Communication
Green Chemistry
(2012CB821600), and the “111” project (B06005) of the Minis-
try of Education of China for financial support.
Notes and references
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Scheme 3 Proposed mechanism for the hydrogenation of esters to
alcohols catalyzed by complex 1a.
5 For selected recent papers, see: (a) J. Zhang, G. Leitus,
Y. Ben-David and D. Milstein, Angew. Chem., Int. Ed., 2006,
45, 1113; (b) L. A. Saudan, C. M. Saudan, C. Debieux and
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an aromatic dihydride intermediate D. Subsequent hydride
transfer to the coordinated aldehyde leads to a Ru–alkoxide
intermediate E. Deprotonation of the benzylic proton next to
the PtBu2 group of the ligand by the alkoxy ligand generates
the product alcohol and a dearomatized intermediate F. The
addition of H2 to F regenerates catalyst 7. The overall catalytic
cycle does not involve a change in the metal oxidation state.
We postulate that the key to the success of catalyst 1a is that
the tetradentate bipyridine ligand 5a could form at least two
five-membered metallacycles with ruthenium in all intermedi-
ates, which is expected to increase the life of the catalyst and
help improve the catalytic activity. The much lower catalytic
activity of catalyst 1c is perhaps due to the difficulty in
opening the chelate ring to provide a vacant coordination site
for ester coordination, to generate an intermediate analogous
to A.
In conclusion, we have developed a new type of ruthenium
complexes with tetradentate bipyridine ligands, which were
judged as outstanding catalysts for ester hydrogenation. A high
efficiency was observed for the hydrogenation of a series of
aromatic and aliphatic carboxylic esters and lactones using as
low as 10 ppm catalyst under mild conditions. Particularly,
biomass derived GVL was hydrogenated to useful 1,4-PDO with
excellent TON (91 000) and TOF (1896 h−1). Further mechanis-
tic studies and applications of these highly efficient ruthenium
catalysts to other polar bond hydrogenations are currently
being investigated in our laboratory.
6 (a) C. Liu, J.-H. Xie, Y.-L. Li, J.-Q. Chen and Q.-L. Zhou,
Angew. Chem., Int. Ed., 2013, 52, 593; (b) X.-H. Yang,
J.-H. Xie, W.-P. Liu and Q.-L. Zhou, Angew. Chem., Int. Ed.,
2013, 52, 7833.
7 W. Li, J.-H. Xie, H. Lin and Q.-L. Zhou, Green Chem., 2012,
14, 2388.
8 D. F. Aycock, Org. Process Res. Dev., 2007, 11, 156.
9 G. W. Huber, S. Iborra and A. Corma, Chem. Rev., 2006,
106, 4044.
Acknowledgements
We thank the National Natural Science Foundation of China,
the
National Basic Research Program of
China
4084 | Green Chem., 2014, 16, 4081–4085
This journal is © The Royal Society of Chemistry 2014