbonyl
group of the κ1-N-amidate ligand
fixes the water molecule in the vicinity of the C(sp) atom of the
nitrile.