Versatile O- and S-functionalized 1,2,3-triazoliums: Ionic liquids for the Baylis-Hillman reaction and ligand precursors for stable MIC-transition metal complexes

Article abstract of DOI:10.1039/c4nj02076f

The efficient synthesis of O- and S-functionalized 1,2,3-triazoliums is reported. Owing to their physical properties, these cations are efficient ionic liquids for Baylis-Hillman addition under mild reaction conditions. Simultaneously, the functionalizati

Full text of DOI:10.1039/c4nj02076f

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Versatile O- and S-functionalized 1,2,3-
triazoliums: ionic liquids for the Baylis–Hillman
reaction and ligand precursors for stable
MIC-transition metal complexes†
Cite this:DOI: 10.1039/c4nj02076f
Received (in Porto Alegre, Brazil)
18th November 2014,
Accepted 15th January 2015
Daniel Mendoza-Espinosa,*a Rodrigo Gonzalez-Olvera,a Cecilia Osornio,a
´
Guillermo E. Negron-Silva*a and Rosa Santillanb
´
DOI: 10.1039/c4nj02076f
www.rsc.org/njc
The efficient synthesis of O- and S-functionalized 1,2,3-triazoliums herein the synthesis of O- and S-functionalized 1,2,3-triazoliums
is reported. Owing to their physical properties, these cations are which are efficient ionic liquids for the Baylis–Hillman reaction
efficient ionic liquids for Baylis–Hillman addition under mild reaction and additionally tolerate the selective metallation at the C-5
conditions. Simultaneously, the functionalization of the triazolium position providing highly stable triazol-5-ylidene rhodium(I),
rings allows for the in situ C-5 metallation providing air stable gold(I) and palladium(II) complexes. To our knowledge, this work
triazol-5-ylidene Rh(I) Au(I), and Pd(II) complexes. The present work represents a rare demonstration of single triazolium salts success-
constitutes a rare example of versatile triazolium salts capable of fully applied in two unrelated synthetic procedures.
serving in two unrelated synthetic procedures.
The synthesis of triazoliums I and II comprises two main
steps namely the CuAAC of the benzyloxy- and thiophenoxy-
1,2,3-Triazolium salts are an important class of frameworks alkynes with sodium azide (in the presence of 4-chlorobenzyl
in organic chemistry. The convenient access to 1,2,3-triazole chloride) to produce the respective 1,2,3-triazoles,7 followed by
precursors via the Cu(I)-catalyzed azide–alkyne process (CuAAC) N-alkylation with methyl iodide (Scheme 1). After column
and their relatively easy N-alkylation has resulted in the modular chromatography purification, the triazolium salts I and II were
synthesis of a wide library of functionalized triazolium cations.1 obtained as air and moisture stable yellow liquids in high yields
Although the most recognized uses of these derivatives include (90–94%). Interestingly, I and II slowly (one week) solidified as
organocatalysis,2 molecular machines and supramolecular waxy materials and melted at 35 and 53 1C, respectively. After
chemistry,3 their application as ionic liquids (ILs),1,4 and as they melted, they remained as liquids for several days.8 The
ligand precursors for mesoionic carbene (MIC) complexes has new cations are highly soluble in most halogenated solvents,
been recently discovered.5 According to a specific process, the acetonitrile and THF, and they are immiscible in aliphatic
triazolium moiety can be designed to feature key characteristics derivatives and water.
such as flexibility, hydrophilicity, aromaticity, acidity, etc. For
The application of ILs as ‘‘green’’ solvents has increased
instance, asymmetric organocatalysts based on triazoliums interest in both academia and industry. Their unique proper-
require a chiral moiety,1b,c,6 whilst MIC precursors in most ties such as low melting points, viscosity and solubility, open a
cases contain rigid structures with voluminous substituents great deal of opportunities in many synthetic procedures with
next to the carbene center.5 Despite all the advantages in the great potential of substituting the commonly used volatile
design of triazolium cations, once their functionalization has organic solvents.9 During the last decade, most of the research
been directed to a specific process, their possible use in another
non-related practice is deterred. Interested in the design and
preparation of multifunctional triazole based molecules, we report
a
´
´
Departamento de Ciencias Basicas, Universidad Autonoma Metropolitana-Azcapotzalco,
´
Av. San Pablo No. 180, Mexico D.F., 02200, Mexico. E-mail: gns@correo.azc.uam.mx,
danielme1982@gmail.com
b
´
´
Departamento de Quımica, Centro de Investigacion y de Estudios Avanzados del
´
´
Instituto Politecnico Nacional, Apartado Postal 14-740, 07000 Mexico D.F., Mexico
† Electronic supplementary information (ESI) available: Details of experimental
procedures, spectroscopic characterization, and 1H and 13C NMR spectra of new
compounds. See DOI: 10.1039/c4nj02076f
Scheme 1 Synthesis of triazolium salts I and II.
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Table 2 Baylis–Hillman reaction of various aldehydes with methyl acrylate in
the presence of IL (I)
Fig. 1 Ionic liquids based on 1,2,3-triazole scaffolds.
Entry
1
RCHO
Product
Yielda (%)
85
Time (h)
20
1a
in ionic liquids has been conquered by imidazolium salts with
successful application in several fields.10 However, in basic
media, the acidic C-2 proton of the imidazolium cation can
be removed and the so obtained carbene derivative inhibits
transformations such as Claisen–Schmidth and Knoevenagel
condensations, and the Baylis–Hillman reaction.11
To date, very few examples of the application of 1,2,3-
triazolium salts as ionic liquids have been known. For instance,
Ryu and coworkers reported that C-4 and C-5 unsubstituted
ionic liquid (A) were efficient solvent media for the Baylis–
Hillman reaction showing improved yields when compared to
ILs based on imidazolium systems.12 This positive effect on the
conversion to Baylis–Hillman products has been attributed to
the stabilization of zwitterionic intermediates generated from
the Michael addition of a nucleophilic Lewis base to an activated
alkene.13 As the intermediate can be maintained in high concen-
tration in the IL, the attack on the aldehyde can be accelerated.14
In addition to (A), bicyclic (B)15 and spiro triazolium cations (C)16
showed their potential as inert media for natural product synthesis
and asymmetric catalysis, respectively (Fig. 1).
2
3
1b
1c
98
85
20
0.3
4
5
1d
1e
81
77
12
16
6
7
1f
83
95
60
1g
1.5
Driven by the physical nature of I and II and their structural
similitude to ionic liquid A, we decided to test the efficiency of
our new triazolium salts in the addition of benzaldehyde to
methyl acrylate (MA) in the presence of DABCO. To ensure
the liquid state of the triazolium salts in the Baylis–Hillman
reaction, we exclusively used freshly prepared (or just melted) I
and II in the reaction tests. The initial results displayed in
Table 1 indicate that the yields of the addition product are
similar to those obtained under the same reaction conditions
employed by Ryu12 and coworkers (entries 1–3). Encouraged by
Reaction conditions: aldehyde (1.0 mmol), MA (1.2 mmol), DABCO
(1.2 mmol), I (100 mg), room temperature. Reaction times assigned as
a
maximum conversion to the product was reached. Isolated yields.
these successful results, we further demonstrate that only 1.2
equivalents of MA and DABCO delivered the Baylis–Hillman
addition products in high yields without the need of time
increase (entries 4–7). As a final screening test, we carried out
the addition reaction at 40 and 60 1C (above melting point
temperatures for I and II, respectively) observing that no
significant improvement in the isolated yields is achieved
(entries 8–12). Furthermore, it is noticeable that triazolium salt
I which features a more flexible C4-moiety (ArCH2O–) than II
(ArS–) provided the best yields of the series.
Table 1 Screening trials of ILs in the Baylis–Hillman reaction
Established by the screening data, we extended the reaction
scope by treating a series of p-substituted benzaldehydes and
aliphatic aldehydes in the presence of IL (I). As observed in
Table 2, most of the addition reactions proceeded smoothly
under mild reaction conditions. Not surprisingly, those sub-
strates containing electron withdrawing groups (p-NO2 and
p-CN) underwent faster reaction rates providing high yields
(95–98%), whilst less activated substrates such as isobutyraldehyde
or 3,4,5-trimethoxybenzaldehyde registered the lower performances
of the series (yields of 77–83%).
Entry
IL
MA (eq.)
DABCO (eq.)
Yielda (%)
Temp. (1C)
1
2
3
4
5
6
7
8
A
I
II
I
II
I
II
A
I
2.0
2.0
2.0
1.5
1.5
1.2
1.2
2.0
1.2
1.2
1.2
1.2
2.0
2.0
2.0
1.5
1.5
1.2
1.2
2.0
1.2
1.2
1.2
1.2
81
87
83
85
80
85
79
83
87
80
89
83
25
25
25
25
25
25
25
40
40
40
60
60
9
10
11
12
II
I
II
To get more insight in the potential of IL (I) in the Baylis–
Hillman reaction, we explored next effects such as sterics and
electronics in the Michael acceptor. Thus, we carried out the
addition of aldehydes to the bulkier tert-butyl acrylate (tBuA)
Reaction conditions: benzaldehyde (1.0 mmol), IL (100 mg), 24 h.
a
Isolated yields.
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Table 3 Baylis–Hillman reaction of various aldehydes with tert-butyl
acrylate (tBuA) in the presence of IL (I)
could be repeated for at least three more times with no
significant loss in the yield (96–98%).
Overall, the results displayed by IL (I) in the Baylis–Hillman
reaction are comparable to those observed with the previously
reported ionic liquid A. However, it is important to remark that in
contrast to IL (I) (which contains an iodine counter ion), IL A
required previous exchange from iodine to a bigger counter ion
(NTf2 or PF6) to execute the addition under optimal conditions.
Additionally, the fact that only 1.2 equivalents of DABCO are
enough for optimal reaction conditions makes the recyclability of
(I) an easy task as the contamination with base excess is avoided.
Interested in the further application of the new ILs, we noticed
that I and II possessed a single acidic proton at position C-5. The
latter feature, which is observed in most MIC precursors together
with the presence of aromatic rings at N-1 and the heteroatom
availability at C-4, provided a suitable environment for the
selective deprotonation and the subsequent generation of 1,2,3-
triazol-5-ylidene ligands. To test the suitability of I and II as
precursors for 1,2,3-triazol-5-ylidenes, the treatment of the cationic
salts with equimolar amounts of KHMDS was carried out in THF at
À78 1C. After work up, the expected mesoionic carbenes were not
isolated due to their fast decomposition in solution.
Entry
RCHO
Product
Yielda (%)
Time (h)
1
2
3
4
5
6
7
a
b
c
d
e
f
2a
2b
2c
2d
2e
2f
83
93
86
86
72
80
84
24
24
2.5
18
20
72
3.5
g
2g
t
Reaction conditions: aldehyde (1.0 mmol), BuA (2.0 mmol), DABCO
(2.0 mmol), I (100 mg), room temperature. Reaction times assigned as
maximum conversion to the product was reached. Isolated yields.
a
Table 4 Baylis–Hillman reaction of various aldehydes with acrylonitrile
(AN) in the presence of IL (I)
Entry
RCHO
Product
Yielda (%)
Time (h)
As no free ligand could be obtained, we proceeded then to an
in situ metallation process. The one pot reaction of precursors I
and II with KHMDS (or KOtBu) and half 0.5 equivalents of
[Pd(allyl)Cl]2 or [Rh(COD)Cl]2 in THF provided readily the transi-
tion metal complexes 4a–b and 5a–b in high yields (93–95%,
Scheme 2). The presence of the MIC complexes was corroborated
in 1H NMR spectroscopy by the disappearance of the acidic
proton observed in the triazolium precursors at 9.00 and
9.02 ppm for I and II, respectively. Additionally, when equimolar
amounts of KHMDS and AuCl(SMe2) were reacted with
1
2
3
4
5
6
7
a
b
c
d
e
f
3a
3b
3c
3d
3e
3f
96
99
94
92
86
81
98
12
2.5
0.2
6
10
4
g
3g
0.1
Reaction conditions: aldehyde (1.0 mmol), AN (1.2 mmol), DABCO
(1.2 mmol), I (100 mg), room temperature. Reaction times assigned
as maximum conversion to the product was reached. Isolated yields.
a
and then to acrylonitrile (AN). The results are presented in precursors I and II, the respective gold(I) complexes 6 and 7
Tables 3 and 4, respectively. were obtained in 89 and 92% yields (Scheme 2) as white solids.
Compared to methyl acrylate, the formation of the respective The presence of sharp peaks for all the methylene groups
adducts with the tert-butyl analogue requires two equivalents of (equivalent protons) in 6 and 7 suggests the lack of coordina-
DABCO, two equivalents of the Michael acceptor, and longer tion through the thio- or ether moieties, and confirms the usual
reaction times in most cases. Although the yields under these linear geometry around the gold center.17 The 13C NMR chemical
optimized conditions are not low, the hindrance effect of the shifts of the carbene carbon of PdII (165.2 and 166.1 ppm), AuI
bulky tert-butyl group is easily noticeable. In contrast, reaction (169.9 and 170.2 ppm), and RhI (doublets at 171.5 and 174.0 ppm)
with the smaller and highly activated acrylonitrile delivers
products in high yields (81–99%) and shorter reaction times,
regardless of the substrate (Table 4).
Acknowledging their chemical stability, negligible vapor
pressure, and high polarity, ionic liquids are prone to be reused
after an appropriate recovery process. Consequently, we tested
the recyclability of IL (I) in the addition of p-cyanobenzaldehyde
to acrylonitrile. After the first run under the optimal reaction
conditions and product isolation (Table 4, entry 7), the ionic
liquid was easily recovered by column chromatography using
a DCM/ethanol (97 : 3) eluent mixture. The recovered IL (I)
displayed high purity by 1H and 13C NMR spectroscopy and
conserved its original appearance. A second batch of p-cyano-
benzaldehyde with acrylonitrile in the presence of DABCO was
added to the recovered IL (I) and after work up and isolation, no
Scheme 2 Synthesis of metal complexes 4a–b, 5a–b, 6 and 7.
change in the yield or reaction time was observed. The process
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melting points lower than 100 1C, which makes them interesting
precursors for processes such as chemical vapor depositions or
metallic based ionic liquids. Research on further applications and
expansion of the class of bifunctional triazolium cations are
underway in our laboratory.
Scheme 3 Synthesis of dicarbonyl rhodium(I) complexes 8 and 9.
Table 5 a-Arylation of propiophenone with aryl bromides
Acknowledgements
´
We are grateful to Consejo Nacional de Ciencia y Tecnologıa,
CONACyT (project 181448). DME, GNS, RGO, and RS wish to
acknowledge the SNI for the distinction and the stipend
received.
Entry
Ar
Cat.
Yielda (%)
Notes and references
1
2
3
4
5
6
7
8
9
Ph
Ph
4a
5a
4a
5a
4a
5a
4a
5a
4a
5a
87
85
82
83
90
88
85
86
81
83
(4-Me)C6H4
(4-Me)C6H4
(4-CN)C6CH4
(4-CN)C6CH4
(4-MeO)C6H4
(4-MeO)C6H4
(4-NO2)C6H4
(4-NO2)C6H4
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a
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complexes are in the range observed for previously reported metal
complexes containing MIC ligands,17 and as expected, in much
higher fields compared to classical NHC analogues.18
To evaluate the donor properties of the new triazol-5-ylidene
ligands, the corresponding rhodium(I) dicarbonyl chloride
complexes 8 and 9 were synthesized by the treatment of
complexes 4b and 5b with an excess of carbon monoxide in
dichloromethane (Scheme 3). Complexes 8 and 9 display in 13
C
NMR spectroscopy two new pairs of two doublets in the range
of 180.6–186.6 ppm ( J = 53 and 79 Hz average), indicating the
complete cyclooctadiene removal and the cis arrangement of
the CO ligands around the tetracoordinated metal center.
The chemical shift of the MIC-rhodium peaks in the carbony-
lated complexes moved upfield (B160 ppm) in comparison to the
cyclooctadiene precursors (B173 ppm), due to the more electron
poor nature of the carbonylated metal center. The average CO
vibration frequency for 8 and 9 (vav = 2027 cmÀ1) indicates that the
electron donor capacity of the new triazol-5-ylidenes is superior to
those of conventional NHCs (vav = 2039–2041 cmÀ1 19
and cyclic
)
(alkyl)(amino) carbenes (vav = 2036 cmÀ1),20 but inferior to those
for previously reported MIC ligands (vav = 2016–2025 cmÀ1).21
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complexes, we used the palladium derivatives 4a and 5a in the
alpha arylation of propiophenone. As observed in Table 5, both
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