Bimetallic copper/cobalt-cocatalyzed double aerobic phenol oxidation/cyclization toward π-extended benzofuro[2,3-b]indoles as electron donors for electroluminescence

Article abstract of DOI:10.1039/d1gc01658j

Benzofuro[2,3-b]indoles, apart from their attractive biological features, constitute a novel family of electron donors for electroluminescent materials. Reported herein is the establishment of a strategy involving bimetallic catalysis to form such π-conju

Full text of DOI:10.1039/d1gc01658j

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Bimetallic copper/cobalt-cocatalyzed double
aerobic phenol oxidation/cyclization toward
π-extended benzofuro[2,3-b]indoles as electron
donors for electroluminescence†
Cite this: Green Chem., 2021, 23,
5031
Received 13th May 2021,
Accepted 15th June 2021
Muhammad Adnan Bashir,‡a Yulong Zhang,‡a Huaibin Yu,a Bofei Wang,a
DOI: 10.1039/d1gc01658j
a
rsc.li/greenchem
Weining Zhaob and Fangrui Zhong
*
Benzofuro[2,3-b]indoles, apart from their attractive biological fea- showed that 6H-benzofuro[2,3-b]indole is a novel donor for the
tures, constitute a novel family of electron donors for electrolumi- efficient thermally activated delayed fluorescence (TADF)
nescent materials. Reported herein is the establishment of a strat- emitter BFITrz with external quantum efficiency (EQE) values
egy involving bimetallic catalysis to form such π-conjugated of over 12.0% and a current efficiency (CE) of 63.6 cd A−1
extended frameworks, specifically via the CoCl2/Cu(OTf)2-cata- (Fig. 1A).3d
lyzed double aerobic oxidative cycloaddition of indoles with
In this context, synthetic strategies allowing straightforward
p-hydroquinone ester. The crucial and distinct roles of the two assembly of a π-extended benzofuro[2,3-b]indole are highly
metals salts and the alcohol solvent were clarified. This oxidation desirable. Unfortunately, existing methods to access this poly-
system enabled the disruption of the well-established acid- cyclic heteroaromatic framework typically require tedious
catalyzed quinone–indole [3 + 2] cycloaddition reaction favoring syntheses from pre-functionalized starting materials via tran-
benzofuro[2,3-b]indolines and is superior to previously known sition metal-catalyzed cross-coupling reactions3–5 (Fig. 1B),
multi-step synthetic methods involving Pd-catalyzed cross- thus substantially compromising synthetic practicality. One-step
coupling with pre-functionalized (pseudo)halides. Moreover, the cycloaddition using readily available indole feedstocks is thus
photophysical properties of the product were preliminarily investi- in high demand, but a corresponding robust catalytic system
gated, the results of which suggested the potential of these poly- and appropriate coupling partners have yet to be identified.
cyclic heteroaromatics to be used in efficient deep-blue organic Nevertheless, expedient copper-catalyzed [3 + 2] annulations of
light-emitting diodes.
Structural fusion of a benzofuran with an indole is syntheti-
cally interesting because the resulting polycyclic heterocycles
often exhibit fascinating pharmaceutical or photophysical pro-
perties.1 In this regard, synthetic efforts have been predomi-
nantly focused on the innovative preparation of benzofuro-
indolines (e.g., dihydrobenzofuro[2,3-b]indole), considering
their extensive biological profiles.2 In recent years, their
π-conjugated counterparts, i.e., benzofuroindoles, have been
identified as promising electroluminescent materials for
organic electronic devices.3 For instance, Lee and co-workers
aHubei Engineering Research Center for Biomaterials and Medical Protective
Materials, Hubei Key Laboratory of Bioinorganic Chemistry & Materia Medica,
School of Chemistry and Chemical Engineering, Huazhong University of Science and
Technology (HUST), Wuhan 430074, China.. E-mail: chemzfr@hust.edu.cn;
Fax: +(0)86-(0)27-8754 3632
bCollege of Pharmacy, Shenzhen Technology University, Shenzhen 518118, China
†Electronic supplementary information (ESI) available. CCDC 2017936. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
Fig. 1 An overview of benzofuroindolines versus benzofuroindoles:
d1gc01658j
‡These authors contributed equally.
synthesis and applications.
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N-sulfonylindoles with quinones have been established, allow- in virtually quantitative yield (Table 1, entry 1). Practically, this
ing for a facile assembly of dihydrobenzofuro[2,3-b]indole discovery provided a route to benzofuroindole 4a wherein
(Fig. 1C).6 Analogous reactions have also been accomplished intramolecular cyclization of intermediate 3a needed to be
by employing chiral phosphoric acid catalysts.7 However, the accomplished. Envisioning that Lewis acid activation might
resulting adducts could not be dehydrogenated under copper- work, compound 3a was isolated and treated with typical
catalyzed aerobic oxidation conditions to give benzofuro[2,3-b] metal salts such as Fe(OTf)3 or Cu(OTf)2. We were delighted
indoles. As part of our continuous interest in sustainable oxi- that compound 4a was obtained in excellent yield (entries 2
dative reactions, we have recently disclosed several bioinspired and 3). However, a one-pot protocol without isolating inter-
oxidative cascade cycloadditions that enable efficient fusion of mediate 3a apparently completely impeded the intramolecular
phenols/anilines with indoles, enamines, alkenes, and hydra- cyclization (entries 4 and 5), while decreasing the amount of
zones under mild conditions.8 Notably, each of these reactions Ag2O to 1.5 eq. facilitated the formation of product 4a (48%
only involve single oxidation. Further hydrogenation of these yield, entry 6). These outcomes suggested the necessity and
cycloadducts, if made to occur, would give rise to electronically sensitivity of Lewis activation for the conversion of inter-
distinct π-extended molecules. In this study, we have developed mediate 3a. In light of the repertoire of copper cofactors for
a prominent bimetallic copper/cobalt cocatalytic system that enzymatic catechol oxidation10 and copper catalysts for
yielded efficient aerobic oxidative [3 + 2] cycloaddition of various aerobic oxidation reactions,11 we evaluated them for
indoles with p-hydroquinone involving each a double dehydro- the model reaction (entry 7 and Table S1†). To our delight,
genation and single cyclization (Fig. 1D). The resulting benzo- the use of 10 mol% of [Cu(CH3CN)4][PF6] gave a mixture of 3a
furo[2,3-b]indoles were found to exhibit attractive photo- and 4a (60% and 10% yields, respectively), and commonly
physical properties and hence could be potentially used for used iron salts were generally ineffective in this regard (entry 8
deep-blue organic light-emitting diodes (OLEDs).
and Table S1†). Subsequent solvent screening revealed a ben-
We initially planned to devise an oxidation system to realize eficial influence of EtOH for intramolecular cyclization of
not only the known phenol–indole [3 + 2] cycloaddition but quinone 3a (entry 9 and Table S2†). However, despite consider-
also the succeeding challenging dehydrogenation of the result- able efforts at optimizing other reaction parameters including
ing dihydrobenzofuro[2,3-b]indole. Keeping in mind the catalyst loading, temperature and reaction time, use of a single
common utilization of Ag2O for the oxidation of p-hydro- metal catalyst unfortunately turned out to be insufficient to
quinone,9 we first employed excess Ag2O (4.0 eq.) in the model accomplish the required dual task, i.e., catalytic oxidation and
reaction of indole 1a with methyl 2,5-dihydroxybenzoate 2a. Lewis acid activation that were demanded for the overall
Unexpectedly, this mixture afforded indolylbenzoquinone 3a process.
Table 1 Optimization of the reaction conditions
Entrya
Cat. A (x)/Cat. B (y)
Solvent
T (°C)
Time (h)
Yieldb (3a, %)
Yieldb (4a, %)
1
Ag2O (400)
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
EtOH
EtOH
EtOH
EtOH
EtOH
25
25
25
25
25
25
25
25
25
25
25
40
50
25
40
40
40
40
24
24
24
24
24
24
48
48
48
48
48
48
48
48
48
48
48
48
98
0
0
0
2c
3c
4
Ag2O (400)/Fe(OTf)3 (10)
Ag2O (400)/Cu(OTf)2 (10)
Ag2O (400)/Fe(OTf)3 (10)
Ag2O (400)/Cu(OTf)2 (10)
Ag2O (150)/Cu(OTf)2 (10)
[Cu(CH3CN)4][PF6] (10)
Fe(OTf)3 (10)
[Cu(CH3CN)4][PF6] (10)
CoCl2 (5)/[Cu(CH3CN)4][PF6] (5)
CoCl2 (5)/Cu(OTf)2 (5)
CoCl2 (5)/Cu(OTf)2 (5)
CoCl2 (5)/Cu(OTf)2 (5)
CoCl2 (5)
95
98
0
94
98
40
60
0
22
44
35
10
0
0
0
5
30
0
5
6
7
8
0
48
10
0
48
55
63
85
63
0
91
42
35
0
9
10
11
12
13
14
15
16
17d
18e
EtOH
EtOH
EtOH
EtOH
CoCl2 (5)/Cu(OTf)2 (10)
CoCl2 (10)/Cu(OTf)2 (5)
CoCl2 (5)/Cu(OTf)2 (10)
CoCl2 (5)/Cu(OTf)2 (10)
EtOH
a Reactions were performed with 1a (0.2 mmol), and 2a (0.2 mmol), catalyst, under O2 (1 atm) in the specified solvent (4.0 mL). b Isolated yield.
c Stepwise operation with intermediate 3a isolated. d Under air. e Under N2.
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We decided to attempt to use bimetallic systems to circum- having tested various conditions, these intermediates could
vent the above dilemma, as synergistic dual transition metal not be made to cyclize into the respective benzofuroindoles.
catalysis has been shown to unlock a plethora of previously This lack of cyclization may have been due to steric congestion
unattainable transformations and novel chemical reactivity.12 between the 4-substituent and the ester group upon rotation
Given the advantages of cobalt catalysts such as their low cost about the C–C axis to the rotamer displaying the intra-
and toxicity, their industrial prospects,13 and their capacity to molecular-cyclization-facilitating
co-planar
relationship
activate molecular oxygen,14 CoCl2 was applied in combination between the indole ring and quinone ring. In addition, other
with copper salts. To our delight, the cocatalysis system functionalized indoles containing an ester, sulfonamide, car-
boosted the oxidation to yield a full conversion of starting bamate, or amide group were identified to be suitable, as seen
materials, and Cu(OTf)2 was a superior partner in terms of the in the smooth formation of compounds 4j–m (60–89% yields).
yield of product 4a (Table 1, entries 10 and 11). A slight When quinone ethyl ester was used, adduct 4n was delivered
elevation of temperature was highly beneficial as it augmented in 66% yield. Next, the tolerance of functionalization at the
the production of 4a (entries 12 and 13). Of note, CoCl2 alone indole N atom was evaluated. Delightfully, the fidelity of this
failed to afford any reactivity (entry 14). Moreover, choosing an catalytic system was found to remain high for indoles with an
appropriate Co/Cu ratio was shown to be essential, as use of N-alkyl group in both the linear and cyclic forms, as well as
excess CoCl2 deteriorated both the oxidation of phenol 2a and N-arylated indoles, affording N-substituted benzofuroindoles
intramolecular cyclization of quinone 3a. The combination of 4o–t in 71–89% yields. It should be noted that p-hydroquinone
5 mol% of CoCl2 and 10 mol% of Cu(OTf)2 was the optimal lacking an ester group was found to not be a suitable sub-
choice, and apparently acted synergistically to deliver the bezo- strate. The carbonyl group was speculated to be important not
furoindole 4a in 91% yield (entry 15). It is noteworthy that only for enhancing the electrophilicity of the quinone moiety
when the reaction was performed under air, a much inferior but also for providing an additional coordination site for
outcome was obtained (entry 16).
copper Lewis acid activation. This speculation was in line with
Having established the optimal reaction conditions, the the mechanism proposed in the literature.6 Due to the limited
substrate generality and limitations of the present oxidative availability of differently substituted p-hydroquinone ester sub-
[3 + 2] cycloaddition were evaluated (Scheme 1). In general, strates as noted in the reference, attempts to make further vari-
indoles containing electronically and sterically different substi- ations of p-hydroquinones were not undertaken. Notably, the
tuents at the C5 or C6 carbon were well tolerated (products synthetic practicality of our protocol was showcased by the
4b–i, 72–94% yields). Reactivity was somewhat compromised gram-scale synthesis of compound 4a with 2.5 mol% Cu(OTf)2
for the ones bearing electronically deactivated groups, particu- and 1.25 mol% CoCl2 in consistently high yield (1.2 g, 85%,
larly for the intramolecular cyclization step. In these cases, see section D in the ESI†).
heating was applied to secure good overall reaction efficiency
A set of additional control experiments were conducted to
levels (products 4f–h, 72–77% yields). Unfortunately, sterically gain more insights into the reaction mechanism, and some
disfavored 4-substituted indoles were not well accommodated, conclusions were made as a result. (1) The present bimetallic
only leading to the formation of indolylquinones. Despite aerobic catalytic system exhibited chemospecificity for oxi-
dation of p-hydroquinone 2a over indole 1a; the latter
remained intact when subjected to the standard reaction con-
ditions (Scheme 2a). (2) Differing from Co(salophen), which
could catalyze aerobic oxidation of p-hydroquinone,15 neither
O2 nor CoCl2/O2 could oxidize p-hydroquinone 2a into
quinone 2b, while the performance of Cu(OTf)2 alone was
fairly limited. These outcomes suggested that Cu2+ was likely
+
responsible for the aerobic phenol oxidation and that Co2
facilitated the turnover of the Cu2+ cation. This notion was in
line with the low yield afforded by CuOTf, but its performance
was significantly enhanced by the addition of CoCl2
(Scheme 2b). (3) Addition of indole 1a to quinone 2b was indi-
cated to be spontaneous but extremely slow in CH2Cl2, and
each of EtOH, CoCl2 and Cu(OTf)2 significantly enhanced the
rate (Scheme 2c). However, the faint detection of adduct 2d
along with excellent isolated yield of quinone 3a (98%) in the
absence of any catalyst suggested that 2d underwent spon-
taneous aerobic oxidation. (4) Quinone 3a was evidently
indicated to be the intermediate to the desired product 4a.
Cu(OTf)2 activation was concluded to be indispensable for
Scheme 1 The substrate scope. Reactions were performed with 1a
(0.2 mmol), 2a (0.2 mmol), Cu(OTf)2 (10 mol%), and CoCl2 (5 mol%),
this transformation while EtOH could provide assistance
(Scheme 2d). By contrast, CoCl2 was ineffective at promoting
under O2 (balloon) in EtOH (4.0 mL) at 40 °C for 48 h. Isolated yield.
a Reaction was performed under reflux.
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largely realized by the Co3+/Co2 cycle, which effectively har-
nesses molecular oxygen.16 Acid activation of quinone 2b
facilitates the nucleophilic attack of indole to give intermediate
A, which further forms indolylhydroquinone B via a proton
transfer that is likely assisted by EtOH. Oxidation of p-hydro-
quinone B occurs spontaneously and rapidly to generate
quinone 3a, with this quinone activated by Cu2+ to undergo
intramolecular cyclization and hence give benzofuroindole 4a.
The proton transfer involved in this particular step probably
benefits from the use of protic EtOH as the reaction medium.
To make a more clear comparison of our protocol with the
literature method, we calculated green metrics based on the
scaled-up synthesis of compound 4a, and found an E-factor17a
of 0.194, 95.8% atom economy, 81.4% atom efficiency, 100%
carbon efficiency, and 75.9% reaction mass efficiency. The
E-factor for the current protocol was found to be much better
than that for the typical multi-step synthesis via Pd-catalyzed
cross-coupling (see section E in the ESI†). Also, we calculated
the EcoScale value17b for this reaction based on parameters
including yield, cost of the reaction components, safety, tech-
nical setup, temperature/time, and workup and purification
(Table S3†). An EcoScale value of 80.5 was obtained. The above
green metrics data showed our revised synthetic method to be
an excellent green synthesis protocol, and much superior to
the otherwise available approach to the synthesis of the title
compound.3–5
+
Scheme 2 Control experiments.
this process. The bimetallic catalysis rather than just copper
catalysis was determined to account for the distinct oxidation
outcome of the present system.6 (5) Due to the thermodynamic
stability of the π-extended structure of 4a, intramolecular cycli-
zation of quinone 3a was concluded to be preferred over conju-
gated addition with a second indole molecule or EtOH.
Neither phenol 5a nor its derivative quinone 6a was observed
(Scheme 2e).
In light of the above experiments, a plausible mechanism
was proposed (Scheme 3). According to this proposal, the reac-
tion commences with the oxidation of p-hydroquinone 2a by
Cu2+ to give quinone 2b. The turnover of copper catalyst is
A few preliminary experiments were conducted to investi-
gate the photophysical properties of the above prepared benzo-
furoindoles. Despite differences in their electronic natures,
sizes, and sites of substituents on the benzofuroindole core,
these compounds showed photoluminescence spectra similar
to each other in toluene (2 × 10−5 M) at 298 K with a maximum
blue-light emission at a wavelength of ca. 426 nm. The exci-
tation and emission spectra of representative compounds 4a,
4q, and 4s are depicted in Fig. 2A. CIE coordinates of 0.156,
0.060 were indicated for model product 4a (Fig. 2B), and
results for the other compounds are summarized in Fig. 2C.
Remarkably, CIEy values of the majority of these compounds
were found to be below 0.070 (deep-blue), a highly desirable
parameter for application as an OLED material.18 The above
photophysical properties suggested a high potential attractive-
ness of the benzofuroindoles provided by the present catalytic
system for highly efficient deep-blue OLEDs.
In summary, we have realized the first direct oxidative
indole–hydroquinone [3 + 2] coupling via devising a method
involving Co/Cu bimetallic catalysis. One distinguishing
feature of this method lies in the aerobic and powerful oxi-
dation conditions, which promote the efficient assembly of
extended π-conjugated benzofuro[2,3-b]indoles. Mechanistic
studies revealed the distinct and complementary roles of the
two metals, which together were shown to accomplish the
chemoselective oxidation and effective Lewis acid activation of
the validated indolyl quinone intermediate. The alcohol
solvent was also found to be important, having been indicated
to likely provide additional acid activation and to facilitate
proton transfer. Finally, the determined photophysical pro-
Scheme 3 A plausible reaction mechanism.
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Fig. 2 The photophysical properties of the prepared benzofuroindoles. (A) Excitation and emission spectra of 4a, 4q, and 4s. (B) The CIE spectrum
of compound 4a. (C) The CIE coordinates of the obtained compounds.
perties of these cycloadducts suggest their potential for
efficient deep-blue OLED applications. Further efforts to apply
these products as electron donor moieties for TADF materials
and to evaluate their respective electroluminescence perform-
ance are currently underway.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We thank the Innovation and Talent Recruitment Base of New
Energy Chemistry and Device (B21003) and Hubei
Technological Innovation Project (2019ACA125). We are also
grateful to the Analytical and Testing Centre of HUST and the
Analytical and Testing Centre of School of Chemistry and
Chemical Engineering (HUST) for access to their facilities.
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