DOI: 10.1002/chem.201304770
Communication
&
Synthetic Methods
Selective Transfer Hydrogenation and Hydrogenation of Ketones
Using a Defined Monofunctional (P^N(Bn)^N(Bn)^P)–RuII Complex
Shih-Fan Hsu and Bernd Plietker*[a]
benzylic CÀH bonds under mild conditions.[12] Both tetraden-
Abstract: A defined (P^N^N^P)–Ru complex possessing
tate ligands possess tertiary amines within the ligand back-
tertiary amines within the ligand backbone proved to be
bone that were shown to bind to the metal center. As com-
highly active both in transfer hydrogenations and hydro-
pared to the existing (PNNP)–Ru-type complexes this structur-
genations of a variety of ketones. As compared to the ex-
al motif is unusual and we became interested in investigating
isting catalytic systems, no bifunctional activation of H2 or
the potential of our monofunctional (P^N^N^P)–RuII complex
of the substrate by the metal center and a secondary
as a hydrogenation catalyst.
amine within the ligand backbone is required to obtain
At the outset of our investigations, we choose the transfer
high activities at catalyst loadings of down to 10 ppm.
hydrogenation as a starting point. Different solvents and addi-
tives were tested in the reduction of benzophenone 2 under
transfer hydrogenation conditions (Table 1).
The selective hydrogenation of carbonyl compounds is one of
the most important transformations in organic chemistry.[1]
Hence a plethora of active catalysts for the selective reduction
of carbonyl compounds to the corresponding alcohols has
been reported up to date.[1,2] When it comes to activity and se-
lectivity, Noyori’s groundbreaking investigations still resemble
the benchmark within this field of catalysis.[3] However, certain
limitations (e.g., reduction of aliphatic ketones, aldehydes,
imines, etc.) do exist and thus there is an ongoing interest in
identifying more active catalysts that are able to address some
of the limitations.
Already initial results indicated complex 1 to possess a high
activity (Table 1). In the presence of only 0.5 mol% catalyst and
2.5 mol% Kt-amyl as a base, almost quantitative yield of alco-
hol 3 were obtained within just 1.5 h. Further optimization
concentrated on a reduction of the catalyst loading. Complex
1 showed a remarkable activity in the test reaction. Even at
Table 1. The Ru-catalyzed transfer hydrogenation system development.[a]
With regard to catalyst loadings, the use of tetradentate
(P^N^N^P) ligands has recently moved into the center of re-
search.[2e,4,5] Important contributions by the groups of Mezzet-
ti[6] and Noyori[7]/Morris,[4b,8] indicated (P^N^N^P) complexes to
maintain their structure throughout the catalytic cycle. Ligand
dissociation processes are usually not observed. Catalyst aging
and hence a decrease of the activity with ongoing conversion
is significantly reduced. Interestingly, both the Mezzetti and
the Morris system rely on either preformed secondary amines
or on bisimines within the ligand backbone.[9] The use of terti-
ary amines as part of the ligand backbone was reported to de-
crease catalyst activity.[10]
Entry
1
Kt-amyl
[mol%]
Solvent
t
[h]
Yield
[%][b]
[mol%]
1
2
3
4
5
6
7
8
9
0.5
0.5
0.5
2.5
2.5
2.5
iPrOH
EtOH
1.5
1.5
1.5
1.5
18
18
18
18
18
18
18
18
98 (97)[c]
11
1<
79
97[d]
98[d]
99[e]
98[e]
40[f]
87[f]
87[f]
75[f]
Recently, our group was able to show that a defined
(N^N^N^N)–Ru complex is a potent catalyst for hydrogen au-
totransfer reactions.[11] At the same time we reported a defined
(P^N^N^P)–RuII complex to catalyze the selective oxidation of
MeOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
0.25
0.125
0.075
0.05
0.025
0.01
0.01
0.005
0.0025
1.25
0.625
0.375
0.25
0.125
0.05
0.20
0.20
0.20
[a] S.-F. Hsu, Prof. Dr. B. Plietker
Institut fꢀr Organische Chemie
Universitꢁt Stuttgart
10
11
12
Pfaffenwaldring 55
70569 Stuttgart (Germany)
Fax: (+49)711-68564285
[a] The reactions were performed on a 1 mmol scale under a N2-atmos-
phere if not otherwise specified. [b] Determined by H NMR analysis using
mesitylene as internal standard. [c] Isolated yield. [d] 5mmol of ketone.
[e] 15 mmol of ketone. [f] 50 mmol of ketone.
1
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304770.
Chem. Eur. J. 2014, 20, 4242 – 4245
4242
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