Anion–π Catalysis on Carbon Nanotubes

Article abstract of DOI:10.1002/anie.201909540

Induced π acidity from polarizability is emerging as the most effective way to stabilize anionic transition states on aromatic π surfaces, that is, anion–π catalysis. To access extreme polarizability, we propose a shift from homogeneous toward heterogeneo

Full text of DOI:10.1002/anie.201909540

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201909540
German Edition: DOI: 10.1002/ange.201909540
Supramolecular Chemistry
Anion–p Catalysis on Carbon Nanotubes
+
+
Anna-Bea Bornhof , Mikiko Vꢀzquez-Nakagawa , Laura Rodrꢁguez-Pꢂrez,
Marꢁa ꢃngeles Herranz, Naomi Sakai, Nazario Martꢁn,* Stefan Matile,* and Javier Lꢄpez-
Andarias*
Abstract: Induced p acidity from polarizability is emerging as
the most effective way to stabilize anionic transition states on
aromatic p surfaces, that is, anion–p catalysis. To access
extreme polarizability, we propose a shift from homogeneous
toward heterogeneous anion–p catalysis on higher carbon
allotropes. According to benchmark enolate addition chemis-
try, multi-walled carbon nanotubes equipped with tertiary
amine bases outperform single-walled carbon nanotubes. This
is consistent with the polarizability of the former not only along
but also between the tubes. Inactivation by p-basic aromatics
and saturation with increasing catalyst concentration support
that catalysis occurs on the p surface of the tubes. Increasing
rate and selectivity of existing anion–p catalysts on the surface
of unmodified nanotubes is consistent with transition-state
stabilization by electron sharing into the tubes, i.e., induced
anion–p interactions. On pristine tubes, anion–p catalysis is
realized by non-covalent interfacing with p-basic pyrenes.
emerged as the reaction of choice to assess the activity of new
anion–p catalysts. The formation of addition product 3 (or A)
occurs in competition with the decarboxylation product 4 (or
[3,5]
[6]
D, Figure 1a).
Recently confirmed by theory, the selec-
tive acceleration of enolate addition by anion–p catalysis has
been attributed to the discrimination of flat and bent enol and
keto tautomers 5 and 6, respectively, on p-acidic surfaces
(Figure 1, red arrow).
For catalyst development, the strength of anion–p inter-
actions is of central importance. Removal of electron density
from aromatic systems for this purpose is limited because of
the onset of catalyst degradation, electron transfer, and
[2,3]
nucleophilic aromatic substitution.
Induced rather than
intrinsic anion–p interactions could ultimately be more
[3]
promising. Induced anion–p interactions originate from
the polarization of the aromatic system in response to the
presence of negative charges, repelling the nearby p electrons
toward the other end of the p system, thus inducing oriented
macrodipoles or rather multipoles that have their focal point
on the inducer, enabling and strengthening anion binding on
T
he term anion–p interaction refers to the binding of anions
on the surface of extended p-conjugated systems, usually
[1–3]
aromatic planes.
Anion–p interactions have received little
attention, presumably because they are counterintuitive and
comparably rare in nature. Catalysis with anion–p interac-
[
3]
tions has been introduced explicitly six years ago. Since
then, anion–p catalysis has been realized for asymmetric
enolate, enamine and iminium chemistry, cascade reactions,
cycloadditions and autocatalytic epoxide opening ether
[
3,4]
cyclizations.
In light of its importance in chemistry and biology, the
addition of malonic half thioester 1 to enolate acceptor 2 has
[
+]
[
*] A.-B. Bornhof, Dr. N. Sakai, Prof. S. Matile, Dr. J. Lꢀpez-Andarias
Department of Organic Chemistry, University of Geneva
1211 Geneva (Switzerland)
E-mail: stefan.matile@unige.ch
javier.lopez@unige.ch
Homepage: http://www.unige.ch/sciences/chiorg/matile/
[
+]
M. Vꢁzquez-Nakagawa, Dr. L. Rodrꢂguez-Pꢃrez,
Dr. M. ꢄngeles Herranz, Prof. N. Martꢂn
Department of Organic Chemistry, Faculty of Chemistry
Universidad Complutense de Madrid
28040 Madrid (Spain)
E-mail: nazmar@ucm.es
Figure 1. a) Benchmark reaction for anion–p catalysis, reporting higher
activity with increasing A/D values, that is the yield of enolate addition
product A (3, red arrow) divided by the yield of decarboxylation
product D (4). b) For anion–p catalysis on MWCNTs 7m, anionic
transition states TS are expected to stabilize themselves by repelling
electron density (red) along as well as between the stacked tubes.
c) TS stabilization on SWCNTs 7s occurs only along this single tube.
PMP=para-methoxyphenyl, S=substrates, P=products, C=catalysts.
Prof. N. Martꢂn
IMDEA-Nanociencia
c/ Faraday 9, Campus Cantoblanco, 28049 Madrid (Spain)
+
[
] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201909540.
Angew. Chem. Int. Ed. 2019, 58, 1 – 5
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angewandte
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the p surface (Figure 1b,c). Anionic transition states emerg-
ing on p surfaces thus generate their own stabilization, create
their own catalyst. This idea has been supported by anion–p
diameters between 6 nm and 13 nm and were functionalized
with 0.64 mmol base per milligram of material in 9m.
[18]
Single-walled carbon nanotubes (SWCNTs) 7s were narrower
(diameter between 0.8 and 1.4 nm) and conjugated to
1.19 mmol base per milligram of material in 9s (Table S1).
Tested in the above mentioned enolate addition reaction
(Figure 1a), suspensions of MWCNTs 7m in [D ]THF/CDCl
[
6]
catalysis on p-stacked foldamers and on [60]fullerene
[7]
[3]
monomers and dimers. From there, a shift of attention
toward heterogeneous anion–p catalysis on higher carbon
allotropes was the obvious next step.
8
3
Since their discovery, carbon nanotubes have been
explored extensively to understand their unique structures
1:1 activated triethylamine (TEA) to A/D = 4.3, a selectivity
only slightly above the intrinsic A/D = 3.4 for TEA under
these conditions (Figure 2, entries 1 and 4). With A/D = 25,
the highest activity was found for MWCNTs 9m (blue,
entry 5). With longer and turned spacers in 10m and 11m,
activities dropped until A/D = 16 (blue, entries 9 and 12). A
decreasing activity with longer spacers was as with the soluble
[
8]
and properties. For their use in materials science, much
emphasis has been conferred to their conducting properties,
including many examples for photoinduced electron transfer
[9]
[10,11]
processes.
Consistent with extreme polarizability,
carbon nanotubes can accept, donate, and conduct both
[
12]
[7]
electrons (e, n) and holes (h, p). Molecular recognition has
attracted attention not only for sensing applications but also
for controlled dispersion, purification, and functionaliza-
fullerene models 9 f and 10 f (striped, entries 8 and 11), and
well explained with increasing entropic losses for transition-
state recognition. Lower activity with rigid turns on mis-
matched 11m was, however, contrary to fullerene models 11 f
(striped, entries 8 and 14). This contrast was meaningful
because the absence of the cyclopropyl turns of Bingel
fullerenes causes the amine base in 11m to end up fixed far
from the p surface. Because of this, according to computa-
[
13]
tion. Most approaches focus on p–p stacking and dispersion
forces, while anion–p and also cation–p interactions with
[
14]
higher carbon allotropes have received little attention,
[10,15]
although they are predicted to be strong.
Contrary to
carbon nanotubes have been explored quite
[3,4,16]
fullerenes,
[7]
extensively in catalysis, mostly as a scaffold to maximize
effective concentrations, occasionally also to modulate redox
tional models, the unique topological matching in 11 f is
ruined for 11m. The activities with the best MWCNT catalyst
9m, also exceeding the best fullerene catalyst 11 f, were
remarkable considering the shift from homogeneous to
heterogeneous catalysis (entries 5 and 14). Indeed, the
reactions stopped as soon as 9m MWCNTs were centrifuged
away and restarted as soon as they were redispersed (Fig-
ure S3). Previous studies on heterogeneous anion–p catalysis
on ITO electrodes gave, without applied voltage, extremely
[12,17]
processes, including the activation of metal catalysts.
The
stabilization of neither anionic nor cationic transitions states
by induced p-system polarization, i.e., neither anion–p nor
cation–p catalysis, have been explored explicitly on carbon
nanotubes.
For this purpose, carbon nanotubes 7 were firstly short-
ened and endowed with carboxylic acids by oxidative treat-
ment (8) and afterwards covalently reacted with tertiary
amine bases bearing different linkers to yield 9–11 (Figure 2
and Schemes S2–S5). The multi-walled carbon nanotubes
[19]
favorable decarboxylation (A/D = 0.08).
Kinetics analysis for MWCNT catalyst 9m confirmed that
addition clearly exceeds decarboxylation (Figure 3a and
Figure S1). The differences in activation energy of addition
(
MWCNTs) 7m used were composed of around 11 tubes with
Figure 2. A/D values for product 3 divided by 4 obtained with substrates 1 (200 mm) and 2 (10 equiv) in [D ]THF/CDCl 1:1 (blue, black), 12
8
3
(
(
pink) or 13 (red) for covalent MWCNT anion–p catalysts 9m–11m compared to SWCNT 9s and controls TEA (black), fullerenes 9 f–11 f
striped) with 20 mol% of catalyst (tertiary amine), with indication of differential transition-state stabilization DDGTS for A vs. D for 9m–11m
[
7]
°
vs. TEA (left), and DA/D values with or without the addition of 7m (9 w/w% of the reaction mixture) to 9p, 10p, 9n, 9 f and TEA (5 mol%, blue)
right). The error of A/D values is estimated to be within Æ10%. m=multi-walled, s=single-walled, n=naphthalenediimide, f=fullerene,
p=pyrene, R=leucylhexylamide, cat=catalyst, in-/act=in-/activator.
(
2
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the tubes with a layer repulsive to the anions, and thus
provided corroborative support for operational anion–p
interactions, i.e., the existence of anion–p catalysis on
MWCNTs (entries 5–7, 9–10, and 12–13).
SWCNTs 9s were with A/D = 6.8 less active than
MWCNTs 9m with A/D = 25 (Figure 2, blue, entries 5, 17)
but could be further inactivated by p-basic competitor 12
(Figure 2, pink, entries 17 and 18). This significant difference
is interesting because it supports that transition states on the
surface of MWCNTs could be stabilized by polarizability
along the tubes but also between the layers, somehow
combining previously explored through-bond contributions
from very large p surfaces beyond fullerenes on the one hand
and from through-space synergistic anion-(p) -p interactions
n
[
3]
on p-stacked foldamers on the other hand (Figure 1b).
[
3]
The binding of anion–p catalyst 9n (Scheme S1) on the
surface of unmodified MWCNTs 7m increased their activity
by maximal DA/D =+ 7.1 (Figure 2, entry 22; Figure 3e, blue
+). The coinciding increase in rate and selectivity with
MWCNT concentrations supported that the central principles
of catalysis also apply for these more complex systems, in this
case increasing transition-state recognition on the naphthale-
nediimide (NDI) surface in 9n by electron sharing with the
carbon nanotubes 7m (Figure 3e, turquoise ~). Activation of
Figure 3. a) Product formation (A, 3, blue
&
; D, 4, blue &) with 9m as
a function of time (10 mol%, 200 mm 1, 10 equiv 2, [D ]THF/CDCl
:1). b) Normalized A/D values with 9m as a function of the concen-
8
3
fullerene anion–p catalysts 9 f was, with up to DA/D =+ 1.4,
1
much weaker perhaps because the convex fullerenes prefer to
tration of inactivators 12 (pink !) and 13 (red !). For calibration, A/D
of TEA in 12 or 13 were used as A/Drel =0. c) A/D with 9m (blue *)
and TEA (pink x) (40 mm of tertiary amine) as a function of the
concentration of substrate 1. Equivalents of 2 were kept constant.
d) A/D with 9m (blue *) and TEA (pink x) as a function of their
[
21]
hide in the concave interior of carbon nanotubes (Figure 2,
entry 23; Figure 3 f, blue ~).
[
17]
The binding of the newly synthesized p-basic pyrene 9p
Scheme S1) by face-to-face p stacking on MWCNTs 7m
(
concentration (200 mm 1, 10 equiv 2). e) DA/D (A/DÀA/D , blue +)
0
slightly increased activity (up to DA/D =+ 1.2, Figure 2,
entry 20; Figure 3 f, red ~). Contrary to 9n, however, catalysis
and conversion after 4 days (turquoise ~) with 9n (5 mol%) as
a function of the concentration of activator 7m (wt% of the reaction
mixture). f) DA/D with 9 f (blue ~), 9p (red ~), 10p (red ~) and TEA
pink x) (5 mol%) as a function of the concentration of activator 7m
with 9p on 7m is unlikely to take place on the more repulsive
pyrene surface. This system thus documented the possibility
of non-covalent interfacing for anion–p catalysis on unmodi-
fied nanotubes 7m, although the obtained activities were
rather weak compared to covalent interfacing in 9m
(Figure 2, entry 20). As with covalent interfacing in 10m,
activities decreased with spacer elongation in the comple-
mentary non-covalent pyrene interfacer 10p on 7m (Figure 2,
entry 21; Figure 3 f, red ~). TEA, without p–p interfacer, was
(
(
wt% of the reaction mixture).
°
and decarboxylation reactions (DGTS ) were compared to
that observed with TEA to give differential transition-state
[3]
°
À1
stabilization DDGTS = À3.1 kJmol (Figure 2). Decreas-
°
À1
°
ing DDGTS = À2.3 kJmol
for 10m and DDGTS =
À1
À2.2 kJmol for 11m reproduced trends from A/D values
very well (blue, entries 5, 9, and 12).
the least sensitive to the presence of MWCNTs 7m (Figure 2,
entry 19; Figure 3 f, pink x).
Comparing bi- against unimolecular transformations, A/D
values increased with substrate concentration (Figure 3c).
They also increased with the concentration of catalyst 9m but
not with TEA (Figure 3d). Saturation behavior characterized
by a formal EC = 11 mm supported the presence of active
sites on MWCNTs, although the complex heterogeneous
system complicates interpretations, a call for caution that
naturally applies for all that follows.
In 1-chloronaphthalene 12, activities of 9m–11m
decreased (Figure 2, pink, entries 7, 10, and 13). Hill analysis
of the dose–response curve gave an IC = 5.9m for the
inactivation of catalyst 9m by 12 (Figure 2, entry 7; Figure 3b,
pink !; Table S2). Stronger inactivation was found for the
In summary, we provide experimental support for the
existence and significance of anion–p catalysis on carbon
nanotubes. Highlights include MWCNTs outperforming
SWCNTs owing to electron sharing within and between the
tubes, thus driving induced anion–p interactions from polar-
izability to the extreme, or the activation of existing anion–p
catalysts on the surface of pristine MWCNTs. The amphoteric
5
0
[12]
nature of MWCNTs suggests that, contrary to results from
[
6]
p-stacked foldamers, the above-mentioned insights should
hold also for the more conventional stabilization of cationic
intermediates on most polarizable p surfaces, i.e., induced
5
0
[
22]
cation–p rather than anion–p catalysis. Additional contri-
butions from the reduced dimensionality in 1D sliding
kinetics to anion–p catalysis, obviously most inviting on
[
20]
more p-basic 1,7-dialkoxynaphthalene (DAN)
a liquid at room temperature (IC = 2.6m, Figure 2, entry 6;
Figure 3b, red !). These results indicated that the p-basic 12
and 13 inactivate the catalysts by covering the p surfaces of
13, also
5
0
[
23]
carbon nanotubes, could deserve future attention.
The
heterogeneous nature of MWCNT anion–p catalysts is
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[
Conflict of interest
The authors declare no conflict of interest.
[
[
Keywords: anion-macrodipole interactions · anion–p catalysis ·
carbon nanotubes · induced p acidity · polarizability
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ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 1 – 5
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Angewandte
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Communications
Supramolecular Chemistry
Polarizability is driven to the extreme and
harnessed for anion–p catalysis. Multi-
walled carbon nanotubes i) outperform
single-walled ones (polarizable beyond
one tube), ii) are inactivated by p-basic
competitors (active sites are on tube
surface), iii) prefer covalent, linker-sensi-
tive interfacing over non-covalent strat-
egies (pyrene), and iv) activate existing
anion-p catalysts by electron sharing
A.-B. Bornhof, M. Vꢁzquez-Nakagawa,
L. Rodrꢂguez-Pꢃrez, M. ꢄngeles Herranz,
N. Sakai, N. Martꢂn,* S. Matile,*
J. Lꢅpez-Andarias*
&&&& — &&&&
Anion–p Catalysis on Carbon Nanotubes
(
NDIs > fullerenes).
Angew. Chem. Int. Ed. 2019, 58, 1 – 5
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!