Acetate Acetylacetonate Ampy Ruthenium(II) Complexes as Efficient Catalysts for Ketone Transfer Hydrogenation

Article abstract of DOI:10.1002/cctc.202000542

The mixed acetate acetylacetonate (acac) ruthenium(II) phosphine complexes Ru(OAc)(acac)P₂ [P₂=(PPh₃)₂, Ph₂P(CH₂)₄PPh₂ (dppb)] were prepared by protonation of Ru(OAc)₂(PPh₃)₂ with acetylacetone in dichloromethane. Reaction of the dppb derivative with 2-(aminomethyl)pyridine (ampy) affords the complex Ru(OAc)(acac)(ampy)(dppb), which converts to [Ru(acac)(ampy)(dppb)](OAc) in toluene at 90 °C. In the former derivative the ampy ligand is monodentate and coordinates through the NH₂-moiety. The isolated acac complexes are active catalysts for the transfer hydrogenation of ketones with loadings as low as 0.01 mol%, the ampy having a strong accelerating effect. Several aromatic and aliphatic ketone substrates are converted to their corresponding alcohols, and different electronic influences through substituents on acetophenone are tolerated.

Full text of DOI:10.1002/cctc.202000542

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Acetate Acetylacetonate Ampy Ruthenium(II) Complexes as
Efficient Catalysts for Ketone Transfer Hydrogenation
Authors: Daniela A. Hey, Michael J. Sauer, Pauline J. Fischer, Eva-
Maria H. J. Esslinger, Fritz Elmar Kühn, and Walter Baratta
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ChemCatChem
10.1002/cctc.202000542
FULL PAPER
Acetate Acetylacetonate Ampy Ruthenium(II) Complexes as
Efficient Catalysts for Ketone Transfer Hydrogenation
[a]
[a]
[a]
[a]
[a]
Daniela A. Hey, Michael J. Sauer, Pauline J. Fischer, Eva-Maria H. J. Esslinger, Fritz E. Kühn*
and Walter Baratta*[b]
[
[
a]
b]
D. A. Hey, M. J. Sauer, P. J. Fischer, E.-M. H. J. Esslinger, Prof. F. E. Kühn
Molecular Catalysis, Catalysis Research Center and Department of Catalysis
Technische Universität München
Lichtenbergstr. 4, Garching b. München, Germany
E-mail: fritz.kuehn@ch.tum.de
Prof. W. Baratta
Dipartimento di Scienze AgroAlimentari, Ambientali e Animali (DI4A)
Università di Udine
Via Cotonificio 108, 33100 Udine, Italy
E-mail: walter.baratta@uniud.it
Supporting information for this article is given via a link at the end of the document.
Abstract: The mixed acetate acetylacetonate (acac) ruthenium(II)
phosphine complexes Ru(OAc)(acac)P [P = (PPh
Ph P(CH
Ru(OAc)
preparation of Ru(acac)
2
P
2
systems for HY reactions has been
2
2
3
)
2
,
reported for
(Figure 1).[
P
2
= dppm, dppe, dppp, dppf and BINAP
6-7]
2
2
)
4
PPh (dppb)] were prepared by protonation of
2
2
(PPh ) with acetylacetone in dichloromethane. Reaction of
3 2
the dppb derivative with 2-(aminomethyl)pyridine (ampy) affords the
complex Ru(OAc)(acac)(ampy)(dppb), which converts to
Ru(acac)(ampy)(dppb)](OAc) in toluene at 90 °C. In the former
derivative the ampy ligand is monodentate and coordinates through
the NH -moiety. The isolated acac complexes are active catalysts for
the transfer hydrogenation of ketones with loadings as low as
.01 mol, the ampy having a strong accelerating effect. Several
[
2
Figure 1. Acetylacetonate diphosphine ruthenium(II) complexes.
0
aromatic and aliphatic ketone substrates are converted to their
corresponding alcohols, and different electronic influences through
substituents on acetophenone are tolerated.
Complexes containing chiral diphosphines have been described
to catalyze the asymmetric HY of alkenes and amides at relatively
high catalyst loading, under high hydrogen pressure and with
moderate turnover frequencies (TOFs).[7-8] Comparison of the
2 2
performance of the acetate Ru(OAc) P vs. the corresponding
Introduction
acetylacetonate Ru(acac) P2 complexes shows that acac
2
derivatives are significantly less active. Interestingly, the catalytic
Ruthenium diphosphine complexes bearing carboxylate ligands
have widely been explored in the catalytic hydrogenation (HY)
activity of Ru(acac) [(S)-BINAP] in olefin HY can be considerably
2
increased by reaction with carboxylic acid under visible light for
several days, leading to the mixed carboxylate and
acetylacetonate species Ru(OCOR)(acac)[(S)-BINAP], which is
[1]
during the last decades with particular attention to their activity.
[
2]
The well-known acetate complexes Ru(OAc)
prepared from the commercially available precursors
RuCl (PPh [RuCl (cod)] (cod 1,5-cyclooctadiene) or
RuCl (p-cymene)] , by exchange of chloride with carboxylate and
2
P
2
are usually
also
more
active
than
the
diacetate
catalyst
[
8b,9]
2
3
)
3
,
2
n
=
Ru(OAc) [(S)-BINAP].
2
[
2
2
reaction with phosphines. The resulting complexes have found
application as catalysts for hydrogenation of olefins and
functionalized ketones, with Noyori’s Ru(OAc) [(S)-BINAP] being
2
one of the most popular systems used in asymmetric HY.[3] In
addition, carboxylate ruthenium(II) catalysts bearing strong
coordinating carbene and pincer ligands have been reported to
display high productivity in HY and transfer hydrogenation (TH) of
carbonyl compounds.[4] Conversely, the related acetylacetonate
phosphine ruthenium(II) complexes are generally synthesized
2
Scheme 1. Light activation of Ru(acac) [(S)-BINAP] for the HY of alkenes.
Considering the high reactivity of ruthenium(II) acetate phosphine
complexes and the contrasting robustness of their
acetylacetonate analogues in catalysis, the heteroleptic
from Ru(acac) [5]
presence of reducing agents (i.e zinc or PR
(acac = acetylacetonate) and phosphines in
).[6] The in situ
3
3
1
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acetate/acetylacetonate ruthenium(II) complexes were assumed
to offer reactivity as well as stability, both beneficial for catalysis.
Lability of an anionic ligand is crucial to open a free coordination
site for the substrate, while the stability of the catalyst’s backbone
is important to reach high turnover numbers (TONs). In this way
a very efficient and at the same time highly robust catalyst can be
achieved without requiring the time-consuming activation process
that had to be performed previously.[8b]
Reaction of the related diacetate ruthenium(II) diphosphine
complex
Ru(OAc)
2
(dppb) (2)
with
acetylacetone
in
dichloromethane at room temperature for 3 h affords complex 4
in 63% yield (Eq. 1). In the 31P{ H} spectrum, 4 exhibits two
1
2
doublets at δ = 66.8 and 52.6 ppm ( J(PP) = 43.2 Hz), in
accordance with the two chemically non-equivalent phosphine
moieties in trans position to the acetate and acac ligand,
1
respectively. The H NMR spectrum shows three distinct signals
Notably, mixed acetate/acetylacetonate phosphine complexes of
ruthenium(II) are sparingly reported in literature.[8b] It is well known
that addition of amines to ruthenium(II) phosphine complexes
leads to an increase of the performance in HY and transfer
hydrogenation (TH) of carbonyl compounds.[4b,10] In particular,
while ethylenediamine (en) has been employed to increase the
activity in the HY of ketones, 2-(aminomethyl)pyridine (ampy)[2c,11]
affords extremely active catalysts in the TH. In these cases the
presence of an NH function plays a pivotal rule, leading to a rapid
outer sphere mechanism.[12] In addition, ampy based
ruthenium(II) complexes are well-established as active catalysts
for a number of organic transformations (dehydrogenation,
racemization, deuteration, isomerization).[13]
3
for the CH groups of the two acac and the acetate methyl groups
at δ = 1.68, 1.52 and 1.45 ppm.
The structure of complex 4 is further confirmed by SC-XRD
measurements (Figure 2). Accordingly, the ruthenium(II) center is
coordinated pseudo-octahedrally by the chelating dppb, acetate
and acac ligands.
In order to obtain highly active and productive catalysts with
retarded deactivation, the design of complexes combining both
labile and robust ligands is crucial. In addition, a straightforward
preparation of the catalysts, avoiding multiple steps, tedious and
costly ligand preparation,[14] appears to be fundamental for
applications.
In this work a simple synthetic route for the preparation of mixed
acetate/acetylacetonate ruthenium phosphine complexes
2
Ru(OAc)(acac)P and their reaction with the ampy ligand is
reported. The isolated and in situ formed ampy derivatives display
Figure 2. ORTEP-style drawing of 4 with ellipsoids at 50% probability level.
Hydrogen atoms omitted for clarity. For selected bond lengths and angles see
Supporting Information.
high activity in the TH of ketones.
Results and Discussion
As expected, the Ru-O bond lengths in trans position to the
phosphine moieties are slightly longer (Ru1-O1 = 2.232(3) Å,
Ru1-O4 = 2.110(3) Å) compared to those trans to another oxygen
atom (Ru1-O2 = 2.128(3) Å, Ru1-O3 = 2.062(3) Å) due to the
strong trans influence of the phosphine. The shorter Ru-O bond
lengths of acac vs. OAc and their smaller variations
Syntheses
Reaction of the diacetate ruthenium(II) triphenylphosphine
complex Ru(OAc)
.5 equiv.) in dichloromethane at room temperature for 3 h affords
the mixed acetate/acetylacetonate complex 3 in 68% yield (Eq. 1).
2 3 2
(PPh ) (1) with acetylacetone (Hacac,
1
(
Ru1-O4 - Ru1-O3 = 0.048 Å vs. Ru1-O1 - Ru1-O2 = 0.104 Å)
indicate that acac is coordinated stronger with respect to OAc,
which is more susceptible to the electronic properties of the trans
ligand. Electron delocalization of the acac ligand is manifested by
bond lengths for the carbon backbone that lie in the range of
aromatic C-C bonds (C3-C4 = 1.393(7) Å, C4-C5 = 1.403(7) Å).
The di-acac complex 5 was synthesized by adding an excess of
Hacac (6 equiv.) to 2 at room temperature in dichloromethane for
3 d and isolated in 58% yield (Eq. 2). The formation of 5 instead
of 4 occurs due to the high excess of Hacac, as well as to the
prolonged reaction time, which results in the gradual substitution
of both OAc ligands from 2.
3
1
1
In CDCl
3
, 3 causes two doublets in the P{ H} spectrum at
2
δ = 64.9 and 53.5 ppm ( J(PP) = 36.4 Hz), in accordance with the
two chemically non-equivalent phosphine moieties in trans
position to the acetate and acac ligand. The chemical shifts
indicate the position of the phosphine. Thus, the moiety in trans
position to the acetate is low-field shifted in comparison to the one
trans to acac, as inferred by comparison of the chemical shifts of
analogue di-acetate and -acac complexes (vide infra and
refs.[1d,15]). The 1H NMR spectrum shows three distinct CH
3
signals for the two acac and the acetate methyl groups at δ = 1.69, In solution, 5 exhibits a sharp singlet at δ = 52.9 ppm in the
3
1
1
1
.59 and 1.47 ppm.
P{ H} NMR spectrum for the symmetrically substituted dppb
ligand. The H and 13C spectra further confirm the formation of a
1
2
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3
single product with two distinct CH moieties at δ = 1.65 and
1
.46 ppm in the proton spectrum and a CH signal at δ = 98.8 ppm
for the tertiary carbon atom of the enol form of the acac ligands.
The SC-XRD structure of 5 (Figure 3) shows the ruthenium(II)
center in a pseudo-octahedral coordination geometry, with the
chelating dppb and two acac ligands. Similar to complex 4, the
acac ligand coordinates strongly to the ruthenium center,
indicated by the relatively short Ru-O bonds.
Figure 4. ORTEP-style drawing of 6 with ellipsoids at 50% probability level. All
2 2
hydrogen atoms except H1N and H2N, and co-crystallized CH Cl are omitted
for clarity. Hydrogen atoms could not be located in the difference Fourier Maps
and were calculated in ideal positions (riding model). For selected bond lengths
and angles see Supporting Information.
Figure 3. ORTEP-style drawing of 5 with ellipsoids at 50% probability level.
Hydrogen atoms omitted for clarity. For selected bond lengths and angles see
Supporting Information.
The Ru-O bonds to acac in 6 are slightly longer than in complex 4
(
vide supra), while the Ru-O bond length to acetate decreases
moderately for 6 (Ru1-O1 = 2.1061 Å). Two of the phenyl rings of
dppb exhibit a planar arrangement to each other (C24-C29 and
C36-C41), indicating π-stacking between the aromatic rings. An
intramolecular hydrogen bond between the acetate C=O and the
ampy NH stabilizes complex 6 via a six-membered ring (O2-H1N
distance = 2.172 Å).[ At room temperature, 6 dissociates in
Treatment of
4 with an excess of ampy (6 equiv.) in
dichloromethane at room temperature affords complex 6 in 77%
isolated yield via an equilibrium reaction (Scheme 2).
16]
2 2
CD Cl to 4 and ampy through an equilibrium reaction (Scheme 2)
with a 6:4 molar ratio of about 5, whereas in toluene-d
8
the 6:4
ratio is 10, as inferred by 31P{ H} NMR measurements (see
1
Supporting Information). At temperatures lower than -10 °C in
toluene-d , 6 does practically not dissociate, while upon heating
8
Scheme 2. Synthesis of 6 and 7 from 4 with ampy.
above 50 °C, the new species 7 forms (Scheme 2 and Supporting
Information). The cationic complex 7 was isolated in 85% yield by
heating 6 at 90 °C in toluene for 1 h. The 31P{ H} NMR spectrum
In toluene-d
8
, 6 exhibits two doublets at δ = 53.2 and 45.9 ppm
(2
J(PP) = 38.2 Hz) in the 31P{ H} NMR spectrum for the phosphine
1
1
moieties in trans position to the acac and amine ligand,
respectively. In the H NMR spectrum, two singlets at δ = 1.64
of 7 in toluene-d
J(PP) = 36.8 Hz) for the phosphine moieties in trans position to
the acac and pyridine ligand, respectively. In the 1H NMR
8
shows two doublets at δ = 51.0 and 45.6 ppm
1
(2
and 1.33 ppm evidence the CH
acetate-CH appears downfield-shifted at δ = 2.16 ppm, while the
NH protons lead to signals at δ = 3.98 and 6.92 - 6.97 ppm, the
3
moieties of the acac ligand. The
3
spectrum, two singlets at δ = 1.94 and 1.02 ppm are in
2
accordance with the acac-CH
acetate anion occurs downfield shifted at δ = 2.53 ppm. The
diastereotopic CH NH protons of the ampy ligand appear at
3 3
moieties. The CH group of the
latter being involved in hydrogen bonding to the acetate. The
multiplets at δ = 3.77 and 3.37 ppm are in accord with the
diastereotopic CH NH protons.
2 2
2
2
2
3
δ = 3.74 ppm (m) and 3.53 ppm (dd, J = 18.18 Hz, J = 8.34 Hz),
while the multiplets at δ = 3.33 and 8.56 ppm evidence the amine
protons, the latter signal being ascribed to hydrogen bonding of
one amine proton to an oxygen atom of the acetate. Bidentate
coordination of ampy in 7 results in a high-field shift of the pyridine
proton signals in comparison to its monodentate analogue 6
The SC-XRD structure of 6 (Figure 4) shows the ruthenium(II)
center in a pseudo-octahedral coordination geometry, with
bidentate phosphine and acac ligands and monodentate
coordination modes of the acetate and ampy moieties.
(
Δδmax = 0.87 ppm). The SC-XRD structure of 7 shows that the
ruthenium(II) center is pseudo-octahedrally coordinated by
bidentate dppb, acac and ampy ligands with acetate as counterion
(
Figure 5). By difference to 6, the NH
2
moiety of 7 is coordinated
forms from 6 through
trans to oxygen, indicating that
7
3
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reorganization of the ampy ligand. The short NH-O
distance (H2N-O1 = 2.039 Å) indicates a strong hydrogen bond
between the outer sphere acetate and the cationic complex, in
accordance with the NMR data in solution.
triphenylphosphine complex 1, although none of these catalyst
derivatives leads to full conversion of a. The comparison of the
activity of the mixed acetate/acac complexes 3 and 4 with PPh
3
and dppb ancillary ligands (Table 1, entries 3 and 4) highlights the
beneficial effect of the chelating dppb with respect to the
monodentate PPh
3
. Thus, 3 converts a maximum of 28% a, while
4
reaches 75% conversion in 8 h. The low conversion with 3 is
attributed to catalyst deactivation upon dissociation of the
phosphine, inhibited in 4 by the bidentate phosphine. The di-acac
complex 5 is virtually inactive, converting only 1% a within 8 h
(
Table 1, entry 5). This is likely due to the strong bonding of acac
to ruthenium, preventing the generation of a free coordination site
and formation of the active catalytic species.
Table 1. Transfer hydrogenation of a with complexes 1 - 5.[a]
Entry
Catalyst
Additive[b]
Conversion[c]
%]
Time
[min]
TOF[d] [h-1]
[
1
2
3
4
5
6
7
8
9
1
2
3
4
5
1
2
3
4
5
-
38
53
28
75
1
480
480
480
480
480
60
n.d.
Figure 5. ORTEP-style drawing of 7 with ellipsoids at 50% probability level. All
hydrogen atoms except H1N and H2N are omitted for clarity. Hydrogen atoms
could not be located in the difference Fourier Maps and were calculated in ideal
positions (riding model). For selected bond lengths and angles see Supporting
Information.
-
100
-
n.d.
-
930
-
n.d.
Thus, the syntheses of ruthenium(II) diphosphine complexes with
diacetylacetonate and mixed acetate/acetylacetonate anionic
ligands were accomplished by protonation of the diacetate
phosphine precursors. Addition of ampy to the mixed complex
leads to mono- or bidentate binding of ampy through opening or
de-coordination of acetate.
ampy
ampy
ampy
ampy
ampy
91
97
93
98
4
3,300
2,200
15,000
26,000
n.d.
60
60
5
Catalytic transfer hydrogenation
10
480
The catalytic activities of the diacetate, diacetylacetonate and
i
[
a] Conditions: iPrOH, 1 mmol a, 2 mol% NaOPr, 0.1 mol% catalyst,
mixed acetate/acetylacetonate complexes 1 - 7 were investigated
S/C/B = 100/0.1/2. [b] 10 equiv. additive with respect to the catalyst. [c]
Maximum conversion, determined by GC; conversion corresponds to yield of
i
i
in the TH of acetophenone (a) in PrOH with NaOPr as base
Scheme 3).
1
-phenylethanol. [d] Calculated at 50% conversion.
(
Upon addition of 10 equiv. ampy to the reaction mixture, 1 - 4
reach quantitative conversion within 60 min (1 - 3, Table 1,
entries 6-8) and 5 min (4, Table 1, entry 9), with turnover
frequencies (TOFs) of 3,300 h (1), 2,200 h-1 (2), 15,000 h (3)
and 26,000 h-1 (4). The fastest conversion is obtained with the
mixed acetate/acac complex 4, while both, the diacetate
-
1
-1
complexes 1 and 2, and the PPh
3
analogue 3, fall short in
comparison. The di-acac complex 5 still shows no considerable
conversion upon addition of ampy (4% conversion within 8 h,
entry 10). Control 31P{ H} NMR measurements show no reaction
1
i
of 5 with ampy at 90 °C in PrOH. In absence of ruthenium
i
catalysts, the base NaOPr (2 mol%) is not capable of converting
the substrate (1% conversion within 8 h). Without base, the most
active catalyst 4 shows no activity, while ampy (1 mol%) is further
inactive, reaching 3% conversion within 8 h. These results
indicate that basic conditions are crucial for the formation of the
active species.
i
Scheme 3. Transfer hydrogenation of ketones a - j catalyzed by 1 - 7 in PrOH
i
at 90 °C oil bath temperature with NaOPr.
At 0.1 mol% catalyst loading, the diacetate complexes 1 and 2
exhibit moderate catalytic performance, reaching 38 and 53%
conversion of a in 8 h, respectively (Table 1, entries 1 and 2).
Complex 2 with the bidentate phosphine is more active than the
Primary amines, like benzylamine (bza) or ethylenediamine (en),
also exhibit considerable impact on catalyst 4 (Figure 6). Bza (▲
-1
green) (10 equiv.) slows down the reaction (TOF = 700 h ), and
low conversion is observed (57% conversion in 60 min). Addition
4
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of en (■ orange) considerably pushes the catalyst’s performance
0.01 mol% of 6, 87% conversion of a is achieved in 10 min,
reaching a TOF of 105,300 h-1 (Table 2, entry 5). By contrast to
the high activity observed with the monodentate ampy complex 6,
the related bidentate ampy derivative 7 is practically inactive in
the TH of a, converting only 4% of the substrate in 4 h (Table 2,
entry 6). The low activity is likely due to the lack of a labile ligand
in 7, hindering the formation of the catalytically active hydride
species via ruthenium isopropoxides.
-1
(
TOF = 2,900 h ), although the performance is lower with respect
-1
to the one obtained with ampy (x blue) (TOF = 26,000 h ).
To evaluate the substrate scope of the best-performing catalyst 6,
a variety of ketones was examined in the TH with 0.03 mol% 6
(
Scheme 3, Table 3). In addition to a (Table 3, entry 1), the para-
substituted acetophenone derivatives b, c and d with R = Me,
MeO and Cl are fully converted in 10 min with TOFs between
5
5,000 and 75,300 h-1 (Table 3, entries 2-4). Derivatives c and d
with electron-withdrawing groups (Table 3, entries 3 and 4)
exhibit slightly higher TOFs than the electron-donating b (Table 3,
entry 2). Ortho-substitution of acetophenone with a chloride
moiety (substrate e) leads to comparable performance as with the
unsubstituted derivative a (Table 3, entry 5). The order of activity
of 6 in TH of acetophenone derivatives is thus a > e > b > c > d
Figure 6. Transfer hydrogenation of acetophenone with complex 4 and
i
1
0 equiv. additives, ampy, en, bza. Conditions: PrOH, 1 mmol a, 2 mol%
i
NaOPr, 0.1 mol% 4, 1.0 mol% additives, S/C/B = 100/0.1/2. x ampy, ■ en,
▲
bza.
(
R = Me > MeO > Cl). The diaryl substrate benzophenone f is
-1
converted in 94% within 20 min as well (TOF = 55,600 h , Table 3,
entry 6). The slower rate is attributed to the steric hindrance of the
substrate. Cyclic aliphatic ketones g and h are fully converted
At lower loading of
(
(
4
(0.03 mol%) in presence of ampy
2-10 equiv.) a is quantitatively converted to 1-phenylethanol
Table 2, entries 1 and 2).
-1
within 10 min, affording TOFs of 105,800 and 52,800 h ,
respectively (Table 3, entries 7 and 8). The linear ketone i is
-1
hydrogenated in 99% within 10 min (TOF = 43,800 h , Table 3,
entry 9), whereas the unsaturated ketone j is selectively reduced
at the carbonyl functionality without formation of the saturated
alcohol (Table 3, entry 10).
Table 2. Transfer hydrogenation of a with complexes [4 + ampy], 6 and 7.[a]
[d]
Entry
Catalyst
Catalyst
loading
ampy
[equiv.][
Coversion[c]
[%]
Time
[min]
TOF
[h ]
b]
-1
[
mol%]
Table 3. Transfer hydrogenation of ketone substrates a - j with complex 6.[a]
1
2
3
4
5
6
4
4
6
6
6
7
0.03
0.03
0.03
0.02
0.01
0.03
2
10
-
96
96
99
98
87
4
10
5
100,000
100,000
125,000
103,000
Entry
Substrate
Coversion[b] [%]
Time [min]
TOF[c] [h-1]
125,000
75,300
62,300
55,000
116,300
55,600
105,800
52,800
43,800
57,100
5
1
2
3
4
5
6
7
8
9
a
b
c
d
e
f
99
96
90
99
98
94
98
97
99
80
5
-
5
10
10
5
105,300
-
10
240
-
n.d.
10
20
10
10
10
10
i
i
[
a] Conditions: PrOH, 1 mmol a, 2 mol% NaOPr, S/B = 100/2. [b] Maximum
conversion, determined by GC; conversion corresponds to yield of
-phenylethanol. [c] Calculated at 50% conversion.
1
g
h
i
To investigate the impact of ampy, different equivalents of this
additive with respect to the catalyst were applied. With 2 and
0 equiv. ampy, 96% conversion of a is reached within 10 and
min, respectively (Table 2, TOF = 100,000 h-1 for both entries 1
1
5
10
j
and 2). However, with 10 equiv. ampy, the conversion decreases
i
if the base NaOPr is added to the refluxing mixture after 2 min,
i
i
[
a] Conditions: PrOH, 1 mmol ketone substrate, 2 mol% NaOPr, 0.03 mol% 6,
while consecutive addition of 10 equiv. ampy-catalyst-base
produces the same high catalytic activities as observed with
S/C/B = 100/0.03/2. [b] Maximum conversion, determined by GC; conversion
corresponds to yield of 1-phenylethanol. [c] Calculated at 50% conversion.
2
equiv. ampy.
On account of the accelerating effect of ampy on catalyst 4, the
isolated ampy derivative 6 was examined at 0.03 mol% catalyst
loading. Similar to the in situ system, 6 converts 98% a within
In analogy to results obtained by Noyori et al. for ruthenium(II)
a bifunctional outer sphere
mechanism is proposed for the catalytic TH of ketone derivatives
primary amine complexes[12]
5
min with TOFs of 125,000 h-1 (Table 2, entry 3). At 0.02 mol%
loading of 6, full conversion is achieved after 5 min, with slightly
with complex 6 (Scheme 4).
-1
lower rate (TOF = 103,000 h , Table 2, entry 4), whereas at
5
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ChemCatChem
10.1002/cctc.202000542
FULL PAPER
mole sieves (3 and 4 Å). Ketone substrates for catalytic reactions were
degassed prior to use. Ruthenium precursors were obtained from Johnson
Matthey Ltd. and all other chemicals were purchased from Sigma Aldrich,
Merck and abcr. Ru(OAc) (PPh ) (1) and Ru(OAc) (dppb) (2) were
2 3 2 2
synthesized following adapted literature procedures.[1d,15] NMR
measurements were recorded on Bruker AV-400 and AC-200 instruments.
Chemical shifts in ppm are reported relative to the stated solvent for 1
H
and 13C spectra and relative to H
PO
4
for 31P spectra. Elemental analyses
3
(C, H, N) were carried out on a Varian SpectrAA-400 instrument and on a
Flash EA1112 elemental analyzer from Carlo Erba. GC analyses were
performed on an Agilent 7890B gas chromatograph equipped with an HP-5
column with 30 m length, column pressure 5 psi, Ar as carrier gas, and a
flame ionization detector (FID). The injector and detector temperatures
were 300 °C, with initial T = 80 °C ramped to 138 °C at 8 °C/min and then
to 300 °C at 30 °C/min.
Syntheses of ruthenium(II) diphosphine complexes
n
Scheme 4. Proposed catalytic cycle for ketone TH with 6. L = dppb, acac.
Synthesis of Ru(OAc)(acac)(PPh
Ru(OAc) (PPh (1) (1.0 equiv., 0.134 mmol) in 3 mL dichloromethane,
1 μL of pentane-2,4-dione (20.0 mg, 1.5 equiv., 0.202 mmol) are added.
3 2
) (3). To a solution of 100 mg of
2
3 2
)
2
Accordingly, dissociation of acetate from 6 and deprotonation of
ampy leads to the five-coordinated species A with a free
coordination site that can be occupied by isopropanol through a
six-membered transition state B. Oxidation of isopropanol and
release of acetone results in the ruthenium hydride species C,
which with the ketone substrate forms transition state D.
Reduction of the substrate then regenerates species A. This
proposed mechanism is in accordance with the high catalytic
activity of ampy complex 6, observed also for other ruthenium
amine complexes that operate through bifunctional catalysis.[17]
The inactivity of catalyst precursors 5 and 7 is attributed to strong
coordination of the bidentate ligands that inhibits the formation of
a free coordination site as in A. Baratta et al. confirmed that an
excess of isopropanol plays a crucial role for the formation of B
and that the alcohol decreases the energy of the transition state,
thus facilitating hydrogen transfer to the substrate.[11a] Current
studies are directed to the isolation of catalytic intermediate
species from 6 in isopropanol.
The mixture is stirred at 30 °C for 3 h and then concentrated to ca. 0.5 mL
and the product precipitated in 3 mL pentane. Filtration and washing with
pentane (3 x 0.5 mL) affords 68% of complex 3 (71.3 mg, 0.091 mmol) as
an orange solid. El. Anal. Calcd. for C43
H
40
O
4
P
2
Ru: C, 65.89; H, 5.14.
, 293 K): δ (ppm) =
.39 - 7.68 (m, 30H, aromatic protons), 5.10 (s, 1H, CH), 1.69 (s, 3H, CH ),
, 293
C), 133.74
135.23 (m, aromatic carbon atoms), 128.7 (d, J(CP) = 2.3 Hz, aromatic
Found: C, 65.90; H, 5.15. 1H-NMR (200 MHz, CDCl
3
6
1
3
1
.59 (s, 3H, CH
3
), 1.47 (s, 3H, CH
3
). 13C{ H}-NMR (50 MHz, CDCl
3
K): δ (ppm) = 187.6 (s, COCH
3
), 187.0 (s, COCH
3
), 185.8 (s, O
2
3
-
3
carbon atoms), 127.3 (d, J(CP) = 9.2 Hz, aromatic carbon atoms),100.0
(s, CH), 27.6 (s, CH
K): δ (ppm) = 64.9 (d, J(PP) = 36.4 Hz), 53.5 (d, J(PP) = 36.4 Hz).
1
3
), 23.9 (s, CH
3
). 31P{ H}-NMR (81 MHz, CDCl
3
, 293
2
2
Synthesis of Ru(OAc)(acac)(dppb) (4). To a solution of 300 mg of
Ru(OAc) (dppb) (2) (1.0 equiv., 0.465 mmol) in 9 mL dichloromethane,
86 μL of pentane-2,4-dione (279 mg, 6.0 equiv., 2.79 mmol) are added.
2
2
The mixture is stirred at 30 °C for 3 h and then concentrated to ca. 0.5 mL
and precipitated in 8 mL pentane. Decantation and washing with pentane
(
5 x 3 mL) affords 63% of complex 4 (199 mg, 0.290 mmol) as a yellow
solid. Crystals are obtained by dilution of the solid in dichloromethane and
storing under n-heptane atmosphere. El. Anal. Calcd. for C35 Ru:
C, 61.31; H, 5.59. Found: C, 61.68; H, 5.67. 1H-NMR (200 MHz, CDCl
38 4 2
H O P
3
,
2
93 K): δ (ppm) = 6.43 - 8.12 (m, 20H, aromatic protons), 5.05 (s, 1H, CH),
Conclusion
1
CH
.83 - 3.03 (m, 8H, CH
2
), 1.68 (s, 3H, CH
, 293 K): δ (ppm) = 187.0 (s, O
), 124.4 - 135.20 (m, aromatic carbon
atoms), 99.8 (s, CH), 27.7 - 28.9 (m, CH ), 27.5 (s, CH ), 24.0 (s, CH ),
). 31P{ H}-NMR (81 MHz, CDCl
, 293 K): δ
J(PP) = 43.2 Hz), 52.6 (d, J(PP) = 43.2 Hz).
3
), 1.52 (s, 3H, CH
3
), 1.45 (s, 3H,
). 13C{ H}-NMR (50 MHz, CDCl
1
3
3
2
C),
The mixed ruthenium(II) acetate/acetylacetonate phosphine
complexes Ru(OAc)(acac)P (P = (PPh ) , dppb) can be easily
2 2 3 2
3 3
186.2 (s, COCH ), 185.6 (s, COCH
2
3
3
synthesized and are examined in catalytic ketone transfer
hydrogenation reactions with iso-propanol. In comparison to the
corresponding diacetate complexes, the performance of the
mixed catalysts is superior in terms of stability and activity, while
the diacetylacetonate Ru(acac)
activity is found for
Ru(OAc)(acac)(ampy)(dppb) (6), reaching TOFs of 125,000 h
for the reduction of a number of aliphatic and aromatic ketones at
loadings as low as 0.01 mol%. Ongoing studies focus on the
development of easily accessible carboxylate ruthenium
complexes in combination with 2-(aminomethyl)pyridine for
catalytic organic transformations.
2
1
23.3 (d, J(CP) = 41.8 Hz, CH
2
3
2
2
(ppm) = 66.8 (d,
Synthesis of Ru(acac)
(2) (1.0 equiv., 0.116 mmol) and 72 µL of pentane-2,4-dione (6.0 equiv.,
0 mg, 0.697 mmol) is stirred in 3 mL dichloromethane at 30 °C for 3 d.
The solution is concentrated to ca. 0.5 mL and the product precipitated in
mL pentane. Decantation with pentane (5 x 0.5 mL) and drying in vacuo
affords 58% of complex 5 (49 mg, 0.067 mmol) as an orange solid. El.
Anal. Calcd. for C38 Ru: C, 62.89; H, 5.83. Found: C, 62.65; H, 5.84.
1H-NMR (200 MHz, CD
Cl 293 K): δ (ppm) = 7.51 - 8.11 (m, 4H,
aromatic protons), 7.24 - 7.48 (m, 6H, aromatic protons), 7.04 - 7.22 (m,
2 2
(dppb) (5). A solution of 75 mg of Ru(OAc) (dppb)
2
(dppb) appears not active. Higher
7
the ampy derivative
-1
2
42 4 2
H O P
2
2
,
10H, aromatic protons), 4.92 (s, 2H, CH), 2.00 - 2.48 (m, 8H, CH
2
), 1.65
, 293 K):
), 139.0 - 141.2 (m,
aromatic carbon atoms), 136.6 - 138.7 (m, aromatic carbon atoms), 134.3
t, 1J(CP) = 4.4 Hz, aromatic carbon atoms), 132.4 (t, 1J(CP) = 4.4 Hz,
). 13C{ H}-NMR (50 MHz, CDCl
1
(s, 6H, CH
3
), 1.46 (s, 6H, CH
3
3
δ (ppm) = 185.5 (s, COCH
), 184.3 (s, COCH
3 3
Experimental Section
(
aromatic carbon atoms), 128.9 (s, aromatic carbon atoms), 127.9 (s,
All reactions were carried out under argon atmosphere using standard
Schlenk techniques. Solvents were used after distillation or taken from a
solvent purification system (SPS) from MBraun, degassed and stored over
aromatic carbon atoms), 127.2 (dt, 1J(CC) = 8.8 Hz, 4.4 Hz, aromatic
2 3
carbon atoms), 98.8 (s, CH), 28.1 - 29.6 (m, CH ), 27.9 (s, CH ),
6
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ChemCatChem
10.1002/cctc.202000542
FULL PAPER
). 31P{ H}-NMR (81 MHz, CDCl
1
reaction mixture under constant argon flow, and diethyl ether was added
to the sample (1/1, v/v). The solution was filtered over a short silica pad
2
6.6 - 27.8 (m, CH
2
), 23.5 (s, CH
3
3
, 293 K):
δ (ppm) = 52.9 (s).
and the conversion determined by GC analysis (Ru 0.1-0.01 mol%, ampy
i
1
-0.02 mol%, NaOPr 2 mol%, acetophenone 0.1 M).
Synthesis of Ru(OAc)(acac)(ampy)(dppb) (6).
Ru(OAc)(acac)(dppb) (4) (1.0 equiv., 0.073 mmol) and 45 µL
-(aminomethyl)pyridine (6.0 equiv., 47 mg, 0.438 mmol) in 2 mL
dichloromethane is stirred at room temperature for 6 h. The solution is then
concentrated to ca. 0.5 mL and orange crystals of 6 * CH Cl are obtained
in 77% yield (49 mg, 0.056 mmol) by slow diffusion of 3 mL pentane to the
solution at 4 °C. El. Anal. Calcd. for C42 Ru: C, 57.41; H, 5.51;
N, 3.19. Found: C, 57.54; H, 5.69; N, 3.46. H-NMR (400 MHz, tol-d
A solution of 50 mg
2
Acknowledgements
2
2
The authors thank Marike Drexler, Wenyi Zeng and Alexandra
Walter for help with catalytic experiments. The TUM Graduate
Schools of Chemistry and MSE are gratefully acknowledged for
financial support for D.A.H., E.-M.H.J.E. and P.J.F.
H
2 2 4 2
48Cl N O P
1
8
,
3
4
2
93 K): δ (ppm) = 8.19 (dt, J = 4.80 Hz, J = 1.46 Hz, 1H, py-H
6
), 8.03 (td,
3
4
J = 8.52 Hz, J = 1.42 Hz, 2H, HPh), 7.83 - 7.69 (m, 2H, HPh), 7.69 - 7.49
(
6
=
3
m, 2H, HPh), 7.34 - 6.87 (m, 15H, py-H
4
, HPh), 6.92 – 6.97 (m, 1H, NH
2
),
3
3
3
4
.80 (d, J = 7.83 Hz, 1H, py-H
3
), 6.50 (ddd, J = 7.62 Hz, J = 4.80 Hz, J
), 4.92 (s, 1H, CH), 4.30 (s, 2H, CH Cl ), 3.98 (t,
), 3.77 (ddd, 2J = 15.23 Hz, 3J = 11.72 Hz, 4J =
), 3.37 (ddd, 2J = 15.23 Hz, 3J = 11.72 Hz, 4J =
), 2.85 - 2.36 (m, 3H, CH CCH ),
dppb), 2.16 (s, 3H, O
), 1.61 - 1.34 (m, 3H,
, 293 K):
C), 161.4 (s,
Conflict of Interest
1.19 Hz, 1H, py-H
J = 11.72 Hz, 1H, NH
5
2
2
2
3
3
1
.76 Hz, 1H, CH
.12 Hz, 1H, CH NH
.90 - 1.70 (m, 2H, CH
2
NH
2
The authors declare no conflicts of interest.
2
2
2
2
3
dppb
2
), 1.64 (s, 3H, COCH
3
Keywords: acetylacetonate • 2-(aminomethyl)pyridine •
1
CH
2
dppb), 1.33 (s, 3H, COCH
3
). 13C{ H}-NMR (101 MHz, tol-d
8
hydrogenation • ruthenium complexes • transfer hydrogenation
δ (ppm) = 185.8 (s, COCH
py-C
1
3
3
), 185.5 (s, COCH
3
), 181.2 (s, O
), 135.2 (d, J(CP) = 8.9 Hz, CPh),
2
3
2
), 149.0 (s, py-C
6
), 135.8 (s, py-C
4
3
3
34.4 (d, J(CP) = 8.9 Hz, CPh), 133.6 (d, J(CP) = 8.9 Hz, CPh), 133.0 (d,
[1]
a) M. Bianchi, P. Frediani, U. Matteoli, G. Menchi, F. Piacenti, G. Petrucci,
J. Organomet. Chem. 1983, 259, 207-214; b) D. A. Hey, P. J. Fischer, W.
Baratta, F. E. Kühn, Dalton Trans. 2019, 48, 4625-4635; c) M. Kitamura,
T. Ohkuma, S. Inoue, N. Sayo, H. Kumobayashi, S. Akutagawa, T. Ohta,
H. Takaya, R. Noyori, J. Am. Chem. Soc. 1988, 110, 629-631; d) R. W.
Mitchell, A. Spencer, G. Wilkinson, Dalton Trans. 1973, 846-854; eM.
Naruto, S. Saito, Nature Comm. 2015, 6, 8140.
J(CP) = 8.9 Hz, CPh), 127.3 (t, 4J(CP) = 7.7 Hz, CPh), 121.3 (s, py-C3,
), 100.3 (s, CH), 53.3 (s, CH Cl ), 47.4 (s, CH NH ), 27.7 - 27.8 (m,
), 26.1 (s, O CCH ), 24.6 (s, CH
dppb), 22.3 (s,
263 K): δ (ppm) = 53.2 (d,
py-C
COCH
CH
2
5
2
2
2
2
3
), 27.3 (s, COCH
3
2
3
2
dppb). P{ H}-NMR (162 MHz, tol-d
31
1
2
8
,
2
J(PP) = 38.2 Hz), 45.9 (d, J(PP) = 38.2 Hz).
[
[
2]
3]
a) M. Kitamura, M. Tsukamoto, Y. Bessho, M. Yoshimura, U. Kobs, M.
Widhalm, R. Noyori, J. Am. Chem. Soc. 2002, 124, 6649-6667; b) L.
Pardatscher, M. J. Bitzer, C. Jandl, J. W. Kück, R. M. Reich, F. E. Kühn,
W. Baratta, Dalton Trans. 2019, 48, 79-89; c) J. Witt, A. Pöthig, F. E.
Kühn, W. Baratta, Organometallics 2013, 32, 4042-4045.
Synthesis of [Ru(acac)(ampy)(dppb)](OAc) (7). A solution of 100 mg
Ru(OAc)(acac)(ampy)(dppb) * CH Cl (6) (1.0 equiv., 0.114 mmol) in
mL toluene is heated to 90 °C for 1 h. After cooling to room temperature,
the solution is concentrated to ca. 0.5 mL and coated with 4 mL pentane.
Cooling to 2 °C for 2 d affords 7 as an orange-brown precipitate in 85%
2
2
4
T. Ohta, H. Takaya, R. Noyori, Inorg. Chem. 1988, 27, 566-569.
a) S. Giboulot, S. Baldino, M. Ballico, R. Figliolia, A. Pöthig, S. Zhang, D.
Zuccaccia, W. Baratta, Organometallics 2019, 38, 1127-1142; b) D. A.
Hey, R. M. Reich, W. Baratta, F. E. Kühn, Coord. Chem. Rev. 2018, 374,
114-132; c) L. Pardatscher, B. J. Hofmann, P. J. Fischer, S. M. Hölzl, R.
M. Reich, F. E. Kühn, W. Baratta, ACS Catal. 2019, 9, 11302-11306.
a) A. K. Gupta, R. K. Poddar, Indian J. Chem. A 2000, 39, 457-460; b)
M. A. Bennett, M. J. Byrnes, G. Chung, A. J. Edwards, A. C. Willis, Inorg.
Chim. Acta 2005, 358, 1692-1708.
yield (77 mg, 0.097 mmol). El. Anal. Calcd. for C41
H, 5.84; N, 3.53. Found: C, 61.91; H, 6.23; N, 3.54. H-NMR (400 MHz,
H
46
N
2 4 2
O P Ru: C, 62.03;
1
[4]
3
tol-d
8
, 293 K): δ (ppm) = 8.63-8.46 (m, 1H, NH), 8.12 (t, J = 8.48 Hz, 2H,
3
H
Ph), 8.08-8.01 (m, 1H, py-H
6
), 7.74 (t, J = 8.27 Hz, 2H, HPh), 7.56 (ddd,
3
J = 9.70 Hz, 3J = 8.19 Hz, 4J = 1.64 Hz, 2H, HPh), 7.40 (t, J = 8.27 Hz,
3
3
3
4
2
=
1
1
H, HPh), 7.30-7.13 (m, 5H, HPh), 6.94 (ddd, J = 8.48 Hz, J = 6.25 Hz, J
1.64 Hz, 4H, HPh), 6.83-6.65 (m, 3H, HPh), 6.46 (td, 3J = 7.77 Hz, 4J =
[5]
[6]
3
3
.60 Hz, 1H, py-H
), 4.96 (s, 1H, CH), 4.15 (tt, 2J = 12.05 Hz, 3J = 5.76 Hz, 1H,
H, py-H
dppb), 3.99-3.84 (m, 1H, CH dppb), 3.84-3.66 (m, 1H, CH
NH ), 3.53 (dd,
NH ), 3.41-3.26 (m, 1H, NH),
4 5
), 6.21 (t, J = 6.59 Hz, 1H, py-H ), 5.91 (d, J = 7.77 Hz,
3
a) M. A. Bennett, G. Chung, D. C. R. Hockless, H. Neumann, A. C. Willis,
Dalton Trans. 1999, 3451-3462; b) R. D. Ernst, E. Melendez, L. Stahl, M.
L. Ziegler, Organometallics 1991, 10, 3635-3642; c) C. Grunwald, M.
Laubender, J. Wolf, H. Werner, Dalton Trans. 1998, 5, 833-839.
T. Manimaran, T. C. Wu, W. D. Klobucar, C. H. Kolich, G. P. Stahly, F.
R. Fronczek, S. E. Watkins, Organometallics 1993, 12, 1467-1470.
a) J. R. Cabrero-Antonino, E. Alberico, K. Junge, H. Junge, M. Beller,
Chem. Sci. 2016, 7, 3432-3442; b) C.-C. Chen, T.-T. Huang, C.-W. Lin,
R. Cao, A. S. C. Chan, W. T. Wong, Inorg. Chim. Acta 1998, 270, 247-
251.
CH
2
2
2
2
2J = 18.18 Hz, 3J = 8.34 Hz, 1H, CH
2
2
dppb), 2.53 (s, 3H, O
2.82-2.53 (m, 2H, CH
2
2
CCH
3
), 1.94 (s, 3H, COCH
3
),
dppb), 1.02 (s, 3H, COCH
). 13C{ H}-NMR (101 MHz,
1
1.60-1.09 (m, 4H, CH
2
3
[
7]
tol-d
8
, 293 K): δ (ppm) = 185.8 (s, COCH
3
), 185.4 (s, COCH
3
), 176.9 (s,
2
2
O C), 165.0 (s, py-C
2
6
), 146.9 (s, py-C ), 135.2 (s, py-C
4
), 135.2 (t, J(CP) =
3
3
[8]
10.7 Hz), 134.8 (d, J(CP) = 9.4 Hz, CPh), 132.2 (d, J(CP) = 8.3 Hz, CPh),
3
131.6 (d, J(CP) = 8.3 Hz, CPh), 129.5 (m, CPh), 121.3 (s, py-C
5
), 120.6 (s,
dppb
py-C
7.4 (s, COCH
162 MHz, tol-d
3
), 100.1 (s, CH), 52.0 (s, CH
2
NH
2
), 28.7 (s, CH
2
), 28.4 (s, COCH
3
),
1
2
3
), 25.7 (s, O CCH
2
3
), 25.5 (s, CH
2
dppb). 31P{ H}-NMR
, 293 K): δ (ppm) = 51.0 (d, 2J(PP) = 36.8 Hz), 45.6 (d,
J(PP) = 36.8 Hz).
[9]
T. Ohta, H. Takaya, M. Kitamura, K. Nagai, R. Noyori, J. Org. Chem.
1987, 52, 3174-3176.
(
2
8
[
[
10] a) E. Peris, Chem. Rev. 2018, 118, 9988-10031; b) B. Zhao, Z. Han, K.
Ding, Angew. Chem. Int. Ed. 2013, 52, 4744-4788.
Procedure for the catalytic transfer hydrogenation of acetophenone
11] a) W. Baratta, S. Baldino, M. J. Calhorda, P. J. Costa, G. Esposito, E.
Herdtweck, S. Magnolia, C. Mealli, A. Messaoudi, S. A. Mason, L. F.
Veiros, Chem. Eur. J. 2014, 20, 13603-13617; b) W. Baratta, E.
Herdtweck, K. Siega, M. Toniutti, P. Rigo, Organometallics 2005, 24,
Ruthenium complexes (1.0-0.1 μmol) were dissolved in 10 mL iPrOH
under argon. 2-(Aminomethyl)pyridine (15.4 µL) was stirred in 15 mL
i
PrOH. The ketone substrate (1 mmol) was placed in a Schlenk flask
1660-1669; c) W. Baratta, P. Rigo, Eur. J. Inorg. Chem. 2008, 2008,
i
closed by a rubber septum under argon and dissolved in PrOH (final
4041-4053.
volume of the solution 10 mL). The solution was then heated to 90 °C oil
bath temperature under argon. After consecutive addition of the respective
amount of ampy solution (where applicable), catalyst solution and 200 μL
NaOPr in PrOH (0.1 M; 0.02 mmol), the reduction of the ketone started
immediately. The reaction was sampled by removing an aliquot of the
[
[
12] R. Noyori, M. Yamakawa, S. Hashiguchi, J. Org. Chem. 2001, 66, 7931-
944.
7
13] a) S. Baldino, S. Facchetti, A. Zanotti-Gerosa, H. G. Nedden, W. Baratta,
Chem. Cat. Chem. 2016, 8, 2279-2288; b) G. Chelucci, S. Baldino, W.
Baratta, Coord. Chem. Rev. 2015, 300, 29-85; c) S. Giboulot, S. Baldino,
i
i
7
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ChemCatChem
10.1002/cctc.202000542
FULL PAPER
M. Ballico, H. G. Nedden, D. Zuccaccia, W. Baratta, Organometallics
018, 37, 2136-2146.
14] a) Á. Vivancos, C. Segarra, M. Albrecht, Chem. Rev. 2018, 118, 9493-
586; b) Z. Wei, H. Jiao, in Advances in Inorganic Chemistry, Vol. 73
Eds.: R. van Eldik, R. Puchta), Academic Press, 2019, pp. 323-384; c)
2
[
9
(
R. Figliolia, P. Cavigli, C. Comuzzi, A. Del Zotto, D. Lovison, P.
Strazzolini, S. Susmel, D. Zuccaccia, M. Ballico, W. Baratta, Dalton
Trans. 2020, 49, 453-465.
[
[
15] R. Figliolia, S. Baldino, H. G. Nedden, A. Zanotti-Gerosa, W. Baratta,
Chem. Eur. J. 2017, 23, 14416-14419.
16] W. Baratta, M. Ballico, A. Del Zotto, E. Herdtweck, S. Magnolia, R.
Peloso, K. Siega, M. Toniutti, E. Zangrando, P. Rigo, Organometallics
2009, 28, 4421-4430.
[
17] B. Zhao, Z. Han, K. Ding, Angew. Chem. Int. Ed. 2013, 52, 4744-4788.
8
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FULL PAPER
Entry for the Table of Contents
Exploiting ligand properties: The ruthenium(II) complex Ru(OAc)(acac)(ampy)(dppb) with acetate, acetylacetonate,
-(aminomethyl)pyridine and diphosphine ligands is highly active in the transfer hydrogenation of ketones (TOFs up to 125,000 h ).
-1
2
Various substrates can be converted to their respective alcohols in quantitative yield and catalyst loadings as low as 0.01 mol%.
9
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