incorporate an N-heterocyclic carbene into the catalyst structure. As illustrated in
Scheme 7 (Method C), its reaction with a
R PN
H
P ligand can lead to a neutral or
cationic pincer complex depending on the solvent used [63].
3.1.2 Hydrogenation of Esters, Ketones, and Their Derivatives
Both Ru-MACHO and Ru-MACHO-BH are commercially available, and among
the complexes shown in Scheme 7, they are the most extensively studied ones for
catalytic hydrogenation reactions. In 2012, Takasago International Corporation
reported that Ru-MACHO mixed with NaOMe was effective for hydrogenation of
esters to alcohols (Eq. 1) [54]. The catalytic system is amenable to benzyloxy,
Scheme 7 Synthetic routes to ruthenium-based hydrogenation (pre)catalysts
270
D. A. Ekanayake and H. Guan