Zinc-Catalyzed Chemoselective Reduction of Nitriles to N-Silylimines through Hydrosilylation: Insights into the Reaction Mechanism.

The N,N' -chelated conjugated bis-guanidinate (CBG) supported zinc hydride ( Zn-1 ) pre-catalyzed highly challenging chemoselective mono-hydrosilylation of a wide range of nitriles to exclusive N-silylimines and/or N,N' -silyldiimines is reported. Furthermore, the effectiveness of pre-catalyst Zn-1 is compared with another pre-catalyst analogue, i.e., Diethyl NacNac zinc hydride ( Zn-2 ), to know the ligand effect. We observed that pre-catalyst Zn-1 shows high efficiency and better selectivity than pre-catalyst Zn-2 for reducing nitriles to N-silylimines. Mechanistic studies indicate the insertion of the C≡N bond of nitrile into Zn-H to form the zinc vinylidenamido complexes ( Zn-1' and Zn-2' ). The active catalysts Zn-1' and Zn-2' are confirmed by NMR, mass spectrometry, and single-crystal X-ray diffraction analyses. A most plausible catalytic cycle has been explored depending on stoichiometric experiments, active catalysts isolation, and in situ studies. Moreover, the synthetic utility of this protocol was demonstrated.