Ad Hoc Adjustment of Photoredox Properties by the Late-Stage Diversification of Acridinium Photocatalysts

Article abstract of DOI:10.1021/acs.orglett.1c01673

The steadily growing interest in substituting precious-metal photoredox catalysts with organic surrogates is vibrantly sustained by emerging methodologies to vary their photochemical behavior. Herein, we report an ad hoc approach for the preparation of acridinium salts with a particularly broad range of photoredox properties. The method involves an aryne-imine-aryne coupling to a linchpin tetrafluoro acridinium salt for a late-stage diversification by nucleophilic aromatic substitution reactions to form diaminoacridinium and undescribed aza-rhodol photocatalysts. The different functionalities and redox properties of the organic acridinium photocatalysts render them suitable for bifunctional photoredox catalysis and organocatalytic photochemical C-N cross-couplings.

Full text of DOI:10.1021/acs.orglett.1c01673

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Letter
Ad Hoc Adjustment of Photoredox Properties by the Late-Stage
Diversification of Acridinium Photocatalysts
Valeriia Hutskalova and Christof Sparr*
Cite This: Org. Lett. 2021, 23, 5143−5147
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sı Supporting Information
ABSTRACT: The steadily growing interest in substituting pre-
cious-metal photoredox catalysts with organic surrogates is
vibrantly sustained by emerging methodologies to vary their
photochemical behavior. Herein, we report an ad hoc approach for
the preparation of acridinium salts with a particularly broad range
of photoredox properties. The method involves an aryne−imine−
aryne coupling to a linchpin tetrafluoro acridinium salt for a late-
stage diversification by nucleophilic aromatic substitution reactions
to form diaminoacridinium and undescribed aza-rhodol photo-
catalysts. The different functionalities and redox properties of the
organic acridinium photocatalysts render them suitable for bifunctional photoredox catalysis and organocatalytic photochemical C−
N cross-couplings.
2
0
hotoredox catalysis has markedly impacted organic
chemistry as a transformative strategy for molecular
from 1,2-dihydrobenzoazete I as reported by Yoshida et al.
P
coupled with an acridane oxidation and a versatile double late-
stage diversification of tetrafluoro acridinium salts by
nucleophilic aromatic substitutions (S Ar) would enable the
ad hoc adjustment of the photoredox properties of acridinium
catalysts 1 (Scheme 1b). While fluorine groups at the
heterocyclic core were considered ideal as leaving groups for
the S Ar diversifications, it was expected that imines with a
bulky mesityl group both prevent undesired 6π-electro-
1
−3
functionalization.
In particular, the catalytic generation of
open shell intermediates under mild reaction conditions
provides a continuous impetus for conceptual advances in
synthetic methodology. Due to their favorable photophysical
features, ruthenium and iridium polypyridyl complexes thereby
N
4
,5
emerged as the most suitable photocatalysts (PCs).
N
However, their high price and the scarcity of precious metals
20
9
prompt the utilization of organic photocatalysts as a
cyclizations and photobleaching of the anticipated catalysts.
6
−8
sustainable alternative.
In recent findings, acridinium salts,
9,10
To explore the feasibility of our concept, we thus started
with the preparation of the tetrafluorinated acridinium salt 4,
which serves as a linchpin intermediate for the crucial late-
stage diversifications (Scheme 2). Using readily available
compounds 2 and 3, we embarked on optimizing the aryne−
imine−aryne coupling and the ensuing acridane oxidation to
form key intermediate 4 (Table S1; see the Supporting
Information for details).
The highest yields for the aryne−imine−aryne coupling
were achieved with cesium fluoride in acetonitrile, whereas
nitrosonium tetrafluoroborate proved to be the most efficient
reagent for the oxidation step. Interestingly, only a few
examples of the application of nitrosonium salts for acridane
introduced by Fukuzumi and refined by Nicewicz, stand out
as a particularly advantageous class of organic photocatalysts.
To access prototypical acridinium photocatalysts, addition
reactions to acridones (A)
B) are regularly applied (Scheme 1a). To expand the
11
12−14
and Bernsthen-type reactions
15
(
diversity of acridinium catalysts, our group developed a route
starting from esters using 1,5-bifunctional organometallic
1
6,17
reagents (C)
while another recent approach implements
18
the conversion of xanthylium salts (D). Owing to the distinct
modularity of acridinium salts, the photophysical properties
can be significantly tuned by structural modifications using
these catalyst preparation methodologies. In particular,
residues directly attached to the acridinium core (R and
1
9
2
21
oxidations are described in the literature, while the treatment
3
R ), such as amino or methoxy groups, were found to exert
strong electronic effects, thereby suitably impacting the
photoredox behavior of the catalysts. However, contemporary
methods require the early introduction of these functionalities,
necessitating multistep syntheses to alter the photoredox
behavior.
Received: May 18, 2021
Published: June 10, 2021
We therefore envisioned that the elegant aryne−imine−
aryne coupling through aza-o-quinone methides II formed
©
2021 American Chemical Society
https://doi.org/10.1021/acs.orglett.1c01673
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Letter
Scheme 1. Background and Acridinium Diversification
Scheme 3. Substrate Scope
Scheme 2. Substrate Synthesis
of the acridane with chloranil, air, MnO , and Bobbitt’s salt
2
resulted in either decomposition or unreacted starting material.
After optimization, the two steps allowed short reaction times,
an unproblematic isolation, and thus an operationally simple
synthetic protocol toward the tetrafluoroacridinium salt 4
(
Scheme 2).
Having acridinium intermediate 4 in hand, we focused our
attention on the pivotal acridinium late-stage functionalization
reactions. To test our notion of the disposition of the
fluorinated acridinium core to nucleophilic attack, enabling the
ad hoc preparation of photophysically distinct acridinium
derivatives, we began with substitution reactions using
a−g
a
Yields of isolated products. Conditions: 4 (47.0 μmol), R NH
2
b
(
188 μmol), CH Cl (1.4 mL), rt; 4 (47.0 μmol), R NH·HCl (470
2
2
2
c
μmol), Cs CO (470 μmol), CH CN (3.3 mL), rt; 4 (47.0 μmol),
2
3
3
d
NH (188 μmol, 0.5 M, dioxane), THF (1.5 mL); 4 (90.0 μmol),
3
e
R NH (360 μmol); Boc-1i or Boc-1j (20 μmol), TFA (2.0 mL),
2
azetidine (Scheme 3). Gratifyingly, an efficient 2-fold S Ar
f
N
CH Cl (0.5 mL); 4 (47.0 μmol), R NH (188 μmol), NEt (94.0
μmol), CH Cl (6.0 mL)/THF (2.0 mL), rt; 4 (1.9 mmol).
2 2 2 3
g
at the most electrophilic positions (C3 and C6) was observed
within 1 hour at ambient temperature, yielding desired
diazetidine product 1a in 88% yield. Interestingly, neither
reduced amounts of amine nor substantial dilution led to
monosubstitution. To determine the scope of this method,
various amines were next subjected to the reaction. To our
delight, the reactions proceeded smoothly, giving rise to the
desired acridinium photocatalysts in excellent yields. Azeti-
dines with fluorine and methoxy groups, which considerably
alter the electronic properties, were readily introduced (1a−d).
Furthermore, free amino groups were also installed by using
ammonia in dioxane (1e). Nonetheless, all attempts to add
2
2
arylamines resulted in undetectable conversions. On the
contrary, selective reactions were observed for the 2-fold
addition of pyrrolidines to provide 1f or 1g and also a gram-
scale synthesis of diaminoacridinium catalyst 1f was feasible
(85% yield). The nucleophilic aromatic substitution with
morpholine and Boc-piperazine led to a yield of ≥90%, and
Boc deprotection reactions were readily achieved. Acyclic
amines were also efficiently introduced (1k, 91%) even with
propargylic moieties (1l, 88%), and the double substitutions of
5
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Letter
L-β-proline and 7-azabicyclo[2.2.1]heptane both proceeded
with 90% isolated yield. Notably, compounds 1l and 1m
comprise reactive functional groups that allow further
manipulations, giving access to an even broader scope of
applications. Indeed, acridinium salt 1l could be effortlessly
coupled by an azide−alkyne click reaction under standard
Table 1. Spectrophotometric and Electrochemical Data
−
−
E
E1/2(PC/PC )
E1/2(PC*/PC )
0
,0
a
b
1
PC
(eV)
(V)
(V)
τ( PC) (ns)
1
a
b
c
d
e
f
2.40
2.42
2.45
2.50
2.63
2.37
2.48
2.38
2.37
2.38
2.39
2.46
2.37
2.40
2.35
2.35
2.40
2.42
2.40
−0.98
−0.94
−0.89
−0.80
−0.99
−0.97
−0.85
−0.73
−0.73
−0.73
−0.89
−0.75
+1.42
+1.48
+1.56
+1.70
+1.64
+1.40
+1.61
+1.65
+1.64
+1.65
+1.50
+1.71
+1.31
+1.59
+2.02
+1.15
+1.03
+1.01
+1.03
3.7
3.4
3.4
3.2
1
1
1
1
1
1
1
22
conditions (Scheme 4). No other side products were
observed after the 2-fold copper-catalyzed cycloaddition, and
the desired bis-triazole acridinium compound 5 was isolated in
3.9
0.6 (89%)
5.1 (54%)
5.4 (96%)
4.7 (56%)
4.3 (59%)
3.1 (89%)
4.4 (64%)
1.4 (48%)
4.2
5.9 (76%)
5.4
5.02
4.8
5.4 (95%)
7
9% yield.
g
h
Scheme 4. Double Click Reaction with an Acridinium Salt
Boc-1i
Boc-1j
1
1
1
1
4
6
7
7
7
k
l
c
m
n
−1.06
−0.81
−0.33
−1.20
−1.36
−1.41
−1.36
a
b
c
a
λmax(abs) from 458 to 513 nm; λ (em) from 494 to 551 nm; ε
max
Captivatingly, aza-rhodols, a novel class of acridinium
photoredox catalysts, could be prepared likewise by the late-
stage functionalization strategy. A remarkable monohydrolysis
of the linchpin tetrafluoro acridinium intermediate 4 was
observed with aqueous sodium bicarbonate [6, 90% (Scheme
max
3
4
−1
−1
from 1.5 × 10 to 7.2 × 10 L mol cm . See the Supporting
Information for the full photophysical data of the catalysts. Measured
b
−
1
in 0.1 mol L n-Bu NPF in degassed, dry MeCN against SCE.
4
6
c
−1
Measured in 0.1 mol L n-Bu NPF in degassed, dry CH OH
4
6
3
against SCE.
5
)]. This oxygenation of the heterocycle combined with amine
catalysts is significantly reflected by the exceptionally broad
Scheme 5. Synthesis of Aza-Rhodol Photoredox Catalysts
−
range of excited state redox potentials from an E (PC*/P )
1
/2
−
of +1.01 V (7b) to an E1/2(PC*/PC ) of +2.02 V (4). While
diamino-substituted (1a−1n) and unsymmetric acridinium
catalysts (6 and 7a−7c) possess redox potentials similar to
those of the transition-metal-based photocatalysts [Ir[dF-
−
(
CF )ppy] (dtbbpy)]PF [E1/2(PC/PC ) = −1.37 V;
3
2
6
−
6
E1/2(PC*/PC ) = +1.21 V vs SCE], compound 4 shows
redox potentials close to that of the Fukuzumi system
MesMeAcr BF ; E (PC/PC ) = −0.57 V; E1/2(PC*/
+
−
−
[
4
1/2
−
15
PC ) = +2.06 V vs SCE]. It is thus worth pointing out
that an increase in the electron-withdrawing nature of the
substituents at the amine moiety generally leads to a stronger
oxidative character of the excited photocatalysts, as observed
for compounds 1a, 1c, and 1d (Table 1). On the other hand,
−
aza-rhodols 7a−7c possess E (PC*/PC ) and E1/2(PC/
1
/2
−
PC ) values considerably lower than those of prior
a−d
Yields of isolated products. b−dYield over two steps. Reaction
16,19
diaminoacridinium dyes,
making them suitable for trans-
a
conditions: 4 (47.0 μmol), CH Cl (2.0 mL)/NaHCO (2.0 mL), rt,
2
2
3
b
c
formations that were unfeasible with preceding acridinium
photocatalysts.
18 h; 6 (30.0 μmol), R NH (6.00 mmol), THF (4.0 mL), 24 h; 4
2
(
1.10 mmol), CH Cl (50 mL)/NaHCO (100 mL), rt, 18 h, then
2 2 3
d
Since the diversification enabled the preparation of
acridinium photocatalysts containing a variety of functional
groups, including free amines (1i and 1j), we explored the
feasibility of an unprecedented bifunctional photocatalysis
approach. Inspired by the elegant work of Akita and co-
CH Cl (50 mL), R NH (15.7 mmol), 1 h, rt; 6 (30.0 μmol), R NH·
HCl (3.00 mmol), Cs CO , CH CN (4.0 mL).
2
2
2
2
2
3
3
S Ar reactions gave rise to aza-rhodols 7a−7c in high overall
N
23
yields. Moreover, the efficacy of this methodology was
confirmed by the preparation of 7b on a 1.1 mmol scale
workers, we thus studied the α-oxyamination of aldehydes
previously utilizing a combination of amine and photoredox
catalysts. To investigate bifunctional amine/acridinium catal-
(
90% yield).
24
We next examined the photophysical and electrochemical
ysis (Table 2), the activity of 1i was compared to that of 1f in
combination with secondary amines. However, the methyl
groups in 1i and 2,6-dimethylpiperidine strongly hampered
conversion. In contrast, 1f together with morpholine as well as
bifunctional aminoacridinium catalyst 1j showed excellent
catalytic activity. Under optimized conditions, full conversion
properties of acridinium salts 1a−1n, 4, 6, and 7a−7c (Table
1
; see Table S4 for full details). With time-correlated single-
photon counting, we were pleased to observe fluorescence
lifetimes that were sufficient for photoredox catalysis for all
aminoacridinium salts. Remarkably, the unique diversity of the
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Letter
Table 2. Catalytic α-Oxyamination of Aldehydes
Scheme 6. Organophotoredox C−N Cross-Coupling
a
b
entry
PC
amine
conversion (%)
2
3
4
5
1i (8%)
1f (8%)
1f (5%)
1j (5%)
1j (6%)
−
25
28
100
83
2,6-dimethylpiperidine (16%)
morpholine (5%)
−
−
c
d
6
100 (84)
a
hυ, Kessil LED A160WE, 40 W (λmax = 464 nm). Reaction
conditions: aldehyde (50.0 μmol), TEMPO (100 μmol), CH CN
3
b
c
(
0.5 mL), Ar, 18 h. Determined by NMR. At 24 h; hυ, SynLED
d
parallel photoreactor (λ = 465−470 nm). Isolated yield.
Reaction conditions: aryl halide (200 μmol), pyrrolidine (600 μmol),
and an isolated yield of 84% were conclusively obtained,
confirming the aptitude of acridinium 1i to serve as a
bifunctional catalyst.
NiBr ·glyme (10.0 μmol), DABCO (360 μmol), 7a (0.20 μmol),
2
DMA (1.0 mL), Ar, rt, 36 h; hυ, SynLED Parallel Photoreactor (λ =
a
b
465−470 nm). Isolated yields. On a 4 mmol scale; hυ, two Kessil
−
Owing to the suitably low E1/2(PC*/PC ) of aza-rhodol
LED A160WE, 40 W, λmax = 464 nm.
photocatalysts 7a−7c, the scope for acridinium photocatalysis
was distinctly expanded. These favorable properties are
deemed advantageous for various catalytic applications, such
In summary, we have developed a late-stage diversification
strategy toward acridinium salts involving an aryne−imine−
aryne coupling, acridane oxidation, and a versatile nucleophilic
aromatic substitution reaction. The approach thereby provided
20 acridinium catalysts with a particularly broad range of redox
potentials. Because of the mild reaction conditions, various
functional groups were tolerated and high yields were
obtained. The novel route to acridinium salts allowed the
synthesis of bifunctional catalysts to efficiently merge amine
and photoredox catalysis. Furthermore, the late-stage diversi-
fication approach opened access to aza-rhodol photocatalysts
as the photoredox catalytic C−N cross-coupling requiring
−
sufficiently reducing photocatalysts [E (PC/PC ) ≤ −1.26
1
/2
II
I
25−27
V] for precatalyst reduction [E (Ni / Ni ) = −1.43 V].
As
i
28
benign alternatives to Ir and Ru catalysts, aza-rhodols 7a−7c
with suitable redox properties were investigated (Table 3; see
Table 3. Comparison of Photoredox C−N Cross-Couplings
−
with uniquely low E1/2(PC/PC ) values, enabling the
replacement of precious-metal complexes in photoredox C−
N cross-couplings.
a
b
entry
PC
yield (%)
+
−
1
2
3
4
5
di-tBu-Mes-Acr BF (0.1%)
8
85
75
98
15
4
ASSOCIATED CONTENT
sı Supporting Information
■
Ir[dF(CF )ppy)] (dtbpy)PF (0.02%)
3
2
6
*
7a (0.02%)
7a (0.1%)
1f (0.1%)
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01673.
a
hυ, SynLED parallel photoreactor (λ = 465−470 nm). Reaction
conditions: 9a (200 μmol), pyrrolidine (600 μmol), NiBr ·glyme
Experimental details and characterization data (PDF)
2
(
10.0 μmol), DABCO (360 μmol), DMA (1.0 mL), Ar, rt, 36 h.
Determined by NMR using durene as the standard.
AUTHOR INFORMATION
Corresponding Author
b
■
Christof Sparr − Department of Chemistry, University of
Basel, 4056 Basel, Switzerland; orcid.org/0000-0002-
Table S3 for details). In the reaction of aryl bromide 9a with
pyrrolidine, a comparison of photocatalyst (di-tBu-Mes-
Acr BF ), Ir[dF(CF )ppy] (dtbpy)PF , aza-rhodol 7a, and
4213-0941; Email: christof.sparr@unibas.ch
+
−
3
Author
4
3
2
6
diaminoacridinium 1f indicated the suitabilty of 7a to serve as
an alternative to precious metallaphotoredox catalysts.
With optimized conditions in hand, we explored the
practicality and generality of aza-rhodol catalysis using different
substrates (Scheme 6). We were pleased to observe that
products 10a−10e were obtained with good to excellent yields
and that the robustness was confirmed by the synthesis of
compound 10a on a 4.0 mmol scale. Because oxygen and
amino substituents are readily introduced during the final step,
we envisage that precisely adjusted acridinium photoredox
catalysts allow applications for a captivating variety of
photoredox reactions.
Valeriia Hutskalova − Department of Chemistry, University of
Basel, 4056 Basel, Switzerland
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01673
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors acknowledge the Alfred Werner Fund and the
University of Basel for financial support. The authors are
■
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pubs.acs.org/OrgLett
Letter
grateful to Björn Pfund and Prof. Oliver S. Wenger (both
University of Basel) for their advice and equipment for
measuring photophysical and electrochemical data.
(19) Fischer, C.; Kerzig, C.; Zilate, B.; Wenger, O. S.; Sparr, C.
Modulation of Acridinium Organophotoredox Catalysts Guided by
Photophysical Studies. ACS Catal. 2020, 10, 210−215.
(
20) Yoshida, H.; Kuriki, H.; Fujii, S.; Ito, Y.; Osaka, I.; Takaki, K.
Aryne−Imine−Aryne Coupling Reaction via [4 + 2] Cycloaddition
between Aza-o-Quinone Methides and Arynes. Asian. Asian J. Org.
Chem. 2017, 6, 973−976.
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