Visible-Light-Driven External-Reductant-Free Cross-Electrophile Couplings of Tetraalkyl Ammonium Salts

Article abstract of DOI:10.1021/jacs.8b08792

Cross-electrophile couplings between two electrophiles are powerful and economic methods to generate C-C bonds in the presence of stoichiometric external reductants. Herein, we report a novel strategy to realize the first external-reductant-free cross-electrophile coupling via visible-light photoredox catalysis. A variety of tetraalkyl ammonium salts, bearing primary, secondary, and tertiary C-N bonds, undergo selective couplings with aldehydes/ketone and CO₂. Notably, the in situ generated byproduct, trimethylamine, is efficiently utilized as the electron donor. Moreover, this protocol exhibits mild reaction conditions, low catalyst loading, broad substrate scope, good functional group tolerance, and facile scalability. Mechanistic studies indicate that benzyl radicals and anions might be generated as the key intermediates via photocatalysis, providing a new direction for cross-electrophile couplings.

Full text of DOI:10.1021/jacs.8b08792

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Communication
Visible-light-Driven External-Reductant-Free Cross-
Electrophile Couplings of Tetraalkyl Ammonium Salts
Li-Li Liao, Guang-Mei Cao, Jian-Heng Ye, Guo-Quan Sun, Wen-
Jun Zhou, Yong-Yuan Gui, Si-Shun Yan, Guo Shen, and Da-Gang Yu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08792 • Publication Date (Web): 06 Dec 2018
Downloaded from http://pubs.acs.org on December 6, 2018
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Visible-light-Driven External-Reductant-Free Cross-Electrophile
Couplings of Tetraalkyl Ammonium Salts
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Li-Li Liao,† Guang-Mei Cao,† Jian-Heng Ye,† Guo-Quan Sun,† Wen-Jun Zhou,† Yong-Yuan Gui,† Si-Shun Yan,† Guo Shen,†
Da-Gang Yu*,†‡
† Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Sichuan University,
Chengdu 610064, P. R. China.
‡ State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, P. R. China.
Supporting Information Placeholder
(A) Classical transition metal-catalyzed cross-electrophile couplings (Many reports)
ABSTRACT: Cross-electrophile couplings between two
electrophiles are powerful and economic methods to generate C─
[TM] (cat.)
R1 R2
R1
X
R2
X
+
Stoichiometric external reductants
(Mn, Mg, Zn, ...)
C bonds in the presence of stoichiometric external reductants.
Herein, we report a novel strategy to realize the first external-
reductant-free cross-electrophile coupling via visible-light
photoredox catalysis. A variety of tetraalkyl ammonium salts,
bearing primary, secondary, and tertiary C ─ N bonds, undergo
selective couplings with aldehydes/ketone and CO2. Notably, the
in situ generated byproduct, trimethylamine, is efficiently utilized
as the electron donor. Moreover, this protocol exhibits mild
reaction conditions, low catalyst loading, broad substrate scope,
good functional group tolerance, and facile scalability.
Mechanistic studies indicate that benzyl radicals and anions might
be generated as the key intermediates via photocatalysis,
providing a new direction for cross-electrophile couplings.
(B) Visible-light driven cross-electrophile couplings without external reductants (This work)
R
Ar
R3
OH
COOH
R2
R1
ArCOR3
R3 = H, Ph
R1
R2
NMe3OTf
R2
[PC], visible light, base
R
R1
External reductant-free
O
C O
R = aryl, alkenyl
R1, R2 = H, aryl, alkyl
Generated
as electron donor
NMe3
R
Figure 1. Cross-electrophile couplings. TM = transition metal. PC
= photocatalyst.
(A) Transition metal-catalyzed oxidative addition of C N bonds in ammonium triflates

NMe3OTf
[TM]n+2OTf
oxidative addition
R2
R2
NMe3
+
[TM]n (cat.)
R
R1
R
R1
(B) Visible-light-driven single electron reduction of C N bonds in ammonium triflates

R2
NMe3OTf
SET
R2
R
R1
R2
R1
R
+
R1
R
hv
NMe3
Carbon-carbon bonds formation plays a vital role in producing
organic compounds with structural complexity and diversity.
Transition metal-catalyzed cross coupling reactions have become
one of the most useful and important methods to generate C─C
bonds.1 In order to avoid pre-generation and handling of
organometallic reagents, cross-electrophile couplings have been
recently well developed through directly using abundant and
stable electrophiles, therefore featuring easy operation and high
step-economy.2 However, stoichiometric external reductants are
required in such reactions (Figure 1A). Moreover, compared with
organohalides, readily available organo pseudohalides are less
investigated. Herein, we report the first external reductant-free
cross-electrophile couplings of tetraalkyl ammonium salts with
aldehydes/ketone and CO2 via visible-light photoredox catalysis
(Figure 1B). Notably, the trimethylamine, in situ generated as the
byproduct, is utilized as the electron donor.
[PC]n
base
[PC]n+1
[PC]n
[PC]n+1
hv
NMe3
NMe2
or HNMe2
Figure 2. Catalytic cleavage of C ─ N bonds in ammonium
triflates.
reactions amines are released as byproducts, which have never
been reutilized to our knowledge (Figure 2A). Although great
progresses have been achieved in C─halogen bond cleavage via
visible-light photoredox catalysis5 with amines as electron donors,
there is only one example on visible-light photocatalyzed
reductive cleavage of C─N bonds in organoammonium salts.6a In
principle, we could consider tetraalkyl ammonium salts as the
electrophiles bearing
a
built-in reductant (amine). We
hypothesized that the photocatalyzed single electron reduction of
tetraalkyl ammonium salts might afford alkyl radicals and release
amines, which could act as electron donors to reduce the oxidized
or excited photocatalyst (Figure 2B). The generated amine radical
cations could be deprotonated by base to give α-amino radicals,
which might also act as electron donors. Moreover, the alkyl
radicals, such as benzyl radicals, might be further reduced by
photocatalyst to give carbon anions, which underwent facile
nucleophilic attack to other electrophiles to give the desired cross-
electrophile coupling products. If the proof-of-concept is
successful, it would provide a mild and economic method to
The C─N bonds are prevalent in natural products, materials
and bioactive molecules. The selective cleavage and
functionalization of C ─ N bonds is highly important but
challenging due to the low reactivity and selectivity.3,4 Stable and
highly crystalline organoammonium salts are important
substances containing C ─ N bonds and have been utilized as
electrophiles in transition-metal-catalyzed coupling reactions via
C─N bond cleavage.4 In these
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resolve the long-standing metal hazard issue. However, it is
Table 2. Visible-light-driven reductive cross-electrophile
couplings between ammonium salts and 2ca
highly challenging to efficiently use the catalytically generated
amines in low concentration as the reductants. Moreover, the
visible-light photoredox catalysis has rarely been investigated in
C─N cleavage.6
3
4
5
6
R1
F
O
R1
Ir(ppy)2(dtbbpy)•PF6 (3 mol%)
Cs2CO3 (2.0 equiv)
H
NMe3OTf
+
Ar
Ar
DMF (0.2 M), 30 W blue LED, N2, rt, 36 h
then HCl (aq.)
OH
3
F
1
2c
With these challenges in minds, we started to investigate the
cross-electrophile couplings with the air and thermally stable
benzyltrimethyl ammonium triflate 1a as the substrate, which can
be easily synthesized in one-step from readily available benzylic
amines. As well known, the Barbier-type reaction is one of the
most efficient methods to generate alcohols from alkyl halides
with carbonyl-containing compounds in the presence of
stoichiometric metallic reductants, such as Mg, Sn, In, etc.7 We
wondered whether we could realize visible-light-driven Barbier-
type reactions8 in the absence of external reductants. After
systematic screening of the reaction of 1a with benzaldehyde 2a
(see the Supporting Information (SI) for details), we were
delighted to get the desired product 3a in 70% yield in the
absence of external reductant. Control experiments demonstrated
that the photocatalyst, base and visible light were indispensable
(Table S3). Interestingly, in this reaction we did not detect
pinacol-type byproducts through homocoupling of ketyl radical.9
With the optimized conditions in hand, we investigated the
generality of the reaction. As shown in Table 1, aldehydes with
either electron-neutral groups (3b), electron-donating group (EDG,
3d), or electron-withdrawing groups (EWGs, 3c) reacted
smoothly to give the desired alcohols in good yields. This cross-
electrophile coupling was not sensitive to steric hindrance, as 2-
methylbenzaldehyde 2f reacted smoothly. Besides aldehydes, we
were delighted to find benzophenone 2h, which underwent facile
reduction and homocoupling via photocatalysis,9b,10 was also
suitable for this reaction.
7
8
9
3i,
3j,
3k,
3l,
R = H, 64%
F
F
3n,
3o,
3p,
R = CF3, 73%
R = COOMe, 74%
R = OCF3, 63%
R = Me, 53%
R = CF3, 58%
R = COOMe, 66%
R
OH
OH
3m,
R
R = 3-thiophen, 71%
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F
CF3
F
F
F
OH
OH
93%, dr = 1:1
OH
OH
Ph
Me
3q
3r,
3s,
3t,
95%, dr = 1:1
, 73%
82%
Me
F
Me
OH
F
F
Me
Ph
OH
90%, dr = 1:1
OH
Me
3u,
3v,
3w,
80%, dr = 1:1
96%
aThe standard reaction conditions as Table 1.
aldehydes due to its thermodynamic stability and kinetic inertness.
However, due to its abundance, nontoxicity and easy availability,
CO2 has been utilized as an appealing C1 source to synthesize
diverse value-added molecules,11 including important carboxylic
acids.12 Notably, the catalytic reductive carboxylation of organic
(pseudo)halides with CO2 have been developed significantly by
many groups2h,4j,12,13 with the use of external stoichiometric
metallic (ZnEt2, Mn, etc.) or organic reductants (amine, Hantzsch
ester).
With a slight modification of the reaction conditions, we were
delighted to realize the first external reductant-free carboxylations
of organic pseudohalides with CO2. Besides 1a, benzyltriethyl and
benzyldimethylisopropyl ammonium triflates also worked well
(See Table S8 in SI for details). As shown in Table 3, a variety of
ammonium salts with different substituents on the aromatic ring
underwent the carboxylation smoothly. Besides the substrates
with EWGs (4d-4h), those substrates bearing EDGs (4i, 4l, 4r)
also afforded the desired acids in moderate yields. Remarkably, a
wide range of functional groups, such as C─F bond (4d), CF3 (4e,
4p, 4t), nitrile (4f), ester (4g, 4q) and methoxyl (4l, 4r), were all
tolerated well. Moreover, heterocycles, including furan (4j),
benzofuran (4v), benzothiophene (4w), and thiophene (4k, 4x),
also did not deter the carboxylation. Notably, product 4v could
undergo dearomatization to give 2,3-dihydrobenzofuran, which is
an important motif in many bioactive compounds.14 Importantly,
this reaction could be easily scalable, as demonstrated by the gram
scale synthesis of 4a in 68% yield with lower catalyst loading.
Table 1. Visible-light-driven cross-electrophile couplings
between 1a and aldehydes/ketonea
O
Ir(ppy)2(dtbbpy)•PF6 (3 mol%)
R
Ar
Cs2CO3 (2.0 equiv)
R
NMe3OTf
+
Ar
DMF (0.2 M), 30 W blue LED, N2, rt, 36 h
then HCl (aq.)
OH
Ph
Ph
1a
2
3
iPr
F
OPh
OH
, 70%
OH
OH
OH
Ph
Ph
Ph
Ph
3a
3b
3c
3g
, 73%
Me
, 76%
3d, 84%
Me
OMe
F
Ph
OH
OH
OH
, 72%
OH
Ph
Ph
Ph
Ph
3e
3f
3h
, 44%
, 63%
, 63%
a1a (0.4 mmol), 2 (0.2 mmol).
Table 3. Visible-light-driven carboxylation of primary C─
Moreover, various ammonium salts were also investigated with
4-fluorobenzaldehyde 2c due to the importance of fluorine-
containing compounds. Gratifyingly, many kinds of functional
groups were well tolerated, including CF3 (3j, 3o, 3q), ester (3k,
3p), OCF3 (3l), and thiophene (3m). The electron-poor substrates
showed better reactivity than those electron-neutral and electron-
rich ones, which might arise from the easier single electron
reduction of ammonium salts and higher stability of the benzylic
anions. Notably, besides primary benzylic C ─ N bonds, the
secondary ones in ammonium salts could also be selectively
cleaved to afford alcohols (3r-3w) in high yields.
N bondsa
Ir(ppy)2(dtbbpy)•PF6 (1 mol%)
Cs2CO3 (2.0 equiv), Na2CO3 (1.0 equiv)
COOH
NMe3OTf
COOH
Ar
Ar
CO2
+
DMF (0.2 M), 30 W blue LED, rt, 26 h
then HCl (aq.)
1
(1 atm, closed)
4
d
R
4n
4o
4p
4q
, R = Ph, 56%
, R = Me, 60%
4f
, R = CN, 84%
COOH
c
4g
4h
, R = COOMe, 91%
, R = OCF3, 60%
, R = OPh, 65%
R
d
, R = CF3, 69%
b
, R = COOMe, 69%
4a
, R = Ph, 80%, 68% 4i
d
d
d
4r
, R = OMe, 52%
4b
4c
4d
4e
, R = H, 69%
, R = tBu, 62%d
4j
, R = 3-furan, 63%
4k
, R = 3-thiophen, 66%
d
COOH
4s
4t
, R = Ph, 88%
c
, R = F, 72%
4l
4m
, R = 4-MeOC6H4, 66%
, R = CF3, 89%
R
, R = CF3, 82%
, R = 4-MeC6H4, 76%
Based on this success, we further considered whether other
electrophiles, especially more inert ones, could be applied in such
catalytic system. As well-known, CO2 is much less reactive than
COOH
COOH
S
COOH
O
COOH
4v, 77%d
S
e
d
d
4u
, 84%
4x
, 55%
4w
, 86%
a0.2 mmol scale. Gram scale: 1a (8 mmol), [Ir]-catalyst
b
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MeOTf (1.3 equiv)
Ir(ppy)2(dtbbpy)•PF6 (3 mol%)
Cs2CO3 (2.0 equiv), Na2CO3 (1.0 equiv)
(0.5 mol%), Cs2CO3 (1.5 equiv), Na2CO3 (1.0 equiv), DMF
(0.4 M). c[Ir]-catalyst (3 mol%), Na2HPO4 instead of
Na2CO3, 36 h. [Ir]-catalyst (3 mol%), 36 h. 12 h, without
Na2CO3.
NMe2
COOH
(2)
CO2
(1 atm, closed)
+
DMF (0.2 M), 30 W blue LED, rt, 36 h
Ph
Ph
d
e
then HCl (aq.)
5
4a
, 63%
, 0.2 mmol
To gain more insight into these cross-electrophile couplings, we
did a variety of mechanistic studies (Figure 3). As we detected
trace amount of homocoupling byproducts of 1 in some cases, we
first tested the effect of 2,2,6,6-tetramethyl-1-piperdinyloxy
(TEMPO) to the reactions with benzaldehyde and CO2, both of
which were significantly inhibited (Figure 3A). These results were
consistent with above mentioned racemization in the reaction of
1am, indicating that radicals might be involved. Second, isotope-
labeling studies were conducted. No deuterium incorporation was
observed when d7-DMF was used as the solvent. When D2O was
added, up to 97% deuterium incorporation was observed (Figure
3B). These results precluded the hydrogen atom transfer (HAT)
with DMF and indicated the formation of benzylic anion
intermediate. Furthermore, the reaction of deuterated substrate 1a’
afforded 6a in 63% yield with 47% deuterium incorporation
(Figure 3C). This result is in accordance with the hypothesis
(Figure 2B) and indicates that both the deprotonation of amine
radical cations by base and protonation of a benzylic anion might
happen in the reaction mixture.
The successful carboxylation of primary C─N bonds prompted
us to investigate the reactions of secondary and tertiary ones to
synthesize α-substituted aryl acetic acids (Table 4). We were
pleased to find many α-aryl benzyl ammonium salts reacted well.
For example, products 4ac and 4ad were obtained in good yields,
both of which could further undergo facile amidation to
synthesize the antiproliferative and the inhibitors of histone
deacetylase.15 Besides those with α-aryl groups, the substrates
with α-alkyl groups (4ah-4an) were also successfully subjected to
this reaction. Notably, the utility of this method was further
demonstrated by generation of important drugs ibuprofen 4ak and
naproxen 4an in synthetically useful yields. Moreover, starting
from the commercially available chiral benzylic amine (>99% ee),
we prepared the ammonium salt 1am, applied it in the standard
reaction conditions and obtained racemic product 4am in 60%
yield, which suggested the formation of an achiral intermediate.
To our delight, the highly challenging carboxylation of tertiary C
─N bonds also worked to give 4ao in a synthetically useful yield,
which represented the first example of carboxylation of tertiary C
─N bond. Besides benzyl ammonium triflates, to our delight, this
strategy was also amenable to the carboxylation of allylic C─N
bond to provide 4ap in total 71% yield, albeit with low
regioselectivity (eq. 1).
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(A) The effect of common radical scavengers in the reaction
CO2 (1 atm, closed)
(2.0 equiv)
Ir(ppy)2(dtbbpy)•PF6 (1 mol%)
TEMPO
Me
Cs2CO3 (2.0 equiv), Na2CO3 (1.0 equiv)
NMe3OTf
COOH
+
Ph
DMF (0.2 M), 30 W blue LED, rt, 26 h
then HCl (aq.)
Ph
Ph
1a
4a
6a
, n.d.
, 0.2 mmol
, n.d.
Ph
(B) The effect of D2O in the reaction
D O
(x equiv)
Ir(ppy)2(dtbbpy)•PF6 (1 mol%)
2
Table 4. Visible-light-driven carboxylation of secondary
and tertiary C─N bondsa
H/D
Cs2CO3 (2.0 equiv), Na2CO3 (1.0 equiv)
NMe3OTf
DMF (0.2 M), 30 W blue LED, N2, rt, 26 h
6a
Ph
then HCl (aq.)
D
x = 0, 60% yield, 0%
x = 1, 70% yield, 56% D
1a
, 0.2 mmol
COOH
NMe3OTf
Ir(ppy)2(dtbbpy)•PF6 (3 mol%)
D
x = 20, 80% yield, 97%
(C) The reaction of deuterated substrate 1a'
Ir(ppy)2(dtbbpy)•PF6 (1 mol%)
Cs2CO3 (2.0 equiv), Na2CO3 (1.0 equiv)
R'
R
R'
R
CO2
(1 atm, closed)
COOH
Ar
Ar
+
DMF (0.2 M), 30 W blue LED, rt, 36 h
D
N(C 3)3OTf
Cs2CO3 (2.0 equiv), Na2CO3 (1.0 equiv)
then HCl (aq.)
H/D
1
4
DMF (0.2 M), 30 W blue LED, N2, rt, 26 h
Ph
COOH
COOH
COOH
Ph
6a,
then HCl (aq.)
1a'
, 0.2 mmol
D
63% yield, 47%
Figure 3. Mechanistic studies.
MeO
F
Cl
b
4z,
4aa,
4ab,
64%
COOH
70%
78%
4y
, 79%
COOH
To illustrate the possibility of NMe3 as the electron donor, we
further did Stern-Volmer luminescence studies and
electrochemistry investigation (see the SI for details). The
luminescence of Ir(ppy)2(dtbbpy)·PF6 (EPRed[IrIII/IrII] = -1.80 V
vs Ag/Ag+ in DMF) at max = 570 nm was readily quenched by
R'' COOH
COOH
Ph
Me
Me
Me
4ae,
4af,
R'' = Me, 84%
R'' = F, 89%
4ac,
4ad,
4ag,
84%
96%
76%
COOH
Me
COOH
Me
COOH
COOH
Me
NEt3 (EP = +0.63 V vs Ag/Ag+ in DMF) with a slope of 223.5,
Ox
Me
Me
NC
Me
which was much more significant than ammonium salt 1a (2.80)
and benzaldehyde (1.07). These results suggested that it might
also be readily quenched by in situ generated NMe3 (EPOx = +0.65
c
4ah
4al,
4ai,
4aj,
, 58%
49%
91%
4ak,
54%
COOH
COOH
Me
COOH
COOH
Me
Me
Me
V vs Ag/Ag+ in DMF) from 1a16 or α-amino radical (E1/2
= -
Red
MeO
Ph
d
1.03 V vs SCE).17 Moreover, the benzyl radical (E1/2Red = -1.43 V
vs SCE)18 might be further reduced by IrII-catalyst (E1/2Red[IrIII/IrII]
= -1.51 V vs SCE)19 to give benzylic carbon anion, which
underwent facile nucleophilic attack to aldehydes/ketone or CO2
to give the corresponding products.20 At this stage, we preferred
such nucleophilic attack process based on the control experiments
(See also Table S9 in SI for more details), although we could not
exclude other alternative pathways,21 such as coupling between
benzyl radicals and ketyl radicals.9
4ao,
43%
4am,
4an,
85%
60%
59%
aThe standard reaction conditions as Table 3 except [Ir]-
catalyst (3 mol%), 36 h. b[Ir]-catalyst (1 mol%), 26 h. c[Ir]-
catalyst (4 mol%). dK2CO3 instead of Na2CO3.
CO2 (1 atm, closed)
Ir(ppy)2(dtbbpy)•PF6 (3 mol%)
Cs2CO3 (2.0 equiv), Na2CO3 (1.0 equiv)
COOH
(1)
+
COOH
Ph
NMe3OTf
, 0.2 mmol
Ph
DMF (0.2 M), 30 W blue LED, rt, 36 h
Ph
, 71% (1:1.25)
then HCl (aq.)
1ap
4ap
To avoid the isolation of 1a, we further tested a convenient
In summary, we have developed the first visible-light-driven
external-reductant-free cross-electrophile couplings of tetraalkyl
ammonium salts with aldehydes/ketone and CO2. Importantly, the
in situ generated byproduct, NMe3, might act as the electron
donor. A wide range of benzylic C─N bonds, including primary,
secondary, and tertiary ones, undergo the reactions smoothly.
one-pot reaction of amine 5 with CO2 in the presence of MeOTf
to give 4a in 63% yield (eq. 2). Notably, the absence of MeOTf
led to no conversion.
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1587. (n) J. Hu, H. Sun, W. Cai, X. Pu, Y. Zhang, Z. Shi, J. Org.
These reactions feature mild conditions, low catalyst loading, high
selectivity, broad substrate scope, good functional group tolerance
and facile generation of important products, including ibuprofen
and naproxen. Moreover, one-pot protocol is also realized with
high step economy. Mechanistic studies indicate that the alkyl
radical and anion might act as the key intermediates in such
reactions. Further application of this novel strategy in other
external reductant-free cross-electrophile couplings is underway.
Chem. 2016, 81, 14. (o) Y.-Q.-Q. Yi, W.-C. Yang, D.-D. Zhai, X.-
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(9) It might be difficult to generate ketyl radical in the presence of
catalytically generated amine in low concentration and excess
Cs2CO3 (See Table S8 in SI for more details). For previous reports,
see: (a) Nakajima, M.; Fava, E.; Loescher, S.; Jiang, Z.; Rueping, M.
Angew. Chem., Int. Ed. 2015, 54, 8828. (b) Chen, M.; Zhao, X.;
Yang, C.; Xia, W. Org. Lett. 2017, 19, 3807.
(10) The reductive byproduct diphenylmethanol was isolated in 54%
yields, which might arise from lower reactivity of benzophenone
than benzaldehyde.
(11) (a) Aresta, M. Carbon Dioxide as Chemical Feedstock; Wiley-VCH,
Weinheim, 2010. (b) Huang, K.; Sun, C.-L.; Shi, Z.-J. Chem. Soc.
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(13) For recent examples, see: (a) Julia-Hernandez, F.; Moragas, T.;
Cornella, J.; Martin, R. Nature 2017, 545, 84. (b) Chen,Y.-G.;
Shuai, B.; Ma, C.; Zhang, X.-J.; Fang, P.; Mei, T.-S. Org. Lett.
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Tortajada, A.; Sun, S.-Z.; Okura, K.; Martin, R. Angew. Chem., Int.
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(14) Ramella, V.; He, Z.; Daniliuc, C. G.; Studer, A. Eur. J. Org. Chem.
2016, 2268 and references therein.
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Detailed experimental procedures, spectral data, and analytical
data are available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
dgyu@scu.edu.cn
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank Prof. Ruben Martin (ICIQ) for valuable discussion and
Prof. Jason J. Chruma (SCU) for helpful manuscript revisions.
Financial support provided by the National Natural Science
Foundation of China (21822108, 21772129, 21502124), the “973”
Project from the MOST of China (2015CB856600), the “1000-
Youth Talents Program”, and the Fundamental Research Funds
for the Central Universities.
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(16) Significant reductive quenching by NMe3 was observed and shown
in SI (Figure S1). However, we could not calculate the Stern-
Volmer kinetic rate accurately as NMe3 is gas at room temperature.
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C.; Ross, S. D. J. Am. Chem. Soc. 1959, 81, 2361.
(17) This may explain why no external reductant is necessary in our
system, although 2 equivalents of reductant should be added. For
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49, 1946.
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(20) As radical-radical coupling byproducts diarylethanes were rarely
(21) The benzyl radicals might be reduced by amines or other reductive
components in the reaction mixtures. For example, see: Lin, L.; Bai,
X.; Ye, X.; Zhao, X.; Tan, C.-H.; Jiang, Z. Angew. Chem. Int. Ed.
2017, 56, 13842.
detected, the reduction of carbon radicals might be more favored.
For examples on reduction of carbon radicals by IrII-catalyst, see: (a)
Musacchio, A. J.; Nguyen, L. Q.; Beard, G. H.; Knowles, R. R. J.
Am. Chem. Soc. 2014, 136, 12217. (b) Yatham, V. R.; Shen, Y.;
Martin, R. Angew. Chem., Int. Ed. 2017, 56, 10915.
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R
Ar
R3
OH
COOH
R2
R1
ArCOR3
R3 = H, Ph
R1
R2
NMe3OTf
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[PC], visible light, base
generated
R2
NMe3
as electron donor
R
R1
O C O
R
1°, 2°, 3° C N bonds cleavage
R = aryl, alkenyl; R1, R2 = H, aryl, alkyl

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