Helical Carbenium Ion: A Versatile Organic Photoredox Catalyst for Red-Light-Mediated Reactions

Article abstract of DOI:10.1021/jacs.0c05507

Red light has the advantages of low energy, less health risks, and high penetration depth through various media. Herein, a helical carbenium ion (N,N′-di-n-propyl-1,13-dimethoxyquinacridinium (nPr-DMQA+) tetrafluoroborate) has been used as an organic photoredox catalyst for photoreductions and photooxidations in the presence of red light (λmax = 640 nm). It has catalyzed red-light-mediated dual transition-metal/photo-redox-catalyzed C-H arylation and intermolecular atom-transfer radical addition through oxidative quenching. Moreover, its potential in photooxidation catalysis has also been demonstrated by successful applications in red-light-induced aerobic oxidative hydroxylation of arylboronic acids and benzylic C(sp3)-H oxygenation through reductive quenching. Thus, a versatile organic photoredox catalyst (helical carbenium ion) for red-light-mediated photoredox reactions has been developed.

Full text of DOI:10.1021/jacs.0c05507

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Communication
Helical Carbenium Ion: A Versatile Organic
Photoredox Catalyst for Red-Light-Mediated Reactions
Liangyong l Mei, Jose M. Veleta, and Thomas L. Gianetti
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c05507 • Publication Date (Web): 30 Jun 2020
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Liangyong Mei, José M. Veleta, and Thomas L. Gianetti*
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Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States
Supporting Information
ABSTRACT: Red light has the advantages of low energy, less health risks, and high penetration depth through various media. Herein,
n
+
a helical carbenium ion (N,N’-di-n-propyl-1,13-dimethoxyquinacridinium ( Pr-DMQA ) tetrafluoroborate) has been used as an
organic photoredox catalyst for photoreductions and photooxidations in the presence of red light (λmax = 640 nm). It has catalyzed
red-light-mediated dual transition-metal/photoredox-catalyzed C-H arylation and intermolecular atom transfer radical addition
through oxidative quenching. Moreover, its potential in photooxidation catalysis has also been demonstrated by successful
applications in red-light-induced aerobic oxidative hydroxylation of arylboronic acids and benzylic C(sp )-H oxygenation through
reductive quenching. Thus, a versatile organic photoredox catalyst (helical carbenium ion) for red-light-mediated photoredox
reactions has been developed.
3
Over the past decade, photoredox catalysis has taken the world of
less health risks, and is naturally abundant from sunlight. More
importantly, it penetrates turbid media, and great success have been
made in its biology-related applications. Yet reports on red-light-
mediated synthetic methodologies still remain scarce, and only
handful examples can be found (Scheme 1a). Ferroud and
coworkers introduced red-light-induced methylene blue (2a)-
catalyzed aerobic photooxidation and photocyanation of
hydrazines (Scheme 1a, I). Though more applications of 2a in
photocatalysis have been reported, they all employ blue, green or
white light.
1
synthetic chemistry by a storm. Iridium and ruthenium polypyridyl
2
3
10
complexes such as 1a and 1b are among the most powerful
photoredox catalysts (PCs) due to their unique properties such as
stable and long-lived excited states, large redox windows, and as
effective excited state oxidants and reductants (Figure 1). However,
given the intrinsic disadvantages of transition-metal catalysts such
as high cost, low sustainability and potential toxicity, researchers
have been looking for inexpensive and environmental-friendly
alternatives. In recent years, organic molecules such as acridiniums
1
1
4
,12
Similarly, PCs 2b-2d have also achieved some
(
e.g. 9-mesityl-10-methylacridinium 1c) or xanthene dyes (e.g.
successes in red-light-mediated methodologies, while no further
applications was conducted (Scheme 1a, II-IV). Notably, Rovis
4
13
Eosin Y 1d) have proven to be efficient alternatives (Figure 1).
Despite the general merits of organic PCs, there are disadvantages
to overcome. For example, the most widely used catalyst in this
and Campos have recently introduced an elegant work on near-
infrared light-induced transformations in the absence of a PC, or in
the presence of 1b, 1d, or Rose Bengal via triplet fusion
5
category 1c, first introduced by Fukuzumi and expanded by
4
a,4b,6
14
Nicewicz,
is mainly used for reductive quenching. Similarly,
upconversion, followed by the development of a series of Os(II)-
1
5
the narrow redox window, pH dependence, and susceptibility to
bleaching have also rendered 1d less effective. Therefore,
developing more versatile organic PCs is still highly desirable.
based PCs for infrared photoredox catalysis. These important
examples all suggest that applying red light in photoredox catalysis
is of great significance.
7
+
The helical carbenium ion – dimethoxyquinacridinium (DMQA ),
1
6
Figure 1. Commonly Used Photoredox Catalysts
has been well-studied regarding its photophysical,
electrochemical properties,
1
6b,17
16b,18
and post-functionalization
1
6a
since its discovery by Laursen and coworkers. Applications of
+
19
DMQA have been primarily reserved to the biological arena. In
005, the Lacour group revealed the first and only example of
2
+
photocatalysis using DMQA , which demonstrated the aerobic
photooxidation of benzylamine in the presence of N,N’-di-n-
propyl-1,13-dimethoxyquinacridinium
tetrafluoroborate (3) under photoirradiation (600 W lamp) to afford
n
+
( Pr-DMQA )
2
0
benzylimine in 15% NMR yield at 70 ℃. Based on the reported
extraordinary photophysical and electrochemical properties of 3, as
well as Lacour’s results on photocatalysis, we speculated that 3
could serve as a versatile organic PC. Herein, we report the
photoredox properties of [ Pr-DMQA ][BF ] (3), and its reactivity
4
in a wide range of red-light-mediated reactions including reductive
and oxidative quenching (Scheme 1b).
Despite the impressive progress in photoredox catalysis to date,
most light-induced reactions are accomplished under high-energy
n
+
-
1
,4
blue, green or white light. Besides the health hazards (photo-
8
oxidative damage to retina), blue and green light are also more
prone to induce undesired mixtures due to the possible photon
9
absorptions by reactants or products. In contrast, red light (600-
7
00 nm) has advantages such as lower energy, fewer side reactions,
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Scheme 1. (a) Previous Work on Red-Light-Mediated
Methodologies and (b) a Versatile Organic PC in This Work
catalyzed C(sp )-H arylation (Table 1). Using 4a as model
substrate, electron-neutral, electron-deficient and electron-rich
aryldiazonium salts 5a-5c were tested. Reactions proceeded
smoothly, delivering the desired products 6a-6c in 86-93% yields
1
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6
(
Table 1, entry 1). Substrate 4b containing pyridine as the directing
group (DG) was also well-tolerated, furnishing the corresponding
products 6d-6f in 60-73% yields in reactions with 5a-5c (entry 2).
In addition, the desired C-H arylated product 6g could be obtained
in 57% yield when 4c with pyrimidine as the directing group
reacted with 5a (entry 3). Lastly, oxime 6h was isolated in 64%
yield when 4d was treated with 5a (entry 4). Notably, all the
products 6 were obtained in relatively higher yields compared to
the previous work except for 6d.
experiments in the absence of 3, red light or Pd(OAc)
much lower yields for all substrates (Table S3). The successful
application of in this red-light-mediated C-H arylation
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Furthermore, control
2
resulted in
3
demonstrates that it is capable of engaging in oxidative quenching
during photocatalysis.
n
+
Table 1. Red-Light-Mediated Dual Pd/ Pr-DMQA -
Catalyzed C(sp )-H Arylation
2
a
Before testing the photocatalytic reactivity of 3, we first
calculated the excitation energy (E0,0), excited state oxidation (E1/2
•++ +*
+*
•
(
C
/C )) and reduction potentials (E1/2(C /C )) following
literature protocols (Table S1, see details in Supporting
4
a
Information). To our delight, 3 possesses moderate ground state
•
++
+
oxidation and reduction potentials E1/2(C /C ) = +1.32 V and
+
•
E
1/2(C /C ) = -0.78 V vs. the saturated calomel electrode (SCE), as
well as moderate excited state oxidation and reduction potentials
•
++ +*
+*
•
E
1/2 (C /C ) = -0.61 V and E1/2(C /C ) = +1.15 V in MeCN
(
Table S1). Solvent effects on the properties of 3 were examined
by recording its absorption and emission spectra, as well as CV
curves in four different solvents (MeCN, DCM, DMF and MeOH,
Figure S1-S2). No significant solvatochromism or redox potentials
differences were observed in these solvents (Table S1). These
features render 3 a mild reductant and oxidant whether a reductive
or oxidative quenching is involved during photocatalysis.
1
6b
Moreover, the excited state lifetime (τ = 5.5 ns) is comparable to
those of other commonly used organic PCs (2–20 ns in general,
4
albeit 5.1 μs for 4CzIPN). In particular, the peak absorption (λmax
)
1
6b
around 620 nm, makes our goal of taking advantage of low-
energy red light feasible.
Initial reactivity studies began with a red-light-mediated dual
n
+
2
Pd/ Pr-DMQA -catalyzed C(sp )-H arylation, which was first
2
+
reported by the Sanford group using Ru(bpy)
6 W compact fluorescent light (CFL). In the reported reaction,
Ru(bpy)
3
in the presence of
21
2
2+
*
3
(E1/2(Ru(III)/Ru(II )) = -0.81 V vs. SCE) acted as a
+
+
•
photoreductant by reducing ArN
2
to aryl radical (PhN
2
/Ph = -
2
2
0.10 V vs. SCE)
oxidation of Pd(III) to Pd(IV) to regenerate Ru(bpy)
through oxidative quenching, followed by
a
2
+
Reactions conducted on 0.2 mmol scale. Isolated yields shown.
3
++
+*
b
(
0
E
1/2(Ru(III)/Ru(II)) = +1.29 V vs. SCE). Given E1/2 (C• /C ) = -
The reaction was run for 4 h.
•
++
+
.62 V and E1/2(C /C ) = +1.32 V for 3 in MeOH, this reaction
was deemed suitable to display its photoreduction ability. To our
delight, the reaction between 1-([1,1'-biphenyl]-2-yl)pyrrolidin-2-
one 4a and benzenediazonium tetrafluoroborate 5a proceeded
Next, we turned our attention to photo-induced aerobic oxidative
hydroxylation of arylboronic acids, which is a well-documented
reaction via reductive quenching under white or blue light. The
reaction mainly involves the oxidation of Pr
excited state PC to form the radical cation Pr
23
smoothly in the presence of Pd(OAc)
2
and 3 under red LED (λmax
40 nm), affording the desired product 6a in 95% NMR yield after
hours (Table S2). Consistent with Sanford’s work,21 in the
, significantly lower yields (≤
5%) of 6a were observed (Table S2).
With these promising results in hand, we sought to expand the
=
i
2
NEt (DIPEA) by an
6
4
*
i
•+
2
NEt
i
i
•+
24
(
Pr
2
NEt/ Pr
2
NEt = +0.72 V vs. SCE) and the reduction of O by
2
absence of 3, red light, or Pd(OAc)
2
•-
•-
•-
the PC to generate the superoxide radical anion O
0
=
2
(O
With E1/2(C /C ) = +1.18 V and E1/2(C /C )
-0.74 V in DMF, 3 should be competent to serve as an efficient
2 2
/O = -
2
24a,25
+*
•
+
•
.57 V vs SCE.).
n
+
substrate scope of this red-light-mediated dual Pd/ Pr-DMQA -
PC for this reaction. Gratifyingly, phenol 8a was obtained in 87%
2
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NMR and 83% isolated yield when 7a was treated with DIPEA and
3 in the presence of air and red light for 24 hours (Table S4). In
accordance with reported results, in the absence of PC or red light,
little or no conversion was observed (Table S4).
Substrate scope results for oxidative hydroxylation of arylboronic
acids are presented in Table 2. A wide range of arylboronic acids 7
with diverse useful functional groups such as halide (7c), nitrile
further support the essential roles of 3 and red light for these
reactions.
1
2
3
4
5
6
7
8
9
23
n
Scheme 2. Additional Examples of Red-Light-Induced Pr-
+
a
DMQA -Catalyzed Reactions
(
7d), aldehyde (7e), ester (7f), carboxylic acid (7g), and nitro (7k)
were well-tolerated, affording the desired phenols 8a-8l in
moderate to high yields. Additionally, the substitution pattern on
the phenyl ring or electronic properties of 7 did not have much
influence over the reaction outcome. For example, 8b-8g were
isolated in 41-87% yields when 7b-7g with different electron-
donating groups or electron-withdrawing groups at para position
were examined. Substrates 7h-7k with diverse substituents at ortho
or meta position also provided the corresponding phenols 8h-8k in
55-73% yields. 2-Naphthylboronic acid 7l was also suitable for this
oxidative hydroxylation, providing naphthalen-2-ol 8l in 65%
1
1
1
1
1
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+
yield. This red-light-induced Pr-DMQA -catalyzed aerobic
oxidative hydroxylation shows that 3 is an efficient PC for
photocatalysis involving reductive quenching.
aReactions conducted on 0.2 mmol scale. Isolated yields shown.
n
+
Table 2.
Pr-DMQA -Catalyzed Aerobic Oxidative
a
Hydroxylation of Arylboronic Acids under Red Light
Based on previous work,21,23 two plausible catalytic cycles
involving in oxidative or reductive quenching are proposed
(
Scheme 3). Scheme 3-I shows a proposed mechanism for dual
n
+
2
21
Pd/ Pr-DMQA -catalyzed
photoexcitation of 3 generates Pr-DMQA , which reduces 5 to
form an aryl radical and Pr-DMQA
C(sp )-H
arylation.
Firstly,
n
+*
•++
n
through oxidative
quenching. Then, addition of the aryl radical to the Pd(II)
intermediate A (generated by C-H activation of substrate 4) affords
the Pd(III) species B, followed by an one-electron oxidation with
n
•++
n
+
Pr-DMQA
to regenerate Pr-DMQA and form the Pd(IV)
intermediate C. Lastly, reductive elimination of the intermediate C
n
furnishes the arylated product 6. A possible mechanism for Pr-
+
DMQA -catalyzed aerobic oxidative hydroxylation is presented in
2
3 i
Scheme 3-II. Pr
2
NEt is first oxidized to an ammonium radical
cation by the excited state of 3, along with formation of a helicene
n
•
radical Pr-DMQA through reductive quenching. Then, the
1
7
helicene radical, which has been observed and studied by Laursen
and our group, further reacts with oxygen to regenerate 3 and
form an O
29
•-
2
. Finally, follow-up oxidative attack onto substrates 7
aReactions conducted on 0.5 mmol scale. Isolated yields shown.
and hydrolysis afford phenols 8.
2
To further illustrate the generality of 3 as a PC, additional red-
light-induced transformations were investigated (Scheme 2). As
another classic example of photocatalysis involving in reductive
Scheme 3. Plausible Catalytic Cycles For (I) C(sp )-H
Arylation, and (II) Aerobic Oxidative Hydroxylation
•
-
3
2
quenching and O , visible-light-mediated aerobic benzylic C(sp )-
H oxygenation using oxygen as the oxidant was extensively
26
studied. When a tertiary amine 9 was treated with 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) and 3 in the presence of air
and red light, amide 10 was isolated in 92% yield (Scheme 2a).
Atom transfer radical addition (ATRA) of organic halides to olefins
serves as an atom-economical approach for simultaneously forming
2
7
C-C and C-X bonds. As presented in Scheme 2b, in the presence
of LiBr and 3, a red-light-induced reaction between 4-nitrobenzyl
bromide 11 and styrene 12 was realized, affording the desired
adduct 13 in 59% yield (see Table S5 and S6 for more optimization
and Scheme S1 for mechanism). Finally, 3 could also act as a viable
alternative for Glorius’s dual gold/photoredox-catalyzed C(sp)-H
2
8
arylation of terminal alkyne 14 with 5a, providing the desired
product 15 in 62% yield (Scheme 2c, see more optimization in
Table S7). Control experiments in the absence of 3, red light, or
other reagents such as DBU, LiBr or Au(PPh
3
)Cl were also
performed (Scheme S2). The reduced yields of 10, 13, and 15
3
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n
In summary, we have disclosed a helical carbenium ion – [ Pr-
Visible Light Photocatalysis. Science 2014, 343, 1239176. (c) Skubi, K. L.;
+
-
Blum, T. R.; Yoon, T. P. Dual Catalysis Strategies in Photochemical
Synthesis. Chem. Rev. 2016, 116, 10035–10074. (d) Hopkinson, M. N.;
Tlahuext-Aca, A.; Glorius, F. Merging Visible Light Photoredox and Gold
Catalysis. Acc. Chem. Res. 2016, 49, 2261–2272. (e) Lang, X.; Zhao, J.;
Chen, X. Cooperative Photoredox Catalysis. Chem. Soc. Rev. 2016, 45,
1
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DMQA ][BF
4
]
(3), which catalyzes photoreductions and
photooxidations in the presence of low-energy red light. The role
of 3 as an efficient PC in oxidative and reductive quenching were
n
+
evaluated by transition-metal/ Pr-DMQA -catalyzed C-H
arylations and intermolecular ATRA (oxidative quenching), as well
as aerobic oxidative hydroxylation of arylboronic acids and
3
026–3038. (f) Twilton, J.; Le, C. C.; Zhang, P.; Shaw, M. H.; Evans, R.
W.; MacMillan, D. W. C. The Merger of Transition Metal and
Photocatalysis. Nat. Rev. Chem. 2017, 1, 0052. (g) Parasram, M.;
Gevorgyan, V. Visible Light-Induced Transition Metal-Catalyzed
Transformations: Beyond Conventional Photosensitizers. Chem. Soc. Rev.
3
benzylic C(sp )-H oxygenation (reductive quenching). Eight
diverse substrates were well-tolerated for red-light-mediated dual
n
+
2
Pd/ Pr-DMQA -catalyzed C(sp )-H arylation. Meanwhile, with
twelve different arylboronic acids as the substrates, red-light-
2
017, 46, 6227–6240. (h) Marzo, L.; Pagire, S. K.; Reiser, O.; König, B.
n
+
Visible-Light Photocatalysis: Does It Make a Difference in Organic
Synthesis? Angew. Chem., Int. Ed. 2018, 57, 10034–10072. (i) Kelly, C. B.;
Patel, N. R.; Primer, D. N.; Jouffroy, M.; Tellis, J. C.; Molander, G. A.
Preparation of Visible-Light-Activated Metal Complexes and Their Use in
Photoredox/Nickel Dual Catalysis. Nat. Protoc. 2017, 12, 472–492.
induced Pr-DMQA -catalyzed aerobic oxidative hydroxylation
proceeded smoothly as well. The successful applications of 3 in
these red-light-mediated reactions have established its role as a
versatile organic PC, which can serve as a lower-energy and
therefore milder, option for current white, blue or green-light-
mediated photocatalysis. Further investigations on the applications
of 3 towards more challenging photoredox catalysis are in progress.
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Thomas L. Gianetti
- Department of Chemistry and
Biochemistry, University of Arizona, Tucson, Arizona 85721,
United States; https://orcid.org/0000-0002-3892-3893
Email: tgianetti@arizona.edu.
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Liangyong Mei - Department of Chemistry and Biochemistry,
University of Arizona, Tucson, Arizona 85721, United States;
https://orcid.org/0000-0002-4704-4030
Methylacridinium Ion with a Much Longer Lifetime and Higher Energy
Than That of the Natural Photosynthetic Reaction Center. J. Am. Chem.
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Email: lymei@arizona.edu.
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José M. Veleta - Department of Chemistry and Biochemistry,
University of Arizona, Tucson, Arizona 85721, United States;
https://orcid.org/0000-0002-5272-2131
Email: jmveleta@arizona.edu.
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Complete contact information is available at:
https://pubs.acs.org/xxx
The authors declare no competing financial interest.
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We are grateful for financial support from the University of
Arizona for this work. We thank Dr. Moutet from the Gianetti
group for his help in recording cyclic voltammetry, and Prof. Jewett
and Mr. Davis from the University of Arizona for their assistance
in recording emission spectra. We thank Profs. Tomat, Huxter, and
Njardarson from the University of Arizona, and Prof. Bergman
from UC Berkeley for helpful discussions.
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Table of Content
Reductive Quenching
H2
Oxidative Quenching
BF4
nPr
O
Au(PPh3)Cl
C
Ph
Ph
N
nPr
Ar1
Ar1
Ar
H
N
N
N
Ar1 = p-tolyl
Ar = Ph,
C+
9
2% yield
62% yield
O2 (Air)
Ar N2
O O
Ar
O2
Red LED
Red LED
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[nPr-DMQA ][BF4 ]
+
OH
Ar
Organic photocatalyst
Low-energy red light
H
DG
Pd(OAc)2
DG
R
B(OH)2
1
2 examples,
up to 87% yield
Reductive and
oxidative quenching
8 examples,
up to 93% yield
R
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