Transfer Hydrogenation of Ketones and Activated Olefins Using Chelating NHC Ruthenium Complexes

• Published in: European Journal of Inorganic Chemistry Vol. 2011; no. 18; pp. 2863 - 2868
• Main Authors: Horn, Sabine, Gandolfi, Claudio, Albrecht, Martin
• Format: Book Review Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01-06-2011; WILEY‐VCH Verlag
• Subjects: Chemoselectivity; Hydrogenation; Ketones; N-Heterocyclic ­carbene; Ruthenium; Transfer hydrogenation
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201100143

N‐Heterocyclic carbene (NHC) ruthenium complexes consisting of different donor substituents attached to the NHC ligand efficiently catalyse the transfer hydrogenation of ketones and of activated olefins in α,β‐unsaturated ketones to give saturated alcohols. The most active catalyst precursor contains a tethered olefin as a hemilabile donor site. This complex also converts nitriles and, depending on the reaction conditions, either benzylamines are produced by means of transfer hydrogenation, or amides from formal addition of H2O. Kinetic analysis of the double hydrogenation of α,β‐unsaturated ketones indicates fast isomerisation of the enol intermediate to its saturated ketone tautomer prior to the second hydrogenation.
The catalytic activity of N‐heterocycliccarbene ruthenium complexes in transfer hydrogenation is strongly influenced by the donor functionality at the carbene ligand, with olefins imparting an optimal balance between lability (catalyst precursor activation) and stability (avoiding catalyst decomposition) thereby allowing ketones and activated olefins to be transfer hydrogenated efficiently.