Zincocene and dizincocene N-heterocyclic carbene complexes and catalytic hydrogenation of imines and ketones

Article abstract of DOI:10.1002/chem.201402875

The N-heterocyclic carbene (NHC) adducts Zn(Cp^R) ₂(NHC)] (Cp^R=C₅HMe₄, C ₅H₄SiMe₃; NHC=ItBu, IDipp (Dipp=2,6- diisopropylphenyl), IMes (Mes=mesityl), SIMes) were prepared and shown to be active catalysts for the hydrogenation of imines, whereas decamethylzincocene [ZnCp*₂] is highly active for the hydrogenation of ketones in the presence of noncoordinating NHCs. The abnormal carbene complex [Zn(OCHPh₂)₂(aItBu)]₂ was formed from spontaneous rearrangement of the ItBu ligand during incomplete hydrogenation of benzophenone. Two isolated Zn^I adducts [Zn₂Cp* ₂(NHC)] (NHC=ItBu, SIMes) are presented and characterized as weak adducts on the basis of ¹³C NMR spectroscopic and X-ray diffraction experiments. A mechanistic proposal for the reduction of [ZnCp* ₂] with H₂ to give [Zn₂Cp*₂] is discussed.