Anion–π Interactions in Light-Induced Reactions: Role in the Amidation of (Hetero)aromatic Systems with Activated N-Aryloxyamides

Article abstract of DOI:10.1002/chem.201903055

The importance of anion–π interactions as a driving force for chemical and biological processes is increasingly being recognized. In this communication, we describe for the first time its key participation in light-induced reactions. We show, in particular, how transient complexes formed through noncovalent anion–π interactions between electron-poor N-aryloxyamides and multiply-charged anions (such as carbonate or phosphate) can undergo facile light-promoted N?O cleavage, affording amidyl radicals that can subsequently be trapped by (hetero)aromatics.

Full text of DOI:10.1002/chem.201903055

A Journal of
Accepted Article
Title: Anion-π interactions in light-induced reactions. Their role in
the amidation of (hetero)aromatic systems with activated N-
aryloxyamides
Authors: Miquel A. Pericàs, Laura Buglioni, Marco M. Mastandrea, and
Antonio Frontera
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To be cited as: Chem. Eur. J. 10.1002/chem.201903055
Link to VoR: http://dx.doi.org/10.1002/chem.201903055
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COMMUNICATION
Anion-π interactions in light-induced reactions. Their role in the
amidation of (hetero)aromatic systems with activated N-
aryloxyamides
Laura Buglioni,‡,[a] Marco M. Mastandrea,‡,[a] Antonio Frontera,*,[b] Miquel A. Pericàs*,[a],[c]
synthetic applicability of amidyl radicals for building up molecular
complexity. In this communication, we present evidences on (i)
the formation of an anion-π complex between carbonate and an
electron-poor N-aryloxyamide, (ii) the weakening of the N-O bond
resulting from this interaction and (iii) the direct cleavage of this
bond after single electron transfer (SET) induced by visible light,
resulting in an amidyl radical that can be trapped in either
intermolecular or intramolecular fashion by electron-rich aromatic
substrates. As the first test of our hypothesis, we studied the
behavior of the 2,4-dinitro substituted aryloxy amide 1a in the
presence of K2CO3 in acetonitrile (ACN) solution. Interestingly,
the UV-visible spectra showed the appearance of a dramatic
enhancement in the absorption between 425 and 450 nm) (Figure
1, red trace). We next studied the light-promoted reaction (4.5 W
blue LED, emission centered at 460 nm, see Supporting
Information for further details) between 1a and N-methyl indole
(2a) in the presence of K2CO3 as the sole additive (Table 1).
Abstract: The importance of anion-π interactions as a driving force
for chemical and biological processes is increasingly recognized. In
this communication, we describe for the first time its key participation
in light-induced reactions. We show, in particular, how transient
complexes formed through non-covalent anion-π interactions
between electron-poor N-aryloxyamides and multiply charged anions
(like carbonate or phosphate) can experience facile light-promoted N-
O cleavage, affording amidyl radicals that be subsequently trapped by
(hetero)aromatics.
The contemporary development of light-promoted reactions has
mostly relied on the use of different types of photocatalysts, such
[3]
as metal complexes,[1] organic dyes[2] and semiconductors.
However, the presence of a photocatalyst in light-mediated
processes is not always required. One common catalyst-free
activation mode involves the formation of charge-transfer (CT)
complexes, with electron donor-acceptor (EDA) complexes as the
best known example[4] whose relevance in organic synthesis has
been documented by a plethora of reports.[5] These CT complexes
can arise from different non-covalent interaction modes,[6] as for
[ 8 ]
instance hydrogen bonds[ 7 ] and π-π interactions.
Anion-π
complexes, in turn, are transient species resulting from
interactions[9] between electron-poor aromatic systems[10] and
anions. First described in 2002,[11] their unexpected stability[12] has
been proven to derive from a combination of electrostatics and
polarization induced by the anion. In spite of the importance they
have in supramolecular chemistry[ 13 ] as well as in biological
processes,[14] the role of anion-π interactions in (photo)catalysis
remains largely unexplored.[15],[16] In the last years, Leonori and
coworkers have reported how hydroxylamine derivatives work
efficiently as nitrogen-centered radical precursors[17] in photo-
chemical transformations,[ 18 ] which are mediated either by
different kinds of photocatalyst or by the formation of EDA
complexes,[ 19 ] sometimes in the presence of inorganic base
(K2CO3).[20] Since multiply charged anions, like carbonate, are
privileged actors in anion-π interactions, we wondered whether
an anion-π interaction might enable the light-mediated formation
of amidyl radicals. Conceptuality and practicality aside, this new
activation mode might offer a new opportunity to harness the
Figure 1. UV-visible absorption studies.
Table 1. Screening and optimization of reaction conditions for the amidation of
N-methylindole (2a) with 1a.
[a]
[b]
Dr. L. Buglioni, M. M. Mastandrea, Prof. Dr. M. A. Pericàs
Institute of Chemical Research of Catalonia (ICIQ), The Barcelona
Institute of Science and Technology,
Avda. Països Catalans 16, E-43007 Tarragona, Spain
E-mail:
NO2
K2CO3 (2.0 equiv)
Ac
O
Ac
N
N
+
N
Me
3a
N
Me
Me
ACN (0.1 M)
blue LEDs
overnight
Prof. A. Frontera
Me
O2N
Department de Química, Universitat de les Illes Balears,
Crta. de Valldemossa km 7.5, E-07122 Palma de Mallorca, Baleares
(Spain)
1a
2a
E-mail:toni.frontera@uib.es
[c]
‡
Prof. Dr. M. A. Pericàs
Departament de Química Inorganica i Orgànica, Universitat de
Barcelona,
Martí i Franqués 1-11, 08028 Barcelona, Spain
These authors contributed equally.
Entry
Variation from standard conditions[a]
None
Yield (%)[b]
1
2
3
4
5
75
Acetone instead of ACN
DMSO instead of ACN
1.5 equiv. of 2a
63
68
72
<5[c]
Supporting information for this article is given via a link at the end of
the document.
23 W CFL instead of blue LEDs
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The scope of this transformation was next investigated under the
optimal reaction conditions (Scheme 1). Modifying the
substituents of either the amide in 1 or of the N-substituted indole
did not affect the reaction outcome. In fact, the corresponding
amidated products 3a-3i were isolated in good to high yields, and
the same stands for 3j, derived from N-phenylpyrrole. When
6
Green LEDs instead of blue LEDs
29[d]
37[d]
18[d]
<5[c]
25[d]
31[d]
15
7
White LEDs instead of blue LEDs
No K2CO3, 48 h
8
9
No light
10
11
12
13
14
15
16
17
NaH2PO4 instead of K2CO3
Na(PhO)2PO2 instead of K2CO3
(NEt4)HCO3 instead of K2CO3
K2HPO4 instead of K2CO3
Na2(PhO)PO3 instead of K2CO3
K3PO4 instead of K2CO3
Et3N instead of K2CO3
switching to different aromatic carbocycles as partners,
a
decrease in yield was generally observed (3k-3n), with the
notable exception of azulene, which afforded the amidated
product 3o in high yield. Starting from 1,2-dimethyl indole and
1,2,3-trimethyl indole, the light-promoted amidation proceeds
efficiently at C-3 (products 3p and 3q), leading in the second case
to dearomatization. When 1,3,3-trimethyl-2-methyleneindoline
(Fischer base) is used as radical acceptor, the amidation took
place at the exo double bond, affording 3r in moderate yield and
good diastereoselectivity. We next explored the possibility of an
intramolecular version of the light-promoted amidation reaction,
which would afford pharmaceutically valuable pyrroloindoline
motifs 5 (Scheme 2).[21] Inspired by a previous work by Wang and
co-workers,[22] we synthesized compound 4, which was irradiated
with blue LEDs in the sole presence of potassium carbonate
(Scheme 2A). Although complete consumption of the starting
material was observed, the desired compound 5a was not
obtained. This experiment strongly suggests that the putative
tertiary radical intermediate that could be originated by N-O
cleavage from 4 is not able to reduce the starting material and,
thus, the radical chain propagation step cannot take place. To
circumvent this limitation, tert-butyl thiol and TEMPO were added
to the reaction mixture as radical scavengers, affording the
desired cyclized products 5b and 5c in 60% and 79% yield,
respectively (Scheme 2B and 2C). Taking all these results into
account, the tentative mechanistic proposal shown in Scheme 3
was formulated for an anion-π driven, light-induced amidation.
Complex A, an anion-π complex generated by the interaction of
the electron-poor aryloxyamide 1a with K2CO3, can be excited to
A* by blue LED-light irradiation (Figure 1). This excited state can
then undergo an intramolecular single electron transfer (SET)
event, leading to the disassembly of the complex and generating
the radical anion C, which readily fragments into the amidyl radical
D and the phenolate E. The former can attack the N-methyl indole,
affording radical F. This radical can contribute to the chain
propagation[23] by reducing 1a (Ered= -0.26 V vs SCE) to the
readily cleavable intermediate C[18b] and giving rise to the carbo-
cation species G, whose base-induced deprotonation leads to the
amidated product 3a. Another intriguing possibility involves the
reduction of radical B, which affords G from F regenerating the
carbonate species.[24]
75
75
75
7
Cs2CO3 in acetone instead of K2CO3
50
[a] Standard conditions: 1a (0.2 mmol), 2a (0.4 mmol), K2CO3 (0.4 mmol), ACN
(2.0 mL) blue LEDs (4.5 W) r.t., overnight; [b] After flash column
chromatography; [c] By 1H NMR of the reaction crude. [d] Incomplete reaction
(30% conversion).
We were pleased to observe that working under the standard
conditions of Table 1 the reaction proceeded to completion over-
night, affording the amidated indole 3a in 75% yield (entry 1). The
substitution of acetonitrile with other solvents such as acetone
(entry 2) or DMSO (entry 3) lowered the yield (63 and 68%,
respectively). Decreasing the ratio between 1a and 2a led to a
slight erosion of the yield, as well (entry 4). The nature of the light
source revealed to be determinant for the reaction outcome of the
transformation. In fact, when either a compact fluorescence lamp
(CFL), or green or white LEDs were employed, the reaction
became much slower with consequent lower yields (entries 5, 6
and 7). Importantly, both the presence of carbonate and of a light
source proved to be necessary for the success of the
transformation. (entries 8 and 9, respectively).
R1
N
NO2
R1
K2CO3 (2.0 equiv.)
O
H
R2
N
R2
+
Ar
Ar
ACN (0.1 M)
Blue LEDs
overnight
O2N
1
2
3
3a, R1 = Me,
3b, R1 = Me,
3c, R1 = Me,
3d, R1 = Me,
3e, R1 = Me,
3f, R1 = Me,
3g, R1 = Bn,
R2 = Ac, 75%
R2 = CO2Me, 79%
R2 = COPh, 68%
R2 = CO2Bn, 61%
R2 = dmTroc, 82%
R2 = Boc, 60%
R2 = Ac, 73%
R1
N
R2
Ac
N
Me
N
Bn
N
(A) K2CO3 (2.0 equiv.)
O
Me
3h, 65%
N
Blue LEDs ACN (0.1 M)[a]
N
Me
Ac
OMe Ac
Me
Me
Ac
N
Ac
N
N
N
Me
OMe
N
N
Bn
Me
Br
5a
Me
Ph
NO2
MeO
Me
H
3i, 50%
3j, 84%
3k, 45%
OMe
Ac
3l, 47%
(B) K2CO3 (2.0 equiv.)
O
N
O
OMe Me
N
N
NO2
Blue LEDs ACN (0.1 M)
tBuSH (2.0 equiv.)
O
N
Me
H
Ac
N
Ac
N
Me
N
Me
Me
3n, 40%
Me
4
5b, 60%
3m, 48%
Ac
3o, 87%
Me
Me
(C) K2CO3 (2.0 equiv.)
Me
Me
Me
N
N
Ac
Me
Me
N
O
Me
O
Ac
N
Me
Blue LEDs ACN (0.1 M)
TEMPO (2.0 equiv.)
Me
Me
N
N
N
Me
N
Me
Me
3q, 57%
N
Me
H
3p, 76%
3r, 35%, d.r. = 5:1
Me
5c, 79%
Scheme 1. Intermolecular light-promoted amidaton
Scheme 2. Intramolecular light-promoted amidaton. Reaction conditions: blue
LEDs (4.5 W), r.t., overnight. Yields measured after column chromatography.
[a] The only isolated product was 1-Methylindole-3-carboxaldehyde (20% yield).
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10.1002/chem.201903055
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component, since the use of caesium carbonate in acetone
O
resulted deleterious (entry 17). However, when solutions of 1a
NO2
O2N
NO2
[ 25 ]
N-methyl
indole
O
Ac
and (Bu4N)3PO4
in acetonitrile were mixed in an inert
Ac
E
N
N
environment (glovebox), as well as in the dark, an immediate and
complete decomposition of 1a took place, suggesting an extreme
photochemical and thermal lability of A. To confirm these
qualitative observations on the formation and lability of anion-π
complex A, the ability of the different bases to form complexes
with the substrate 1a was analysed by DFT calcu-lations at the
B3LYP-D/6-311+G* level of theory. Firstly, the molecular
electrostatic potential (MEP) was computed, plotted onto the Van
der Waals surface in order to know which are the most
electrophilic and nucleophilic parts of the substrate (Figure 2). The
most negative region corresponds to the O atom of the carbonyl
group (–30 kcal/mol, red colored). On the other hand, the
electrostatic potential over the aromatic ring is positive (+23.8
kcal/mol, blue colored) due to the electron-withdrawing effect of
both nitro groups. Therefore, the substrate is well suited to
establish anion–π interactions. In addition, the H atom of the N-
bonded methyl group that points to the aromatic surface also
exhibits a large and positive MEP value (+30.7 kcal/mol, see
Figure 1a). In summary, this MEP analysis anticipates that anionic
bases will interact with the substrate by means of a combination
of H-bonding and anion–π interactions.
Me
Me
Ac
N
O2N
N
Me
2-
CO3
Me
1a
D
F
Radical
chain
NO2
2-
CO3
CO3
Me
1a
O
Ac
N
N
O
Ac
B
Me
O2N
2-
O2N
CO3
NO2
C
A
SET
Ac
N
Me
Base
CO3
hν
B
N
Me
2-
*
CO3
Me
N
Ac
G
O2N
O
Ac
N
Me
NO2
A*
N
Me
3a
Scheme 3. Proposed mechanism of the anion-π driven photo-amidation.
Although the anion-π interaction looks very relevant for the
outcome of this transformation, an alternative mechanism could
in principle be also operate in the observed transformation
involving the homolytic cleavage of the N-O bond in 1a due to the
absorption of light of the proper wavelength (see also the
Supporting Information). The possible light-induced homolysis of
1a was tested (Scheme 4) by irradiating 1a in the standard
conditions of this study (blue LED, 4.5 W) and in the presence of
1,4-cyclohexadiene (6). The formation of the homolysis products
7 and 8 under these conditions was observed by NMR, proving
that also the homolytic cleavage could have some relevance in
the reaction outcome. However, it is important to note that the
possible contribution of this initiation mode appears to be rather
unimportant, as shown by the results of the amidation reaction
performed in the absence of base (Table 1, entry 8), where only
a 18% yield of 3a is achieved after 48 h reaction.
Figure 2. Closed (a) and open (b) MEP surfaces of 1a.
d3-ACN (0.1 M)
4,5 W Blue LEDS
NO2
NO2
Overnight
O
Me
HO
Me
Ac
N
N
Next, the EDA complexes of the 1a with monoanionic
(hydrogencarbonate) and dianionic (carbonate) bases were
computed both in the gas phase and taking into consideration
solvent effects (using a polarized continuum model).[ 26 ] Key
geometric features of the optimized geometries and interaction
energies of the CT complexes are shown in Figure 3. As predicted
by the MEP analysis, the complexes are stabilized by the
concurrent formation of anion–π and H-bonding interactions in all
cases (see Supporting Information). The corresponding
equilibrium distances are shorter for the dianionic molecules
because the electrostatic forces are stronger. Consequently, the
interaction energies are significantly greater in absolute value for
the dianionic bases. Thus, for all monoanionic complexes the
interaction energies in acetonitrile range from –1.7 to –0.9
kcal/mol (see also Supporting Information), while for the dianionic
complexes the interaction energies are all very similar (around –
5 kcal/mol). Interestingly, this correlates well with the ability of the
different bases (Table 1) to promote the light-induced amidation
and, in particular, with the observation that essentially the same
yield (ca. 75%) is recorded for all the studied dianionic bases.
From the geometrical point of view, it is important to remark that
the N–O distance increases upon formation of the EDA complex.
The N–O distance in 1a is 1.42 Å, which goes up to 1.43 Å in all
EDA complexes, thus revealing a weakening of the N–O bond
upon complexation.
H
Ac
O2N
NO2
1a
6
7
8
5.0 equiv
50% Conversion
Scheme 4. Study on the homolytical fragmentation of 1a. Reaction conditions:
1a (0.2 mmol), 6 (1.0 mmol), deuterated-acetonitrile (2.0 mL), blue LEDs (4.5
W), r.t. overnight. The conversion was measured by 1H NMR.
At this point, it became important to assess the influence of the
base used in the amidation reaction, for its fundamental role in the
generation of the anion-π complex. From the observation of the
results summarized in Table 1, entries 10-16, we can deduce that:
i) the use of a neutral base, such as trimethylamine is deleterious
for the transformation (entry 16), ii) the use of singly charged
anions has a negative effect on the outcome of the reaction
(entries 10-12), and iii) doubly or triply charged anions are those
giving the best results (entries 13-15). Interestingly, these
observations perfectly correlate with the expected strength of
anion-π interactions.
In an attempt to perform photochemical studies on A, which would
be facilitated by a high concentration of the complex, soluble
tetrabutylammonium phosphate was used as the anionic
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of 1a increases the probability of excitation and decreases the
absorption energy (see Supporting Information).
Figure 3. B3LYP-D/6-311+G* optimized geometries and energies of the
bicarbonate (a) and carbonate (b) complexes of 1a. Distances in Å.
A key point in our mechanistic hypothesis is the absorption of one
photon by complex A leading to the generation of the excited state
A*, where an internal SET can take place. When the frontier
orbitals of complex A are analyzed, it can be observed (Figure 4)
that the HOMO is located in the anion region (a), while the LUMO
is concentrated in the nitro groups and one carbon atom of the
aromatic group. The very small computed HOMO-LUMO
difference (1.83 eV) anticipates that low-lying excited states are
available for this structure, and that internal single electron
transfer should be quite feasible.
Figure 5. Optimized structure of C (distance in Å).
Second, when the homolysis of the N–O bond required for the
generation of the amidyl radical D is considered, it is found that
the direct homolysis of A yielding carbonate dianion and radicals
H and D is not energetically favorable (52.2 kcal/mol), being
almost identical to the calculated for 1a (54.7 kcal/mol, Supporting
Information). In contrast, the transformation of A to C with the
concomitant formation of [CO3·–] is more achievable [ΔGr(A→C)
= 6.1 kcal/mol] and, remarkably, the subsequent homolysis of the
transient intermediate C, yielding D and E is predicted to be
exergonic [ΔGr(C → D + E)= –10.6 kcal/mol]. The calculated
activation energy for this homolysis step is very low, i.e. ΔGact
=
3.2 kcal/mol, thus confirming the feasibility of this mechanistic
route (see Supporting Information for further details).
2-
Figure 4. Plots of the HOMO (a) and LUMO (b) of complex A. (c) Spin density
plot of the biradical open singlet configuration of A.
NO2
CO3
Me
N Ac
O
CO3
Me
N
O
hν + SET
- CO3
O
Ac
Ac
N
O2N
O2N
Me
O2N
NO2
NO2
A
Unstable
C
Moreover, since small HOMO-LUMO gaps are indicative of
biradical or biradicaloid character, we examined the presence of
this character in A. To do so, the electronic structure of the
complex was re-calculated using an open-shell singlet
configuration, this leading to an energy drop of 1.4 kcal/mol (i.e.;
the biradical is predicted to be more stable than the closed-shell
system). The spin density plot of this configuration is depicted in
Figure 4c and it clearly shows that the alpha electron (blue) is
located basically in the aromatic ring and nitro groups, whereas
the beta electron is located in the carbonate. It thus corresponds
to a situation where an internal SET has taken place from the
putative complex A*. It is worth to comment that the attempted
geometrical optimization of this configuration leads to the
dissociation of the complex because of the mutual electrostatic
NO2
NO2
ΔGr
ΔGr
=
=
6.1 kcal/mol
A
C
C
D
+
+
CO3
O
N
Ac
O
Me
N
Ac
-10.6 kcal/mol
= 3.2 kcal/mol
Me
E
O2N
O2N
ΔGact
C
E
D
Scheme 5. Energetics of the calculated generation of amidyl radical D from
anion–π complex A.
In conclusion, anion-π interactions have been identified for the
first as an enabling phenomenon in light-mediated processes.
Anion-π interactions involving carbonate anion have been
described by spectroscopic and theoretical methods as the
enabling step in the light-promoted amidation of arenes with
activated aryloxyamides. The available evidence supports the
hypothesis that π–interactions between carbonate and the
electron-poor aryloxyamide facilitate the absorption of visible light
(blue LED) promoting the generation of a low-lying excited state
that spontaneously experiences internal SET and disintegration
of the complex, providing a low-energy pathway for the generation
of highly reactive amidyl radical. Our results suggest that other
·–
repulsion of two negative charges [CO3 ····1a·–]. Interestingly,
the optimized structure of C (1a·–) exhibits an elongation of the N–
O bond with respect to 1a (Figure 5).
Additional data from two different origins further confirm the
relevance of the anion-π complex in the light-mediated cleavage
of 1a. First, the results of time-dependent DFT calculations show
that the presence in A of the anion interacting with the π-system
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10.1002/chem.201903055
Chemistry - A European Journal
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poorly understood light-promoted processes involving the use of
inorganic bases can also result from overlooked anion-π
interactions.
product was purified by silica gel column chromatography (Petroleum
ether/EtOAc 9:1 to 4:1).
Acknowledgements
Experimental Section
This work was funded by MINECO (grant CTQ2015-69136-R,
MINECO/FEDER) and the CERCA Programme/Generalitat de
Catalunya. We also thank the Severo Ochoa Excellence
Accredita-tion 2014-2018 (SEV-2013-0319). L.B. acknowledges
the COFUND-Marie Curie action of the European Union’s FP7
(Ref. 291787 - ICIQ-IPMP) for a postdoctoral fellowship.
The corresponding aryloxyamide 1 (0.2 mmol) was mixed in a 10 mL vial
with K2CO3 (0.4 mmol, 55 mg) and an aromatic compound 2 (if solid) (0.4
mmol). Then the vial was sealed and three cycles of vacuum-refill with Ar
were performed. After adding ACN (2.0 mL) and degassing with a flow of
argon during 10 minutes, the aromatic compound 2 (0.4 mmol) was added
with a syringe (when liquid) and the mixture was stirred under blue LEDs
(4.5 W) irradiation overnight. Afterward, it was washed with water (5.0 mL),
extracted with EtOAc (3x4.0 mL) and dried over MgSO4. The reaction
mixture was then filtered and concentrated under vacuum. The desired
Keywords: non-covalent interactions • anion-π complex •
photocatalysis • amidyl radical
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10.1002/chem.201903055
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COMMUNICATION
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COMMUNICATION
Largely
formation
complexes triggers light-
mediated processes by
overlooked,
the
Laura Buglioni, Marco M.
of anion-p
Mastandrea, Antonio Frontera,*
Miquel A. Pericàs*
Page No. – Page No.
Anion-π interactions in light-
induced reactions. Their role
facilitating photon absorption
and intramolecular single
electron transfer
in
the
amidation
of
(hetero)aromatic
systems
with activated N-aryloxyamide
Anion-π complexes: Hidden landmarks on
the way to photogenerated amidyl-radicals
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