(Enantio)selective Hydrogen Autotransfer: Ruthenium-Catalyzed Synthesis of Oxazolidin-2-ones from Urea and Diols

Article abstract of DOI:10.1002/anie.201600698

A novel strategy for the synthesis of oxazolidin-2-ones from vicinal diols and urea is described. In this heterocycle synthesis, two different C?O and C?N bonds are sequentially formed in a domino process consisting of nucleophilic substitution and alcohol amination. The use of readily available starting materials and the good atom economy render this process environmentally benign. While this transformation is already highly chemo- and regioselective, we also developed the first asymmetric version of this method using (R)-(+)-MeO-BIPHEP as the chiral ligand.

Full text of DOI:10.1002/anie.201600698

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International Edition: DOI: 10.1002/anie.201600698
German Edition: DOI: 10.1002/ange.201600698
Ruthenium Catalysis Very Important Paper
(Enantio)selective Hydrogen Autotransfer: Ruthenium-Catalyzed
Synthesis of Oxazolidin-2-ones from Urea and Diols
Miguel PeÇa-López, Helfried Neumann, and Matthias Beller*
Abstract: A novel strategy for the synthesis of oxazolidin-2-
ones from vicinal diols and urea is described. In this hetero-
À
À
cycle synthesis, two different C O and C N bonds are
sequentially formed in a domino process consisting of
nucleophilic substitution and alcohol amination. The use of
readily available starting materials and the good atom
economy render this process environmentally benign. While
this transformation is already highly chemo- and regioselective,
we also developed the first asymmetric version of this method
using (R)-(+)-MeO-BIPHEP as the chiral ligand.
A
plethora of biologically active compounds contain a het-
erocyclic system as a central skeleton.[1] Therefore, many
methods aimed at the construction of such frameworks have
been developed.[2] Traditionally, most of these methods rely
on classic nucleophilic substitution reactions and thus result in
the formation of significant amounts of waste. Hence, the
discovery of more efficient and environmentally friendly
processes constitutes an important goal. In this context,
metal-catalyzed dehydrogenative coupling reactions offer
several benefits. More specifically, the so-called hydrogen
autotransfer (or “borrowing hydrogen”) approach allows the
Scheme 1. Dehydrogenative metal-catalyzed reaction of urea with vici-
nal diols.
challenge because both starting materials behave as nucleo-
philes and electrophiles. This heterocycle is highly relevant in
organic chemistry. For example, it can be used as precursor for
the synthesis of naturally occurring amino acids[6] and as
a chiral auxiliary in asymmetric synthesis (Evansꢀ oxazolidi-
nones).[7] Furthermore, it is present in several antimicrobials
and antibiotics.[8] Although several synthetic strategies have
already been published,[9] the further development of new,
efficient, and low-cost catalytic procedures remains to be
particularly interesting.
À
À
efficient formation of C C and C N bonds from non-
activated alcohols.[3] In the last decade, several heterocyclic
compounds have also been synthesized by applying such
reactions in a sequential manner.[4] Advantageously, water is
formed as the only byproduct, and the possibility of using
renewable alcohols as the starting materials makes these
transformations sustainable.
On this basis and following our previous experience, we
herein propose the use of ubiquitously available substrates
(urea and diols) for the synthesis of valuable heterocycles.[5]
Vicinal diols display high versatility in such transformations
as they are susceptible to more than one metal-catalyzed
dehydrogenation. In general, the starting diol is considered to
be a dinucleophile, whereas successive oxidations give rise to
electrophilic carbonyl compounds (Scheme 1). Subsequent
reactions of such intermediates with urea potentially lead to
the formation of different heterocycles, such as dioxolanones,
oxazolidinones, and imidazolidinones.
Based on our knowledge in the area of dehydrogenative
processes,[4] we envisioned a novel (chemo)selective synthesis
À
of oxazolidin-2-ones. As shown in Scheme 2, a C O bond
would initially be formed by nucleophilic substitution of urea
with one hydroxy group of the diol. Next, a ruthenium-
catalyzed dehydrogenation of the second alcohol moiety
should provide the corresponding carbonyl compound, which
would undergo a condensation with the remaining amino
À
group of the urea moiety, affording the desired C N bond
after reduction with the hydrogen that was initially extracted.
It should be noted that this heterocycle synthesis could also
take place in reverse order depending on which step is faster
Within these possibilities, the selective synthesis of
oxazolidin-2-ones (cyclic urethanes) constitutes a particular
[*] Dr. M. PeÇa-López, Dr. H. Neumann, Prof. Dr. M. Beller
Leibniz-Institut für Katalyse e.V. an der Universität Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: Matthias.Beller@catalysis.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201600698.
Scheme 2. Ruthenium-catalyzed synthesis of oxazolidin-2-ones from
urea and vicinal diols.
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Table 1: Ruthenium-catalyzed synthesis of 4-substituted oxazolidin-2-ones (3a–3g)
from ureas or carbamates (1, 4, 5) and terminal diols (2a–2h).[a]
under the applied reaction conditions. Obviously,
a high degree of selectivity is required owing to the
nucleophilic and electrophilic character of both
substrates, which could lead to different heterocyclic
compounds and oligomers or polymers. In this
context, we herein describe the first highly selective
ruthenium-catalyzed synthesis of oxazolidinones
from inexpensive urea and vicinal diols by hydrogen
autotransfer.
Our investigations began with the optimization
of the reaction conditions for the model reaction of
urea (1, 1 mmol) with 1-phenylethane-1,2-diol (2a,
1 mmol), a substrate with two different hydroxy
Entry
XR1
R2
Product
Yield [%][b]
1
2
3
4
NH2 (1)
NH2 (1)
NH2 (1)
NH2 (1)
phenyl (2a)
(3a)
(3b)
(3c)
(3d)
76
71
65
73
p-ClC6H5 (2b)
p-F3CC6H5 (2c)
p-MeOC6H5 (2d)
5
6
NH2 (1)
NH2 (1)
benzyl (2e)
n-butyl (2 f)
(3e)
(3 f)
64
69
groups
(see
the
Supporting
Information,
Scheme S1). Based on previous studies by our
group,[10] we initially used [Ru3(CO)12] as the catalyst
in combination with different phosphines. Only
traces of the desired product were observed under
ligand-free conditions, whereas the use of different
mono- and bidentate phosphines afforded 4-phenyl-
oxazolidin-2-one (3a) selectively in moderate yields
(20–52%). Based on these first results, we performed
a more detailed optimization of the best ligand
7
8
NH2 (1)
NH2 (1)
n-hexyl (2g)
5-hexenyl (2h)
(3g)
(3g)
66
65[c]
9[d]
NHMe (4a)
NHEt (4b)
NHPh (4c)
NHBn (4d)
OMe (5a)
OtBu (5b)
OPh (5c)
phenyl (2a)
phenyl (2a)
phenyl (2a)
phenyl (2a)
phenyl (2a)
phenyl (2a)
phenyl (2a)
phenyl (2a)
(3a)
(3a)
(3a)
(3a)
(3a)
(3a)
(3a)
(3a)
56
54
49
45
62
71
69
72
10[d]
11[d]
12[d]
13[d]
14[d]
15[d]
16[d]
system
(dppf = 1,1’-bis(diphenylphosphino)ferro-
cene). As shown in the Supporting Information,
Table S1, we studied different ruthenium precursors,
catalyst loadings, ratios of 1/2a, and additives and
varied other critical parameters, such as the solvent,
temperature, and reaction time. Finally, we found
that the reaction of urea (1, 1 mmol) and 2a
(0.5 mmol) was most efficient in the presence of
[Ru3(CO)12] (0.01 mmol) and dppf (0.03 mmol) in
1,4-dioxane at 1508C for 22 h, which enabled the
isolation of 3a in 76% yield (Table 1, entry 1).
Considering that at least four steps are involved in
this reaction sequence, the optimized yield indicates
OBn (5d)
[a] Unless otherwise specified, all reactions were carried out with 1 (1.0 mmol), 2
(0.5 mmol), Ru3(CO)12 (0.01 mmol), and dppf (0.03 mmol) in 1,4-dioxane (1 mL) at
1508C for 22 h. [b] Yields of isolated products. [c] The maximum amount of product
would be 0.25 mmol. In addition, the formation of unsaturated isomers in 10% yield
was observed. [d] Urea or carbamate (4 or 5, 1.5 mmol).
that each individual transformation proceeds with high
efficiency, while the formation of water and ammonia as the
only byproducts renders this process atom-efficient and
environmentally benign. Gratifyingly, the reaction proceeded
with chemoselectivity as 3a was almost exclusively obtained
with only traces of the cyclic carbonate (8% yield, see
Scheme 1). Furthermore, the 4-substituted product was
formed as the only regioisomer, which suggests that nucleo-
philic substitution preferably occurs with the terminal alcohol
and intramolecular amination with the internal one.
1,2-diol (2 f and 2g) under the initially established conditions
led to the desired products in similar yields (69 and 66%;
entries 6 and 7). Finally, we were interested in studying the
behavior of unsaturated oct-7-ene-1,2-diol (2h). In this case,
=
reduction of the terminal C C bond was also observed,
providing the saturated oxazolidin-2-one 3g in 65% yield
(entry 8). The well-known ability of ruthenium complexes to
promote the isomerization or reduction of multiple bonds[11]
implies that the hydrogen extracted in the first step is used in
=
both imine and C C bond reduction.
To expand the scope of this method, we next evaluated the
reactivity of different terminal diols. Electron-deficient
styrene diol derivatives, such as compounds 2b and 2c, were
effectively converted into the corresponding oxazolidin-2-
ones 3b and 3c (71 and 65% yield, respectively; Table 1,
entries 2 and 3). Analogously, this reaction can also be
extended to aryl diols with electron-donating groups such as
2d, which afforded 3d in 73% yield (entry 4). On the other
hand, the reaction of benzyl derivative 2e also took place
regioselectively to give 4-benzyloxazolidin-2-one (3e) in
a good yield of 64% (entry 5). Aside from aromatic
substrates, this reaction also worked well with aliphatic
terminal diols. Thus, the reactions of hexane- and octane-
At this point, our catalytic system was applied to the
reaction of 1-phenylethane-1,2-diol (2a) with different sub-
stituted ureas and carbamates (Table 1). Here, our initial
objective was the synthesis of N-protected oxazolidin-2-ones
by using N-monosubstituted ureas as the starting materials.
Unfortunately, the ruthenium-catalyzed reaction of 4a–4d
using the conditions previously developed provided the free
(NH) oxazolidin-2-one 3a in moderate yields (45–56%;
entries 9–12). These results suggest that the primary amine
is a better leaving group than ammonia in the initial
nucleophilic substitution step, and that the amination of the
secondary alcohol takes place preferably with the free NH2
group. The presence of the primary amine in the reaction
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Table 2: Ruthenium-catalyzed synthesis of 4,5-disubstituted oxazolidin-2-ones (7a–
7g) from urea (1) and internal diols (6a–6g).[a]
medium also promotes further side reactions; there-
fore, higher amounts of urea were required to
achieve acceptable yields. Next, we performed the
reaction with carbamates as urea analogues. The
reaction of 2a with 5a–5d provided the heterocycle
3a in good yields (62–72%; entries 13–16). Here, the
alcohol resulting from substitution does not partic-
ipate in undesired reactions that decrease the yield.
To demonstrate the broad applicability of this
synthetic strategy, our method was tested with
internal diols. Unlike in the previous study, here
the similar reactivity of both hydroxy groups could
lead to a lack of selectivity, resulting in a mixture of
compounds as shown in Scheme 1. On this basis, we
initially performed the reaction of urea (1, 1 mmol)
with butane-2,3-diol (6a, 0.5 mmol) in the presence
of [Ru3(CO)12] (0.01 mmol) and dppf (0.03 mmol) in
1,4-dioxane at 1508C. Satisfactorily, the desired
product, 4,5-dimethyloxazolidin-2-one (7a), was iso-
lated in 65% yield as a mixture of diastereoisomers
(60:40 d.r.), while the corresponding 1,3-dioxolan-2-
one and imidazolidin-2-one were formed in only 4
and 10% yield, respectively (Table 2, entry 1). We
then decided to study the reactivity of cyclic diols
such as 6b and 6c. In both cases, the oxazolidin-2-
ones 7b and 7c were formed as the major products
and with good diastereoselectivity (48 and 56%;
entries 2 and 3), although the amount of the men-
tioned side products increased. Alternatively, experi-
ments with the long-chain substrate 6d and (1R,2R)-
1-phenylpropane-1,2-diol (6e) gave rise to the 4,5-
disubstituted heterocycles 7d and 7e as complex
mixtures of regio- and diastereoisomers in moderate
yields (45 and 42%, respectively; entries 4 and 5).
Entry Diol
Product
Yield [%][b]
1
(6a)
(7a) 65 (4/10)[c,d]
2
3
(6b, n=1)
(6c, n=2)
(7b) 48 (15/19)[c]
(7c) 56 (3/21)[c]
4
(6d)
(7d) 45 (23)[e,f]
5
6
(6e)
(6 f)
(7e) 42 (21)[e,f]
(7 f) 43 (32)[e]
7
(6g)
(7g) 45 (41)[e]
[a] Unless otherwise specified, all reactions were carried out with 1 (1.0 mmol), 6
Here, cyclic carbonates were observed only as side (0.5 mmol), Ru3(CO)12 (0.01 mmol), and dppf (0.03 mmol) in 1,4-dioxane (1 mL) at
1508C for 22 h. [b] Yields of isolated products. [c] The isolated yield of the
corresponding 1,3-dioxolan-2-one/imidazolidin-2-one is given in parentheses.
[d] 7a: 60:40 d.r. [e] The yield of isolated cyclic carbonate is given in parentheses.
[f] The oxazolidinones were isolated as complex mixtures of regio- and diastereo-
products. However, the reaction of urea (1) with
(R,R)-hydrobenzoin (6 f) provided the unsaturated
oxazolidinone 7 f in 43% yield (entry 6). The double
bond formed after the condensation reaction isomer-
izes to a conjugated olefin, which is not reduced
under these conditions. Lastly, we studied the
isomers.
reactivity of an internal diol with a tertiary alcohol in its
structure, thereby avoiding the formation of imidazolidin-2-
ones owing to the inability of that moiety to undergo
dehydrogenative reactions. The reaction of terpene diol 6g
resulted in the formation of cyclic urethane 7g and the
corresponding carbonate in moderate yields (45 and 41%,
respectively; entry 7). The structure of the only diastereoiso-
mer formed in this novel cyclization process was confirmed by
X-ray crystallography (see the Supporting Information).
Finally, in an effort to render this procedure more
synthetically relevant, we considered the development of
the corresponding asymmetric version. One of the most
important applications of oxazolidin-2-ones in organic
chemistry is their use as chiral auxiliaries.[7] For this purpose,
these compounds need to be available in enantiopure form,
and despite the synthetic strategies published thus far,[12] the
discovery of straightforward, selective, sustainable, and more
efficient methods is still of high importance. Based on the
postulated mechanism (Scheme 2), the key step for the
generation of the stereogenic carbon center would be the
reduction of the intermediate imine with the hydrogen that is
initially extracted. Chiral variants of hydrogen autotransfer
processes have rarely been reported,[13] which is probably due
to the high temperatures that are required for such reactions,
but usually lead to a decrease of the enantiomeric excess.
To develop such an asymmetric variant, we considered the
reaction of urea (1) with racemic 1-phenylethane-1,2-diol
(2a) as a model transformation as an asymmetric center is
formed at the C4 position. As shown in Scheme 3, different
chiral ligands were studied in combination with [Ru3(CO)12].
Obviously, the previously used dppf ligand provided
4-phenyloxazolidin-2-one (3a) as a racemic mixture. In line
with this example, chiral ferrocenyl phosphines were tested
and found to lead to similar yields but moderate enantiose-
lectivities (5–62% ee). Encouraged by these results, we
analyzed the effects of ligands with axial chirality. Among
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and 92% ee, Scheme 4). These results suggest that highly
stable catalytic species are involved in this reaction that
enables the enantioselective formation of stereogenic carbon
centers even at such high temperatures.
In conclusion, we have described a novel domino process
for the ruthenium-catalyzed synthesis of oxazolidin-2-ones
from urea and vicinal diols. This atom-efficient method allows
À
À
the sequential formation of C O and C N bonds in good
yields using a commercially available catalytic system (Ru3-
(CO)12/dppf). The use of simple substrates and the formation
of water and ammonia as the only byproducts render this
straightforward process environmentally benign. While these
transformations generally proceed with high chemo- and
regioselectivity, the use of an appropriate chiral ligand
enabled the development of an asymmetric version.
Acknowledgements
Scheme 3. Ligand screen for the enantioselective ruthenium-catalyzed
synthesis of 4-phenyloxazolidin-3-one (3a). Yields of isolated products
are given. [a] (R)-3a was formed as the major enantiomer. [b] (S)-3a
was formed as the major enantiomer.
We thank the state of Mecklenburg-Vorpommern and the
Bundesministerium für Bildung und Forschung (BMBF) for
financial support. M.P.-L. thanks the European Commission
for a Marie Curie Intra-European Fellowship (PIEF-GA-
2012-328500).
various readily available diphosphines, such as (R)-
(+)-BINAP
and
(R)-(+)-SEGPHOS,
(R)-(+)-MeO-
Keywords: diols · domino reactions · enantioselectivity ·
BIPHEP enabled the formation of (S)-3a at 1508C in 53%
yield and excellent 88% ee (94:6 enantiomeric ratio,
Scheme 3).
oxazolidinones · ruthenium catalysis
How to cite: Angew. Chem. Int. Ed. 2016, 55, 7826–7830
Angew. Chem. 2016, 128, 7957–7961
With a suitable chiral catalytic system in hand, we studied
the enantioselective reaction of urea (1) with other diols.
Gratifyingly, the oxazolidin-2-one 3d was obtained in a good
yield of 64% and 82% ee, whereas the benzyl derivative gave
similar results (3e, 66% yield, 87% ee; Scheme 4). Further-
more, this asymmetric reaction can also be applied to terminal
alkyl-substituted diols as shown by the formation of products
3 f and 3g (59 and 57% yield, 93 and 90% ee). Finally, an
internal diol was also tested. The amination reaction afforded
the disubstituted heterocycle 7a in 56% yield. Whereas the
two diastereoisomers were formed in equal amounts (1:1 syn/
anti), the enantioselectivity of the reaction was excellent (90
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Received: January 25, 2016
Published online: April 13, 2016
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