Remote Control of Anion–π Catalysis on Fullerene-Centered Catalytic Triads

Article abstract of DOI:10.1002/anie.201804092

The design, synthesis and evaluation of catalytic triads composed of a central C₆₀ fullerene with an amine base on one side and polarizability enhancers on the other side are reported. According to an enolate addition benchmark reaction, fullerene–fullerene–amine triads display the highest selectivity in anion–π catalysis observed so far, whereas NDI–fullerene–amine triads are not much better than fullerene–amine controls (NDI=naphthalenediimide). These large differences in activity are in conflict with the small differences in intrinsic π acidity, that is, LUMO energy levels and π holes on the central fullerene. However, they are in agreement with the high polarizability of fullerene–fullerene–amine triads. Activation and deactivation of the fullerene-centered triads by intercalators and computational data on anion binding further indicate that for functional relevance, intrinsic π acidity is less important than induced π acidity, that is, the size of the oriented macrodipole of polarizable π systems that emerges only in response to the interaction with anions and anionic transition states. The resulting transformation is thus self-induced, the anionic intermediates and transition states create their own anion–π catalyst.

Full text of DOI:10.1002/anie.201804092

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201804092
German Edition: DOI: 10.1002/ange.201804092
Anion–p Catalysis Very Important Paper
Remote Control of Anion–p Catalysis on Fullerene-Centered Catalytic
Triads
Javier Lꢀpez-Andarias, Antonio Bauzꢁ, Naomi Sakai, Antonio Frontera,* and Stefan Matile*
Abstract: The design, synthesis and evaluation of catalytic
triads composed of a central C60 fullerene with an amine base
on one side and polarizability enhancers on the other side are
reported. According to an enolate addition benchmark reac-
tion, fullerene–fullerene–amine triads display the highest
selectivity in anion–p catalysis observed so far, whereas
NDI–fullerene–amine triads are not much better than full-
erene–amine controls (NDI = naphthalenediimide). These
large differences in activity are in conflict with the small
differences in intrinsic p acidity, that is, LUMO energy levels
and p holes on the central fullerene. However, they are in
agreement with the high polarizability of fullerene–fullerene–
amine triads. Activation and deactivation of the fullerene-
centered triads by intercalators and computational data on
anion binding further indicate that for functional relevance,
intrinsic p acidity is less important than induced p acidity, that
is, the size of the oriented macrodipole of polarizable p systems
that emerges only in response to the interaction with anions and
anionic transition states. The resulting transformation is thus
self-induced, the anionic intermediates and transition states
create their own anion–p catalyst.
Figure 1. MEP surface of a) C60 stacked to Me NDI and b) a C dimer,
2
60
optimized geometries of chloride complexes with c) C ···C (with
6
0
60
cartoon illustrating the anion-induced macrodipole), d) C60 and
e) NDI···C , and comparative summary of f) polarizability a, g) inter-
action energies Eint with Cl , h) Cl –p distances and i) deepest p holes
on MEP surfaces.
6
0
[
1]
A
nion–p interactions refer to the attraction between
À
À
anions and aromatic surfaces. This is opposite to the conven-
[
2]
tional cation–p interaction between a positive charge and
a p-basic aromatic surface, and much less recognized because
it is counterintuitive. Their comparable relevance is generally
recognized only if the interactions are too strong and continue
into either nucleophilic and electrophilic aromatic substitu-
tions or the generation of radical anions and cations,
respectively. Whereas the exact nature of anion–p interac-
tions is still under debate, intrinsic p acidity, that is, low-lying
LUMO energies, p holes (areas with positive molecular
electrostatic potential (MEP), Figures 1a,b), positive quadru-
pole moments perpendicular to the aromatic plane or in-
plane dipoles from electron-withdrawing substituents, have
received much attention, resulting in much interest in small,
compact p surfaces such as, for instance, in hexafluoroben-
[
1,3–5]
zene or naphthalenediimides (NDIs).
Despite much
[6]
encouragement from pioneering theoretical studies,
induced anion–p interactions have been largely ignored in
practice. However, very recent results from anion–p catalysis,
that is, the stabilization of anionic transition states on
[3]
aromatic surfaces, revealed that activities increase in
[
4]
response to electric fields, with increasing length of p-
[5]
[7]
stacked foldamers, and on fullerenes. The discovery of
anion–p catalysis on fullerenes was of particular interest
because this most popular carbon allotrope has received
little attention with regard to both anion–p interactions and
[
8]
[
9]
[10]
[11]
catalysis.
This emerging power of polarizability
for
anion–p catalysis called for expanded p surfaces beyond
fullerenes to access induced macrodipoles as large as possible
in response to anion binding (Figure 1c). As a first step in this
direction, we here report the design, synthesis and evaluation
of catalytic triads composed of a central fullerene with an
active site on one side and a polarizability enhancer on the
other side.
[
*] Dr. J. Lꢀpez-Andarias, Dr. N. Sakai, Prof. S. Matile
Department of Organic Chemistry, University of Geneva
Geneva (Switzerland)
E-mail: stefan.matile@unige.ch
Homepage: http://www.unige.ch/sciences/chiorg/matile/
A. Bauzꢁ, Prof. A. Frontera
Department de Quꢂmica, Universitat de les Illes Balears
Palma de Mallorca, Baleares (Spain)
E-mail: toni.frontera@uib.es
Electronic communication in the ground state in full-
[
12]
[13]
[14]
erene–fullerene, fullerene–NDI and many other dyads
has been explored extensively. In our theoretical calculations
on the BP86-D3/def2-TZVP level with dispersion correction
in THF continuum, we found that the deepest p hole on the
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.201804092.
Angew. Chem. Int. Ed. 2018, 57, 1 – 6
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1
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molecular electrostatic potential (MEP) surface increases by
at most only 4.4 kJmol in the presence of an NDI or
a fullerene p stacked to the other side of a fullerene
rials (Scheme 1, Schemes S1–S5). Removal of the tert-butyl
protecting group afforded acid 9. From this key intermediate,
control 6 was obtained by coupling with amine 10. For triad 1,
malonamide 11 with an adamantyl solubilizer was prepared
first. Bingel reaction with fullerene 7, followed by tert-butyl
deprotection, coupling with 10 and further Boc removal
afforded amine 12, which was transformed into the target
molecule by coupling with acid 9. The series of NDIs 13–16
was prepared following previously reported procedures. In
separate reactions, the four NDIs were then first deprotected,
and the obtained amines were reacted with acid 9 to afford
triads 2–5.
À1
(
Figures 1a,b,i). However, the polarizability of fullerene–
fullerene but not NDI–fullerene dyads more than doubled
compared to C60 (Figure 1 f). In chloride complexes, full-
erene–fullerene dyads gave the largest binding energies and
shortest anion···fullerene distances, thus indicating that
induced p acidity is more important than intrinsic p acidity
[
3,4]
(
Figures 1c–i).
In the catalytic triad 1, a fullerene–fullerene dyad is
combined with a tertiary amine base (Scheme 1). The
positioning of this base is important to turn on anion–p
interactions as soon as the negative charge is injected into
The UV/Vis absorption spectra of 2–5 were characterized
by a hypochromic effect in the NDI region around 380 nm
(Figure 2a, Figure S5). The circular dichroism (CD) spectra
[
7]
a substrate by proton transfer. Theoretical models demon-
strated that a shape-persistent cyclohexyldiamine bridge
would also be suitable to bring the two fullerenes into close
contact (d = 3.09 ꢀ, vide infra). The fullerene–fullerene–
amine triad 1 was complemented by NDI–fullerene–amine
[15]
triads 2–5 with NDIs of different p acidity, and fullerene–
amine dyad 6 as a negative control (Scheme 1, Figures S1–S3).
The synthesis of all six anion–p catalysts started with
a Bingel reaction of fullerene 7 with malonamide 8, which was
readily accessible from commercially available starting mate-
Figure 2. a) UV/Vis absorption spectra of 2 (cyan), 6 (black) and 13
(
dotted cyan) in CHCl . b) CD spectra of 2 (cyan), 6 (black) and 13
3
(
dotted cyan) in CHCl . c) CD spectra of 1 (purple), 6 (black); inset:
3
CD of 1 at 430 nm with increasing concentrations of 21. d) DPV of 2
(cyan), 6 (black) and 13 (dotted cyan). e) DPV of 4 (red), 6 (black), 15
(dotted red), with pertinent part of the energy-minimized structure of
4
. f) DPV of 1 (purple) and 6 (black). DPV was performed in CH Cl2
2
with tetrabutylammonium hexafluorophosphate (TBAPF , 0.1m) as the
6
+
supporting electrolyte and Fc /Fc as the internal reference.
of 1–5 contained strong positive and negative CD Cotton
effects from 350 nm up to 700 nm (Figure 2b,c, Figure S6).
The p-stacked immobilization of the NDIs and the second
[
16]
fullerene in the (+)-sector
of the central fullerene con-
ceivably accounted for the distinct positive CD Cotton effect
at 430 nm, whereas Boc and amine termini were insufficiently
immobilized on the fullerene surface to afford a strong
induced CD for 6. All these spectroscopic characteristics
supported strong electronic coupling between the two chro-
mophores in the respective dyads.
Scheme 1. a) 3-tert-Butoxy-3-oxopropanoic acid, (R,R)-N,N-dimethyl-
cyclohexane-1,2-diamine, HBTU, TEA, CH Cl , 20 min, RT, 87%;
2
2
b) 1. I , DBU, toluene, 2 h, RT, 57%; 2. TFA, CH Cl , RT, overnight,
2
2
2
quant.; c) 3-tert-butoxy-3-oxopropanoic acid, 1-adamantanemethyl-
amine, HBTU, TEA, CH Cl , 1 h, RT, 95%; d) 1. I , DBU, toluene, 2 h,
2
2
2
RT, 41%; 2. TFA, CH Cl , RT, overnight, 97%; 3. 10, HBTU, TEA,
The redox properties were determined by cyclic (CV) and
differential pulse voltammetry (DPV) versus Fc /Fc as
2
2
CH Cl , 3 h, RT, 88%; 4. TFA, CH Cl , RT, 2 h, quant.; e) HBTU, TEA,
+
2
2
2
2
CH Cl , RT, overnight, 55%; f) HBTU, TEA, CH Cl , 4 h, RT, 90%;
2
2
2
2
internal standard. The first irreversible reduction potential
of 1 appeared + 40 mV compared to 6 (Figure 2 f, Figures S7
and S8, Table 1). Results from fullerene dimers in the
g) see the Supporting Information; h) 1. TFA, CH Cl , RT, 2 h, quant.;
2
2
2
. HBTU, TEA, CH Cl , RT, 4 h, 73% (2), 57% (3), 56% (4), 49% (5).
2 2
HBTU=2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexa-
fluorophosphate, DBU=1,8-diazabicyclo[5.4.0]undec-7-ene,
TFA=trifluoroacetic acid.
[12]
literature
confirmed that electron sharing accounts for
this lowering of the LUMO energy.
2
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Table 1: Characteristics of catalysts.
[
a]
[e]
[f]
Entry
Cat.
DV
MEP
[kJmol
a
A/D1
A/D2
[
b]
À1 [c]
[d]
[
mV]
]
[a.u.]
1
2
3
4
5
6
7
TEA
6
2
3
4
–
0
+30
0
–
–
903
0.6
2.3
2.7
1.6
2.2
2.0
5.2
1.8
8.2
9.2
6.1
7.6
13.8
16.6
14.2
15.7
17.0
13.9
1016
1130
1141
1127
1499
À50
5
1
À140
+40
6.3
22.5
[a] Catalysts, see Scheme 1; TEA=triethylamine. [b] Difference between
the first reduction potential of fullerene within the catalysts and the first
reduction potential of control 6 (from DPV experiments). [c] Highest
positive potential on the MEP surfaces of the central fullerenes.
[d] Computed polarizability. [e] Yield of addition/yield of decarboxylation.
Reactions were conducted with 20 mol% catalyst 1–6, 200 mm 17 and
1
2
.0m 18 and at 208C in [D ]THF, and monitored by H NMR
8
spectroscopy. [f] As with [e] in [D ]THF/CDCl 1:1.
8
3
In NDI–fullerene 2, lowered and raised LUMO levels for
fullerenes (irreversible, + 30 mV) and NDIs (reversible,
À50 mV), respectively, were consistent with the increasing
intrinsic p acidity of the fullerene due to the transfer of
electron density to the NDI (Figure 2d, Figures S7 and S8,
Table 1). In 3, the NDI has two sulfide donors in the core.
Figure 3. a) The base-catalyzed reaction between MAHT 17 (PMP: p-
methoxyphenyl) and enolate acceptor 18 to afford addition product 19
A) or decarboxylation product 20 (D). b) Energy-minimized cutaway
(
[15]
Consistent with the weaker p acidity of this NDI,
the
structure of triad 1 (without adamantyl solubilizer), with indication of
the equilibrium control between reactive intermediates RI and RI on
LUMO level of the fullerene in 3 did not decrease (Figures S7
and S8). Conversion of the sulfide donors in 3 to sulfoxide and
sulfone acceptors in the NDIs in 4 and 5 shifted the first
fullerene reduction potential by À50 mV and À140 mV,
respectively (Figure 2e, Figures S7 and S8). These shifts
were possibly caused by repulsive NDI radical anions.
Positive shifts of the NDI reduction potentials suggested
that electron density is transferred from the NDI through
back-donating lonepair–p interactions from the SÀO donors
A
D
À
the central fullerene by the remote fullerene and intercalators 21 (PF
6
salt) and 22. c) A/D values of catalysts 1 and 6 (40 mm) in the
R
absence and presence of 21 or 22 (800 mm; 200 mm 17, 2m 18,
[
D ]THF/CDCl 1:1; A/D =A/D(1+21)/ A/D(1), etc). d–g) Comparative
8 3 R
summary of d) polarizability a, e) catalysis (A/D values), f) first full-
erene reduction potential relative to 6 and g) deepest p holes on MEP
surfaces for catalysts 1, 2 and 6 (compare Table 1).
À1
to the fullerene acceptors (Figure 2e, Figure S4).
+ 30 mV, deeper p holes; + 2.8 kJmol ) and the induced p
The addition of malonic acid half thioester (MAHT) 17 to
nitroolefin 18 was selected to probe anion–p catalysis with
acidity (higher polarizability; + 113.4 a.u.) on the active
fullerene surface. As expected for the sulfide donors in the
[3–5]
triads 1–5 (Figure 3a).
This transformation has emerged as
NDI core, anion–p catalysis by 3 dropped to A/D = 1.6 and
1
the benchmark reaction. Recent computational studies have
confirmed that selective recognition of the planar, charge-
A/D = 5.6 (entry 4). Partial recovery of activity by triad 4
2
suggested that higher fullerene LUMO levels (À50 mV) are
compensated by higher polarizability (+ 238 a.u., entry 5).
The poorer activity of triad 5 was consistent with stronger
back-donating lonepair–p interactions from sulfone oxygens
in the NDI core (À140 mV, entry 6).
delocalized “enol” tautomer in reactive intermediate RI on
A
p-acidic surfaces promotes the formation of the biologically
relevant but disfavored addition product 19 (A), while weaker
anion–p interactions with the bent, charge-localized “keto”
intermediate hinder decarboxylation before enolate addition
and thus the formation of the irrelevant product 20 (D,
With fullerene-fullerene-amine triad 1, anion–p catalysis
increased to A/D = 5.2 and A/D = 22.5 (entry 7). This
1
2
[5,7]
Figure 3a,b).
activity is outstanding not only in the context of this work,
The fullerene–amine dyad 6 conceived here as a starting
point was identified as a powerful anion–p catalyst that
inverts the preference for decarboxylation with triethylamine
it is a new record for anion–p interactions in organocatalysis
[
3–5,7]
(excluding anion–p enzymes).
The activity found for triad
1 exceeded expectations from the lowering LUMO levels of
fullerenes from 6 to 2 and 1 clearly (Figures 3e,f) and
disagreed with predictions based on p holes on the central
fullerene (Figures 3e,g). However, the exceptional activity of
fullerene–fullerene–amine triad 1 corresponded well with the
exceptional polarizability of triad 1 (Figures 3d,e; triads 3–5
could not be compared because the NDI core substituents
complicate the situation, vide supra). This correlation of
activity with polarizability was consistent with the functional
(
TEA) in [D ]THF (A/D = 0.6) to a preference for enolate
8 1
addition (A/D = 2.3, Table 1, entries 1 and 2; A/D: Yield of
1
1
9 divided by yield of 20). For the catalytic NDI–fullerene–
amine triad 2, this selectivity increased to A/D = 2.7 in
1
[
D ]THF and from A/D = 8.2 for 6 to A/D = 9.2 for 2 in the
8 2 2
more hydrophobic [D ]THF/CDCl3 1:1 (entries 2 and 3).
8
These results were in agreement with contributions from the
remote NDI to increase the intrinsic (lower LUMO levels;
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3
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Angewandte
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relevance of the oriented macrodipoles that are formed only
on contact with anions and anionic transition states, i.e.,
induced p acidity (Figure 1c).
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5
0
ure S6), thus attracting electron density from the active site
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[18]
the presence of 2,7-dimethoxynaphthalene 22
decreased
the activity of triad 1 but not control 6 by A/D(1+22)/A/D(1)
=
0
.79 (Figure 3c). This inactivation was consistent with the
complementary increase in electron density at the active site
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most active anion–p catalyst known so far but also unprece-
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tributions from polarizability outweigh static contributions to
anion–p interactions by far. In other words, induced p acidity
from the interaction of anions and anionic transition states
with the giant oriented macrodipole that is generated only by
their presence in polarizable p systems affords anion–p
interactions with highest functional relevance. This lesson
learned from fullerene-centered catalytic triads calls for
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LUMOs as low, p holes as deep and quadrupoles as positive as
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1
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8
Keywords: anion–p catalysis · fullerenes · induced p acidity ·
polarizability
1
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5
Manuscript received: April 6, 2018
Version of record online: && &&, &&&&
Angew. Chem. Int. Ed. 2018, 57, 1 – 6
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www.angewandte.org
5
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Angewandte
Communications
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Communications
Anion–p Catalysis
Self-induced transition-state stabilization
on polarizable p systems by giant ori-
ented dipoles that emerge only on con-
tact with the transforming substrates
affords highest catalytic activities: This
lesson learned from the direct compar-
ison of intrinsic and induced p acidity on
fullerene-centered catalytic triads calls for
a shift of paradigm in anion–p catalysis
from small p surfaces with high intrinsic
p acidity to p surfaces as large as
possible.
J. Lꢁpez-Andarias, A. Bauzꢂ, N. Sakai,
A. Frontera,* S. Matile*
&&&& — &&&&
Remote Control of Anion–p Catalysis on
Fullerene-Centered Catalytic Triads
6
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Angew. Chem. Int. Ed. 2018, 57, 1 – 6
These are not the final page numbers!