Calix[8]arene nanoreactor for Cu(i)-catalysed C-S coupling

Article abstract of DOI:10.1039/c5cc09232a

The catalytic activity of Cu(i) confined within a phenanthroline-containing calix[8]arene derivative was tested in C-S cross-coupling reactions. The substrate selectivity, solvent dependence, and catalyst loading differ significantly from those reported for bimetallic molecular systems. The putative monomeric complex represents the first calix[8]arene functionalised for endo-oriented metal chelation used as a nanoreactor.

Full text of DOI:10.1039/c5cc09232a

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Received 00th January
2
0xx,
Calix[8]arene Nanoreactor for Cu(I)-Catalysed C–S Coupling†
Edmundo Guzmán-Percástegui, David J. Hernández, and Ivan Castillo*
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The catalytic activity of Cu(I) confined within a phenanthroline- = 4,6) have been also used as platforms for catalysis, most
containing calix[8]arene derivative was tested in C–S cross- available systems result from exo-oriented metal-coordination
coupling reactions. The substrate selectivity, solvent dependence, with reactivity outside the cavity, and offer limited inner space
7
,8
and catalyst loading differ significantly from those reported for for
substrate-binding.
To
date,
only
a
few
bimetallic molecular systems. The putative monomeric complex metallocalix[6]arenes exhibit adequate space for internal
represents the first calix[8]arene functionalised for endo-oriented binding of small molecules for ligand-exchange and dioxygen-
9
metal chelation used as a nanoreactor.
activation relevant to Cu-monooxygenases. Conversely, the
integration of the host and size attributes of the larger
calix[8]arene should result in an effective tool in catalysis;
The small compartments in metalloenzymatic active sites are
the prototypes of well-defined confined environments around
metal ions and substrates that are chemically distinct from the
environment in bulk solvent. In synthetic systems designed for
however,
little
work
has
been
reported
on
metallocalix[8]arene catalysts, and the sole reports are
restricted to exo-coordinated systems via the phenolic
7
,10
metal confinement within
a supramolecular container,
units.
In contrast, the use of calix[8]arenes bearing endo-
dramatic alterations of catalytic performance relative to
1
analogous molecular catalyst have been observed. As a result,
coordinative residues to enforce metal binding and catalytic
activity to occur internally remains elusive, probably due to the
difficulty to introduce functional groups in a regioselective
manner.
supramolecular catalysts with enhanced activity can be
obtained, which may stabilise reactive intermediates and
enable effective collisions by increasing the local concentration
2
a) Molecular N,N'-Cu catalysts
of reactants. These reaction pockets may also give rise to
R
H
H
Ar
Ph
p-Tol
B
B
B
1
2
3
enhanced or unusual selectivity by imposing steric restrictions,
exclusion affinity for substrates, and promoting alternative
reaction pathways. Additionally, the opening process on the
container appears to be essential to control the outcome of
Type A
Type B
R
Ar
R
Cu
R
CH3 p-Tol
S
N
N
I
I
N
N
N
N
N
N
Cu
Cu
Cu
R
S
Ar
chemical transformations by modulation of the encapsulation b) Supramolecular N,N'-Cu catalysts
3
of precursors and the subsequent dissociation of products. In
the context of non-covalent interactions that control these
processes, the rational design of supramolecular catalysts
remains a challenging task. Nonetheless, greater insights on
such factors will represent a significant advancement towards
the development of superior catalysts with tuneable features.
LCuCl-THF (1)
LCuSPh (2)
N
N
O
N
N
Cu
O
O
O
Cu
SPh
O
Cl
OH
OH OH
OH
HO
OH
HO
HO
OH OHHO
OH
4
Customarily, self-assembled nanocages and cavitands, like
5
6
cyclodextrins and cucurbiturils, operate as nanoreactors in
analogy to binding sites in enzymes. Although calix[n]arenes (n
Chart 1 Molecular Cu(I) catalysts A and B, and supramolecular analogues 1 and 2
Instituto de Química, Universidad Nacional Autónoma de México, Circuito
Exterior, Ciudad Universitaria, México D.F. 04510, México.
Given the importance of aryl sulfides and related fine
chemicals in the pharmaceutical and materials industries,
several Cu(I)-based protocols have been developed for C–S
†
Electronic Supplementary Informaꢀon (ESI) available: Detailed information of the
synthetic procedures as well as characterisation of supramolecular catalysts and
, and coupled products is provided. See DOI: 10.1039/x0xx00000x
1
2
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1
coupling reactions. Significant contributions to this area have thiophenolates
1
are
of
competent
haloarenes
catalysts
through
in
the
1
been reported by the groups of Venkataraman and Hartwig
2
13
thioetherification
1
Cu(III)
3
regarding the thioetherification of haloarenes (Chart 1). Similar intermediates. Such mechanism also agrees with calculations
to these systems, our approach entails an endo-oriented predicting that neutral LCu(I)–SPh are the active catalysts in
1
phenanthroline motif appended to a calix[8]arene in the 1,5- Ullmann S–arylations, especially in non-polar toluene.
5
phenolic positions as ligand
L
for Cu(I)-catalysed C–S coupling
a
Table 1 Thioetherification of aryl halides catalysed by in situ formed LCuCl
within the macrocyclic cavity as nanoreactor. To our
knowledge, no prior examples of supramolecular approaches
for this purpose have been informed. Recently, we reported
LCuCl
N
N
O
Cu
O
Hal
2.5% mol L
1
4
SNa
+
S
the preparation and conformational aspects of
employ the complexes LCuCl ) (isolated as the C
LCuCl-THF) and LCuSPh (2) (Chart 1) formulated as monomers
L
;
herein, we
2.5% mol CuCl
OH Cl HO
OH OH
Toluene, 110°C
(
1
s
-symmetric
HO
R
OH
R
based on VT-NMR, DOSY-NMR, and MS data (see ESI†) in
Ullmann-type C–S coupling reactions. We chose this particular
reaction as proof-of-concept to assess the suitability and
repercussions of metal-confinement on catalytic performance;
the reported molecular analogues serve as benchmarks for
proper comparisons to discern the role of the calixarene
cavitand. Covalent attachment of reactive sites to
supramolecular scaffolds to generate robust frameworks may
b
Entry Aryl halide
Product
Time (h) Yield(%)
I
S
1
15
15
15
12
12
88
95
95
93
95
Br
S
2
be difficult; compounds
1 and 2 represent rare examples of
Br
S
S
systems engineered to operate through the synergy of the
recognition properties of the calix[8]arene, and the catalytic
3
attributes
of
Cu(I)-phenanthroline
complexes.
Our
I
4
observations highlight the advantages of metal confinement
and may provide a general strategy to carry out diverse
organic reactions within calix[8]arene nanoreactors.
S
S
Br
5
Typically, the reaction was carried out in toluene at 110°C
with equimolar amounts of sodium thiophenolate and an aryl
I
6
12
10
87
95
halide. Additionally, 2.5% of
L and 2.5% of CuCl were added
O
O
O
(Table 1). The protocol is compatible with both electron-
S
S
Br
deficient and electron-rich aryl halides, resulting in good to
excellent isolated yields of the substituted arylthioethers.
Control experiments with CuCl or CuI (5% mol) were carried
out; we did not detect significant amounts of coupled products
after 24 h in refluxing toluene. As additional control
7
O
OH
O
I
8
9
16
16
70
80
OH
experiments, the use of isolated LCuCl-THF
afforded virtually identical results in terms of reaction time
and yield of coupled products as those obtained when and
(1) as catalyst
I
S
S
O
L
CuCl were added separately. This indicates that in situ
formation of the pre-catalyst LCuCl must occur rapidly. To
Br
Cl
1
1
1
0
1
2
3
16
20
10
12
94
92
95
94
CN
CN
CN
CN
confirm the role of
isolated the corresponding thiophenolate LCuSPh
reaction of equimolar amounts of and sodium thiophenolate.
reacts efficiently with one equivalent of haloarene to
quantitatively form the corresponding aryl thioether in a few
hours (Scheme 1). These results suggest that may be an
1
as precursor of the active catalyst
2, we
S
S
S
(
2) from the
1
2
Br
NO
2
NO2
NO2
2
Cl
intermediate in the catalytic production of aryl sulfides.
Further insight into the mechanism of thioetherification may
1
NO
2
be obtained from the reaction of
1 and 2 with o-
a
This species should not contain THF, in contrast with LCuCl-THF (1) prepared in
(allyloxy)iodobenzene (entry 9, Table 1 and Scheme 1). The
b
THF. Isolated yields after column chromatography
corresponding C–S coupling product was isolated in over 80%
yield, without evidence of the cyclic product expected if a
radical mechanism was involved. This outcome parallels the
observations by Hartwig et al., which indicate that the absence
of cyclic products imply that C–S coupling occurs without
intermediacy of aryl radicals, and confirms that Cu-
Our results are intriguing since most reports on Cu(I)-mediated
C–S coupling reactions usually suffer from limitations such as
the requirement of high polarity solvents (HMPA, DMF, DMSO,
dioxane, NMP), high temperatures, sub-stoichiometric catalyst
11
loadings (10%–20%), and excess of aryl halides. Moreover,
2
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the efficiency of the reaction tends to be highly dependent on reactions of one equivalent of
COMMUNICATION
2
with a mixture of haloarenes
the functional groups on the aryl halide; for instance, products (one equivalent of each halide, Scheme 1). These results
derived from electron-rich haloarenes are difficult to obtain in suggest that the calixarene imposes steric restrictions around
good yields (see entries 6–9, Table 1). Thus, the great the catalytic centre, and the less congested aryl halides access
performance of supramolecular catalyst
1 could be directly more easily the Cu(I) ion. In this sense, the calix[8]arene
ascribed to the benefits of active-site confinement within the pocket kinetically discriminates between substrates and
calix[8]arene hydrophobic cavity. Unlike the smaller induces selectivity effects, which may not be easily attained
calix[n]arene congeners (n = 4,6), calix[8]arene seems to through modifications on molecular catalysts
A
and B.
provide a suitable hydrophobic pocket to encapsulate guests;
I
S
Br
Br
in this scenario, the calixarene framework should experience
conformational variations in order to correctly bind substrates
S
Toluene
110°C, 4h
95%
70%
and release products. As substrates are sequestered by
1 or 2
S
O
I
from bulk solvent, their encapsulation may be translated into
increased likelihood of effective collisions and stabilisation of
reactive intermediates (Fig. 1). Additionally, confinement of
the catalytic Cu(I)-centre allows site-isolation for higher
reactivity: dimer formation prevalent in related molecular
systems is prevented by the calixarene framework. This also
Toluene
O
1
10°C
S
S
N
N
20%
Toluene
O
Cu
SPh
HO
O
1
10°C, 3h
83%
OH
OH OH
OH
HO
Br
Br
NO
2
S
I
O
NO2
NO2
90%
O
Toluene
1
10°C, 3h
90%
Toluene
10°C
Br
precludes degradation and deactivation of the active site;
and maintain their integrity for extended periods of time
even when exposed to air. These attributes are further
1
LCuSPh (2)
1
NO2
0.3 equiv.
2
Br
NO
2
I
S
evidenced by comparison of the catalytic performance of
1
NO2
Toluene
110°C, 2.5h
NO2
0.7 equiv.
1
2
(Chart 1). The use of
85%
and its molecular analogue
A
1
led to
high conversions of haloarenes into the corresponding Scheme 1 Selected C–S coupling reactions of LCuSPh (2) (left) and substrate-selectivity
experiments (right)
thioethers in less time and with lower catalyst loadings,
decreasing from 10% of
was only effective with iodoarenes,
bromo-, and some activated chloroarenes (Table 1). Notably,
the protocol for exhibits modest results with CuCl as
precursor, and CuI was used instead; in contrast, our system
A
to 2.5% mol of
1. While catalyst A
Various solvents were screened to test the catalytic activity
1 under the conditions in Table 1. Methanol and THF
1
was efficient with iodo-,
of
promoted low yields of the target products (around 50%), and
formation of disulfide. The use of more polar solvent mixtures
such as water:MeOH (2:1), DMF, and DMSO led to even lower
yields of the target thioethers (ca. 30%), together with starting
materials, and undesired phenyl disulfide. Hence, the less
polar toluene was confirmed to be the most effective medium
for this reaction. Interestingly, this outcome differs from the
polar and high-boiling point solvents generally used for C–S
A
works efficiently with the normally less active CuCl. Likewise,
is a more effective precursor when compared to type-
thiophenolates. Table S1 and Scheme S1 in the ESI† compare
2
B
1
and
2 versus their molecular counterparts A and B, clearly
attesting to the advantages of the supramolecular platform.
1
1
coupling reactions. On the basis of the aforementioned
observations, we reasoned that the solvent-dependent
catalytic behaviour of supramolecular compounds
1 and 2
could be associated to an opening-closing mechanism of the t-
Bu-rich calixarene upper rim that modulates the traffic of
substrates and products in and out of the cavity. Considering
that access to the catalytic centre and release of products
must occur through the gate defined by the eight t-Bu groups,
our hypothesis is that the polar solvents may enforce grouping
of t-Bu moieties by hydrophobic interactions. This effectively
shuts access to the macrocyclic cavity by creating a t-Bu
Fig. 1 Simulation of the inclusion of 4-Bromobenzonitrile within the calixarene pocket
of LCuSPh (2) (Energy-minimised simulations; MMFF, Spartan)
Although the effect is subtle, the calixarene pocket
apparently affects the selectivity for the halogen substrate.
The reactivity of arylbromides was generally superior to that of
aryliodides, as evidenced by the slightly higher yields and
shorter reaction times (Table 1); the latter observation is also
“
seam”, thus compromising the host properties of 1 and 2.
Since efficient catalysis requires that the product readily
dissociates from the calixarene structure, easy access and
release from the cavity by opening the hydrophobic t-Bu seam
may be favoured by the non-polar environment in toluene.
Further support of this idea was obtained in our previous
interesting since the analogous
A is only efficient for C–S
coupling of aryliodides. Thus, the calixarene enables a higher
reactivity for the smaller arylbromides, overruling the usual
studies on the dynamic nature of calixarene L, which revealed
that cone conformers predominate at high temperature (393
K) in the low polarity C D Cl . Accordingly, the stable cone
1
1
trend of superior reactivity for iodides. Further insight into
size- and shape-selectivity may be obtained from competition
2
2
4
conformation of
L
is constructed by two ¾-cone clefts whose
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1
4
eight t-Bu groups are oriented syn,
with an open
Armspach, D. Semeril, W. Oberhauser, D. Matt and L.
Toupet, Angew. Chem. Int. Ed., 2014, 53, 3937; (c) M. Guitet,
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Fensterbank, Y. Zhang, S. Roland, M. Ménand and M.
Sollogoub, Angew. Chem. Int. Ed., 2013, 52, 7213.
conformation as observed in related 1,5-functionalised
6
calix[8]arenes. Interestingly, the corresponding open cone
conformation in and appears to be rigid due to the reduced
1
1
2
conformational mobility imposed by the CuCl-THF and CuSPh
moieties inside the cavity; this is evident from the presence of
an AB system for the methylene linkers of the phenanthroline
group. VT-NMR showed no coalescence for any of the signals
6
(a) L. Zheng, S. Sonzini, M. Ambarwati, E. Rosta, O. A.
Scherman, A. Herrmann, Angew. Chem. Int. Ed., 2015, 44
,
1
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4
up to 390 K in C
2
D
2
Cl
4
(see Fig. S3 in ESI†); this array is
and
Sivaguru, Chem. Eur. J., 2012, 18, 12178.
R. Gramage-Doria, D. Armspach and D. Matt, Coord. Chem.
Rev., 2013, 257, 776.
foreseen to meet the nanoreactor conditions for
1
2.
7
8
In summary, we have presented the first case of a
calix[8]arene cavitand employed as nanoreactor for metal
(a) D. M. Homden and C. Redshaw, Chem. Rev., 2008, 108
086; (b) C. Jeunesse, D. Armspach and D. Matt, Chem.
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007, 3303.
,
5
confinement in C–S catalytic coupling. Calixarene
suitable platform for supramolecular complexes
L
served as a
and that
1
2
2
efficiently promoted the formation of aryl sulfides in toluene.
Catalysts and present experimental advantages that can be
9
1
N. Le Poul, Y. Le Mest, I. Jabin and O. Reinaud, Acc. Chem.
Res., 2015, 48, 2097–2106.
0 (a) E. Hoppe, C. Limberg and B. Ziemer, Inorg. Chem., 2006,
1
2
translated into an improved catalytic performance when
compared to their molecular analogues. The calixarene pocket
also imposes steric restrictions that affect the selectivity for
substrates: it appears that the less hindered bromoarenes
react faster than iodoarenes, in contrast with their usually
4
2
5, 8308; (b) J. Ling, Z. Shen and W Zhu, J. Polym. Sci. A,
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observed
reactivity.
Furthermore,
the
calixarene
conformations seem to be affected by the nature of the
solvents, leading to differing activities likely due to
hydrophobic effects. Our observations clearly prove the
supramolecular effects of calix[8]arene on the catalytic activity
of confined Cu(I) centres, substrate selectivity, and solvent-
induced reactivity. This supramolecular strategy represents a
novel approach for transition metal catalysed transformations
within calix[8]arene derivatives, which may pave the road for
their further development, along with analogous
supramolecular scaffolds. Investigation of this new class of
supramolecular catalysts will provide insights into catalysis in
confined space, and shed light on cavitand features relevant to
fine tuning for the rational design of advanced catalysts.
1
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