Manganese-catalyzed homogeneous hydrogenation of ketones and conjugate reduction of α,β-unsaturated carboxylic acid derivatives: A chemoselective, robust, and phosphine-free in situ-protocol

Article abstract of DOI:10.1016/j.apcata.2021.118280

We communicate a user-friendly and glove-box-free catalytic protocol for the manganese-catalyzed hydrogenation of ketones and conjugated C[dbnd]C[sbnd]bonds of esters and nitriles. The respective catalyst is readily assembled in situ from the privileged [Mn(CO)₅Br] precursor and cheap 2-picolylamine. The catalytic transformations were performed in the presence of t-BuOK whereby the corresponding hydrogenation products were obtained in good to excellent yields. The described system offers a brisk and atom-efficient access to both secondary alcohols and saturated esters avoiding the use of oxygen-sensitive and expensive phosphine-based ligands.

Full text of DOI:10.1016/j.apcata.2021.118280

Applied Catalysis A, General 623 (2021) 118280
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Manganese-catalyzed homogeneous hydrogenation of ketones and
conjugate reduction of α,β-unsaturated carboxylic acid derivatives: A
chemoselective, robust, and phosphine-free in situ-protocol
Thomas Vielhaber, Christoph Topf*
Johannes Kepler University (JKU), Institute of Catalysis (INCA), Linz, 4040, Austria
A R T I C L E I N F O
A B S T R A C T
Keywords:
We communicate a user-friendly and glove-box-free catalytic protocol for the manganese-catalyzed hydroge-
–
Homogeneous hydrogenation
Conjugate reduction
Manganese
–
C bonds of esters and nitriles. The respective catalyst is readily assembled
nation of ketones and conjugated C
–
in situ from the privileged [Mn(CO)5Br] precursor and cheap 2-picolylamine. The catalytic transformations were
performed in the presence of t-BuOK whereby the corresponding hydrogenation products were obtained in good
to excellent yields. The described system offers a brisk and atom-efficient access to both secondary alcohols and
saturated esters avoiding the use of oxygen-sensitive and expensive phosphine-based ligands.
Picolylamine
Chelates
1. Introduction
amenable to chiral modification(s) thus enabling asymmetric reduction
of selected substrates [47].
The first report of a manganese-based pincer complex that effects the
homogeneous hydrogenation of carbonyl compounds and nitriles by
Beller and coworkers in 2016 [1] has triggered a fast-developing
research process in the field of Mn-mediated redox transformations
[2–6]. Besides (de)hydrogenation catalysts that incorporate the classic
PNP motif in the ligand framework [7–19], active complexes that
contain unsymmetrical PNP’ [20,21] or (chiral) PNN [22–26] scaffolds,
all-nitrogen NNN donor sets [27,28], and mixed donor arrangements
such as SNP [29] and CNP [30] are also well known and found wide-
spread applications in a variety organic synthesis protocols.
Strikingly, the popular [Mn(CO)5Br] precursor compound can also
function as a decent catalyst in its own right and as such it enables the
efficient hydrogenation of quinolines under very mild conditions [48,
49].
Given the vast number of Mn-catalyzed hydrogenation protocols
which rely on H2 gas as the principal reductant [50], it is quite con-
spicuous that reports on the highly relevant and rewarding conjugate
reduction of
α,β-unsaturated carbonyl compounds deploying
manganese-based catalysts and molecular hydrogen [32] are still scarce
in the literature.
However, it was soon demonstrated that the redox-activity of man-
ganese catalysts featuring a planar tridentate ligand architecture is
faithfully reproduced in their molecularly less complicated PN [31–33],
PC [34]-, and PP-ligated [35–38] congeners amongst which the former
are readily developed into chiral entities that permit access to the
enantioselective synthesis of pertinent secondary alcohols through
Mn-catalyzed transfer hydrogenation of ketones [39]. In the same vein,
phosphorus-free oxamide ligands have also been demonstrated to enable
the syntheses of optically pure alcohols [40]. Rewardingly, the reactivity
of these phosphine-based assemblies carries over to manganese coordi-
nation compounds that rely on bidentate NN ligands [41–46] which are
rather cost-effective and not prone to detrimental oxidation as compared
to their PN congeners. These N-ligated manganese species are also
Traditionally, the conjugate reduction of α,β-unsaturated esters and
related compounds is well centered about the use of copper in combi-
nation with phosphine [51] or NHC [52–55] ligands, yet other metals
such as titanium have already been found to facilitate the given trans-
formation [56]. In this context, Stryker’s classic reagent [Cu(PPh3)H]6
[57,58] and ligand-modified versions thereof have to be mentioned, too
[59,60].
* Corresponding author.
E-mail address: christoph.topf@jku.at (C. Topf).
https://doi.org/10.1016/j.apcata.2021.118280
Received 29 March 2021; Received in revised form 24 June 2021; Accepted 29 June 2021
Available online 2 July 2021
0926-860X/© 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

T. Vielhaber and C. Topf
Applied Catalysis A, General 623 (2021) 118280
2. Results and discussion
2.1. Catalytic activity of the pre-synthesized manganese(I) complex
versus the in situ-system
Based on a recent report by Sortais and co-workers [41] who
described the very efficient transfer hydrogenation of various carbonyl
compounds with iso-propanol enabled by a simple picolylamine-tagged
manganese complex [Mn(NN)(CO)3Br], we initially tested the same
catalyst in the related and easy-to-envisage reduction of acetophenone
using H2 gas, t-BuOK as base additive, and THF as the solvent (Table 1).
Although the pre-synthesized catalyst displayed activity in the given
hydrogenation process, the overall transformation was sluggish and the
desired product, 1-phenylethanol, was obtained in only minute quanti-
ties (Table 1, entries 1-2). Notably, the catalyst performance was not
significantly increased upon solvent variation and/or changing the na-
ture of the base (Table S1 in the supporting information). The autoclave
charging procedure was then performed in the glovebox under an
Ar-atmosphere to rule out any possible degradation routes of the single
catalytic components due to the exposure to inevitable O2 and H2O of an
ordinary laboratory atmosphere, but this approach did not improve the
catalytic result either (Table 1, entry 3). Notwithstanding these disap-
pointing results, we conducted the same hydrogenation as an in
situ-experiment by simply combining the components in the reaction
vessel prior to charging the autoclave with the needed H2 pressure and,
to our delight, we observed complete substrate conversion and almost
quantitative yield of the secondary alcohol in this case (Table 1, entry 5).
Noteworthy, applying the picolylamine-ligand in an over-
stoichiometric amount versus the manganese caused a steep decline of
both conversion and yield whereas full collapse of the catalytic perfor-
mance was observed when the given transformation was carried out
without any ligand. Furthermore, it was borne out by experiment that
the addition of a base is indispensable for the Mn-complex to develop
proper hydrogenation activity (Table 1, entries 6-8).
Fig. 1. Catalytic activity of the in situ-system versus the pre-synthesized
manganese(I) complex: Yield/time diagram for the hydrogenation of aceto-
phenone to the corresponding alcohol. Reaction conditions: 0.5 mmol aceto-
phenone, 3 mol% [Mn(NN)(CO)3Br] or [Mn(CO)5Br]/2-picolylamine, 3 mol% t-
BuOK, 2 ml THF, 30 bar H2, 120 ◦C, and 12 h. The yields were determined by
GC analyses using n-dodecane as the internal standard.
case of the pre-synthesized well-defined manganese complex [Mn(NN)
(CO)3Br] the catalytic activity developed very slowly and remained poor
throughout the given reaction interval.
Surprised by the marked difference in catalytic activity of the pre-
synthesized and the in situ-generated picolylamine-manganese com-
plex, we continued with the investigation of our Mn-based in situ system
by standard spectroscopic techniques, viz. NMR and IR spectroscopy.
Initially, we aimed to prove the formation of the NN-tagged manganese
complex [Mn(NN)(CO)3Br] under hydrogenation conditions upon pair-
ing the manganese precursor [Mn(CO)5Br] with 2-picolylamine inside
the autoclave. Indeed, the characteristic signals of the IR and NMR
spectra of the thus-obtained organometallic product match those of the
pre-synthesized manganese(I) complex and are well in accordance with
the literature values (supporting information, Section 8.1) [41]. Sec-
ondly, a control experiment was performed to probe the reactivity of the
precursor [Mn(CO)5Br] with one equivalent of t-BuOK under the
authentic reaction conditions. The IR spectrum of the isolated com-
pound was compared to [Mn(CO)5Br] and clearly, a different carbonyl
species (Fig. 2, red curve) was formed upon base treatment. Finally, the
outcome of the reaction of an equimolar mixture of [Mn(CO)5Br],
In order to substantiate the homogeneous nature of the in situ-
formed manganese chelate in combination with t-BuOK, the reaction
was performed in the presence of a few mercury drops [61,62]. Since
both substrate-conversion and yield were unaffected by the
amalgam-forming agent, we infer that the catalyst acts in a homoge-
neous manner (Table 1, entry 9).
The yield/time trace recorded for the Mn-catalyzed hydrogenation of
acetophenone (Fig. 1) revealed that the active catalyst species is formed
after a short induction phase of approximately 30 min. resulting in
almost quantitative formation of the desired secondary alcohol. In the
Table 1
Comparison of the catalytic hydrogenations of acetophenone to afford 1-phenylethanol facilitated by the pre-synthesized complex [Mn(NN)(CO)3Br] and the same in
situ-generated species in the presence of t-BuOKa.
Entry
Catalyst (mol%)
t-BuOK (mol%)
t (h)
Conversion (%)
Yield (%)
1
[Mn(NN)(CO)3Br] (3)
6
3
3
6
3
3
0
5
3
18
18
18
12
12
12
12
12
12
6
6
2
[Mn(NN)(CO)3Br] (3)
8
8
3b
4
[Mn(NN)(CO)3Br] (3)
10
98
>99
25
0
9
[Mn(CO)5Br] (3) + 2-picolylamine (3)
[Mn(CO)5Br] (3) + 2-picolylamine (3)
[Mn(CO)5Br] (3) + 2-picolylamine (6)
[Mn(CO)5Br] (3) + 2-picolylamine (3)
[Mn(CO)5Br] (3)
98
99
24
0
5c
6c
7
8
0
0
9c,d
[Mn(CO)5Br] (3) + 2-picolylamine (3)
>99
99
a
0.5 mmol of acetophenone were used. Conversions and yields were determined by GC analysis using n-dodecane as internal standard.
The reaction vial was charged in the glovebox under an Ar-atmosphere.
b
c
Applied pressure: 40 bar H2. dMercury drops were added to the reaction solution.
2

T. Vielhaber and C. Topf
Applied Catalysis A, General 623 (2021) 118280
(CO)3Br] is known to form a binuclear coordination compound upon
reaction with t-BuOK [41], we were interested as to whether this dimer
is also generated when the given picolylamine-tagged Mn complex is
generated in situ. For this purpose, we applied a slightly modified
literature procedure using 1,4-dioxane instead of toluene to increase the
solubility of t-BuOK in that solvent (supporting information, Section
8.8). Analysis of the isolated product by IR spectroscopy and X-ray
crystallography confirmed that the dimer fac-[Mn(NN)(CO)3]2 can also
be prepared directly from [Mn(CO)5Br] and 2-picolylamine in the
presence of a strong base (Fig. 4).
Curious about the behavior of the synthesized dimer under the given
hydrogenation conditions without substrate, the binuclear compound
was brought to reaction with H2 gas (0.016 mmol [Mn(NN)(CO)3]2,
1.0 ml THF, 30 bar H2, 120 ◦C, 12 h) and the isolated compound was
then analyzed by IR spectroscopy. Very interestingly, it seems that the
isolated species has the same structure as those substances obtained
from the reaction of [Mn(NN)(CO)3Br] with base or from the combi-
nation of equimolar amounts of [Mn(CO)5Br], 2-picolylamine, and base
since the recorded IR spectra are almost identical in the CO region (see
Fig. S32).
The binuclear manganese complex was then tested for its ability to
catalyze the reduction of acetophenone without the addition of external
base under standard hydrogenation conditions (3 mol% [Mn(NN)
(CO)3]2, 0.5 mmol acetophenone, 2 ml THF, 30 bar H2, 120 ◦C, 12 h). In
this case, 1-phenylethanol was obtained in a rather modest yield of only
55 %.
To conclude, IR study on the reaction products under hydrogenation
conditions indicated that the same carbonyl species were formed
regardless of whether the dimer [Mn(NN)(CO)3]2 or the combinations
[Mn(NN)(CO)3Br]/t-BuOK and [Mn(CO)5Br]/2-picolylamine/t-BuOK,
respectively, were used as the hydrogenation reaction setup. However,
1
the H NMR spectrum of the reaction solution pertinent to the in situ
system turned out to be rather complicated on comparison with the
spectrum obtained from the reaction mixture that corresponds to the
application of the well-defined Mn-complex. Notably, this intricate
pattern is due to the presence of several picolylamine-based species.
Fig. 2. ATR-IR spectra of [Mn(CO)5Br] (orange) and isolated products from
reactions under hydrogenation conditions (30 bar H2, 120 ◦C, 12 h): [Mn
(CO)5Br] + 1 equiv. 2-picolylamine (blue), [Mn(CO)5Br] + 1 equiv. t-BuOK
(red); [Mn(CO)5Br] + 1 equiv. 2-picolylamine + 1 equiv. t-BuOK (green). (For
interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article).
2.2. Optimization of the reaction conditions
2-picolylamine, and t-BuOK in the presence of molecular hydrogen
(30 bar H2, 120 ◦C, 12 h) was investigated by IR spectroscopy (Fig. 2,
green trace). In this case, the formation of another distinct metal
carbonyl compound, that turned out to be exceedingly air-sensitive in
solution, was observed. The corresponding IR spectra of all relevant Mn
species that were obtained in this experiment series are shown in Fig. 2.
To get a glimpse into the understanding of the different catalytic
performance of the in situ system and the pre-synthesized catalyst [Mn
(NN)(CO)3Br], NMR experiments were performed under the typical re-
action conditions without substrate (0.073 mmol [Mn(CO)5Br]/2-pico-
lylamine or [Mn(NN)(CO)3Br], 0.073 mmol t-BuOK, 0.7 ml THF-d8,
30 bar H2, 120 ◦C, and 12 h). In the case of the Mn in situ system, the 1H
spectrum (Fig. 3, blue spectrum) of the reaction solution showed the
presence of several picolylamine species but, unfortunately, it was not
possible to detect any hydride species. Notably, a strong spectroscopic
feature at 2.60 ppm was observed. The existence of various
picolylamine-tagged species was further confirmed by 13C NMR spec-
troscopy (supporting information, Section 8.3) while also a CO signal
was detected at a chemical shift of 239 ppm.
To improve upon the results obtained in THF, we first embarked on a
solvent variation study to find proper alternatives with respect to the
reaction medium. Indeed, when the title transformation was conducted
in 1,4-dioxane we obtained results which are similar to those observed
in THF (see Table S5 in the supporting information for details). How-
ever, as the dioxane did not significantly outperform its smaller ring
congener, we adhered to the use of THF owing to its lower price and
advantageous boiling point that allows for convenient removal from the
reaction mixture.
We then substituted the t-BuOK additive by other strong bases, viz.
MeONa, t-BuONa, and Cs2CO3, but since this approach resulted in a
steep decline of the catalyst activity (Table S4 in the supporting infor-
mation), we decided to use the potassium alcoholate throughout.
Noteworthy, the in situ-system tolerates a slight excess of t-BuOK
without loss of activity but upon adding more than two equivalents
versus the Mn-catalyst a pronounced decrease of the catalytic perfor-
mance was observed (Table 2, entries 1-2). In fact, this is a rather un-
usual behavior considering that related catalytically active metal centers
held by deprotonable NH motifs perform significantly better on
increasing the base-loading [31,63]. On the other hand, applying less
than one equivalent of t-BuOK resulted in a decrease of the both
substrate-conversion and yield (Table 2, entry 3). Consequently, the
activity of the [Mn(CO)5Br]-picolylamine assembly depends on a deli-
cate balance of base, metal, and the ligand.
By contrast, NMR studies of the pre-synthesized complex disclosed
the presence of one dominant picolylamine species (Fig. 3, red trace)
and again, 13C NMR spectroscopy confirmed these findings (supporting
information, Section 8.3). Moreover, a CO signal located at identical
shift as compared to the in situ system was observed. In addition, the IR
spectrum of the isolated product displayed the same absorption pattern
in the CO region (see Fig. S29).
Having optimized the metal-to-base-ratio as well as the physical
parameters, viz. temperature and H2-pressure (Table 2, entries 4-7), we
finally sought to reduce the catalyst amount. Regrettably, a prompt and
Since the well-defined NN-ligated manganese(I) complex [Mn(NN)
3

T. Vielhaber and C. Topf
Applied Catalysis A, General 623 (2021) 118280
Fig. 3. 1H NMR spectra of the reaction mixtures in the presence of H2 (30 bar, 12 h, 120 ◦C) in THF-d8: [Mn(CO)5Br] + 2-picolylamine + t-BuOK (blue); [Mn(NN)
(CO)3Br] + t-BuOK (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 4. Formation of the binuclear Mn-carbonyl compound from the pre-synthesized [Mn(NN)(CO)3Br] upon reaction with t-BuOK in THF [41]. The reaction also
occurs when the respective NN-ligated manganese(I) complex is prepared in situ.
marked decrease of the catalytic performance resulted if the [Mn
(CO)5Br] precursor was used in quantities less than 3 mol% (entries 8-
11).
desired secondary alcohols giving rise to excellent yields in each case.
Noteworthy, the position of the appendant CH3-group did not have a
measurable impact on the catalyst performance.
With respect to the halide-functionalized ketones a11-a21 the cat-
alytic system sharply discriminates between two sets. The hydrogena-
tion of the fluoro- and chloro-substituted acetophenones a11-a17 as
well as the disubstituted substrate a14 featuring both an electron-
withdrawing- and electron-donating group produced the desired tag-
ged alcohols in quantitative yield, albeit with a slight modification of the
standard reduction protocol for ortho-chloroacetophenone a15. On the
other hand, all tested brominated acetophenone derivatives a18-a20
were not amenable to reduction under the optimized reaction condi-
tions. Consequently, both H2-pressure and temperature were increased
(50 bar, 120 ◦C) whereby prolonging the reaction time (20 h) but the
conversion still remained inferior, especially for the ortho- and meta-
substituted bromoacetophenones b18 and b19. The same negative
result was obtained for the related iodo-compound a21 which proved to
be almost unreactive in the given hydrogenation process. The reactivity
of halo-acetophenones thus seems to be strongly dominated by the
2.3. Catalytic hydrogenation of ketones
After the systematic variation of the reaction conditions, we went
straight ahead to elaborate the scope and limitations of the given Mn-
based hydrogenation protocol (Table 3). First, we probed the aptitude
of the given catalytic system to cope with steric hindrance imposed by
the substrate substitution pattern and for that purpose we introduced
α
-branching at the CH3-group of the parent acetophenone a1; in a
related experiment the pertinent methyl group was substituted for the
phenyl motif. To our delight, substrates a2-a5 were fully converted into
their corresponding alcohols, indicating that the presence of additional
or longer alkyl chains as well as arene moieties do not hamper the
catalyst.
The bulky kindred naphthalene derivatives a6 and a7 as well as the
methyl acetophenones a8-a10 were also neatly transformed into the
4

T. Vielhaber and C. Topf
Applied Catalysis A, General 623 (2021) 118280
combined Mn-mediated dehydrogenation-hydrogenation sequence is
also conceivable [64]. Indeed, when a pristine portion of commercial
d36 was subjected to the reaction conditions from Fig. 5, but in the
absence of H2 gas, the given allyl alcohol was completely converted to
the ketone c36 [65]Fig. 5.
Table 2
Optimization of the reaction conditions for the reduction of acetophenone
catalyzed by the in situ formed manganese catalysta.
2.4. Hydrogenation of cinnamic acid ester derivatives
Entry
[Mn(CO)5Br]
(mol%)
t-BuOK
t
p
Conversion
(%)
Yield
(%)
The pleasing results obtained in the hydrogenation of trans-chalcone
a36 encouraged us to test the in situ-generated [Mn(NN)(CO)3Br] for its
ability to selectively reduce cinnamate derivatives (Table 4). Applying
the optimized reaction conditions established for the acetophenone
hydrogenation resulted in virtually full conversion of the methyl cin-
namate c1 (95 %, Table S6 in the supporting information). On
exchanging THF for 1,4-dioxane as solvent we actually achieved com-
plete substrate conversion while the corresponding saturated ester
product was isolated in very good yield (86 %). The alkyl cinnamates c2-
c4 gave rise to the same excellent yields, in fact irrespective of the de-
gree of branching in the esterifying alcohol. Rather bulky substrates
such as c5 and c6 did not hamper the course of the catalytic trans-
formation either, allowing the synthesis of d5 and d6 in almost quan-
titative yields without debenzylation taking place in the ester part. Quite
remarkably, the in situ-prepared Mn-catalyst smoothly converted allyl
cinnamate c7 into the targeted hydrogenation product without
(mol%)
(h)
(bar)
1
3
9
12
12
12
12
12
5
50
50
40
40
30
20
10
40
40
50
50
67
>99
89
98
>99
42
26
18
79
27
4
67
99
89
97
99
42
25
17
79
27
4
2
3
4
3
3
2
4b
5
3
3
3
3
6
3
3
7
3
3
16
12
12
18
18
8
2.5
2.5
2
5
9
2.5
2
10
11
1
2
a
0.5 mmol of acetophenone were used. Conversions and yields were deter-
mined by GC analysis using n-dodecane as internal standard.
b
The reaction temperature was 100 ◦C. M:L denotes the molar ratio of the
manganese versus the picolylamine-ligand.
–
–
C bond of the allyl motif. However, a higher
compromising the C
–
electronegativity of the attached halide species which is, however,
intrinsically related to the size of the respective atom.
catalyst loading (5 mol% versus 3 mol%) and slightly reinforced reaction
conditions had to be applied in order to achieve this important result
(50 bar H2 and 20 h reaction time versus the standard values of 30 bar H2
and 12 h).
Our Mn-based catalytic system also mediated the reduction of
pharmaceutically relevant CF3-tagged acetophenones, viz. a22 and its
disubstituted congener a24, to generate the corresponding
trifluoromethyl-alcohols in excellent yields. Interestingly, we observed a
subtle but decisive effect of the alkyl chain length that is in direct
proximity of the ketone group. If we subjected the immediate homolo-
gous propiophenones a23 and a25 to the same hydrogenation process,
the catalytic transformation was sluggish and the alcohols b23 and b25
were produced in very low yields (17 % and 23 %, respectively).
Along with aromatic ketones, (benzannulated) cyclic substrates that
incorporate the carbonyl group directly in the ring (b26-b30) were also
completely converted to afford the corresponding secondary alcohol in
very good yields. Notably, the ring size affected the catalyst activity in
that six-membered rings required higher H2 pressure and prolonged
reaction time (b26 and b27 versus b28).
Placing electron-releasing groups on the phenyl ring of the cinnamic
acid ester did not markedly affect the catalyst performance. Yet, in case
of the methyl cinnamates c8 and c9 the conditions had to be modified
(50 bar H2, 100 ◦C and 20 h) so as to maintain full substrate conversion
and decent yield.
Regarding the halide-functionalized esters c11-c14 we did not
observe any unwanted hydro-dehalogenation processes for the fluoro-
and chloro-compounds whereby the corresponding products were iso-
lated in good yields ranging from 80 % to 89 %. Most notably, the
respective bromo-derivatives were also amenable to the given hydro-
genation which is in stark contrast to the results obtained for the
brominated acetophenones a18-a20 (vide supra). However, the reaction
conditions had to be adjusted (6 mol% catalyst loading, 50 bar, 24 h,
100 ◦C) which again demonstrated a somewhat detrimental decreased-
electronegativity-effect of the appendant bromide substituent. With
respect to d13 and d14 we detected 3-phenyl methyl cinnamate in the
GC–MS spectra which is indicative of concomitant hydro-debromination
that occurred during the course of the Mn-catalyzed hydrogenation.
The introduction of a cyano group on the arene ring was not as well
accommodated as was the case with the related CN-labelled acetophe-
none a33 (vide infra) and hence cinnamate d15 was only isolated in a
mediocre yield of 75 %. Diesters c16 and c17 displayed considerable
reactivity towards the conjugate hydrogenation of their C = C-bonds
though the yield of the former was surprisingly low considering its
rather simple molecular architecture.
To our regret, methyl ketones with an attached pyridine core were
either not at all (a31) or poorly (a32, a33) converted into the desired
products b31-b33 presumably due to inhibition of the active Mn-center
through the strongly ligating sp2 nitrogen atom of the pertinent
heterocycle.
Finally, the chemoselectivity of the manganese in situ-system was
examined by testing substrates that incorporate one additional reducible
motif, viz. CN-functionalized acetophenone a34, isophorone a35, and
the aldehyde-tagged ketone derivative a37 (see Section 4 in the sup-
porting information). With the exception of the latter, the ketone group
was selectively reduced to produce the target alcohols b34 and b35 in
very good yields. On the contrary, the aldehyde-bearing acetophenone
a37 was exhaustively hydrogenated to the bis-alcohol b37. Quite
remarkably, on applying benzaldehyde derivatives devoid of an acetyl
group on the arene moiety as substrates, we observed either only traces
of the respective primary alcohol or no product at all.
The most surprising feature of the in situ-prepared [Mn(NN)(CO)3Br]
complex is its ability to even reduce tetra-substituted C = C-bonds at
100 ◦C in crowded esters such as c18 and c19. Upon applying a H2-
pressure of 50 bar and a reaction time of 20 h the isolated yields of both
desired products were well above 70 %. Accordingly, the conjugated
In stark contrast to the selectivity observed in the case of the
reduction of isophorone 34a, the hydrogenation of trans-chalcone a36
produced a mixture of saturated alcohol b36 and ketone c36 (Fig. 5).
Since no evidence for the formation of allyl alcohol d36 was found, it is
tempting to assume that the given Mn-catalyzed transformation is
considerably biased towards the reduction of the C = C-bond. However,
the intermediate formation of d36 through hydrogenation of the ketone
group in enone a36 and its direct isomerization to the ketone c36 via a
C = C-bond of the less sterically congested α-phenyl cinnamyl nitrile c20
was fully hydrogenated upon applying lower H2-pressure (30 bar) and a
shorter reaction time (12 h).
To further explore the limitations of the Mn-based in situ-system
presented herein, we expanded the substrate panoply by ethyl-
β-methyl cinnamate c21 (Table 5). Regrettably, we soon realized that
this compound cannot compete with those listed in Table 4 with respect
5

T. Vielhaber and C. Topf
Applied Catalysis A, General 623 (2021) 118280
Table 3
Scope and limitation of the ketone-hydrogenation catalyzed by in situ-prepared [Mn(NN)(CO)3Br]a.
a
A 0.5 mmol amount of the ketone was used. Reaction conditions: A: 30 bar H2, 120 ◦C, 12 h; B: 50 bar H2, 100 ◦C, 12 h; C: 50 bar H2, 120 ◦C, 20 h; D: 50 bar H2,
120 ◦C, 24 h. Conversions (upper value) and yields (lower value) were determined by GC analysis using n-dodecane as internal standard. In case of b8-b10 n-hex-
adecane was used as the standard. Isolated yields are given in parentheses. bIsolated yield.
6

T. Vielhaber and C. Topf
Applied Catalysis A, General 623 (2021) 118280
Fig. 5. Hydrogenation of trans-chalcone (0.5 mmol) using H2 gas effected by the in situ-formed manganese catalyst. A mixture that was solely composed of saturated
alcohol b36 and ketone c36 was obtained. The substituted allyl alcohol d36 was not detected, presumably due to its quantitative Mn-catalyzed isomerization to yield
ketone c36 [65]. Conversion and yields were determined by GC analysis using n-dodecane as internal standard.
to catalytic activity. On applying the optimized reaction conditions
found for the methyl cinnamate reduction (vide supra), the substrate-
conversion was as low as 34 %. Even on doubling the H2-pressure and
using significantly higher catalyst loadings we only achieved 87 %
conversion after a reaction time of 48 h. Thus, we did not pursue the
reduction of ethyl-β-methyl cinnamate any further in this work. The fact
that the congested olefins c18 and c19 do exhibit considerable activity is
explained by the presents of the appendant CN-groups that exert a strong
electron-withdrawing effect that eventually activates the C = C-motif
towards nucleophilic attack of a hydride.
picolylamine, a 10 μl Hamilton Gastight® syringe (Model 1701 RN) was
used. Hydrogenation reactions were performed in a 300 ml autoclave
from Parr Instruments GmbH and the employed hydrogen was pur-
chased from Linde Gas GmbH (5.0 purity). Dry solvents were either
purchased from Acros Organics or drawn off from a MB-SPS-7 solvent
system (M. Braun GmbH). GC–MS analyses were carried out on a Shi-
madzu GC–MS QP-2020 with helium carrier gas from Linde Gas GmbH
(5.0 purity). Infrared spectroscopy was performed in the solid state on a
Bruker Alpha II and high-resolution mass spectra were recorded on a
Thermo Fisher Scientific LTQ Orbitrap XL. NMR measurements were
carried out on a Bruker Avance 300 MHz and 500 MHz spectrometer.
Spectra for different cores were recorded as follows: 300 MHz for 1H
NMR, 75.5 MHz for 13C NMR, and 470.5 MHz for 19F NMR. Chemical
shifts are listed in parts per million (ppm) on the delta scale (δ). Axis
calibration of the 1H and 13C NMR spectra is based on the non-
deuterated solvent as reference [66]. The active [Mn(NN)(CO)3Br]
complex and the manganese dimer [Mn(NN)(CO)3]2 were prepared in
accordance with the literature procedure [41]. Vacuum distillation for
product isolation in the scale-up experiment was performed with the
Glass-Oven B-585 from Büchi.
In order to verify whether the method reported herein is applicable
to synthesize larger quantities of hydrogenation products a scale-up
experiment (5 mmol) was performed using methyl cinnamate as sub-
strate. Indeed, the respective ester product was obtained in good yield
(79 %) and was easily separated from the reaction mixture by vacuum
distillation (see Section 5 in the supporting information). Hence, the
given catalytic protocol allows a scale-up by at least a factor of ten, thus
rendering this operationally simple system a sound method for the
synthesis of various saturated esters.
3. Conclusion
4.2. Safety statement concerning high-pressure hydrogenation
We presented a convenient, robust, and phosphine-free Mn-based in
situ-protocol for the chemoselective hydrogenation of selected ketones
and cinnamate derivatives using molecular H2. The key constituents, viz.
the organometallic Mn-precursor and the picolylamine ligand, are
readily available through commercial channels whereby the catalytic
assembly is readily prepared on simply combining the components in the
reaction vessel under an ordinary laboratory atmosphere. Notably, the
in situ-system heavily outperforms the analogous method which relies
on the pertinent pre-synthesized Mn-catalyst.
The H2 pressure tank (200 bar, 50 l) was placed in a safety storage
cabinet with an installed tapping unit whereby the gas container was
connected to a control panel that allowed for fine adjustment of the H2
pressure used for the hydrogenation reactions. The autoclave charging
procedure was performed in a fume hood that was equipped with a
sensor which was wired to a magnetic valve. The latter instantaneously
stops the gas supply in case of any H2 leakage that might occur during
the filling procedure. Furthermore, both optical and acoustic alarm
signals are triggered whenever free flammable gas is detected inside the
hood.
The catalytic hydrogenation of ketones was well-compatible with the
presence of phenyl F- and Cl-substituents in that detrimental hydro-
dehalogenation was not observed in both cases. However, the reduc-
tion of the carbonyl group failed to occur in related compounds con-
taining Br- and I-motifs while potentially reducible CN-groups are also
tolerated by the given system.
4.3. Representative procedure for the catalytic hydrogenation reactions
A 4 ml glass vial was sequentially charged with solid [Mn(CO)5Br]
(0.015ꢀ 0.030 mmol), the substrate (0.5 mmol), 2-picolylamine
(0.015ꢀ 0.030 mmol), and a magnetic stirring bar. The reaction com-
ponents were then dissolved in THF (2 ml) or 1,4-dioxane (2 ml)
whereupon the resulting yellow solution was then gently stirred
(200 rpm) for a period of 5 min. Whilst stirring, the glass vial was sealed
with the septum cap. Hereafter, solid t-BuOK (0.015ꢀ 0.030 mmol) was
added to the reaction mixture upon which the reaction vessel was again
sealed with a septum cap which was then penetrated with a needle.
Notably, the base addition was carried out without stirring. After that,
the glass vial was placed in a drilled aluminum liner which was promptly
transferred into the 300 ml autoclave. Once tightly sealed, the latter was
purged five times with H2 (20 bar per cycle) before being pressurized to
the desired value. The autoclave was then placed on a pre-heated stir-
ring plate and heated up to the required reaction temperature. On
completion of the hydrogenation reaction, the autoclave was allowed to
As to the cinnamate hydrogenation, a vast array of saturated esters
was accessible including those which are derived from challenging tetra-
substituted starting materials containing a CN-group in the α-position.
Given the initial results on the reduction of ethyl-β-methyl cinnamate,
the introduced Mn-in situ-system is likely to be developed into a more
active (chiral) version that enables effective (enantioselective) syntheses
of branched saturated esters.
4. Experimental section
4.1. General information
All chemicals were purchased from Acros Organics, Alfa Aesar,
Merck (including Sigma Aldrich), Roth, TCI, VWR, or Fluorochem, and
were used as received without further purification. For the addition of 2-
7

T. Vielhaber and C. Topf
Applied Catalysis A, General 623 (2021) 118280
Table 4
Scope and limitation of the cinnamate hydrogenation mediated by the in situ-formed [Mn(NN)(CO3)Br]a.
a
0.5 mmol of the substrate were used. Reaction conditions: A: 30 bar H2, 12 h, 120 ◦C; B: 50 bar H2, 20 h, 100 ◦C; C: 50 bar H2, 24 h, 100 ◦C; D: 50 bar H2, 24 h,
120 ◦C. Conversions were determined by GC analysis using n-hexadecane as internal standard. Isolated yields are given in parentheses. The amount x of [Mn(CO)5Br],
2-picolylamine, and t-BuOK is given in mol% directly under the short description of the corresponding product.
8

T. Vielhaber and C. Topf
Applied Catalysis A, General 623 (2021) 118280
Table 5
Catalytic performance of the Mn-based in situ-system for ethyl-β-methyl cinnamatea.
Entry
[Mn(CO)5Br] (mol%)
T / ◦
C
p (bar)
t (h)
Conversion (%)
1
2
3
4
3
5
5
7
120
100
120
120
30
50
40
60
12
20
48
48
34
41
71
87
a
0.5 mmol of the ester were used. Conversions were determined by GC analysis using n-hexadecane as internal standard. The hydrogenation product was verified by
GC analysis. M:L and M:B denotes the molar ratio of the manganese versus the picolylamine-ligand and t-BuOK, respectively.
reach room temperature. Afterwards, the remaining gas was slowly
released upon which the reaction mixture was degassed through briefly
stirring on air. Finally, n-dodecane (12 mg) or n-hexadecane (20 mg)
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2021.118280.
were added and an aliquot of 30 μl was taken from the solution, mixed
with acetone (1 ml) whereupon the resulting solution was analyzed by
GC.
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