Merging Molecular Recognition and Gold(I) Catalysis with Triphoscalix[6]arene Ligands

Article abstract of DOI:10.1002/chem.202101323

We report the synthesis and characterization of novel triphosphine calix[6]arene ligands. These supramolecular wheels, with recognition features governed by the hydrogen-bonding domain, were employed to synthesize multitasking trinuclear gold(I) complexes as a new platform for the synthesis of interwoven (pseudo)rotaxane species. In parallel, the multivalent, metal-bonded upper rim displayed catalytic features promoting highly selective gold-catalyzed cycloisomerization reactions of 1,6-enynes.

Full text of DOI:10.1002/chem.202101323

Accepted Article
Accepted Article
Title: Merging Molecular Recognition and Gold(I) Catalysis with
Triphoscalix[6]arene Ligands
Authors: Gianpiero Cera, Andrea Secchi, Arturo Arduini, and Gabriele
Giovanardi
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.202101323
Link to VoR: https://doi.org/10.1002/chem.202101323
01/2020

10.1002/chem.202101323
Chemistry - A European Journal
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Merging Molecular Recognition and Gold(I) Catalysis with
Triphoscalix[6]arene Ligands
Gianpiero Cera,*[a] Gabriele Giovanardi,[a] Andrea Secchi *[a] and Arturo Arduini *[a]
Dedicated to Vincenzo Balzani on the occasion of his 85th birthday
[a]
Prof. Gianpiero Cera, MSc. Gabriele Giovanardi, Prof. Andrea Secchi, Prof. Arturo Arduini
Dipartimento di Scienze Chimiche, della Vita e della Sostenibilità Ambientale, Parco Area delle Scienze 17/A, 43124 Parma, Italy.
Università di Parma
Parco Area delle Scienze 17/A, 43124 Parma, Italy
E-mail: gianpiero.cera@unipr.it
andrea.secchi@unipr.it
arturo.arduini@unipr.it
Supporting information for this article is given via a link at the end of the document
Abstract: We report the synthesis and characterization of novel
triphosphine calix[6]arene ligands. These supramolecular wheels,
with recognition features governed by the hydrogen-bonding domain,
were employed to synthesize multitasking trinuclear gold(I)
complexes as a new platform for the synthesis of interwoven
(pseudo)rotaxanes species. Parallelly, the multivalent, metal-bonded
upper-rim displayed catalytic features promoting highly selective gold
catalyzed cycloisomerization reactions of 1,6-enynes.
progress has been recently reached using phosphine-based
cavitands, particularly in the field of gold catalysis,[11-13] the
working mode of these supramolecular species is limited to the
catalytic event itself. Due to our recent interest in the synthesis of
supramolecular multitasking objects able to perform different
functions through the control of the space and binding sites, we
now introduce a novel family of triphoscalix[6]arene derivatives.
Remarkably, the selective functionalization of the scaffold allowed
for the unprecedented synthesis of triphosphine ligands,
eventually coordinated with three gold(I) nuclei, which were
exploited for both molecular recognition and catalysis (Figure 1).
Since many years, the versatility of the calix[6]arene scaffold has
been exploited to construct heteroditopic synthetic receptors to
design interlocked and interwoven species such as
(pseudo)rotaxanes, with the aim to synthesize artificial nano-
machines and devices.[1] A key feature of this class of compounds
is represented by the hydrogen-bonding (HB) donor domain,
present on the upper ring of the calix[6]arene, which is able to
promote the threading of mono- and di-cationic species, in low
polarity solvents.[2] This occurs by weak HB interactions able to
separate the ion pair associated with an organic axle, for instance,
a bipyridinium salt, promoting the threading inside the π-rich
aromatic cavity of the host.[3] In this context, we recently
developed a novel class of calix[6]arene-based heteroditopic
receptors (TSA) that present three sulfonamide moieties at the
macrocycle's upper rim.[4] Here, the HB domain was utilized to
design ion-pair selective receptors for bipyridinium salts.[5]
Particularly, while in the presence of weak ion-pairs, TSAs form
pseudorotaxanes in a typical cone conformation, the use of tight
ion-pairs led to a selective rearrangement that brings the
calix[6]arene in a partial cone (pC) conformation. The reactivity of
these systems is highly influenced by the reaction medium. In fact,
in polar protic solvents, such as methanol or water, TSAs work as
Brönsted acid catalysts, promoting a general Michael addition of
indoles to nitroolefines.[6] Parallelly, another important goal that
has been tackled in supramolecular chemistry is the synthesis of
multivalent organometallic macrocycles for catalysis.[7] Inspired
by nature's enzymes, many researchers aimed to devise artificial
(metal-)catalysts for selective organic transformations.[8] In this
context, cutting-edge contributions demonstrated the ability of
Figure 1. Comparing the reactivity of TSA Brönsted Acids with Triphosphine
calix[6]arene Au(I) complexes.
The synthesis of the novel multivalent calix[6]arene
derivatives A and B (see Figure 1) was accomplished starting
from the known trinitrotrioctyloxy derivative TN. Reduction of the
nitro groups in the presence of hydrazine and catalytic amounts
of Pd/C, led to the triaminoderivative that was subsequently
reacted in the presence of p-iodobenzensulfonyl chloride or p-
iodobenzoyl chloride to afford the corresponding tri(sulfon)amide
scaffolds (83% and 75% respectively). Successively, these
calix[n]arene[9]
and
resorcinarene-based
cavitands,[10]
appropriately functionalized with phosphine ligands, to promote
highly selective metal-catalyzed transformations. Although much
1
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intermediates were submitted to typical cross-coupling conditions
using Pd(OAc)2 (20 mol %) as the catalyst and
diphenylphosphine[14] to yield triphoscalix[6]arene ligands A and
B in moderate yields (51% and 48%) (Scheme 1). Ligand B could
be also delivered in a more convenient manner through a
convergent approach based on an amide-coupling reaction using
an excess of 4-diphenylphosphino benzoic acid (3.5 equiv.) in the
presence of the triaminocalix[6]arene intermediate (see SI for
more details).
indicative of the formation of a pseudorotaxane complex. 1H-NMR
analysis was subsequently performed. We first observed an
extensive up-field shift of the aromatic CH and N-CH2 protons of
the guest, which thus confirmed the formation of an interwoven
species, along with two new downfield-shifted signals for the
methoxy groups ($ and £) in a 1:2 ratio (Figure 3). Similarly, the
three protons of the NH groups of the host, suffered a splitting (1:2
ratio) and a downfield shift to 10.2 and 9.5 ppm as a consequence
of their engagement in HB interactions with the tosylate
counterions (Figure 3).[15] A different pattern for the methylene
groups, analogous to what is usually observed with this family of
heteroditopic TSA receptors, revealed a fair selective formation
(⁓ 3:1) of a pseudorotaxane in a partial cone conformation. 2D
NMR techniques confirmed this finding, and, particularly, HSQC
analysis revealed a significant shift of the 13C-NMR resonances to
δ = 35.7 ppm for the a/a' couple.[16] This shift, typical of an anti-
orientation of the methylene bridging protons, is the prerogative
of an inversion point associated with a ring bearing a methoxy
group ($) (see figure S1 in supporting information). Differently,
triphosamide derivative B was not able to host any bipyridinium
guest, probably due to the low efficiency of the NH amide moieties
to separate the ion pairs associated with the viologen-based salts.
Scheme 1. Synthesis of Triphosphinocalix[6]arene ligands A and B: i) Pd/C
(cat.), N2H4 H2O, EtOH, 100 °C, quant.; ii) 4I-C6H4SO2Cl, TEA, CH2Cl2, 25 °C,
83%; iii) Pd(OAc)2 (cat.), PHPh2; TEA, DMF, 130 °C, 51%; iv) Pd/C (cat.), N2H4
H2O, EtOH, 100 °C, quant.; v) 4I-C6H4COCl, TEA, CH2Cl2, 25 °C, 75%; vi)
Pd(OAc)2 (cat.), PHPh2, TEA, DMF, 130 °C, 48%.
The conformation adopted by A and B in solution was
analyzed by NMR spectroscopy. In CDCl3, A is present in a typical
pseudo cone conformation as reflected by the AX system of two
doublets (J ⁓ 15 Hz) for the six bridging methylene groups and the
up-field shift (⁓2.5 ppm) of the methoxy groups, which points
inside the aromatic cavity of the calix[6]arene scaffold.
Noteworthy, 31P-NMR of triphosphine A was characterized by a
single peak at -5 ppm, indicating the high symmetrical geometry
of the calix[6]arene ligand. A more entangled situation was
observed for B. Here, the major pseudo cone conformer is in
equilibrium, at the NMR time scale (Fig. S1 in SI), with a second
conformation that gives rise to six pair of doublets: three for the
equatorial and three for the axial diasterotopic methylene protons
(vide supra). Such a pattern is suggestive of a "distorted" cone.
The presence of two species in equilibrium was further reflected
by 31P-NMR analysis that presents two close signals at -5.56 and
-5.95 ppm (Figure 2).
We next evaluated the ability of A and B to work as
supramolecular receptor for viologen salts derivatives. Hence, we
equilibrated a solution of A with 1.5 equivalents of DOV•2OTs in
CDCl3 at room temperature. The formation of a deep yellow
mixture suggested the presence of charge-transfer interactions
Figure 2. 1H-ROESY spectra of B showing interchanges (highlighted in green)
between axial methylene protons of "distorted" cone and pseudo-cone
conformation (left); detailed 31P-NMR of B (right).
2
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Figure 3. 1H-NMR spectra (400 MHz, 298 K) of a) DOV•2OTs in CD3CN, b) pseudorotaxane P[A(pC)DOV]2OTs in CDCl3, c) calixarene A in CDCl3. Down-right,
schematic representation of P[A(pC)DOV]2OTs. The color of the ovals/rectangles indicate the relative position of the phenolic substituent with respect to the plane
defined by the bridging methylene groups (hexagon), i.e. black upward, blue downward. The rectangle identifies the phenolic ring substituted with the octyloxy
chains, while the circle those with the methoxy groups.
Intrigued by these features, we next carried out the synthesis
of the corresponding trinuclear gold(I) calix[6]arene complex.
Hence A(AuCl)3 was obtained by simply mixing triphosphine A
with 3 equivalents of Au(DMS)Cl precursor in CH2Cl2 and
characterized by NMR and HR-MS spectroscopy. Interestingly,
31P NMR analysis revealed a significant shift to 33 ppm upon
exhaustive complexation of the phosphines with Au(I) nuclei. This
value, equal to that of PPh3AuCl, suggested that the electronic
and steric properties of A(AuCl)3 are not influenced by the
calix[6]arene scaffold and so could be somehow related to the
ones of the well-established gold(I) catalyst. Furthermore, NMR
analysis of A(AuCl)3 did not show any notable variation in the
geometry of the calix[6]arene macrocycle, which is still present in
a pseudo-cone conformation in low-polarity solvents. For the sake
of comparison, we evaluated the ability of A(AuCl)3 to work as a
host in the presence of bipyridinium salts. We thus operated as
previously described by mixing A(AuCl)3 with an excess of
DOV•2OTs and analyzed the mixture by NMR. In solution, we
observed the formation of an interwoven structure, which could
be easily assigned to the pseudorotaxane in a partial cone
conformation P[A(AuCl)3(pC)DOV]2OTs, in slow exchange, on
the NMR timescale, with the "empty" host. This could be deduced
by the presence of a residual signal at ⁓2.6 ppm for the methoxy
groups inside the aromatic cavity (Figure 4 and SI for a complete
characterization). The described situation could be attributed to a
steric effect, operated by the presence of three gold(I) nuclei,
which depletes somehow the ability of the wheel A(AuCl)3 to
perform hydrogen-bonding interactions with the counterions of the
salt and promoting the threading of the axle from the upper-rim of
the macrocycle. Hence, the results collected so far indicates that
the selective (partial cone vs cone) complexation features of
trisulfonamide calix[6]arene derivatives are completely in charge
of the HB domain of the wheel, independently by the presence of
coordinating substituents, such as a phosphine, present at the
upper rim of the calix[6]arene unit.
We thus wanted to analyze the eventual reactivity of the novel
trinuclear gold(I) calix[6]arene complexes in a model catalytic
reaction. To this end, we chose
a
gold(I)-catalyzed
cycloisomerization of 1,6-enynes.[17-18] By submitting an N-
tethered enyne 1a to A(AuCl)3 (0.5 mol %) in the presence of
catalytic amounts of AgSbF6 as the chloride scavenger,[19] we
observed complete consumption of the starting material after 30
min (91% yields, entry 1, Table 1). NMR analysis of the crude
reaction mixture revealed a quite remarkable selectivity (14:1)
toward the formation of the 6-endo-dig rearranged diene 2a with
respect to 2a', which is instead formed through an initial 5-exo-dig
cyclization.[20] Comparable results were obtained by running the
catalysis with loading as low as 0.25 mol % for A(AuCl)3. The
reaction, although slower, yielded product 2a after 1 h with 85%
yields (entry 2).
We finally tested calixphos amide B as a catalyst. Hence, a
reaction performed by forming in-situ B(AuCl)3 (0.5 mol %) led to
2a in high yields and close selectivity (88%, 13:1 endo:exo entry
3). The results collected so far were compared with data reported
in the literature for mononuclear PPh3AuCl (C) and
JohnPhosAuCl (D) catalysts.[21] Indeed, while A/B(AuCl)3 showed
a reactivity similar to the one of the triphenylphosphine gold(I)
catalyst C, the selectivity was closer to the one of the sterically
more encumbered D (entries 4,5 Table 1).
3
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Figure 4. 1H-NMR spectra (400 MHz, 298 K) of a) DOV•2OTs in CD3CN, b) pseudorotaxane P[A(AuCl)3(pC)DOV]2OTs in CD2Cl2, c) calixarene A(AuCl)3 in CDCl3.
Down-right, schematic representation of P[A(AuCl)3(pC)DOV]2OTs. The color of the ovals/rectangles indicate the relative position of the phenolic substituent with
respect to the plane defined by the bridging methylene groups (hexagon), i.e. black upward, blue downward. The rectangle identifies the phenolic ring substituted
with the octyloxy chains, while the circle those with the methoxy groups.
Table 1. Ligand effect in gold-catalyzed cycloisomerization of 1a.
(78-60%). Substitution at the ortho-position of the cinnamyl
fragment with a methyl group did not impact the catalysis with
product 2g isolated with high yields. Interestingly, in the presence
of electron-withdrawing groups such as fluoride, chloride and
trifluoromethyl, the catalysis required additional time to reach full
conversion. After 1 h, product 2h-j were delivered in very good
Entry
1[a]
L[Au]n
min
30
2a/2a’
14:1
--
2a [%]
91
yields (79-85%). The reactivity was also preserved in the
presence of isoprenyl-substituted enynes. However, while in the
case of 1k, a simple 1,3-diene 2k was isolated with 94% yields, a
different outcome was observed for the geranyl-substituted 1l.
Here, the 5-exo-dig cyclization pathway became more
predominant, and the formation of gold(I)-carbene intermediates
led to a diastereoselective cascade bis-cyclopropanation to form
tetracycle 2l in valuable yields of 79% (Scheme 2, c).[22] This shift
in the selectivity was also observed for enynes bearing a tri-
substituted cinnamyl fragment 1m,n. In accordance with the
literature,[20] the 6-endo pathway became unfavorable, leading to
the formation of dienes 2m,n in 73% and 65% yields, respectively
(Scheme 2, d).
A(AuCl)3 (0.5 mol %)
A(AuCl)3 (0.25 mol %)
B(AuCl)3 (0.5 mol %)
PPh3AuCl (2.0 mol%) (C)
JohnPhosAuCl (2.0 mol %) (D)
2[b]
60
85
3[c]
30
13:1
1:0
87
4[d]
5
100
87
5[d]
120
7:1
[a] Reaction conditions: 1a (0.2 mmol), AgSbF6 (1.5 mol %), isolated yields. [b]
1a (0.2 mmol), AgSbF6 (0.75 mol %). [c] 1a (0.2 mmol), B (0.5 mol %),
Au(DMS)Cl (1.5 mol %), AgSbF6 (1.5 mol %). [d] Reported data, see ref. 20.
Ts = 4-Toluensulfonyl.
Finally, the role of the calix[6]arene scaffold was investigated
running the model catalytic reaction with
a monomeric
In order to verify the general applicability of the transformation,
we synthesized a small family of 1,6-enynes 1 with different
sulfonamide-based N-tethering moieties (Scheme 2, a) and
cinnamyl fragments (Scheme 2, b) and submitted to optimal
conditions. Electron-rich groups such as for 1b,c performed with
comparable high efficacy delivering the corresponding 1,3-dienes
2b,c in high yields (91-87%). Contrarily, for halogen and nitro-
substituted arylsulfonamides 1d-f, we observed a general
decreased performance probably due to the higher affinity of such
FGs toward the gold(I) catalyst. The reaction never reached full
conversion despite a prolonged stirring (up to 2 hours).
Nevertheless, 2d-f could be isolated in synthetically useful yields
sulfonamide-based phosphine E and in the presence of a
competitive binder such as DOV •2OTs (Table 2). In the first case,
we observed a slight drop in the regioselectivity (2a/2a’ = 11:1,
entry 1) which highlighted the cooperative role of three
phosphines implanted in the calix[6]arene ligand in controlling the
selectivity of the catalysis. Contrarily, the reaction with an in situ-
formed pseudorotaxane (entry 2) did not display any substantial
variation with respect to the model one. Although just preliminary
results, this led us to conclude that the catalytic event occurs
outside the aromatic cavity of the trinuclear gold catalyst
A(AuCl)3.[23]
4
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species are both able to perform as catalysts for general
cycloisomerizations of 1,6-enynes, their ability as guests is highly
influenced by the HB domain that promotes the threading of
viologen-based axles inside the aromatic cavity of the wheel. This
study paves the way for more investigations on the reactivity of
these complexes and their (pseudo)rotaxane analogues
particularly on the control of regio- and stereoselectivities in
established gold-catalyzed cascade reactions.[24]
Acknowledgements
The authors thank Centro Interdipartimentale di Misure of the
University of Parma for NMR measurement. This work was
supported by the Italian MIUR (PRIN 20173L7W8 K). This work
was carried out within the COMP-HUB Initiative, funded by the
"Departments of Excellence" program of the Italian Ministry of
Education, University and Research (MIUR, 2018–2020).
Keywords: phosphines • calix[6]arenes • pseudorotaxanes •
molecular recognition • gold catalysis
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1[a]
L[Au]n
additive
--
2a/2a’
11:1
2a [%][a]
93
[9]
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EAuCl (2.0 mol %)
A(AuCl)3 (0.5 mol %)
2[b]
DOV•2OTs
14:1
89
[a] Yields determined using 1,3,5-trimethoxybenzene as the internal standard.
In conclusion, we reported on the synthesis of a novel family
of multitasking calix[6]arene-based phosphine ligands that were
employed to construct trinuclear gold(I) complexes. While these
5
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[23] Catalyst A(AuCl)3 is not able to thread enyne 1a in low polarity solvents
(CDCl3, see Fig S4 in SI).
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10.1002/chem.202101323
Chemistry - A European Journal
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We report the synthesis of novel trinuclear triphosphine-gold(I)-calix[6]arene complexes A,B. These multitasking supramolecular
wheels, which could be employed as a platform for the synthesis of interwoven (pseudo)rotaxanes species, displayed catalytic
features promoting highly selective cycloisomerizations of 1,6-enynes.
Institute and/or researcher Twitter usernames: Gianpiero Cera
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This article is protected by copyright. All rights reserved.