Efficient 1,3-Oxazolidin-2-one Synthesis through Heterogeneous PdII-Catalyzed Intramolecular Hydroamination of Propargylic Carbamates

Article abstract of DOI:10.1002/chem.201900678

Herein, we present an operationally simple protocol for the cycloisomerization of propargylic carbamates in which a heterogeneous catalyst consisting of Pd species immobilized on amino-functionalized siliceous mesocellular foam (Pd^II-AmP-MCF) i

Full text of DOI:10.1002/chem.201900678

A Journal of
Accepted Article
Title: Efficient 1,3-oxazolidin-2-one synthesis through heterogeneous
Pd(II)-catalyzed intramolecular hydroamination of propargylic
carbamates
Authors: Michael Oschmann, Clotilde Placais, Anuja Nagendiran, Jan-
Erling Bäckvall, and Oscar Verho
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201900678
Link to VoR: http://dx.doi.org/10.1002/chem.201900678
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COMMUNICATION
Efficient 1,3-oxazolidin-2-one synthesis through heterogeneous
Pd(II)-catalyzed intramolecular hydroamination of propargylic
carbamates
Michael Oschmann[a], Clotilde Placais[a], Anuja Nagendiran[a], Jan-E. Bäckvall[a]*, Oscar Verho[a]*
Abstract: Herein, we present an operationally simple protocol for the
cycloisomerization of propargylic carbamates in which
Most available methods based on cycloisomerization
chemistry requires the use of high high loadings of homogeneous
catalysts[7a,b,d,f,g] or a strong base.[7c] However, recently Saavedra
et al. reported an example of a heterogeneous Pd(II)-catalyzed
protocol for the cycloisomerization of propargylic carbamates to
the corresponding 1,3-oxazolidin-2-ones.[9] Although this protocol
led to improvements, it still suffered from a few issues that limited
its practicality. For instance, this protocol was only applied to a
limited number of propargyl carbamate substrates and most of the
reactions called for extended reaction times and high reaction
temperatures (5 h−4 days at 50-90 °C). Furthermore, all reactions
were carried out using water as the solvent. Although water is an
attractive solvent from a green chemistry perspective, it is not
ideal for this transformation since certain cyclic carbamates are
susceptible to hydrolysis, which can lead to the formation of ring-
opened products.
a
heterogeneous catalyst consisting of Pd nanoparticles immobilized on
amino-functionalized siliceous mesocellular foam (Pd(II)-AmP-MCF)
is used. This Pd nanocatalyst displayed high efficiency at low catalyst
loading and reaction temperatures, which allowed for the efficient and
mild synthesis of a wide range of 1,3-oxazolidin-2-one derivatives and
related compounds. Moreover, it proved possible to re-use the Pd
nanocatalyst for several reactions, although a gradual decrease in
activity was observed in the subsequent cycles.
Oxazolidinones and their derivatives have been found to
possess a wide range of biological activities, which have made
them a highly important compound class in therapeutics and
agricultural research. These compounds are perhaps most well-
known for their antimicrobial effects,[1] but there also exist
In 2011, we reported on the synthesis of a heterogeneous
Pd(0)-catalyst, consisting of Pd nanoparticles immobilized on
amino-functionalized siliceous mesocellular foam (Pd(0)-AmP-
MCF). It was demonstrated that this catalyst could efficiently
mediate the racemization of amines.[10] Since then, the Pd(0)-
AmP-MCF has been successfully employed in a wide range of
organic transformations, including oxidations,[11] reductions,[12]
Suzuki cross-couplings[13] and various cooperative reaction
protocols.[14] Interestingly, the Pd(II)-precursor of this Pd(0)
nanocatalyst, can also be used as a catalyst for a number of
useful reactions. For example, this Pd(II)-AmP-MCF catalyst has
been employed for the cycloisomerization of alkynoic acids[15]
and for the oxidative carbocyclization-borylation of enallenols.[16]
Additionally, this Pd(II)-AmP-MCF could be utilized as a catalyst
in the chemodivergent syntheses of γ-lactones and γ-lactams via
different oxidative tandem processes.[17]
examples
of
oxazolidinone
derivatives
that
display
immunomodulatory[2] and neuroleptic activities.[3] In the field of
organic synthesis, chiral oxazolidinones are frequently
encountered as a chemical motif in synthetic intermediates, most
often as chemical auxiliaries to assist in different asymmetric
transformations.[4] Because of the many applications of
oxazolidinone derivatives, significant efforts have been dedicated
towards their synthesis. Here, the most common methods to
prepare oxazolidinones have relied primarily on three different
synthetic strategies; (i) the reaction of amino alcohols with
phosgene or chloroformate reagents as the carbonyl precursor,[5]
[6]
(ii) carboxylative cyclizations utilizing CO2 or (iii) catalytic
cyclizations of propargyl carbamates.[7]
Although many methods are known for the efficient
preparation of oxazolidinones, a majority of them suffer from
various drawbacks. When designing new protocols for
oxazolidinone synthesis, it would be attractive if they like many of
the CO2-based cyclisation methods would use an immobilized
catalyst, and in this way simplify catalyst separation and recycling.
However, for practical purposes it would be desirable to avoid a
method that relies on the use of CO2 as a reagent since this
requires special reaction equipment. In this regard, the
cycloisomerization of propargyl carbamates that employs a
heterogeneous catalyst would constitute the ideal approach for
accessing oxazolidinones. Such a procedure would only require
standard experimental set-ups, while benefitting from the typical
advantages that is associated with the use of heterogeneous
catalysts, such as simple catalyst separation and recycling.[8]
Following up on this work, we were interested in studying if
our Pd(II)-AmP-MCF catalyst could also be used for the related
cycloisomerization of propargyl carbamates to the corresponding
1,3-oxazolidin-2-ones. Here, our goal was to develop a more
practical and recyclable heterogeneous system for the
preparation of these valuable compounds (Scheme 1).
Scheme 1. Envisioned heterogeneous Pd(II)-catalyzed protocol for the
cyclization of propargylic carbamates into 1,3-oxazolidin-2-ones.
The cycloisomerization of prop-2-yn-1-yl tosylcarbamate (1a)
into 4-methylene-3-tosyloxazolodin-2-one (2a) using the Pd(II)-
AmP-MCF catalyst was chosen as the model reaction for the
screening of reaction conditions, where the effects of base,
solvent and catalyst loading were investigated. Some selected
entries are presented in Table 1 (for a more detailed presentation
[a]
M. Oschmann, C. Placias, A. Nagendiran, Prof. J.-E. Bäckvall, Dr.
O. Verho
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm
University, SE-106 91 Stockholm, Sweden
E-mail: oscar.verho@su.se, jan.e.backvall@su.se
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201900678
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of these results, see Figure S1-3 and Table S1 in the Supporting
Information, SI). The optimization began with a screening of
different bases using 0.5 mol% Pd(II)-AmP-MCF in CH3CN at
room temperature (rt). Modest reaction performances were
observed when using the organic bases DBU and NEt3, which
gave 39% and 46% yield of oxazolidinone 2a, respectively (Table
1, entries 1−2). With n-Bu4NOAc as base, the yield of the
cycloisomerization reaction could be improved to 62% (Table 1,
entry 3). Full conversion was obtained for these reactions within
one hour (Figure S1); however, at the end of this reaction a clear
color change of the Pd(II)-AmP-MCF had occurred, indicating
reduction to Pd(0). Best results were achieved when using
inorganic and non-coordinative bases, such as K2CO3, CsCO3
and NaOAc, which gave 73%, 69% and 90% yield, respectively
(Table 1, entries 4−6). Interestingly, the reaction also worked well
when 0.25 mol% Pd(II)-AmP-MCF was used, giving 62% yield
(Table 1, entry 7) and full conversion after 30 min.
reduction of the catalyst loading to 0.125 mol% and performing
the reaction under more dilute conditions (0.2 M) gave full
conversion within 3 h (Table 1, entry 14). As acetone exhibits a
better safety and green chemistry profile than MeCN, it was
chosen as the solvent for further studies. No product formation
could be observed in absence of base or Pd(II)-AmP-MCF (entry
not shown in Table 1).
Comparing the performance of our Pd(II)-AmP-MCF to that of
other heterogeneous Pd catalysts showed that it was superior to
the other catalysts tested for this cycloisomerization reaction (see
SI, Figure S4). When using either Pd(0)-AmP-MCF or commercial
Pd/C as catalysts for our model reaction, a significantly lower
yield of product 2a was obtained compared to that obtained with
the Pd(II)-AmP-MCF catalyst. In a previous study we had
observed that additions of BQ to the Pd(II)-AmP-MCF catalyst
gave a highly active catalyst with surface-confined Pd(II)
species;[15] however, this approach did not lead to any
improvements in the present cycloisomerization reaction with the
Pd(II)-AmP-MCF.
Table 1. Base and solvent screening for the cycloisomerization of 1a to 2a.
With the optimized reaction conditions in hand, we next
investigated the substrate scope of our catalytic protocol (Table
12). The work-up procedure were conceivably simple. After
completion of the reactions, the catalyst was filtered off and
washed with acetone, and the collected liquid fractions were
concentrated under reduced pressure. In many cases, the
products were obtained in high purity without the need of further
purifications. Following this work-up procedure, it was for example
possible to isolate product 2a from our initial model reaction, in an
excellent yield of 99% after only 20 min. To our delight, the
catalytic protocol also worked efficiently for the cyclization of
carbamates derived from sec-alcohols, such as 1b−1d, which
gave the corresponding oxazolidinones in 83−91% yield after
0.5−6 h at rt (Entries 2−4). Quaternary substrates could also be
converted to the corresponding products in high yields and short
reactions times (Entries 5−6). Product 2e was found to be
unstable and therefore the yield was determined by NMR against
an internal standard (90% yield after 2 h at rt). On the other hand,
the spirocyclic product 2f was more stable and could be isolated
in 88% yield after 30 min. For substrates with substituents in the
terminal position of the alkyne moiety, elevated temperatures and
extended reaction times were generally required to ensure high
yields of the oxazolidinone products (Entries 7−12). Moreover, for
these reactions, a high selectivity for the expected Z isomer of the
products were typically observed. For the cycloisomerization of
substrate 1g with a terminal methyl substituent, an excellent yield
of 95% and a Z:E ratio of 88:12 was obtained for product 2g within
7 h at 40 °C (Entry 7). In the case of substrate 1h with a terminal
phenyl substituent, the cycloisomerization reaction was found to
favor the formation of the Z conformer of product 2h almost
exclusively (>95%), although a longer reaction time of 20 h was
required for a high yield (94%, Entry 8). The p-CN-phenyl
substituted 2i was found to be unstable, which required yield
determination to be done by NMR against an internal standard.
Product 2i was obtained in 80% yield and in a Z:E ratio of 95:5
after 24h.
Entry
Catalyst
(mol%)
0.5
Base (equiv.)
Solvent
2a (%)a
1
2
DBU (0.1)
NEt3 (0.1)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CPME
PhCH3
DCE
39
46
62
73
69
90
62
<5
<5
35
49
77
>95
16
0.5
0.5
3
n-Bu4NOAc (0.1)
K2CO3 (0.1)
Cs2CO3 (0.25)
NaOAc (0.1)
NaOAc (0.1)
NaOAc (0.1)
NaOAc (0.1)
NaOAc (0.1)
NaOAc (0.1)
NaOAc (0.1)
NaOAc (0.1)
NaOAc (0.1)
4
0.5
5
0.5
6
0.5
7
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.125
8
9
10
11
12
13
14b
THF
EtOAc
Acetone
Acetone
conversion determined by 1H NMR with 1,3,5-trimethoxybenzene as internal
standard. b concentration 0.2 M.
a
The cycloisomerization of 1a was found to proceed slowly in
non-polar solvents such as cyclopentyl methyl ether (CPME) and
toluene, yielding only trace amounts of 2a after 30 min (Table 1,
entries 8−9). These poor results can most likely be ascribed to the
low solubility of NaOAc in these non-polar solvents. The reaction
worked moderately well in DCE and THF, giving 35% and 49%
yield, respectively (Table 1, entries 10−11). EtOAc, on the other
hand, was found to be a good solvent for the reaction, allowing
oxazolidinone 2a to be acquired in 77% yield (Table 1, entry 12).
However, when the reaction was carried out in acetone it was
possible to match the performance of the reactions run in MeCN,
allowing for >95% yield after 20 min (Table 1, entry 13). A further
For substrate 2j with a p-EtO2C-phenyl substituent, an
excellent yield of 88% and exclusive formation of the Z isomer
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were observed after 18 h (Entry 10). In the case of p-Br-phenyl
substituted substrate 1k, a slower reaction was observed and
80% yield was obtained after 48 h (Entry 11). The selectivity
towards the Z isomer of product 2k was excellent (>95%).
Similarly, excellent Z selectivity was also observed for the p-NO2-
phenyl substituted product 2l, which was obtained in 91% yield
after 24 h (Entry 12).
20 h
/ 40
80
8b
[<5:>95]
24 h
/ 40
80a
9b
[5:95]
As demonstrated in Entry 13, the cycloisomerization protocol
could also be extended to propargylic benzylcarbamates when
using n-Bu4NOAc as the base and oxazolidinone 2m was
obtained in 85% yield after 22 h at rt. Unfortunately, propargylic
phenylcarbamate 1n was found to be beyond the scope of our
protocol (Entry 14), which most likely can be ascribed to the lower
acidity of the N–H proton in this substrate. On the other hand,
propargylic tosylureas proved to be suitable substrates(Entry 15),
as exemplified by the cycloisomerization of 1o to the cyclic urea
2o (75% yield after 48 h at rt).
18 h
/ 40
88
10b
11b
12b
[<5:>95]
48 h
/ 40
80
[<5:>95]
Table 2. Synthesis of oxazolidinones and related compounds.
24 h
/ 40
91
[<5:>95]
t /T
entry
1
substrate
product
yield (%)
99
(°C)
22 h
/ rt
13c
85a
nra
75
20
min /
rt
24 h
/ rt
30
min /
rt
14
2
3
91
86
48 h
/ rt
15
6 h /
rt
All yields are isolated if not otherwise stated; a yield determined by 1H-NMR with
1,3,5-trimethoxybenzene as internal standard, due to instability of product; b
0.5mol% catalyst; c 0.1 equiv. n-Bu4NOAc
2 h /
rt
4
5
83
90a
88
An important aspect of a heterogeneous catalyst is the potential
to recycle it. Therefore, to assess the recyclability of our Pd(II)-
AmP-MCF catalyst, it was subjected to three reactions cycles
using substrate 1a (Figure 1) and the formation of product 2a was
followed over time. From this recycling study, we could observe a
gradual decrease of the catalyst’s activity over each cycle.
Comparing to the first reaction which reached completion after 20
min, the subsequent two cycles required longer reaction times to
give comparable results (e.g. 1 h for cycle 2 and 2 h for cycle 3).
To elucidate the cause of this deactivation process and to get
further insights into the mechanism of our Pd(II)-AmP-MCF
catalyst, aliquots were withdrawn from each cycle and studied by
Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-
OES) to determine the extent of Pd leaching. These
measurements revealed a considerable Pd leaching of 22.5 ppm
(15.9% of total Pd) in the first cycle, which decreased to 4.8 ppm
2 h /
rt
30
min /
rt
6
88
7 h /
40
7b
[E:Z
12:88]
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(3.5% of total Pd) in cycle 2 and 2.0 ppm in Cycle 3 (1.4% of total
Pd). This result suggests that the Pd(II)-AmP-MCF can to a
Acknowledgements
certain degree operate through
a
release-and-catch
Financial support for this project from The European
Research Council (ERC AdG 247014), The Swedish Research
Council (2016-03897), the Berzelii Center EXSELENT, and the
Knut and Alice Wallenberg Foundation (KAW 2016.0072) is
gratefully acknowledged. O. V. also wishes to acknowledge the
Wenner-Gren Foundations Fellow Program for career support.
mechanism,[18] in which the Pd(II)-AmP-MCF function as a
reservoir of homogeneous and catalytically-active Pd(II) species
that can contribute in the reaction. However, it seemed unlikely
that all of the observed catalytic activity can be ascribed to
homogeneous Pd species, as the levels of leached Pd(II) did not
correlate linearly with product formation. We therefore carried out
a hot filtration test (see SI, Figure S5), and here we could see that
the homogeneous species only contributed marginally to the
observed reaction. The participation of the heterogeneous
component was further supported by a second recycling study
(see SI), in which the cycloisomerization of 1a was performed
over 5 cycles using 0.5 mol% Pd(II)-AmP-MCF over 30 min at rt.
Here in the fifth cycle, a yield of 90% of 2a was obtained when,
interestingly, no homogeneous Pd(II) could be detected by ICP-
OES. Furthermore, this second recycling study also showed that
if one wish to maintain a satisfactory reaction performance over
multiple cycles with the Pd(II)-AmP-MCF catalyst, despite the
observed catalyst deactivation, it is possible to do so by simply
increasing the catalyst loading to 0.5 mol%. With this increased
catalyst loading it was possible to achieve >90% yield of 2a over
all five cycles within 30 min.
Keywords: heterogeneous catalysis • palladium • mesoporous
silica • cycloisomerization • oxazolidinones
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In summary, we have developed an efficient and operationally
simple protocol for the cycloisomerization of propargylic
carbamates, based on the Pd(II)-AmP-MCF catalyst.
Interestingly, high reaction performances could be observed even
when using very low catalyst loading and mild reaction conditions.
In addition this protocol exhibited excellent functional group
tolerance which made it possible to prepare a wide range of 1,3-
oxazolidin-2-one derivatives in high yields. Moreover, it was found
to be possible to recycle the Pd(II)-AmP-MCF catalyst, although
a gradual decrease in performance was observed upon reuse
when using 0.25 mol% catalyst. Interestingly, the use of 0.5 mol%
catalyst led to more robust performance, which enabled recycling
up to 5 times with >90% yield in all cycles after 30 min. Seen
overall, this Pd(II)-AmP-MCF based protocol constitutes a highly
attractive alternative for preparing various oxazolidinones
derivatives on a laboratory scale.
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10.1002/chem.201900678
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COMMUNICATION
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This article is protected by copyright. All rights reserved.

10.1002/chem.201900678
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
Michael Oschmann, Clotilde Placais,
Anuja Nagendiran, Jan-E. Bäckvall,
Oscar Verho
Page No. – Page No.
Title
Closing the cycle: Herein, we report on a heterogeneous catalyst consisting of
Pd(II)-species immobilized on amino-functionalized mesocellular foam, and its use in
the cycloisomerization of propargylic carbamates and ureas. The developed catalytic
protocol was found to operate efficiently at mild conditions and it was possible to
recycle.
This article is protected by copyright. All rights reserved.