Cascade Functionalization of C(sp3)-Br/C(sp2)-H Bonds: Access to Fused Benzo[ e]isoindole-1,3,5-trione via Visible-Light-Induced Reductive Radical Relay Strategy

Article abstract of DOI:10.1021/acs.orglett.9b02153

A reductive radical relay strategy for the construction of fused benzo[e]isoindole-1,3,5-trione through a reaction of α-bromo ketones with maleimides in the presence of Ir(ppy)₃ under visible-light irradiation is described. The protocol employs very mild reaction conditions and offers satisfactory yields. Moreover, the reaction proceeds through a cascade C(sp³)-Br/C(sp²)-H functionalization, double C-C bond formation, and oxidative aromatization sequence.

Full text of DOI:10.1021/acs.orglett.9b02153

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cascade Functionalization of C(sp3)−Br/C(sp2)−H Bonds: Access to
Fused Benzo[e]isoindole-1,3,5-trione via Visible-Light-Induced
Reductive Radical Relay Strategy
Jia-Nan Zhu, Wen-Kang Wang, Yuan Zhu, Yin-Qiu Hu, and Sheng-Yin Zhao*
College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, People’s Republic of China
S
* Supporting Information
ABSTRACT: A reductive radical relay strategy for the
construction of fused benzo[e]isoindole-1,3,5-trione through
a reaction of α-bromo ketones with maleimides in the
presence of Ir(ppy)3 under visible-light irradiation is
described. The protocol employs very mild reaction
conditions and offers satisfactory yields. Moreover, the
reaction proceeds through a cascade C(sp3)−Br/C(sp2)−H
functionalization, double C−C bond formation, and oxidative
aromatization sequence.
enerally, nitrogen-containing heterocycles were found in
an array of bioactive molecules,1 natural products,2 and
C−H bond activation and the succinimide scaffolds were
obtained in moderate to excellent yields (Scheme 1a).
Subsequently, they13 successfully applied this transformation
for the synthesis of 3-substituted maleimides through
oxidative-Heck reaction of maleimides with ketones by
Rh(III)-catalyzed C−H activation strategy (Scheme 1b).
G
functional materials.3 They thereby play a significant role in
diverse research disciplines including drug discovery, chemical
biology, and material science. They also represent a very
important and fast-developing area in organic synthesis.4
Among them, benzo[e]isoindole-1,3,5-trione derivatives show
glycogen synthase kinase 3β inhibitors,5 α1A adrenoceptor
antagonistic activity,6 and so on7 (Figure 1). However, reports
Scheme 1. Functionalization of Different Positions of
Acetophenones
Figure 1. Representive examples of benzo[e]isoindoles.
on the synthesis of benzo[e]isoindole scaffolds are rarely seen
in the literature8 (Figure 1). Given the usefulness of
benzo[e]isoindole-1,3,5-triones, development of a practical
and efficient avenue to their synthesis is of great significance.
The maleimide frames are ubiquitously found in a large
number of natural products and pharmaceutically active
molecules9 as well as functional materials.10 As a consequence,
the introduction of maleimide skeleton has drawn considerable
attention for chemists. In recent years, transition-metal-
catalyzed C−H bond activation of (hetero)arenes with
maleimides has been the most efficient and convenient
approach for the synthesis of structurally diverse compounds.11
For example, in 2015, the Prabhu group12 reported the 1,4-
addition of ketone with maleimides via Ru(II)-catalyzed direct
Received: June 21, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02153
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Furthermore, the Antonchick group14 described a copper-
catalyzed double C−H bond functionalization reaction at the
α-position of acetophenones with maleimides for the direct
synthesis of cyclopropanes (Scheme 1c). In addition, Maiti’s
group15 developed a copper-catalyzed example with respect to
dihydrofuran synthesis from aryl ketones and aromatic olefins
via radical cyclization strategy (Scheme 1d). To the best of our
knowledge, a cascade functionalization of C(sp3)−Br/C(sp2)−
H of α-bromo ketones with maleimides for the construction of
fused benzo[e]isoindole-1,3,5-triones was not explored so far.
Therefore, development of a direct avenue for the efficient
construction of the benzo[e]isoindole-1,3,5-trione is highly
desirable.
Visible-light-induced photoredox catalysis has achieved
impetus in recent years for the development of nontraditional
bond constructions in organic synthesis. A milestone was
gained in this field on account of the pioneering research by
MacMillan,16 Stephenson,17 Yoon,18 and other groups.19
Indeed, a series of covalent bonds such as C−C, C−X, and
even C−H have been activated into corresponding radical
intermediates,20 and plentiful momentous synthetic trans-
formations have been realized, including radical addition,
cross-dehydrogenative coupling, and radical cyclic reaction.21
Meanwhile, related photoredox catalysis reactions of malei-
mides have been investigated to some extent.22 However, these
reactions are mainly limited to tertiary anilines.22 While
significant advances have been made, the exploration of
photoredox-catalyzed transformation via a radical relay strategy
for the construction of privileged fused benzo[e]isoindole-
1,3,5-triones is still promising and attractive. As part of our
ongoing research on the development of maleimide chem-
istry,23 herein we disclose a cascade functionalization of
C(sp3)−Br/C(sp2)−H bonds for the synthesis of fused
benzo[e]isoindole-1,3,5-triones via visible-light-induced reduc-
tive radical relay.
To test this feasibility, we initiated our investigation by
exploring diverse reaction conditions for this photoredox
cascade protocol of 2-bromoacetophenone 1a and N-tert-
butylmaleimide 2a, and screening results are summarized in
Table 1. To our delight, the desired product 3a was isolated in
32% yield when the reaction proceeded in the presence of 2
mol % of Ir(ppy)3 and 1.0 equiv of NaHCO3 in degassed DMF
under blue LED irradiation, which showed this novel radical
cyclic reaction strategy is feasible. Further screening of solvents
suggested that CH3CN was the best choice, affording the
desired product in 67% yield (entries 2−10). Obviously, the
base played a crucial role in neutralizing the byproduct HBr.
Then a variety of bases were validated, and the results revealed
that none of the other bases was superior to K2HPO4, with an
up to 81% yield (entries 11−20). It should be noted that the
target molecule 3a was not observed because maleimides are
unstable in the presence of strong bases and are subjected to
polymerization and decomposition (entries 19 and 20).24 The
replacement of Ir(ppy)3 with other photocatalysts, including
eosine and Ru(bpy)3Cl2·6H2O, led to no product detection,
probably due to the unfavorable redox potential25 with
substrate 1a (entries 21 and 22). Finally, control experiments
indicated that the base, photocatalyst Ir(ppy)3, and visible light
were all necessary for this transformation (entries 23−25).
Having the optimal reaction conditions in hand (Table 1,
entry 17), the scope and generality of this reductive radical
relay reaction were then explored. The reaction revealed good
functional group compatibility and proved to be a versatile
a
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst
Ir(ppy)3
solvent
base
3a yield (%)
1
2
3
DMF
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
Na2CO3
DBU
Et3N
NaOAc
DABCO
Na2HPO4
K2HPO4
K3PO4
32
trace
23
0
26
48
67
39
21
0
54
11
57
42
56
74
81
69
0
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
Eosine
DMSO
dioxane
THF
c
4
5
6
7
8
CHCl3
acetone
CH3CN
DCE
9
PhCl
10
11
12
13
14
15
16
17
18
19
20
21
22
CH3OH
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
KOH
Cs2CO3
K2HPO4
K2HPO4
0
0
0
Ru(bpy)3Cl·6H2O
Ir(ppy)3
c
23
trace
0
d
24
K2HPO4
K2HPO4
e
25
Ir(ppy)3
0
a
Reactions were carried out with 1a (0.3 mmol), 2a (0.2 mmol),
catalyst (2 mol %), and base (1 equiv) in a solvent (2.0 mL) upon
irradiation with a 7.5 W blue LED under N2 atmosphere at rt for 20 h.
b
c
d
Isolated yield. No base was used. No photocatalyst was used.
e
Performed in the dark.
approach for the construction of desired products (Scheme 2).
First, a variety of N-protected maleimides can react smoothly
under the standard conditions to furnish the products in 70−
81% yield (3a−e). Noteworthily, this transformation was not
confined to N-protected maleimides, but it could also be
extended to the unprotected maleimides supplying the target
products in 52−71% yield (3f−h). The structure of 3g was
confirmed by X-ray single-crystal diffraction. It should be noted
that the reaction is completely selective toward cis config-
uration. Furthermore, either trans-dimethyl fumarate or cis-
diethyl maleate was well compatible, affording the product in
65−74% yield (3i,j). Unfortunately, when maleic anhydride
was utilized as the substrate, no product was detected. A
possible reason is that maleic anhydride is unstable in the
presence of base and is inclined to polymerize or decompose.
Next, we sought to examine the scope with respect to
various 2-bromoacetophenone derivatives (Scheme 3). Both
electron-withdrawing and electron-donating substituents read-
ily underwent the desired radical relay cascade reactions,
providing benzo[e]isoindole-1,3,5-triones in moderate to good
yields (3k−v). The mild conditions can bear a variety of
functional groups, including bromides, chlorides, trifluoro-
methyl, nitriles, ethers, and esters (3k−r, 51−77% yields).
Fortunately, we unexpectedly delivered the target products
B
DOI: 10.1021/acs.orglett.9b02153
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Organic Letters
Letter
a,
b
a,b
Scheme 2. Scope of Electron-Deficient Olefins
Scheme 3. Scope of Various 2-Bromoacetophenones
a
Reactions were carried out with 1 (0.6 mmol), 2 (0.4 mmol),
Ir(ppy)3 (2 mol %), and K2HPO4 (1 equiv) in CH3CN (4.0 mL)
b
under N2 conditions at rt for 20 h. Isolated yield.
a
Reactions were carried out with 1 (0.6 mmol), 2a (0.4 mmol),
Ir(ppy)3 (2 mol %), and K2HPO4 (1 equiv) in CH3CN (4.0 mL)
under N2 conditions at rt for 20 h. Isolated yield.
containing the free hydroxy group in moderate yields, which
can offer the opportunity for further functionalization (3u,v).
In addition, this radical relay protocol was also applicable to
heteroaryl α-bromo ketones in the form of benzofuran,
thiophene, and indole, affording the corresponding products
in 76−84 yields (3w−y). Eventually, the regioselectivity of this
reaction was further demonstrated with meta-functionalized
substrates (Scheme 4). Regrettably, no regioselective products
were achieved. When m-bromo-substituted substrate 1z was
implemented under the standard conditions, the proportion of
3z1 and 3z2 was 1.4:1. In contrast, m-methoxy-substituted
substrate 1A gave the isomers 3A1 and 3A2 in a ratio of 1.8:1.
b
Scheme 4. Examination of Regioselectivity
Scheme 5. Control Experiments
1
All of these isomers were confirmed by H NMR (see the
Supporting Information).
To further demonstrate the mechanism of this photo-
catalytic reaction, radical trapping experiments were performed
(Scheme 5). The radical scavenger TEMPO (2,2,6,6-
tetramethyl-1-piperidinyloxy) was directly introduced into α-
bromoacetophenone under the standard conditions, and the
intermediate TEMPO-adduct 4a was isolated in 86% yield,
confirmed by NMR and HRMS (Scheme 5A). Notably, when
TEMPO was added to the reaction of α-bromoacetophenone
1a and N-tert-butylmaleimide 2a under the standard
conditions, the desired radical relay reaction was completely
shut down, with the formation of the TEMPO-adduct 4a in
81% yield, explicitly indicating that a single-electron-transfer
radical process was involved (Scheme 5B).
irradiation, the photocatalyst IrIII(ppy)3 is converted into the
strongly reducing excited state IrIII*{E1/2 [*IrIII/IrIV] =
red
−1.73 V vs SCE},25a which is oxidatively quenched by 1 with
the formation of carbon-centered radical intermediate 426 and
IrIV. Subsequent C-centered radical inserts into the CC of
maleimides, and cyclization onto the aromatic ring produces
the corresponding cyclohexadienyl radical 5, which undergoes
On the basis of these control experiments and literature
precedent,22a a plausible mechanism for this reductive radical
relay reaction is shown in Scheme 6. Under blue LED
C
DOI: 10.1021/acs.orglett.9b02153
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Scheme 6. Proposed Reaction Mechanism
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a stepwise single-electron oxidation to achieve the carbocation
6 with the regeneration of the photocatalyst IrIII(ppy)3. Further
deprotonation of 6 offers the eventual product 3 in the
presence of base.
In conclusion, we have successfully developed an efficient
and practical strategy for the synthesis of fused benzo[e]-
isoindole-1,3,5-trione from α-bromo ketones and maleimides
via visible-light-induced reductive radical relay. This versatile
protocol proceeds through a cascade C(sp3)−Br/C(sp2)−H
functionalization, double C−C bond formation, and oxidative
aromatization sequence under mild conditions. Further activity
tests of these privileged structures are ongoing in our group.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02153.
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Experimental details, characterization data of all
1
compounds, and copies of H and 13C NMR spectra
(PDF)
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Accession Codes
CCDC 1933382 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail:syzhao8@dhu.edu.cn.
ORCID
Sheng-Yin Zhao: 0000-0002-5778-8068
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge the National Innovation Experiment
Program for University Students (201910255071) and the
Fundamental Research Funds for the Central Universities and
Graduate Student Innovation Fund of Donghua University
(CUSF-DH-D-2018063 and BCZD2019004).
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DOI: 10.1021/acs.orglett.9b02153
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