Ruthenium pincer-catalyzed synthesis of substituted γ-butyrolactones using hydrogen autotransfer methodology

Article abstract of DOI:10.1039/c5cc01708d

The ruthenium pincer-catalyzed synthesis of γ-butyrolactones from 1,2-diols and malonates using borrowing-hydrogen methodology is reported. This regioselective domino-process takes place through catalytic C-C bond formation, followed by intramolecular transesterification. Herein, we show the Ru-MACHO-BH complex as a valuable catalyst in hydrogen autotransfer reactions.

Full text of DOI:10.1039/c5cc01708d

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Ruthenium pincer-catalyzed synthesis of substituted
γ
-butyrolactones using hydrogen autotransfer methodology
Cite this: DOI: 10.1039/x0xx00000x
Miguel Peña-López, Helfried Neumann and Matthias Beller*a
Received 00th January 2014,
Accepted 00th January 2014
DOI: 10.1039/x0xx00000x
www.rsc.org/chemcomm
intermediate carbonylic compound, provided the corresponding C-
alkylated products in a economical and environmentally benign way.
The ruthenium pincer-catalyzed synthesis of γ-butyrolactones
from 1,2-diols and malonates using borrowing-hydrogen
methodology is reported. This regioselective domino-process
takes place through catalytic C-C bond formation, followed
by intramolecular transesterification. Here, we show the Ru-
MACHO-BH complex as a valuable catalyst on hydrogen
Inspired by the metal-catalyzed Knoevenagel condensation with
alcohols, we propose that the introduction of a second appropriate
functional group in the substrate might lead, after the C-C bond
formation, to an intramolecular cyclization providing an heterocyclic
compound (Scheme 1). Taken this premise into account, we planned
to use 1,2-diols and malonates as reagents, which would allow to
obtain the correponding lactones. In agreement with the established
borrowing-hydrogen process for the formation of carbon-carbon
autotransfer reactions
.
γ
-Butyrolactones are a common structural moiety present in a variety
of natural products. They are usually taking part of more complex
frameworks, especifically polycyclic ring systems, which display
broad biological activities including antitumor, antiviral, antibiotic
or antifungal properties among many others.1 This relevance makes
this unit very attractive for the discovery of novel potential drugs.
Hence, the development of new synthetic methodologies for this
class of compounds is of special interest. Altough a number of
bonds, the synthesis of γ-butyrolactones should proceed by the
following domino sequence: temporary removal of hydrogen from
the diol provides the corresponding 1,2-hydroxyketone A, which
undergoes a Knoevenagel reaction with the malonate giving, after
returning of the hydrogen initially extracted, the intermediate
compound C. Finally, decarboxylative reaction followed by
subsequent lactonization in basic medium affords the desired
heterocyclic compound. Promisingly, this reaction cascade would
allow us to generate sequentially for the first time a C-C and a C-O
bond using this hydrogen autotransfer methodology. Under these
assumptions, we present herein the first study into the ruthenium-
classical procedures for the synthesis of γ-butyrolactones are known,
most of them require either specific substrates and/or rigorous
reaction conditions.2 For this reason, during the last years several
research groups were interested to discover more efficient metal-
catalyzed processes.3 In addition, also metal-free strategies have
been published for this purpose. For example, the groups of Glorius4
and Bode5 have succesfully used N-heterocyclic carbene (NHC)
catalyzed synthesis of substituted
and malonates as substrates.
γ-butyrolactones by using 1,2-diols
catalyzing conjugate umpolung of
α,β-unsaturated aldehydes to
synthesize different -butyrolactones. Alternatively, this methodo-
γ
logy was also extended to other coupling partners as well as different
carbene catalysts.6
During the last decade, our research group showed a continuing
interest in the so-called borrowing-hydrogen methodology, also
known as hydrogen autotransfer process.7 This procedure allows the
activation of simple and available alcohols through a temporary
metal-catalyzed oxidation to the corresponding aldehyde or ketone.
Following condensation with amines and final reduction with the
previously removed hydrogen, lead to amination of alcohols in an
atom economy process as H2O is generated as the only by-product.8
Additionally, the synthesis of N-heterocycles by sequential
dehydrogenative coupling processes has also been developed, where
C-N and C-C bonds are simultaneously formed.9 Notably, this
methodology was also elegantly developed in the research groups of
Williams and Yus for the C-C bond formation between C-
nucleophiles and alcohols.10 In this latter case, the Wittig or
Knoevenagel reaction, as well as the aldolic condensation with the
Scheme 1. Synthesis of γ-butyrolactones via ruthenium-catalyzed
carbon-carbon bond formation between 1,2-diols and malonates.
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DOI: 10.1039/C5CC01708D
Table 1. Optimization of the reaction conditions for the synthesis of 12). Adding another equivalent of 2 did not promote a more effective
5-butyldihydrofuran-2(3H)-one (3a) from 1,2-hexanediol (1a) and reaction (Table 1, entry 13). Using higher amounts of base, other
diethyl malonate (2).a
solvents and catalyst loadings, as well as higher reaction
temperatures and times, resulted in even lower yields (Table 1,
entries 14-20). Finally, the reaction without base only gave traces of
the desired product, whereas the experiment in absence of catalyst
showed no conversion (Table 1, entries 21 and 22). Thus, after this
exhaustive screening we selected the reaction of 1,2-diol (1 mmol)
and malonate (2 mmol) with Ru-MACHO-BH (0.02 mmol), K2CO3
(0.1 mmol) in t-amyl alcohol at 150 °C for 18 h as the optimal
catalytic system. It is important to note that the reaction proceed
Entry
Catalyst
Base
Solvent
Yield (%)b
1
Ru3CO12
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
NaOH
NaHMDS
K2CO3
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
—
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
toluene
—
9
2
Ru3CO12/CataCXiumPCy
Ru3CO12/Xantphos
[RuCl2(p-cymene)]2
RuHCl(CO)(PPh3)3
Ru-MACHO
3
—
11
15
27
32
29
28
36
26
61
58
53
57
53
42
48
45
60
7
regioselectively to give exclusively the 5-substituted
γ-
4
butyrolactone. Considering the overall reaction sequence, which
involves at least six different intra- and intermolecular reactions
(dehydrogenation, addition, H2O elimination, hydrogenation,
lactonization, decarboxylation), the optimized yield suggests that
each individual step proceeds with very high efficiency.
5
6
7
Ru-MACHO-BH
Ru-MACHO-BH
Ru-MACHO-BH
Ru-MACHO-BH
Ru-MACHO-BH
Ru-MACHO-BH
Ru-MACHO-BH
Ru-MACHO-BH
Ru-MACHO-BH
Ru-MACHO-BH
Ru-MACHO-BH
Ru-MACHO-BH
Ru-MACHO-BH
Ru-MACHO-BH
Ru-MACHO-BH
—
8
9
Once chosen the best reaction conditions, we examined the
scope and limitations of the optimized catalytic system. As shown in
Table 2, different vicinal diols were effectively converted into the
10
11
12c
13d
14c,e
15c
16c
17c,f
18c,g
19c,h
20c,i
21c
22c
Table 2. Ruthenium-catalyzed synthesis of substituted
γ-butyro-
lactones (3a-i) from 1,2-diols (1a-i) and diethyl malonate (2).a
1,4-dioxane
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
Entry
1
Diol
γ-Butyrolactone
Yield (%)b
61
K2CO3
—
(1a)
(1b)
(1c)
(1d)
(1e)
(1f)
(1g)
(3a)
(3b)
(3c)
(3d)
(3e)
(3f)
(3g)
a
Unless otherwise specified, all reactions were carried out with diol (1a, 1 mmol),
malonate (2, 1.1 mmol), catalyst (0.02 mmol) and base (0.1 mmol) in a solvent (1 mL)
b
at 150 °C for 18 h. GC yields of the crude reaction mixture with hexadecane as
c
e
f
i
internal standard. Malonate (2 mmol).d Malonate (3 mmol). Base (0.2 mmol).
2
3
4
5
6
7
52
56 c
54
g
h
Catalyst (0.01 mmol). Catalyst (0.04 mmol). Reaction temperature: 160 °C.
Reaction time: 24 h.
Our research started by selecting an effective ruthenium-based
catalytic system for the model transformation between 1,2-
hexanediol (1a, 1 mmol) and diethyl malonate (2, 1.1 mmol) in basic
medium. Based on our previous experience,8 we studied the reaction
in the presence of Ru3(CO)12 (0.02 mmol) and tBuOK (0.1 mmol) as
base in t-amyl alcohol at 150 °C. Initially, we observed no reaction
under ligand free conditions, while the use of phosphines as ligands,
well-known as metal reactivity moderators, did not improve too
much the efficiency of the catalyst (9% yield observed with
CataCXiumPCy as ligand, Table 1, entries 1-3). Other ruthenium
sources also provided the corresponding lactone 3a in low yields
(11-15%, Table 1, entries 4 and 5), but complexes bearing aliphatic
non-inocent PNP pincer ligands allowed to increase considerably the
yield to 27 and 32% respectively (Table 1, entries 6 and 7).
Recently, our group proved the utility of Ru-MACHO derivatives in
a variety of (de)hydrogenation reactions.11 Hence, we decided to
explore different reaction conditions by using the commercially
available Ru-MACHO-BH complex as catalyst. The study of several
bases showed us that K2CO3 provided the best result (36%, Table 1,
entries 8-11), while, fortunately, the reaction with 2 equivalents of
diethyl malonate led to 5-butyldihydrofuran-2(3H)-one (3a) as the
only regioisomer in a substantially higher 61% yield (Table 1, entry
22
57
52
8
9
(1h)
(1i)
(3h)
(3i)
63
54
a
Unless otherwise specified, all reactions were carried out with diol (1a-i, 1 mmol),
malonate (2, 2 mmol), Ru-MACHO-BH (0.02 mmol) and K2CO3 (0.1 mmol) in t-amyl
b
c
alcohol (1 mL) at 150 °C for 18 h. Isolated yields. Mixture of isomers with
delocalized double bond.
2 | Chem. Commun., 2014, 00, 1-3
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corresponding γ-butyrolactones. We found that 2-substituted-1,2-
diols containing aliphatic saturated chains such as 1a-b provided the
desired products regioselectively in moderate isolated yields (3a-b,
61-52%, Table 2, entries 1 and 2), while the unsaturated substrate
oct-7-ene-1,2-diol (1c) led to the 5-substituted lactone 3c as a
mixture of isomers with the double bond delocalized along the side
chain (56%, Table 2, entry 3). This result is in agreement with the
well-known ability of ruthenium complexes to promote the
isomerization of terminal alkenes to internal ones.12 Additionally, the
reaction also proceeds with the sterically hindered tert-butyl
derivative 1d, resulting 3d in 54% yield (Table 2, entry 4). Aryl
substrates such as 1-phenylethane-1,2-diol (1e) afforded the
corresponding γ-butyrolactone 3e too, but in lower yield (Table 2,
entry 5). However, conversion of the benzylic substrate 1f took place
cleanly affording 3f in 57% yield (Table 2, entry 6). Apart from
terminal 1,2-diols, we analyzed the reactivity of internal
disubstituted diols for which we chose simple cyclic compounds as
model substrates (Table 2, entries 7-9). As an example, the reaction
of diethyl malonate with cyclohexanediol (1h) led to the bicycle 3h
in a good 63% isolated yield, whereas the 5- and 8-membered ring
substrates provided the desired lactones in slightly lower yields (3g
and 3i, 52 and 54% respectively).
Scheme 2. Ruthenium-catalyzed synthesis of hexahydrobenzofuran-
2(3H)-ones (3-6) from 1,2-cyclohexanediol (1h) and different
malonates (4-5). a In parenthesis, yield of the saturated lactone.
suggests that the reduction of the conjugated benzylic olefine is
easier than in the case of the previous aliphatic substituted
malonates.
After this first study and, in order to demonstrate the general
applicability of this methodology, our protocol was tested by
applying different malonates as coupling partner (Scheme 2). For
this purpose, we selected the cyclohexanediol (1h) as model
substrate because it was shown the most effective diol under the
established conditions. First, we were intrigued about the reactivity
of malonates with different alkoxy groups (4a-e), a relevant aspect
since this methodology implies a final lactonization step so the
leaving group could play an important role.13 In this way, we
observed that the reaction of 1h (1 mmol) with dimethyl malonate
(4a, 2 mmol) in presence of Ru-MACHO-BH (0.02 mmol) and
K2CO3 (0.1 mmol), led to hexahydrobenzofuran-2(3H)-one (3h) in
53% yield, only slightly lower with respect to the diethyl analogue
(63%, Scheme 2). Surprisingly, this transformation also proceeds
effectively with bulky substrates such as di-tert-butyl malonate (4b)
In conclusion, we have described for the first time the ruthenium
pincer-catalyzed synthesis of γ-butyrolactones from easily available
reagents such as 1,2-diols and malonates using borrowing-hydrogen
methodology. This regioselective domino-transformation takes place
through carbon-carbon bond formation by using a dehydrogenation-
hydrogenation sequence, followed by intramolecular transesterifi-
cation. The application of Ru-MACHO-BH as catalyst allowed the
synthesis of a variety of lactones in moderate yields with the high
atom economy typical of this hydrogen autotransfer process.
Notes and references
a
Leibniz-Institute für Katalyse e.V. an der Universität Rostock, Albert-
Einstein-Straße 29a, 18059 Rostock (Germany)
obtaining the same γ-butyrolactone in a good 58% yield, which
E-Mail: Matthias.Beller@catalysis.de; Fax: (+49)381-1281-5000.
demonstrates that lactonization is not a crucial step in this reaction
sequence. Likewise, dibenzyl malonate (4c) afforded the desired
product in 51% yield, however reaction with the monoester (ethyl
hydrogen malonate 4d) did not take place, which highlights the
importance of basic medium for both carbon-carbon bond formation
and lactonization steps.
†
Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/c000000x/
†† Acknowledgements: We thank the state of Mecklenburg-Vorpommern
and the Bundesministerium für Bildung und Forschung (BMBF) for financial
support. M.P.L. thanks European Commission for a Marie Curie Intra-
European Fellowship within the 7th Framework Programme (PIEF-GA-2012-
328500).
Following this analysis, we turned our attention to the use of 2-
substituted malonates 5 in this domino-process, which would allow
to access trisubstituted lactones (Scheme 2).14 With this aim, we
assayed the reaction between the cyclic diol 1h and diethyl
methylmalonate (5a). Indeed, after 18 h at 150 °C we observed the
1
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formation of the α,β-unsaturated lactone 6a in 55% yield. This result
shows that this reaction does not undergo the classical mechanism
for the Knoevenagel-type condensations. Here, due to the
substitution of the malonate, the elimination of H2O only can take
place after the decarboxylation, so the double bond should be formed
in the last steps of the transformation. Additionaly, the
hydrogenation of the tetrasubstituted olefine under our reaction
conditions does not proceed, whereby the hydrogen firstly removed
from the diol would be released as H2 in order to regenerate the
catalytic species. To confirm this result, the reaction with 2-butyl
malonate (5b) was also tested, obtaining the corresponding
2
trisubstituted
of isomers. When we carried out the same experiment with diethyl
phenyl malonate (5c) curiously we obtained both the -unsaturated
α,β-unsaturated lactone 6b in 50% yield as a mixture
α,β
and saturated products in 30 and 56% yield, respectively. This result
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