Zincocene and Dizincocene N-Heterocyclic Carbene Complexes and Catalytic Hydrogenation of Imines and Ketones

• Published in: Chemistry : a European journal Vol. 20; no. 27; pp. 8370 - 8378
• Main Authors: Jochmann, Phillip, Stephan, Douglas W.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01.07.2014; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Adducts; carbene ligands; Carbenes; Catalysis; Catalysts; Chemistry; homogeneous catalysis; Hydrogenation; Imines; Ketones; reaction mechanisms; X-rays; Zinc; reaction mechanisms; homogeneous catalysis; hydrogenation; zinc; carbene ligands
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201402875

The N‐heterocyclic carbene (NHC) adducts Zn(CpR)2(NHC)] (CpR=C5HMe4, C5H4SiMe3; 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 ZnI 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. Reaction mechanisms: A series of N‐heterocyclic carbene (NHC) adducts of zincocenes [Zn(CpR)2(NHC)] (CpR=C5HMe4, C5H4SiMe3; NHC=ItBu, IDipp, IMes, SIMes) was isolated and tested in the catalytic hydrogenation of imines. Catalytic hydrogenation of ketones was achieved with [ZnCp*2] (Cp*=C5Me5)/NHC. Adducts of zinc alkoxides and [Zn2Cp*2] were isolated, and their mechanistic implications are discussed (see scheme).