Photoredox Catalysis of Aromatic β-Ketoesters for in Situ Production of Transient and Persistent Radicals for Organic Transformation

Article abstract of DOI:10.1002/anie.201916423

Radical formation is the initial step for conventional radical chemistry. Reported herein is a unified strategy to generate radicals in situ from aromatic β-ketoesters by using a photocatalyst. Under visible-light irradiation, a small amount of photocatalyst fac-Ir(ppy)₃ generates a transient α-carbonyl radical and persistent ketyl radical in situ. In contrast to the well-established approaches, neither stoichiometric external oxidant nor reductant is required for this reaction. The synthetic utility is demonstrated by pinacol coupling of ketyl radicals and benzannulation of α-carbonyl radicals with alkynes to give a series of highly substituted 1-naphthols in good to excellent yields. The readily available photocatalyst, mild reaction conditions, broad substrate scope, and high functional-group tolerance make this reaction a useful synthetic tool.

Full text of DOI:10.1002/anie.201916423

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Accepted Article
Title: Photoredox Catalysis of Aromatic β-Ketoester in Situ toward
Transient and Persistent Radicals for Organic Transformation
Authors: Li-Zhu Wu, Xiu-Long Yang, Jia-Dong Guo, Hongyan Xiao, Ke
Feng, Bin Chen, and Chen-Ho Tung
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201916423
Angew. Chem. 10.1002/ange.201916423
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10.1002/anie.201916423
Angewandte Chemie International Edition
COMMUNICATION
Photoredox Catalysis of Aromatic β-Ketoester in Situ toward
Transient and Persistent Radicals for Organic Transformation
Xiu-Long Yang, Jia-Dong Guo, Hongyan Xiao, Ke Feng, Bin Chen, Chen-Ho Tung, and Li-Zhu Wu*
Abstract: Radical formation is an initial step for conventional radical
chemistry. Reported herein is a unified strategy to generate radicals
in situ from aromatic β-ketoester only by a photocatalyst. Under
visible light irradiation, a small amount of photocatalyst fac-Ir(ppy)3
succeeds in the generation of transient α-carbonyl radical and
persistent ketyl radical in situ, for the first time. In contrast to the
well-established approaches, neither stoichiometric external oxidant
nor reductant is required for this reaction. The synthetic utility is
demonstrated by pinacol-coupling of ketyl radicals and
benzannulation of α-carbonyl radicals with alkynes to give a series of
highly substituted 1-naphthols in good to excellent yields. Provided
the readily available photocatalyst, extremely mild reaction condition,
broad substrate scope and functionality tolerance offer a unique
perspective in synthetic planning and implementation.
and reductant in one pot reaction.[9] Toward this goal, one needs
not only to satisfy the thermodynamical requirement for radical
generation but also to keep the kinetic reactivity of the
generation radicals for following transformation. What's more,
the intermediates generated in situ with different lifetime under
neutral condition seems even more difficult to couple for
subsequent product formation.
Development of atom- and step-economic approaches to make
useful molecular structures is always an important concern of
organic synthesis.[1] Recent blooming of visible light catalysis
becomes sought after to success of this easy manipulation and
environmental benign transformation.[2] In terms of
a
photocatalyst either reductive-quenching by electron donor
(reductant) or oxidative-quenching by electron acceptor (oxidant),
the generated reactive open-shell radical cations (D•+) and
radical anions (A•−) exhibit rich synthetic reactivity for
subsequent reactions.[3] Over the past decade, these
photocatalytic designs have spurred tremendous research
interest and there have been numerous advances in an array of
photoredox catalytic reactions, for example, cross-coupling
Scheme 1. Photoredox catalysis of aromatic β-ketoester in situ toward
transient and persistent radicals for organic transformation.
With this in mind, we initiated the study to generate radicals
in situ by a photocatalyst under redox-neutral condition. Herein,
1,3-dicarbonyl compound, aromatic β-ketoester, is selected due
to the fact that the activation of its unique molecular skeleton is
difficult to access without the aid of external oxidant or reductant.
It was expected that both electrophilic and nucleophilic
functional groups by keto−enol tautomerism[10] of aromatic β-
reactions,[4]
α-amino/oxy/carbonyl
C(sp3)−H
bond
functionalization,[5] cycloadditions,[6] reductive umpolung of
carbonyl derivatives[7] and dehalogenation reactions.[8] In these
reactions, however, stoichiometric external oxidant or reductant
is often required for subsequent oxidative or reductive reactions.
In light of the fact that a photocatalyst in its excited state is a
stronger oxidant and a stronger reductant than in its ground
state,[3] we wondered whether one could make full use of a
photocatalyst to form radicals without any aid of external oxidant
ketoester is able to interact with photoexcited photocatalyst (PC*)
11]
to afford ketyl radical A[7,
and photocatalyst radical cation
(PC•+).[12] The higher electron affinity and lower LUMO energy
level of aromatic β-ketoesters[13] render the keto form rather than
the enol form of aromatic β-ketoester to act as electron acceptor
(Table S4, supported by the results of the DFT calculation in the
Supporting Information) (Scheme 1). Although photocatalyst
[*]
Dr. X.-L. Yang, J.-D. Guo, Dr. K. Feng, Prof. Dr. Chen, Prof. Dr. C.-
H. Tung, Prof. Dr. L.-Z. Wu
+
ox
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
Beijing, 100190 (China)
and
School of Future Technology
University of Chinese Academy of Sciences
Beijing, 100049 (China)
*fac-Ir(ppy)3 (E1/2 Ir(ppy)3/*Ir(ppy)3 = −1.73 V vs. SCE)[2a] is not
able to take place single electron transfer (SET) with aromatic β-
ketoester 1a (Ered < −1.90 V vs. SCE, Eox > +1.90 V vs. SCE in
CH3CN, Figures S4 and S6), the anion 1a– (E1/2
1a•/1a–
=
ox
+0.66 V vs. SCE in DMF, Figure S5)[14] resulted from partially
deprotonated 1a would be oxidized by +fac-Ir(ppy)3 species
+Ir(ppy)3/Ir(ppy)3
=
+0.77
V
vs. SCE) to have
red
(E1/2
E-mail: lzwu@mail.ipc.ac.cn
Homepage: http://lsp.ipc.ac.cn/
Dr. H. Xiao
Key Laboratory of Bio-Inspired Materials and Interface Sciences
Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
photocatalyst fac-Ir(ppy)3 regenerated and simultaneously
transient α-carbonyl radical B produced.[13] The striking feature
of aromatic β-ketoester is that the enol form enables to eliminate
proton to form its anion, and at the same time, the eliminated
proton can activate keto form to significantly decrease the
reduction potential of 1a. As a result, the persistent ketyl radical
A and transient α-carbonyl radical B are produced only by
Beijing, 100190 (China)
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.201916423
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photocatalyst fac-Ir(ppy)3 under visible light irradiation without
any stoichiometric external oxidant and reductant, which is
unprecedented in the photoredox catalysis.
Indeed, after irradiation of aromatic β-ketoester 1a, 5,5-
dimethyl-1-pyrroline-N-oxide (DMPO) and fac-Ir(ppy)3 in N,N-
dimethylformamide (DMF) for 60 seconds, a characteristic signal
2 or 3 of ketyl radical A or α-carbonyl radical B were detected by
electron spin resonance (ESR) (Scheme 2A). The two radical
inhibitors, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and 2,6-
di-tert-butyl-4-methylphenol (BHT) greatly suppressed the
reaction with the formation of alkoxyamine 4 in about 10% yield
(Scheme 2B).[14-15] Importantly, visible light irradiation of
aromatic β-ketoester 1a to generate persistent ketyl radical A[7, 11]
and transient α-carbonyl radical B[16] can couple with 1,1-
diphenylethylene 5 to afford bridged diethyl heptanedioate 6 with
the persistent radical effect (PRE) principle[17] (Scheme 2C).
Scheme 3. Kinetic isotope effect (KIE) and control experiments.
Scheme 2. Radical capture experiments in support of generation of α-carbonyl
B or ketyl radical A. [a] Reaction conditions: 1a (0.5 mmol) or 5 (0.5 mmol),
fac-Ir(ppy)3 (3 mol%, 0.015 mmol), DMF (5.0 mL, AR), in the argon under blue
LEDs irradiation at room temperature for 48 h. [b] Isolated yield based on 0.25
mmol 1a. [c] The configuration of the diastereomer was determined by 1H
NMR.
reducing *fac-Ir(ppy)3 displayed the best photocatalytic activity.
When ethyny-d-benzene ([D1]-7a) was employed as a substrate,
the fully retained deuteration ratio of the desired product [D1]-8-1
suggested that the C–H bond of alkyne was not cleaved in this
reaction [Eq. (2)]. Intermolecular kinetic isotope effect (KIE) of
kH/kD = 3.0 further ascertained the C−H bond cleavage for the
generation of persistent ketyl radical A and transient α-carbonyl
radical B [Eq. (3)]. All of the results demonstrated the keto form
and enol form of aromatic β-ketoester were activated by the
photocatalyst under visible light irradiation. When the framework
of phenyl group of substrate 1a was changed to methyl
substituted 1y, no any product was observed, suggesting the
redox potential and equilibrium of keto and enol form are critical
for these radical generation [Eq. (4)]. Interestingly, when
benzophenone,[18] a well-known hydrogen and electron acceptor
were added under the standard conditions, another ketyl radical
suppressed the formation of ketyl radical A, leading to the
pinacol coupling of benzophenone 9b+9b' in 64% yield and
Given the difference in reactivity, an alkyne was selected to
intercept the transient α-carbonyl radical B, while the
accumulated persistent ketyl radical A was found to undergo the
pinacol-coupling along with the consumption of transient α-
carbonyl radical B. As shown in Scheme 3, when aromatic β-
ketoester 1a and phenylacetylene 7a were mixed with 3 mol%
fac-Ir(ppy)3 in 5.0 mL DMF for visible light irradiation, the
annulation product 8-1 and pinacol-coupling products 9a/9a'
were obtained in a ratio of 1 : 1 with 75% and 78% yields (based
on the consumption of 1a) in one reaction [Eq. (1)]. Among the
investigated photocatalysts (Table S3, entries 1−11), strongly
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10.1002/anie.201916423
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COMMUNICATION
benzannulation product 8-1 in 72% yield [Eq. (5)]. In contrast to
those reported methods for synthesis of 1-naphthols scaffold[19]
and pinacols,[20] neither excess of external oxidant (Mn(OAc)3,[21]
transient α-carbonyl radical B is very dependent on the form of
aromatic β-ketoester (Scheme 1). The anion enol by eliminating
proton from enol form can be oxidized to produce α-carbonyl
radical B, and the eliminated proton can activate keto form to
generate ketyl radical A. Remarkably, cyclic voltammetry
measurements showed that the reduction of enol form of 1a was
Pd(OAc)2/Cu(OAc)2,[22] Ag salt/Na2S2O8,[23] Br source/TBHP[24]
)
nor reductant (tertiary amines,[25] Hantzsch esters[26]) were
employed. The reaction occurred through photoredox catalysis
to generate α-carbonyl radical B and ketyl radical A for the
formation of 1-naphthols and pinacols in one reaction. Moreover,
the presence of commonly used oxidants (K2S2O8, t-BuOOH and
PhNO2) almost inhibited the reaction completely [Eq. (6)] (Table
S3, entries 23−25).
shifted to lower potentials upon addition of
5
mol%
dimethylamine (Ered = −1.05 V vs. SCE, Eox = +0.32 V vs. SCE in
CH3CN, Figures S6 and S7). Simultaneously, the eliminated
proton from the enol form could activate the keto form of 1a by
generated Lewis acid (+NH2(Me)2). Here, the subtle proton
transfer shuttle[28] of dimethylamine facilitated the formation of
the persistent ketyl radical A and transient α-carbonyl radical B
by fac-Ir(ppy)3 under visible light irradiation. As a result, the
transient α-carbonyl radical B could be selectively intercepted by
alkynes to synthesize 1-naphthols 8-1 and the persistent ketyl
radical A followed dimerization to pinacols 9a+9a', respectively.
To illustrate the generality of the redox-neutral reaction
established, we examined the cyclization of a wide range of
alkynes 7 and aromatic β-ketoesters 1. As shown in Scheme 4,
both electron-donating and electron-withdrawing substituents
were well tolerated. Para-, meta-, and ortho-substituted
arylacetylenes underwent smoothly in the benzannulation to
generate 8-1–8-7 and 8-10–8-15 as single regioisomers in 40–
79% yields. Trace amounts of dimethylamine might generate
NH(Me)2•HX by dehalogenation of 4-Cl and 4-Br substituted
phenylacetylenes, thus resulting in trace desired products
formation. Treatment of 1a with 3-ethynylpyridine and 3-
thienylacetylene furnished the naphthols 8-16 and 8-17 in the
yield of 66% and 40%, respectively. Linear, cyclic and
functionalised aliphatic alkynes also performed well to afford the
corresponding products (8-18–8-31) in 25–70% yields. Due to
the lack of enol anion formation, the alkynic acid bearing an
acidic group -COOH 7ad did not produce any of the desired
product. Internal alkynes, such as 1-phenyl-1-hexyne and ethyl
phenylpropiolate were subjected to the reaction conditions,
albeit with moderate yields (8-31/8-31' and 8-32). Estrone and
cholesterol derivatives were also achieved, highlighting the good
functional group tolerance and potential applications for the late-
stage modification of complex molecules (8-33−8-35).
In the course of investigations, we noted that photocatalyst,
light and DMF (≥ 99.5%, analytical reagent) are essential. A set
of control experiments with water, formic acid, dimethylamine
and other organic or inorganic bases (DABCO (1,4-
Diazabicyclo[2.2.2]octane), DMAP (4-dimethylaminopyridine),
Et3N, K2CO3 and K3PO4) shown in Table 1 revealed that the
small amount of dimethylamine (~ 170 ppm, ~ 3.8 mol% in 5.0
mL DMF based on 0.5 mmol 1a, Table S6) decomposed from
DMF promoted this redox-neutral reaction (Table S5 and
Scheme S1).[27] On the basis aforementioned observation, we
proposed that the formation of persistent ketyl radical A and
Table 1: Control experiments.[a]
Yield [%]
Variation from
Entry
8-1[b]
trace
75
9a+9a'[c]
trace
78
the standard condition
1
DMF (99.8%, extra dry, acroseal)
DMF (≥ 99.5%, analytical reagent)
H2O (5.0/50 μL)
2[d]
3[e]
4[e]
5[e,f]
6[e,f]
7[e,f]
8[g]
9[g]
10
trace
trace
72
trace
trace
75
HCOOH (0.5/5.0 μL)
NH(Me)2 (1.0 μL, 40% wt, aq.)
NH(Me)2 (2.5 μL, 40% wt, aq.)
NH(Me)2 (5.0 μL, 40% wt, aq.)
No photocatalyst
Having achieved the reaction with various alkynes 7, we
shifted our attention to the scope of aromatic β-ketoesters 1 by
reacting with phenylacetylene 7a (Scheme 4). The reaction
tolerated various functional groups including -X, -Me, and -OMe
on the aromatic ring of aromatic β-ketoesters 1 except aromatic
β-ketoesters containing 4-Cl, 4-Br and 4-NO2 because of
dehalogenation and reduction of nitrobenzene, providing the
corresponding products in poor yields (8-37, 8-38 and 8-42).[29]
53
56
46
48
0
0
In dark
0
0
Other solvent
trace
60
trace
61
The aromatic β-ketoesters 1i bearing
a strong electron-
11[h]
DMF (99.8%, extra dry)
withdrawing group 4-CN produced a mixture of regioisomers 8-
43 and 8-43' in a ratio of 2.5:1 and 59% combined yield,
suggesting
a radical mechanism involving ipso cyclization
[a] Reaction conditions: 1a (0.5 mmol), 7a (0.5 mmol), fac-Ir(ppy)3 (3 mol%,
0.015 mmol) and additive in 5.0 mL DMF under argon and irradiation of 3 W
blue LEDs for 48 h at rt. [b] Isolated yield based on 0.167 mmol 1a. [c] 1H
NMR yields based on 0.167 mmol 1a using benzhydrol as internal standard.
[d] >95% conversion of 1a. [e] DMF (99.8%, extra dry, acroseal). [f] NH(Me)2
(40 wt. % in H2O). [g] DMF (≥ 99.5%, analytical reagent). [h] DMF (99.8%,
extra dry, acroseal) was used after sealed heating for 1 h at 140 oC.
followed by rearrangement through the C–C bond cleavage in
the spirocyclohexadienyl radical.[30] When a methyl group was
introduced to the meta-position of phenyl ring, a mixture of
regioisomers 8-44 and 8-44' were obtained in a ratio of 2.9:1
and 65% combined yield. Remarkably, (+)-borneol and isobornyl
substituted aromatic β-ketoesters delivered 8-47 and 8-48 in 53%
and 88% yields. In addition to the COOEt-substituted 1,
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Scheme 4. Scope of alkynes 7 and aromatic β-ketoesters 1. [a] Reaction conditions: 1 (0.5 mmol), 7 (0.5 mmol), fac-Ir(ppy)3 (3 mol%, 0.015 mmol), DMF (5.0 mL,
AR), in the argon under blue LEDs irradiation at room temperature for 48 h. [b] Isolated yield based on 0.167 mmol 1 (average of two trials).
substrates bearing N-phenylamide, -Me and -C6H5 group formed
1-naphthols 8-49−8-51 in 55%–64% yields. The benzannulation
is not limited to the formation of naphthol, as exemplified by
using 2-thienyl β-ketoester 1r to generate 7-hydroxyl-
benzothiophene 8-52, albeit only in the yield of 45%. The ethyl
ester of aromatic β-ketoesters could be replaced by i-propyl, n-
butyl, tert-butyl, cyclohexyl and 2-adamantine esters, which
could proceed smoothly in this transformation delivering the
corresponding products (8-53−8-57) in moderate to excellent
yields. In addition, the reaction was adapted for the
intramolecular benzannulation of 1x leading to products 8-58.
In summary, we have designed a new strategy to generate a
persistent ketyl radical A and a transient α-carbonyl radical B
from aromatic β-ketoester in situ by photoredox catalysis. A
trace amount of dimethylamine is able to facilitate the radical
formation, and then the persistent ketyl radical A can undergo
the pinacol-coupling, while the transient α-carbonyl radical B can
be intercepted by alkyne to afford 1-naphthols. Remarkably, this
photocatalytic method overcomes the most fundamental
difficulty in the presence of stoichiometric external oxidant and
reductant in one reaction vessel for a redox-neutral reaction, and
utilizes commercially available substrates to produce the desired
1-naphthols in good to excellent yields under mild conditions.
The simple generation of radicals to directly synthesize valuable
products provided here opens up new avenues for the utilization
of a catalytic amount of photocatalyst other than stoichiometric
external oxidant or reductant for redox-neutral organic
transformation in atom- and step-economic manner.
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Acknowledgements
This work was supported by the Ministry of Science and
Technology of China (2017YFA0206903), the National Natural
Science Foundation of China (21861132004 and 21473227), the
Strategic Priority Research Program of the Chinese Academy of
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This article is protected by copyright. All rights reserved.

10.1002/anie.201916423
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Text for Table of Contents
X.-L. Yang, J.-D. Guo, H. Xiao, K. Feng,
B. Chen, C.-H. Tung, and L.-Z. Wu*
All-in-one: A photocatalyst succeeds
in the generation of transient α-
carbonyl radical and persistent ketyl
radical from aromatic β-ketoester in
situ for the first time, which realizes
straightforward synthesis of pinacol
and highly substituted 1-naphthol as
well as diethyl heptanedioate in an
atom- and step-economic manner
under extremely mild condition.
Page No. – Page No.
Photoredox Catalysis of Aromatic β-
Ketoester in Situ toward Transient
and Persistent Radicals for Organic
Transformation
This article is protected by copyright. All rights reserved.