Electrophotocatalysis with a Trisaminocyclopropenium Radical Dication

Article abstract of DOI:10.1002/anie.201906381

Visible-light photocatalysis and electrocatalysis are two powerful strategies for the promotion of chemical reactions. Here, these two modalities are combined in an electrophotocatalytic oxidation platform. This chemistry employs a trisaminocyclopropenium (TAC) ion catalyst, which is electrochemically oxidized to form a cyclopropenium radical dication intermediate. The radical dication undergoes photoexcitation with visible light to produce an excited-state species with oxidizing power (3.33 V vs. SCE) sufficient to oxidize benzene and halogenated benzenes via single-electron transfer (SET), resulting in C?H/N?H coupling with azoles. A rationale for the strongly oxidizing behavior of the photoexcited species is provided, while the stability of the catalyst is rationalized by a particular conformation of the cis-2,6-dimethylpiperidine moieties.

Full text of DOI:10.1002/anie.201906381

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Electrophotocatalysis with a Trisaminocyclopropenium Radical
Dication
Authors: He Huang, Zack M Strater, Michael Rauch, James Shee,
Thomas Sisto, Colin Nuckolls, and Tristan Hayes Lambert
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201906381
Angew. Chem. 10.1002/ange.201906381
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10.1002/anie.201906381
Angewandte Chemie International Edition
COMMUNICATION
Electrophotocatalysis with a Trisaminocyclopropenium Radical
Dication.
He Huang,[a] Zack M. Strater,[b] Michael Rauch,[b] James Shee,[b] Thomas J. Sisto,[b] Colin Nuckolls,[b]
and Tristan H. Lambert*[a,b]
Abstract: Visible light photocatalysis and electrocatalysis are two
powerful strategies for the promotion of chemical reactions. Here,
these two modalities are combined in an electrophotocatalytic
electrochemical and photochemical processes were reported. For
example, Xu demonstrated the alkylation of arenes with
organotrifluoroborates using photoredox catalysis coupled with
anodic re-oxidation,[14] and Stahl developed a Hofmann-Löffler-
Freytag type C–H amination reaction using electrochemical
iodination paired with photochemical homolysis.[15] Meanwhile,
Grätzel and Hu have shown that the photoelectric effect can be
utilized for the coupling of electron-rich arenes with azoles.[16]
oxidation
platform.
This
chemistry
employs
which
a
trisaminocyclopropenium
(TAC)
ion catalyst,
is
electrochemically oxidized to form a cyclopropenium radical dication
intermediate. The radical dication undergoes photoexcitation with
visible light to produce an excited state species with oxidizing power
(3.33 V vs SCE) sufficient to oxidize benzene and halogenated
benzenes via single electron transfer (SET), resulting in C–H/N–H
coupling with azoles. A rationale for the strongly oxidizing behaviour
of the photoexcited species is provided, while the stability of the
catalyst is rationalized by a particular conformation of the cis-2,6-
dimethylpiperidine moieties.
A. Photocatalytic oxidation
B. Electrocatalytic oxidation
e–
PC
EC
light
photocatalyst
electrocatalyst
subox
electricity
oxidant
PC*
photoexcited
PCred
e–
sub
Due to the unique reactivity patterns of free radicals, single-
electron oxidations offer a powerful means to induce chemical
transformations.[1, 2] In recent years, vigorous activity in the areas
of visible light photocatalysis[3–6] and electrocatalysis[7–10] have
reduced
ECOX
oxidized
subox sub
enabled
a
range of powerful oxidative methods. With
C. Trisaminocyclopropenium (TAC) ion electrophotocatalytic oxidation
photocatalysis, the energy of visible light is leveraged to convert
a ground-state photocatalyst (PC) into a strongly oxidizing
photoexcited species, the potency of which can far exceed that of
whatever terminal oxidant is employed (Fig. 1A). With
electrocatalysis, an applied potential effects the oxidation of a
substrate through the intermediacy of an electrocatalyst (EC) (Fig.
1B), which obviates the need for external oxidants and can
improve the kinetics of electron transfer, prevent electrode
passivation, and provide catalyst control for selective
transformations. Given the substantial benefits of these two broad
paradigms, processes that combine photocatalysis and
electrocatalysis could be especially attractive. Nevertheless, the
merger of electro- and photocatalysis in organic chemistry has
rarely been achieved. Moutet and Reverdy demonstrated[11] the
electrophotocatalytic oxidation of benzyl alcohol to benzaldehyde,
albeit with only three turnovers and no reported yield. Meanwhile,
Scheffold achieved the nucleophilic acylation of electron-deficient
olefins using vitamin B12 as a reductive electrophotocatalyst.[12, 13]
More recently, several elegant examples of the combination of
subox
Me
Me
N
Me
Me
electricity
E1/2= 1.26V vs SCE
e–
N
N
MeMe
sub
1
cation
(colorless)
2+
Me
Me
N
Me
Me
Me
Me
N
Me
Me
+
N
N
N
N
MeMe
MeMe
3
2
radical
dication
photoexcited
radical
dication
"
m ax = 450, 500, 550 nm
Eox= 3.33V vs SCE
!
light
Figure 1. Generic paradigms for (A) photocatalytic and (B) electrocatalytic
oxidations. (C) Electrophotocatalytic oxidation with TAC ion 1.
We envisioned the merger of light and electrical energy via
the design of new single-component “electrophotocatalysts”. The
major challenge in this endeavor resides in the identification of a
species that (a) can undergo a facile one-electron redox event to
yield a stable intermediate, (b) absorbs visible light once oxidized,
(c) has a sufficiently long-lived excited state to interact with a
substrate, and (d) is stable throughout the catalytic cycle. In this
Communication, we show that trisaminocyclopropenium (TAC)
[a]
[b]
Dr. H. Huang, Prof. T. H. Lambert
Department of Chemistry and Chemical Biology
Cornell University, Ithaca, NY 14853, USA
E-mail: Tristan.lambert@cornell.edu
Z. M. Strater, M. Rauch, J. Shee, Dr. T. J. Sisto, Prof. C. Nuckolls,
Prof. T. H. Lambert
Department of Chemistry
Columbia University, New York, NY 10027, USA
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201906381
Angewandte Chemie International Edition
COMMUNICATION
A
ion 1[17] meets all of these criteria and can operate as a potent yet
selective oxidative electrophotocatalyst (Figure 1C).
O
O
OEt
H
OEt
8 mol% 1
+
H2
+
N
Trisaminocyclopropenium (TAC) ions[17] are unusual in that
they can be reversibly oxidized to the corresponding stable radical
dications. In 1971, Yoshida showed that these radical dications
could be generated from the corresponding TAC monocations by
H
N
AcOH, LiClO4
C (+) | Pt (–)
N
N
4
6
5
EOX = 2.48 V
65% yield
Ecell = 1.5 V
23 W CFL
CH3CN
–ClO4
18
]
electrochemical oxidation.[
Johnson later measured the
Me
Me
Me
Me
N
without 1: 0% yield
without current: 0% yield
without light: 0% yield
oxidation potential of a permethylated TAC to be only 1.12 V (vs.
standard calomel electrode, SCE).[19] In 1975, Weiss reported that
TAC radical dications generated by chemical oxidation formed
deep red crystals that were stable enough for single crystal X-ray
analysis.[20] Recently, Sanford demonstrated that TACs can serve
as robust electrolytes, with possible applications in non-aqueous
flow batteries.[21]
N
N
direct electrolysis up to 3.0 V: ≤17% yield,
MeMe
formation of polymeric material
1
B
(1)
In our own work, we have found that trisaminocyclopropenium
(TAC) ion 1 can be electrochemically oxidized (E1/2 = +1.26 V vs
SCE) to the corresponding air-stable radical dication 2.[18–21]
Given the deep red color of this species (lmax = 550, 500, 450 nm),
we suspected it might function as a visible light photooxidant and
therefore be a candidate for a potent new electrophotocatalyst.
Indeed, based on the oxidation potential of 1 and the long
wavelength absorption tail (600 nm) of 2,[22] the photoexcited TAC
radical dication 3 has a calculated excited state reduction
potential of E*1/2 = 3.33 V (vs SCE). For comparison, this value
exceeds the excited state potentials of 9-mesityl-10-
methylacridinium (E*1/2 = 2.06 V),[5] 3-cyano-1-methylquinolinium
(E*1/2 = 2.72 V),[5] and even 2,6-dichloro-3,5-dicyanoquinone
(DDQ) (E*1/2 = 3.18 V).[5] As such, we expected that 3 should be
capable of oxidizing a variety of challenging substrates.
(2)
(3)
(4)
C
To test this idea, we first targeted the Nicewicz-type oxidative
coupling of benzene (Fig. 2).[23] With an oxidation potential of 2.48
V (vs SCE), the controlled functionalization of benzene presents
a major challenge for both electrochemistry[24] and photochemistry.
electrophotocatalytic
direct electrolysis (3.0 V)
[25, 26]
Previous instances of this type of coupling reaction by
Nicewicz,[23] Lei,[27] and Grätzel and Hu[16] have been conducted
with more easily oxidized arenes using the 9-mesityl-10-
methylacridinium photocatalyst; however, the oxidative
functionalization of benzene remains a challenge. We found that,
Figure 2. (A) Electrophotocatalytic C–H/N–H coupling of benzene and pyrazole
4. (B) 1H NMR spectra of (1) the reaction shown in Figure 2A; (2) the same
conditions without catalyst 1 showing no reaction occurred; (3) the same
reaction under direct electrolysis at 3.0 V; and (4) direct electrolysis of benzene
at 3.0 V. (C) Visual comparison of attempted direct electrolysis and
electrophotocatalytic reactions.
28
using a cell voltage of 1.5 V (Eanode = 1.4 V)[ ] and irradiation with
a 23 W compact fluorescent light (CFL) in acetonitrile with acetic
acid,[ 29 ] 8 mol% TAC 1 catalyzed the oxidative coupling of
benzene (4) and pyrazole (5) to furnish product 6 cleanly in 65%
yield (Fig. 2A). The counterelectrode product, hydrogen gas,
could be seen as bubbles formed at the cathode. Crucially,
control experiments showed that no reaction occurred without the
catalyst (1), light, or current. Furthermore, direct electrolysis at
higher potentials up to 3.0 V resulted in very low yield and the
visible formation of polymeric material. The stark difference
between direct electrolysis and the electrophotocatalytic
approach can be appreciated by comparison of the 1H NMR
spectra of the electrophotocatalytic reaction versus that of direct
electrolysis (Figure 2B, spectra 1 and 3 respectively), and
especially by visual comparison of the two (Figure 2C).
An investigation of reaction scope revealed that, in addition to
the carboxyl group (Table 1, entry 1), aldehyde and ketone
functional groups were well tolerated (entries 2 and 3). Oxidative
phenylation of 4-chloropyrazole was also feasible (entry 4).
Notably, we found that catalyst 1 was also capable of the oxidative
functionalization of chlorobenzene (entries 5-9), and
bromobenzene (entry 10). In these cases, the regioselectivity of
the coupling was significantly influenced by the nature of the azole
partner. In further probing the oxidizing capacity of this catalyst,
we found that all three isomers of dichlorobenzene could also be
made to engage in azole coupling (entries 11-13). Again,
potentially sensitive carbonyl functionalities were found to be
compatible with this process. The reaction of 1,2-
dibromobenzene also gave rise to the coupled product (entry 14).
In an attempt to find the limits of the oxidizing power of this
catalyst, we examined the reaction of fluorobenzene and
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10.1002/anie.201906381
Angewandte Chemie International Edition
COMMUNICATION
trifluorotoluene. Reaction of the former indeed led to the formation
of the arylated adduct as a mixture of regioisomers (entry 15). On
the other hand, trifluorotoluene did not undergo any observable
reaction (entry 16), suggesting the trifluoromethyl group is too
strongly deactivating for this catalyst. In addition to these
challenging arenes, this method also works with more easily
oxidized substrates such as mesitylene and other alkylated
benzenes (entries 17-36).
Notably, whereas reactions of benzene and the halogenated
benzenes were optimally conducted in a divided cell, reactions
with the alkylated benzenes could be performed in an undivided
cell. We measured the consumed current for a reaction in the
divided cell as 2.05 F/mol, which accords well with the theoretical
value. On the other hand, an undivided cell reaction was found
to consume 3.44 F/mol.
We believe the excess charge
consumption is due to the competing, unproductive reduction of
the TAC radical dication 2 back to TAC 1. Additionally, we note
Table 1. Electrophotocatalytic coupling of arenes and nitrogen heteroaromatics.[a]
–ClO4
R
N
N
Me
Me
N
Me
Me
R
8 mol% 1
N
+
R
HN
R
AcOH, LiClO4
C (+) | Pt (–)
N
N
7
8
9
Ecell = 1.5 V
MeMe
23 W CFL
CH3CN
1
(1)
(2)
(5)
(8)
(3)
O
N
(19)
Br
(20)
I
(21)
Br
CO2Et
CHO
Me
Me
Me
Me
Me
N
N
N
N
N
N
N
N
N
N
N
Me
Me
Me
Me
Me
Me
6
10
11
27
28
29
69% yield
O
58% yield
83% yield
O
61% yield (+25% s.m.)
CHO
60% yield (+20% s.m.)
CO2Et
65% yield (+18% s.m.)
Cl
O
(4)
(6)
(22)
(23)
(24)
OEt
Me
H
Me
Me
Me
N
N
N
N
N
N
N
N
N
N
N
N
12
Cl
13
Cl
14
30
31
32
Me
Me
Me
Me
Me
Me
71% yield
75% yield
67% yield
45% yield (+30% s.m.)
56% yield (+30% s.m.)
46% yield (+30% s.m.)
p:o:m = 4:1:1
p:o = 10:1
O
(7)
(9)
(25)
(26)
(27)
Me
2
2
N
N
N
N
N
N
Me
N
N
Me
N
Me
N
1
N
1
N
N
N
N
N
N
Cl
Br
35
Me
Me
Me
Me
Me
Me
Cl
15
16
17
33
34
Cl
Cl
55% yield (+30% s.m.)
31% yield (+55% s.m.)
35% yield (+50% s.m.)
65% yield
51% yield
1:1 N1:N2
75% yield
1:1 N1:N2
p:o:m = 8:3:1
p:o = 3:1
p:o = 10:1
O
Cl
(10)
(11)
(12)
(28)
(29)
(30)
CHO
Me
N
N
N
N
N
Me
N
N
N
A
N
Cl
N
N
N
N
A
N
F
N
36
Br
Me
Me
Me
Me
Cl
B
B
Cl
18
19
20
37
38
Me
Me
Cl
Cl
42% yield (+49% s.m.)
40% yield (+45% s.m.)
35% yield (+55% s.m.)
50% yield
82 % yield
6:1 CA:CB
87 % yield
10:1 CA:CB
p:o = 3:1
O
Cl
(13)
(14)
(15)
(31)
(32)
(33)
O
CHO
Cl
Me
Me
Br
Cl
N
A
N
N
N
Me
N
N
Br
N
N
N
Cl
N
N
N
Me
Me
B
Me
Me
F
Me
21
22
23
39
40
41
36% yield (+40% s.m.)
30% yield (+60% s.m.)
31% yield (+55% s.m.)
71 % yield
11:1 CA:CB
61% yield
56% yield
p:o:m = 5:1:1
Cl
Cl
Cl
Cl
(16)
CHO (17)
(18)
(34)
(35)
(36)
Me
Me
N
N
N
N
N
N
N
N
N
N
N
N
Me
Me
Me
Me
Me
Me
Me
Me
F3C
Me
24
no reaction
25
26
42
43
44
Me
82% yield
80% yield
55% yield (+33% s.m.)
45% yield (+25% s.m.)
62% yield
p:o = 7:1
p:o = 6:1
p:o = 4:1
[a] Entries 1-16 and 34-36 were conducted in a divided cell, while entries 17-33 were conducted in an undivided cell. Yields reflect isolated and purified
material. Reactions were performed on a 0.4 mmol scale. Reaction times were generally 48-60 h. See supplementary materials for experimental details.
This article is protected by copyright. All rights reserved.

10.1002/anie.201906381
Angewandte Chemie International Edition
COMMUNICATION
that the measured current typically begins at 4-5 mA, but quickly
drops to and remains at £1 mA for the rest of the reaction. Such
a current profile is expected, assuming that the photooxidation or
some subsequent step is rate-limiting, because once all of the
catalyst is oxidized only a very low steady state amount of TAC 1
would remain to be oxidized.
transitions result in an excited state SOMO-HOMO level
inversion,[31] with the resulting hole residing in a sub-frontier orbital.
The energy gained from repopulation of this low-lying orbital helps
to explain the exceptionally potent oxidizing power of this
photoexcited species.
A
A
mechanistic rationale for these electrophotocatalytic
HOMO*
SOMO
reactions is shown in Figure 3. Electrochemical oxidation of the
colorless TAC cation 1 generates the radical dication 2.
Photoexcitation then leads to the reactive intermediate 3, which
can oxidize benzene (4) via SET to produce the radical cation 45
with concomitant direct regeneration of 1. Nucleophilic trapping
of 45 by pyrazole 5 followed by deprotonation leads to the radical
46. Further oxidation of this radical by 2 or directly at the anode,
followed by rearomatization via loss of proton then reveals the
coupled product 6. Although this rationale is in line with previous
reports of this type of arene-azole coupling,[16, 17, 27] it is plausible
that in some cases the pyrazole substrates, rather than the arenes,
undergo initial oxidation given the higher oxidation potentials of
many of the arenes in our study. We view this as a less likely
possibility because the arene is used in vast excess, the
chemistry works even with strongly deactivated pyrazoles (e.g. 5),
we have observed small amounts of oligomeric products arising
from benzene oxidation, and the attempted reaction of pyrazole
by itself resulted in no conversion. However, the mechanistic
possibilities for this reaction are clearly complex, and further study
is warranted.
photon
level
inversion
sub
E
HOMO-1
HOMO-2
SOMO*
photoexcited
radical
dication
radical
dication
doublet photocatalyst
B
SOMO
HOMO-1
HOMO-2
C
*
–
ClO4
+
CO2Et
Me
Me
N
Me
Me
2+
H3C
CH3
4
5
N
+
N
N
N
N
H
N
N
CH3
CH3
photon
MeMe
CH3
45
H+
CH3
3
–
ClO4
2+
2•(ClO4)2
CO2Et
Me
Me
N
Me
Me
Me
Me
N
Me
Me
H
N
N
46
D
i)
5 equiv.
–ClO4
Me
Me
N
N
N
N
48
Cl
Me
Me
N
MeMe
MeMe
Me
N
H
Me
Cl
Cl
Cl
Cl
2
2
1
Et3N
ii) LiClO4
N
N
H+
1
47
MeMe
65% yield, 6.2 g
1/2 H2
H+
CO2Et
1
e–
N
–
N
+
Figure 4. (A) Level inversion of a photoexcited TAC radical dication. (B) TD-
DFT calculated orbitals of TAC radical dication 2. (C) X-ray structure of TAC
radical dication 2. (D) Synthesis of TAC catalyst 1.
6
Figure 3. Mechanistic rationale.
Despite such a strongly oxidizing character, the TAC catalyst
is remarkably stable, remaining >95% intact during a typical
electrophotocatalytic reaction. To explain this stability, we were
able to obtain X-ray quality crystals of the TAC radical dication 2
Notably, radical dication 2 represents an unusual example of
an open-shell, doublet photocatalyst (Figure 4A).[30] We used
time-dependent density functional theory (TD-DFT) to
characterize the low-lying excited electronic states of the TAC
radical dication 2. As predicted,[20] the two lowest energy
absorptions result from transitions from the nearly degenerate
HOMO-1 and HOMO-2 orbitals to the SOMO (Figure 4B). These
32
as the bis-perchlorate salt (Figure 4C). The most conspicuous
aspect of this structure is the conformational bias of the three
piperidine units, which places all six methyl substituents in the
axial position to avoid what would otherwise be a severe allylic-
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10.1002/anie.201906381
Angewandte Chemie International Edition
COMMUNICATION
type strain with the cyclopropenium ring. This conformation thus
places the piperidine a-hydrogens orthogonal to the TAC p-
system, rendering them unavailable for deprotonation or
hydrogen atom abstraction, the common destructive process of
aminyl radical cations. In support of this claim, we have
calculated that the conformation in which one of the piperdine
rings has the methyl substituents equatorial is 13.4 kcal higher in
energy. We have also found that the TAC radical dications
derived from 2-methylpiperidine or piperidine rapidly decompose
upon irradiation.
Acknowledgements
We thank Prof. Ged Parkin (Columbia University) for use of the X-ray
diffractometer, Profs. Richard Friesner and David Reichman (Columbia
University) for providing computational resources, and Prof. Song Lin
(Cornell University) for helpful discussions. Funding: Research
reported in this publication was supported by the National Institutes of
Health under R35 GM127135. The electrochemical studies of T.J.S. and
C.N. were based upon work supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences, under
Award Number DE-SC001944. M.R. acknowledges the National
Science Foundation for a Graduate Research Fellowship (DGE-16-
44869). We thank the National Science Foundation (CHE-0619638) for
acquisition of an X-ray diffractometer.
Finally, we note that the TAC catalyst 1 is a bench stable solid
that
can
be
synthesized
by
the
reaction
of
pentachlorocyclopropane (47) and cis-2,6-dimethylpiperidine (48)
(both commercially available) in a one-pot procedure on a multi-
gram scale and with no column purification (Figure 4D). The TAC
Keywords: Electrophotocatalysis • radical dication •
trisaminocyclopropenium ion • oxidation • C–H functionalization
catalyst
1
should thus be readily available for further
investigations of electrophotocatalysis.
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10.1002/anie.201906381
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
!
TAC●2+*
A
He Huang, Zack M. Strater,
Michael Rauch, James Shee,
Thomas J. Sisto, Colin
light
trisaminocyclopropenium
(TAC) ion is used as a
potent
Me
Me
N
Me
Me
Nuckolls, Tristan H. Lambert
TAC●2+
Eox= 3.33V
N
N
electrophotocatalyst for
the selective oxidative
coupling of benzene and
other arenes. The
potent oxidizing power
derives from the open-
shell character of the
intermediate TAC radical
dication.
Me
Me
TAC
electrophotocatalyst
Page No. – Page No.
electricity
TAC+
Title
e–
This article is protected by copyright. All rights reserved.