Mechanochemical Palladium-Catalyzed Carbonylative Reactions Using Mo(CO)6

Article abstract of DOI:10.1002/chem.201904528

Esters and amides were mechanochemically prepared by palladium-catalyzed carbonylative reactions of aryl iodides by using molybdenum hexacarbonyl as a convenient solid carbonyl source and avoiding a direct handling of gaseous carbon monoxide. Real-time monitoring of the mechanochemical reaction by in situ pressure sensing revealed that CO is rapidly transferred from Mo(CO)₆ to the active catalytic system without significant release of molecular carbon monoxide.

Full text of DOI:10.1002/chem.201904528

A Journal of
Accepted Article
Title: Mechanochemical Palladium-Catalyzed Carbonylative Reactions
Using Mo(CO)6
Authors: Pit van Bonn, Carsten Bolm, and José G. Hernández
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To be cited as: Chem. Eur. J. 10.1002/chem.201904528
Link to VoR: http://dx.doi.org/10.1002/chem.201904528
Supported by

10.1002/chem.201904528
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COMMUNICATION
Mechanochemical Palladium-Catalyzed Carbonylative Reactions
Using Mo(CO)6
Pit van Bonn, Carsten Bolm* and José G. Hernández*
a) Hydroformylation of olefins using syngas in a ball mill.
Abstract: Esters and amides were mechanochemically prepared by
CO/H2 (15 bar)
Rh catalyst, PPh3
saccharide additive
O
H
O
palladium-catalyzed carbonylative reactions of aryl iodides using
molybdenum hexacarbonyl as a convenient solid carbonyl source
avoiding a direct handling of gaseous carbon monoxide. Real-time
monitoring of the mechanochemical reaction by in situ pressure
sensing revealed that CO is rapidly transferred from Mo(CO)6 to the
active catalytic system without significant release of molecular carbon
monoxide.
+
H
ball milling
850 rpm, 4 h
branched (b)
linear (l)
conversion: 38 - 100%
b/l selectivity: 2 - 10
b) This work: Carbonylation of aryl iodides using M(CO)6 by ball milling.
O
Pd catalyst
ligand, base
I
Nu
R1
+
M(CO)6
+
Nu-H
R1
ball milling
2
3
Ball milling techniques are largely used in academia and in
industry for advanced comminution and mixing of solids.[1] Due to
the operational simplicity and especially to the effectiveness of
ball milling to transduce mechanical loads to the sample being
milled, a significant growth in the use of ball mills to mechanically
induce chemical reactions has recently been experienced.[2]
Intuitively, mechanochemical activation by ball milling appears
more appropriate for solid samples. However, mechanical
treatment of soft organic materials, liquids and even gaseous
reactants has proven equally effective.[3]
In the case of mechanochemical reactions involving gaseous
components, gases such as H2,[4] O2,[5] CO,[6] CO2,[7] HCN,[8]
C3H6,[9] CH4,[10] among others,[11] have been reported to undergo
clean chemical transformations inside ball mills in various areas
of synthetic chemistry. For example, in 2017 Hapiot and
1
Nu-H = R2OH, R2R3NH
M = Cr, Mo, W
Scheme 1. Mechanochemical carbonylation reactions by ball milling.
These precedents made us wondering if solid metal carbonyl
complexes such as M(CO)6 (M = Cr, Mo, W) could be activated
by ball milling allowing mechanochemical carbonylations.
Specifically, the use of metal carbonyls as one-carbon building
blocks in palladium-catalyzed carbonylation reactions such as
alkoxycarbonylations and aminocarbonylations was envisaged
(Scheme 1b).[15] Herein we report the findings on our investigation,
which include the development of a solvent-free protocol for the
palladium-catalyzed carbonylation by ball milling, the in situ
pressure monitoring of the reaction, and the application of the
mechanochemical approach to the synthesis of series of esters
and amides.
coworkers reported
a rhodium-catalyzed mechanochemical
Initially, the carbonylation reaction between iodobenzene (1a), n-
butanol (2a), M(CO)6 (M = Cr, Mo, W), K3PO4, and a catalytic
system composed by Pd(OAc)2/triphenylphosphine in a mixer mill
was tested.[16] After 90 min of milling the crude mixtures of these
experiments were directly analyzed by 1H NMR spectroscopy. In
all experiments, formation of butyl benzoate (3aa) was confirmed,
and out of the three metal carbonyl complexes tested
molybdenum hexacarbonyl proved to be the most active one for
the carbonylation reaction (Table 1, entry 2). In the reactions with
Cr(CO)6 and W(CO)6 a competitive oxidative homo-coupling
reaction of 2a led to the formation of butyl butyrate (3aa’).
hydroformylation of styrene derivatives in a planetary ball mill
using a mixture of CO/H2 (Scheme 1a).[12] While the reactions led
to high conversions of substrates such as 2-vinylnaphthalene
without the need for external heating, they required a 15 bar
pressure of carbon monoxide and hydrogen inside a pressurized
milling vessel.[12] Noteworthy, however, such high pressure is not
an intrinsic requirement of the mechanochemical approach. In
general, most carbonylation reactions rely on the use of special
high-pressure laboratory equipment and involve the handling of
odorless, toxic, and flammable CO gas. To mitigate this problem,
carbonylation procedures were developed that improved both
reaction performance and overall safety. Recent examples
include the introduction of flow chemistry protocols to react
gaseous CO,[13a] and the use of reagents with the ability to release
or to form carbon monoxide in situ during the reaction.[13b-c]
Among the latter, metal carbonyls such as M(CO)6 (M = Cr, Mo,
W) and Co2(CO)8 have received large attention as convenient
carbonyl sources avoiding the need for directly handling gaseous
carbon monoxide.[13b, 14] Although, the toxicity of metal carbonyls
always depends on their stability, volatility and on the harmfulness
of the given metal.
Table 1. Screening of metal carbonyls M(CO)6 (M = Cr, Mo, W) in the
mechanochemical carbonylation of 1a with n-butanol 2a.[a]
M(CO)6 (1 equiv)
O
Pd(OAc)2 (10 mol%)
PPh3 (20 mol%)
K3PO4 (3 equiv)
I
O
nBu
+
+
nBuOH
O
nBu
O
30 Hz, 90 min
1a
2a
3aa
3aa’
Entry
M(CO)6
Yield 3aa [%][b]
Yield 3aa’ [%][b]
1
2
3
Cr(CO)6
Mo(CO)6
W(CO)6
13
99
22
26
<1
26
[a]
P. van Bonn, Prof. Dr. C. Bolm, Dr. J. G. Hernández
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
E-mail: carsten.bolm@oc.rwth-aachen.de
E-mail: jose.hernandez@oc.rwth-aachen.de
Supporting information for this article is given via a link at the end of
the document.
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), M(CO)6 (0.2 mmol),
Pd(OAc)2 (0.02 mmol), PPh3 (0.04 mmol), and K3PO4 (0.6 mmol) were
milled in a 5 mL SS milling jar with one 10 mm milling ball of the same
material. [b] Determined by 1H NMR spectroscopy using ethylbenzene as
internal standard.
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Having observed chemical reactivity towards the carbonylative
pathway for all the group VI hexacarbonyl complexes, we
wondered if the mechanical treatment of these complexes had
promoted the release of molecular carbon monoxide inside the
ball mill. To test this hypothesis, M(CO)6 (M = Cr, Mo, W) were
milled under air using a planetary ball mill suitable for sensing
changes in pressure and temperature during milling (for details
see ESI). The experiments showed that other than the calculated
rise in pressure due to the expansion of the air inside the milling
jar by frictional heating, the pressure inside the milling container
stayed essentially constant (Inset, Figure 1a). On the other hand,
repeating the milling experiments with M(CO)6 in the presence of
K3PO4 (1 equiv) produced almost immediately gaseous carbon
monoxide as indicated by the corresponding pressure increase
(Figure 1a).[17] This indicated that the K3PO4 used in the
carbonylation reactions could have two roles: First, as base for
the deprotonation of 2a, and second, as a promoter for the release
of carbon monoxide from the metal hexacarbonyl sources. The
mechanism by which K3PO4 facilitates the release of CO is not
completely elucidated, but 31P NMR analysis of the milled
samples revealed the presence of new peaks in the 31P NMR
spectrum (Figure S5), suggesting that K3PO4 might have
participated in a ligand displacement with M(CO)6. After 4 h of
milling, Cr(CO)6 had produce the highest CO pressure inside the
milling vessel. However, as noted before, the carbonylation of 1a
with Cr(CO)6 and 2a had been sluggish (Table 1), and this result
led us doubt that the carbonylation really involved gaseous carbon
monoxide formed from the reaction of M(CO)6 and K3PO4. Hence,
subsequent experiments were performed to evaluate if an
alternative mechanism was operational by ball milling.
of 3 equiv of K3PO4 promoted a fast buildup of pressure upon
milling, which after 110 min of milling gradually dropped (Figure
1b). Such a drop in pressure was detected to occur earlier and to
reduce the CO pressure more rapidly when 4 equiv of K3PO4 were
used (see Figure S4 in ESI).
A
similar feedback
mechanochemical reaction was recently reported in experiments
between CO2 and zeolitic imidazolate frameworks.[19] However,
repeating our experiments under argon was observed to offer a
protective effect leading to higher pressure values while avoiding
the drop in pressure upon milling (see Figure S6 in ESI).
Even more surprisingly was the pressure monitoring profile of the
palladium-catalyzed carbonylation between 1a, 2a, Mo(CO)6 and
K3PO4 (2 equiv), which did not reveal any significant buildup of
pressure upon milling (Figure 1b). This result hinted at the
possibility for the carbonylation reaction to involve a fast CO
transfer from Mo(CO)6 to the active palladium species during the
catalytic cycle, rather than the release of molecular CO followed
by its consumption during the reaction. This hypothesis was
further tested after carrying out the reaction using 3 equiv of
K3PO4. This time, an increase in pressure was detected after the
carbonylation had finished (after 90 min of milling), which can be
interpreted as resulting from a reaction between the remaining
Mo(CO)6 and K3PO4 (see Figure S7 in ESI). Similar gas-free
carbonylative reactions using metal carbonyls complexes have
precedence.
For
instance,
recent
palladium-catalyzed
carbonylative Suzuki reactions using Mo(CO)6,[20a] and
carbonylation reactions using Co2(CO)8 under palladium
catalysis,[20b] were proposed to involve cooperative rapid CO
insertion into the catalytic cycle directly from the metal carbonyl
source. Despite this likely scenario it was important to evaluate if
the mechanochemical carbonylation could also take place using
gaseous carbon monoxide. Thus, the palladium catalysis was
repeated by ball milling 1a, 2a and K3PO4 (2 equiv) in the absence
of Mo(CO)6 but under a CO atmosphere (1 atm), and experiments
in both a mixer mill and a planetary ball mill confirmed the
formation of butyl benzoate (3aa) (Scheme 2).
First, the most active M(CO)6 (M = Mo) in the carbonylation
reaction was milled for 4 h with various amounts of K3PO4 (1-3
equiv), and the reaction was followed by sensing changes in
pressure (Figure 1b). Clearly, the extent of the gas evolution
depended on the amount of K3PO4 used. For example, 1.05 equiv
of CO were released after milling of a mixture of Mo(CO)6 and
K3PO4 (1 equiv) for 4 h. Whereas the same experiment with 2
equiv of K3PO4 afforded 1.66 equiv of CO.[18] Moreover, the use
Figure 1. a) Pressure monitoring of milling experiments of M(CO)6 and K3PO4 (1 equiv). The inset shows the control experiments in the absence of K3PO4. For
pressure and temperature monitoring profiles, see Figure S2. b) Pressure monitoring of milling experiments between Mo(CO)6 and K3PO4 (1-3 equiv) and of the
mechanochemical palladium-catalyzed carbonylation reaction between 1a, 2a (2 equiv), Mo(CO)6 (1 equiv), and K3PO4 (2 equiv). Milling parameters: planetary ball
mill operated at 800 rpm using a ZrO2 milling vessel charged with 5 milling balls (10 mm in diameter), for additional experimental details, see ESI.
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CO (1 atm)
Mo(CO)6 (1 equiv)
Mixer mill
1.5 h, 30 Hz, 3aa:1a (90:10)
O
O
Pd(OAc)2 (10 mol%)
PPh3 (20 mol%)
K3PO4 (2 equiv)
Pd(OAc)2 (10 mol%)
Xantphos (20 mol%)
K3PO4 (3 equiv)
I
I
R2
nBu
R2OH
+
O
R1
+
nBuOH
O
R1
Planetary ball mill
2 h, 800 rpm, 3aa:1a (95:5)
2a-f
(2 equiv)
30 Hz, 90 min
ball milling
1a-e
3
1a
2a
3aa
O
O
O
Scheme 2. Mechanochemical carbonylation reaction using gaseous CO by ball
nBu
O
O
milling.
O
Ph MeO
Ph
These results reveal that in ball mills both mechanistic alternatives
are feasible: First, a gas-free carbonylation pathway for which the
CO is rapidly transferred from Mo(CO)6 to the active catalytic
system as demonstrated by in situ pressure monitoring of the
mechanochemical alkoxycarbonylation reaction and, second, a
direct uptake of gaseous CO by the palladium catalyst during the
ball milling reaction.
3aa, yield = 87%
3ab, yield = 70%
3bb, yield = 75%
O
O
O
O
Me
Me
O
O
Me
MeO
3bc, yield = 67% (94%)
MeO
MeO
3bd, yield = 32% (73%)
3be, yield = 94%
3af, R = H
91%
After having studied the mechanochemical alkoxycarbonylation
by ball milling between 1a and 2a using Mo(CO)6/K3PO4, we set
to evaluate the applicability of the protocol to other substrates. For
this, the standard conditions were applied with the exception of
using xantphos instead triphenylphosphine to facilitate the
isolation by column chromatography. First, iodobenzene (1a) was
reacted with solid biphenyl-4-methanol (2b) (mp. 96-100 ºC). After
purification by column chromatography ester 3ab was isolated in
70% yield (Scheme 3). Then, solid 4-iodoanisole (1b) (mp. 50-53
ºC) and solid biphenyl-4-methanol (2b) were milled under the
optimized reaction conditions affording the corresponding product
3bb in 75% yield (Scheme 3). Also methanol (2c), isopropanol
(2d), phenol (2e) and benzyl alcohol (2f) were tested as
nucleophiles in the mechanochemical carbonylation reactions.
The results with 4-iodoanisole (1b) as reaction partner showed
that all alkoxycarbonylations leading to 3bc-3bf proceeded
smoothly within 90 min of milling. However, the high volatility of
some of the products during post-processing vacuum drying
prevented the isolation of the esters in higher yields after
separation by column chromatography (Scheme 3). Reacting
iodobenzene (1a) and 4-iodotoluene (1c) with benzyl alcohol (2f)
led to esters 3af and 3cf in 91% yield and 80% yield, respectively
(Scheme 3). To our surprise, the reaction of p-iodonitrobenzene
(1d) under the standard reaction conditions [i.e., Mo(CO)6 (1
equiv)] gave ester 3df in only 23% yield. Realizing that Mo(CO)6
has the capability of reducing nitro groups,[21] the reaction was
repeated with only 0.2 equiv of Mo(CO)6. This small change led
to an increase in the yield of ester 3df to 42% yield after the same
milling time. Using such substoichiometric amounts of Mo(CO)6
for other substrates produced lower yields, indicating that the
initially applied quantity (1 equiv) was preferable.
O
3bf, R = 4-OMe 93%
3cf, R = 4-Me 80%
3df, R = 4-NO2 42%
O
R
(with of 0.2 equiv of Mo(CO)6)
Scheme 3. Mechanochemical alkoxycarbonylation reactions using Mo(CO)6 by
ball milling (yields after column chromatography; in parentheses, yields as
determined by 1H NMR spectroscopy).
nucleophiles leading to amides 5ba-5bd in yields ranging from
58% to 86% (Scheme 4a). Finally, the mechanochemical
carbonylation
protocol
was
applied
to
the
sulfoximinocarbonylation of iodobenzene (1a) with S-methyl-S-
phenyl- sulfoximine (6aa), which gave the corresponding N-aroyl
sulfoximine 7aa in 78% yield (Scheme 4b).
a)
Mo(CO)6 (1 equiv)
O
Pd(OAc)2 (10 mol%)
Xantphos (20 mol%)
K3PO4 (3 equiv)
I
R2
R1R2NH
+
N
R1
MeO
4a-d
(2 equiv)
30 Hz, 90 min
MeO
1b
5
O
O
N
Me
N
H
H
MeO
MeO
5ba, yield = 86%
5bb, yield = 76%
O
N
O
Me
Me
N
H
MeO
MeO
5bc, yield = 58%
5bd, yield = 67%
b)
O
S
Me
I
O
Me
O
S
Same conditions
as above
+
N
Attempts to carry out the mechanochemical alkoxycarbonylation
of benzyl alcohol (2f) with 4-bromoanisole or phenyl triflate
instead of iodobenzene (1a) showed that the latter substrate (1a)
was superior over the formed two. Thus, with 4-bromoanisole
product 3bf was obtained in only 15% yield, and the use of phenyl
triflate gave ester 3af in 86% yield.[22] For comparison, with
substrates 1b and 1a, 3bf had been obtained in 93% yield and
3af in 91% yield (Scheme 3). Then, it was examined if the
mechanochemical carbonylation procedure using molybdenum
hexacarbonyl could be extended to palladium-catalyzed
aminocarbonylation reactions. For this, 4-iodoanisole (1b) was
reacted with primary and secondary amines under the standard
milling conditions (Scheme 4a). Among them, n-butylamine (4a),
benzyl amine (4b), diethyl amine (4c) and aniline (4d) proved to
be suitable as
HN
1a
6a
7aa
yield = 78%
(2 equiv)
Scheme 4. (a) Mechanochemical aminocarbonylation reactions and (b)
sulfoximinocarbonylation of 1a using Mo(CO)6 by ball milling. Yields after
column chromatography.
In summary, we have developed a mechanochemical protocol to
carry
out
palladium-catalyzed
carbonylative
reactions
(alkoxycarbonylations and aminocarbonylations) in ball mills
using molybdenum hexacarbonyl as a one-carbon building block.
The model alkoxycarbonylation reaction was found to proceed
mostly through a gas-free mechanism as evidenced by the real-
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time monitoring of the mechanochemical reaction using in situ
pressure sensing during ball milling. This result suggests that CO
is rapidly transferred from Mo(CO)6 to the active catalytic system
without significant release of molecular carbon monoxide.
However, if preferred, gaseous CO can be applied in the
mechanochemical alkoxycarbonylation reaction as well. From a
more general perspective, this study reinforces the concept of in
situ generation and consumption of gaseous reactants by
mechanochemistry, which reduces the direct handling and
exposition to toxic or highly reactive gaseous substances.[3]
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Acknowledgements
The authors are grateful to RWTH Aachen University for financial
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Keywords: ball milling • carbonylation • mechanochemistry •
molybdenum hexacarbonyl • real-time monitoring
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This article is protected by copyright. All rights reserved.

10.1002/chem.201904528
Chemistry - A European Journal
COMMUNICATION
This article is protected by copyright. All rights reserved.

10.1002/chem.201904528
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
Pit van Bonn, Carsten Bolm* and José
G. Hernández*
O
Pd(OAc)2
P-ligand, K3PO4
I
Nu
R1
+
Mo(CO)6
+
Nu-H
R1
ball milling
Page No. – Page No.
Nu-H = R2OH, R2R3NH
yield = 32-94%
- Solid CO-surrogate - Mechanical activation - Solvent-free - No external heating
- No high-pressure equipment
Mechanochemical Palladium-
Catalyzed Carbonylative Reactions
Using Mo(CO)6
A protocol for carrying out mechanochemical palladium-catalyzed carbonylative
reactions in ball mills is reported. The reaction uses molybdenum hexacarbonyl as a
convenient solid carbonyl source that precludes the need for a direct handling of CO
gas.
This article is protected by copyright. All rights reserved.