Trisulfonamide calix[6]arene-catalysed Michael addition to nitroalkenes

Article abstract of DOI:10.1039/d0ob01319f

We describe the application of a novel family of trisulfonamide (TSA) calix[6]arenes in general acid catalysis. Hydrogen-bonding interactions between acidic TSA and methanol boosted the reactivity of the Michael addition of indoles to nitroalkene derivati

Full text of DOI:10.1039/d0ob01319f

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Trisulfonamide Calix[6]arene-Catalysed Michael Addition to
Nitroalkenes
Gianpiero Cera,*a Davide Balestri,a Margherita Bazzoni,a Luciano Marchiò,a Andrea Secchi*a and
Received 00th January 20xx,
Accepted 00th January 20xx
Arturo Arduini*a
DOI: 10.1039/x0xx00000x
We describe the application of a novel family of trisulfonamide reaction”, has been so far less exploited.[7] In this context, our
calix[6]arenes TSA in general acid catalysis. Hydrogen-bonding group recently reported on the synthesis of a new generation of
interactions between acidic TSA and methanol boosted the heteroditopic trisulfonamide calix[6]arenes TSA receptors.[8]
reactivity of
a Michael addition of indoles to nitroalkene Interestingly, we found that their binding ability is highly
derivatives. The transformation occurs at a low catalyst loading of influenced by H-bonding interactions performed by the acidic
5 mol % allowing for the synthesis of nitroalkanes in good yields NH sulfonamide moiety with the counterions of dialkylviologen-
and functional group tolerance.
based guests. Prompted by these findings, we reasoned on the
possibility to employ for the first time, the same features that
Calix[n]arenes represent a key structural motif in organic govern threading processes for TSA calix[6]arenes in a general
chemistry, whose properties have been broadly investigated acid catalysis model reaction.[9]
particularly with the aim to synthesize synthetic receptors and At the outset of the investigation, we probed the reactivity of
working devices for the development of nanotechnologies.[1] TSA calixarenes in a well-established Michael addition of
Most of these systems, intensively studied for the preparation electron-rich heteroarenes such as indole 2a to a nitroalkene
of rotaxane and catenane systems, exploited the presence of H- 1a.[10] Hence, using 5 mol % of TSA A in low-polarity solvents,
bond donor moieties such as ureido groups anchored onto their where these are present in a typical cone conformation, low
upper rim.[2] In low-polar media, they are able to efficiently conversions were observed (Table 1, entries 1,2). Differently,
separate the ion pair of mono- and dicationic salts, promoting with an aprotic polar solvent such as DMSO, the reactivity was
the threading of these axles inside the π-rich aromatic cavity.[3] completely shut down (entry 3). Using MeOH (pKa = 15.5), under
Another application, where their ability to establish non- otherwise identical conditions, we observed an increased
covalent interactions has found broad success, is their use as amount of product 3aa (22%, entry 4).[11] However, running the
catalysts. Indeed, calix[4]arene derivatives have been reaction without any TSA calixarene, at the working
successfully employed as a “platform” to synthesize ligands for temperature of 50 °C, comparable low conversion and yield
supramolecular organometallic catalysis or to link established were observed, highlighting the presence of a background
organocatalysts.[4,5] Parallely, the dimensions and the reaction (26 %, entry 5). We next probed the reactivity in the
hydrophobic character of tailored-designed calix[6]arenes has presence of differently substituted TSA, C and D presenting a
been only recently exploited to accelerate organic reactions methoxy and chlorine substituent on the para position of the
under “on water” conditions.[6] The design of these novel sulfonamide moiety, respectively. However, in all these cases
“supramolecular” entities often allowed for the development of the outcome of the reaction followed suit (entries 6-8). On the
more selective and higher performing catalytic systems. other hand, increasing the H-donor ability of the calixarene with
Despite this considerable progress, the use of calix[6]arenes as a nitro group as for E, a not trivial increment in the reactivity
catalyst that means a “substance that increases the rate of a was recorded (42%, entry 9). Since many examples of thio- and
urea catalysed Michael addition are reported to date,[12] we
probed the reactivity also in the presence of catalytic amounts
of TPU calixarene F. However, we did not notice any significant
improvement (entry 10). To test the effect of the
supramolecular wheel E, we synthesised its monomeric
analogue G and used in the Michael addition with a loading of
15 mol %. Hence, a lower efficacy in the reaction was observed
a. Università di Parma, Dipartimento di Scienze Chimiche, della Vita e della
Sostenibilità Ambientale, Parco Area delle Scienze 17/A, 43124 Parma, Italy. E-
mail: gianpiero.cera@unipr.it., andrea.secchi@unipr.it., arturo.arduini@unipr.it.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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(27%, entry 11). Finally, by just increasing the amount of reaction outcome was not influenced by the VpierweAsreticnlecOenlionef
DOI: 10.1039/D0OB01319F
nucleophile (3 equiv) and the concentration up to 0.4 M, halogen functional groups as for 3ca and 3da. Differently
product 3aa could be obtained in good yields (entry 12). For the substituted indole derivatives could be applied as well. Hence,
sake of comparison, a similar reaction performed in MeOH, in indoles with EDGs such as ethers and hydroxyl (3eb-ec)
the absence of any calixarene, led to the formation of 3aa in performed in high yields (88-91%), while the inherently less
lower yields (58%, entry 13). Noteworthy, employing t-BuOH reactive fluorine, bromine-substituted ones 2d-e delivered 3ed-
(pKa = 18.5) as the solvent, the reaction in the presence of E 3ae in synthetically useful yields (53-57%). Finally, the
performed worse than the background one, highlighting how methodology could be applied also in the presence of N-
the acidity of the medium plays an important role under present protected indole derivative 3ef (86%) (Table 2).
conditions (entries 14,15).[13] Finally, H2O was found as a
suitable solvent for the TSA-catalysed Michael addition, yielding Table 2. Scope for the catalytic Michael addition
3aa with comparable efficacy (82%, entry 16).
R
R1
NO2
E
(5 mol %)
MeOH
Table 1. Optimisation of Reaction Conditions
R1
R
+
NO2
MeO
N
H
50 °C, 24 h
N
H
1
2
3
NO2
calix
(5 mol %)
+
NO2
Cl
Br
Me
MeO
N
solvent, 50 °C
MeO
H
N
1a
2a
3aa
H
NO2
NO2
NO2
Entry[a]
calix
A
A
A
A
--
B
C
D
E
F
G
E
--
--
E
Solvent
toluene
CHCl3
Conv (%)
8
Yields (%)
N
N
N
1
2
3
4
5
6
7
8
9
10
11[b]
12[c]
13[c]
14[c]
15[c]
16[c]
6
7
--
H
H
H
9
--
3da
: 83%
3ba
3ca
: 79%
: 77%
DMSO
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
t-BuOH
t-BuOH
H2O
23
28
26
28
32
43
36
33
86
68
36
25
85
22
26
25
22
30
42
33
27
83 (81)
58
31
20
82
NO2
NO2
NO2
MeO
HO
N
H
N
H
N
H
3eb
: 88%
3ea
: 80%
(in H2O, 84%)
3ec
: 91%
(in H2O, 88%)
MeO
NO2
NO2
F
NO2
Br
N
H
N
N
H
Me
E
3ef
: 86%
3ed
: 53%
3ae
: 57%
a) Reaction conditions: 1a (0.1 mmol), 2a (0.15 mmol), calix (5 mol
%), solvent (0.5 ml, 0.2 M), 50 °C, 24 hs. Yields calculated by 1H-NMR
integration using 1,3,5-trimethoxybenzene as internal standard. b) G
(15 mol%) was employed. c) 2a (0.3 mmol), solvent (0.25 ml, 0.4 M).
In brackets, isolated yields.
In order to get more insights on the features of calixarene E in
the reaction and to compare its effect with respect to the
background reaction, we analysed the kinetic profile of parallel
reactions conducted in the presence of catalytic amounts of E
and in sole methanol (Figure 1).
O2N
R
R
R
HN
NH
HN
O
S
NH
O
OMe
O
20
15
10
5
MeO
OMe
O
R1
R1
O
R1
OMe
R1 = n-Oct
R1 = (CH2)2OEt (B)
2 R1 = n-Oct
R1 = n-Oct
R1 = n-Oct
R1 = n-Oct
(
)
(
R = 4-MeC6H4SO2
R = 4-MeC6H4SO2
R = 4-MeOC6H4SO
R = 4-ClC6H4SO2
R = 4-NO2C6H4SO2
R = C6H5NH(CO)
A
G
)
C
D
(
(
(
(
)
)
)
)
E
F
Time (min)
Attracted by the features of the protocol and to probe a general
applicability of the methodology, we synthesised a small library
of nitroalkane derivatives of type 3. Particularly, substituted and
unsubstituted nitrolefines 1a-e were converted smoothly in to
the corresponding products 3aa-3ea (83-77%). Noteworthy, the
0
0
50
100
150
200
Figure 1. Kinetic profile of parallel reactions (MeOH, 0.02 M) in the
presence of E (red line) and without E (blue line)
2 | J. Name., 2012, 00, 1-3
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For both cases, we obtained a linear slope that has not reached nitroalkane 3aa (Figure 2, b). This supported once more that the
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its plateau yet, observing a moderate ky /ky2 ≈ 1.5. This finding interplay between TSA E and MeOH resulDteOdI:c1r0u.1c0i3a9l/tDo0eOnBh01a3n1c9eF
1
proved the catalytic role of E in boosting the reactivity of the the catalytic process. Finally, to probe a possible hydrophobic
Michael addition not only in terms of chemical yields (see effect, a 1:1.5 mixture of TSA E and nitroalkene 1a was
1
entries 12, 13 in Table 1). Subsequently, to investigate on the evaluated by H-NMR. No shift of the proton resonances of 1a
role of methanol in the catalytic process, we performed a was observed, excluding at this stage any role of the electron-
control experiment by running a model reaction in a sealed rich calixarene cavity (see Fig. S2-S3 in SI).[14] Since the results
glass tube, using toluene as the solvent (see figure S1 in SI). acquired are suggestive of a necessary cooperation between
After stirring the mixture for 1 h, a small aliquot of the crude TSA calixarenes and MeOH to improve the performances of the
was analysed by NMR. Reasonably, no product formation was catalytic reaction, we first investigated on the possible
observed. We thus added 1 equiv of MeOH keeping the mixture conformations assumed by our model TSA A in this protic
at 50 °C for an additional hour. After that, the crude mixture was solvent. We previously observed that TSA calixarenes adopt, in
analysed, highlighting trace formation of 3aa (Figure 2, a).
apolar solvents such as CDCl3 and d6-benzene, a pseudo-cone
conformation. This geometry was confirmed by the upfield shift
of the 1H NMR resonances of methoxy signals and the AX system
of two doublets at δ = 4.4 and 3.4 (2J = 15.5 Hz), related to the
bridging pseudo-axial and pseudo-equatorial methylene groups
of the macrocycle (Figure 3, a). Furthermore, the absence of NH
signals of the sulfonamide moieties, lead us to hypothesise the
presence of an homodromic H-bonding domain existing on the
upper rim of the cavity.[15] This was supported by a preliminary
geometry optimisation of TSA A, carried out at the PM6-DH+
level12 using the Mopac 2016 program. Indeed, intrannular NH-
--O bonds (2.765-2.817 Å) resulted crucial to keep TSA
calixarenes in their typical cone conformation, in apolar
solvents (see Fig. S4 in SI). Interestingly, the situation
completely changed when we analysed the conformation of TSA
A in d4-methanol.
MeO
MeO
a)
NO2
NO2
MeOH
(1 equiv)
E (5 mol %)
toluene, 50 °C
1 h
toluene, 50 °C
1 h
NO2
N
N
H
H
MeO
+
X
MeO
N
H
NO2
MeOH
(1 equiv)
toluene, 50 °C
2 h
b)
N
H
X
Figure 2. Comparing the effect of MeOH
To compare this result, we performed a second reaction, in the
presence of 1 equiv of MeOH, without E. After 2hs, analysis of
the crude by NMR did not reveal any significant presence of
a)
Me
Me
Me
O
S
O
O
S
S
O
O
HN
NH
HN
O
£'
A'
A
3OMe
OMe
MeO
O
O
OMe
O
$
£
6
6
6
b)
£'
III
c/c'
II
c/c'
b/b'
I
$
IV
c
b
b/b'
a/a'
a
a'
VI
a/a'
c'b'
£
V
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
f1 (ppm)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Figure 3. 1H NMR spectra (400 MHz, 298 K) of a) calixarene A in CDCl3, b) calixarene A in MeOH-d4. At the bottom right are schematic
representations of A. The colors of the ovals and rectangles indicate the relative position of the phenolic substituent with respect to
the plane defined by the bridging methylene groups (hexagon) (black, upward; white outward). The rectangle identifies the phenolic
ring substituted with the octyloxy chains, while the circle those with the methoxy groups.
This journal is © The Royal Society of Chemistry 20xx
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Its 1H-NMR clearly showed a “flipping” of one phenolic ring with molecules (Table 3). The donor-acceptor distances NH---Owater
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DOI: 10.1039/D0OB01319F
methoxy substituent (£) that was pushed outside from the π- ranging from 2.807(8) to 2.943(5) Å could be categorise as
rich aromatic cavity. This was highlighted by a “down-field” shift “moderate, most electrostatic” according to Jeffrey’s
of its proton signal (£, 0.8 ppm) along with an “up-field” shift of classification.[18] The conformational rearrangement observed
its parental t-Bu (£’, 0.3 ppm). The different conformation was in solution, along with the information acquired by solid state
further established by a new pattern for the methylene bridging analysis, supported the ability a TSA to perform H-bonding with
protons. The two doublets (ax+eq, A+A′) present in the protic solvents. Based on these studies, we hypothesised H-
pseudocone conformation, split in six doublets, with geminal bonding interactions between TSA and methanol as responsible
coupling, three for the axial and three for the equatorial, in a for the observed boosted reactivity of the Michael addition to
1:1 ratio (Figure 3, b). The appearance of two signals (2:1 ratio) nitrolefines. It is a matter of fact these secondary interactions
for the acidic NH bonds of A were suggestive of the break of the produce several modifications of the physical and chemical
homodromic domain and their engagement in intermolecular properties of organic molecules, including acidity. Although
interactions with the protic solvent. This “distorted” cone subtle, we could parallel this effect to the one investigated for
conformation was finally confirmed through a complete NMR polyols, where the stabilisation of the charged oxygen centre by
analysis (see Fig. S5-S10 in SI for more details).
H-bonding network, induced an acidity enhancement that was
The ability of TSA calixarenes to perform intermolecular H- exploited, as we herein reported, for Brönsted acid-catalysed
bonding interactions was further investigated in the solid state. Michael addition reactions.[19] Thus, a plausible catalytic cycle
Indeed, we succeeded in obtaining a single crystal of TSA B, could be depicted.[20] The H-bonding interactions between
which present three ethylethyloxy alkyl chains, through slow catalytic amounts of TSA E and MeOH induce an acidity
evaporation of a methanol solution (Figure 4).
amplification of the former, enough to work as a Brönsted acid
catalyst. The activation of nitroolefin 1a, enables a subsequent
nucleophilic attack by the indole. Finally, intermediate 3aa’
III
V
undergoes
a hydrogen transfer process to deliver the
II
corresponding Michael product 3aa (Figure 5).
I
IV
Ar
VI
O
H
O
S
N
5
+
y
[MeOH]x
1a 2a
+
Me
calix
O
H
z
H
O
X
Ar
H
X
N
O
Ar
O
N
O
N
N
H
H
3aa
Figure 4. X-ray molecular structure of TSA B. The colours of the
ovals and rectangles indicate the relative position of the phenolic
substituent with respect to the plane (black, upward; white,
outward, blue, downward).
H
X
Ar
H
O
H
X
N
O
HN
Table 3: a) NH tosylamide as hydrogen bond donor, b) O tosylamide
as hydrogen bond acceptor, c) O ethoxy group as acceptor.
3aa'
Figure 5. Proposed mechanism for Michael addition
Interaction
Distance D---A
(Å)
Distance DH---A
(Å)
Angle D-H---A
(°)
Conclusions
a
N1S1---O1W1
N1S2---O1W2
N1S3---O1W3
O1S1---O1W2
O1S2---O1W3
2.843(6)
2.807(8)
2.943(5)
2.874(8)
3.057(7)
2.828(5)
2.877(6)
1.96(5)
1.98(7)
164.(6)
153.(5)
a
We offered a proof of principle concerning the application of
TSA calixarenes in general acid catalysis. Mechanistic hints
supported a supramolecular cooperation between TSA and
methanol, established by H-bonding interactions, able to
improve the catalytic performances of a Michael addition to
nitroalkenes. At the same time, we demonstrated how the
solvent plays an important role in dictating the application of
this class of calix[6]arenes. While in apolar solvents, the H-
bonding domain has been exploited for the synthesis of
supramolecular receptors, here the use of a protic solvent
a
b
b
c
2.083(4)
2.064(6)
2.168(5)
1.990(3)
1.984(5)
165.6(3)
146.9(3)
165.0(3)
162.0(4)
170.7(4)
O2E1---O1W1
O2E2---O1W2
c
The molecular structure of B, determined via microfocus X-ray
diffraction, assumes a geometry compatible with a “distorted”
1,2,3-alternate conformation (see Fig. S11-18 in SI).[16,17]
Interestingly, each NH moiety of the tosylamide groups is
engaged in H-bonding interactions with crystallisation water
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6669; c) F. J. Parlevliet, M. A. Zuideveld, C. Kiener, H.
opened access to the unprecedented application of these
systems in catalysis.
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Conflicts of interest
“There are no conflicts to declare”.
8
9
G. Cera, M. Bazzoni, A. Arduini and A. Secchi, Org. Lett., 2020,
22, 3702–3705.
For selected examples of sulfonamide catalysts, see: a) H.
Tanaka, K. Sakai, A. Kawamura, K. Oisaki and M. Kanai, Chem.
Commun., 2018, 54, 3215; b) S. Takata, Y. Endo, M. Shaid Ullah
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Acknowledgements
The authors thank Centro Interdipartimentale di Misure of the
University of Parma for NMR measurements. Chiesi
Farmaceutici SpA is acknowledged for the support for the D8
Venture. This work was supported by the Italian MIUR (PRIN
20173L7W8K). This work has been carried out within the COMP-
HUB Initiative, funded by the “Departments of Excellence”
program of the Italian Ministry for Education, University and
Research (MIUR, 2018-2022).
10 For a recent review, see: D. A. Alonso, A. Baeza, R. Chinchilla,
C. Gómez, G. Guillena, I. M. Pastor and D. J. Ramón, Molecules,
2017, 22, 895.
11 For catalytic Michael additions performed in methanol, see:
a) N. Martín, M. Dusselier, D. E. De Vos and F. G. Cirujano, ACS
Catal. 2019, 9, 44; b) I. A. Bhat, A. Devaraj, P. Howlader, K.-W.
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12 M. N. Grayson and K. N. Houk, J. Am. Chem. Soc., 2016, 138,
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13 For selected examples of HFIP-mediated FC reactions, see: a)
R.-J. Tang, T. Milcent and B. Crousse, RSC Adv., 2018, 8, 10314;
b) R. H. Vekariya, J. Aube, Org. Lett., 2016, 18, 3534; c) N.
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14 A model reaction performed in MeOH in the presence of a
competitive binder (dioctylviologen ditosylate, DOV•2OTs),
yielded 3aa with comparable efficacy (74%) with respect to
standard conditions, excluding the ability of TSA E to catalyse
Michael addition inside its cavity, see S3 in SI for further
details.
Notes and references
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the matter
under discussion, limited experimental and spectral data, and
crystallographic data.
1
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