Pd(II)-Catalyzed Direct Dehydrogenative Mono-and Diolefination of Selenophenes

Article abstract of DOI:10.1021/acs.orglett.0c00506

Pd(II)-catalyzed dehydrogenative Heck olefination of selenophenes with a broad olefinic substrate scope and high functional group tolerance is demonstrated. Carbonyl-substituted and phenyl-substituted olefins with electron-donating (D) and electron-accepting (A) groups can be regioselectively installed at C2 of the selenophene. The 2-olefinated selenophenes can subsequently undergo a second oxidative olefination to rapidly produce a new class of symmetrical D-π-D or unsymmetrical D-?€-A 2,5-diolefinated selenophene materials.

Full text of DOI:10.1021/acs.orglett.0c00506

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Letter
Pd(II)-Catalyzed Direct Dehydrogenative Mono- and Diolefination of
Selenophenes
Shi-Yen Chen, Santosh K. Sahoo, Ching-Li Huang, Tung-Hsien Chan, and Yen-Ju Cheng*
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ABSTRACT: Pd(II)-catalyzed dehydrogenative Heck olefination of
selenophenes with a broad olefinic substrate scope and high
functional group tolerance is demonstrated. Carbonyl-substituted
and phenyl-substituted olefins with electron-donating (D) and
electron-accepting (A) groups can be regioselectively installed at
C2 of the selenophene. The 2-olefinated selenophenes can subsequently undergo a second oxidative olefination to rapidly produce a
new class of symmetrical D−π−D or unsymmetrical D−π−A 2,5-diolefinated selenophene materials.
Table 1. Direct Olefination of Selenophene with Methyl
Acrylate
lefination of arenes or heteroarenes to extend π
Oconjugation is an important transformation in synthesiz-
ing organic semiconducting materials and bioactive molecules.1
Although the traditional palladium-catalyzed Heck reaction has
been widely used to install carbon−carbon double bonds, the
direct cross-dehydrogenative Heck reaction (DHR) coupling
of an arene with an alkene, first discovered by Fujiwara and
Moritani,2 is the most powerful and cost-effective strategy, as it
does not require the preparation of a halogenated arene.3
Thiophene is the most ubiquitous component in enormous
conjugated molecules for organic optoelectronic applications.4
Selenophene in the chalcogenophene family has emerged as a
promising alternative to traditional thiophene for property
optimization. Selenophene has a higher polarizability and lower
aromaticity than thiophene.5 Therefore, the selenophene-
incorporated materials usually exhibit higher crystallinity and
narrower band gaps, which are desirable characteristics for
achieving high-performance semiconducting properties.6 As a
result, π extension of selenophenes by C−H activation
methodology has attracted increasing interest in recent
years.7 DHR of heteroarenes, including C2 olefination of
thiophene, has been previously described.8 Nevertheless, C−H
arylation of selenophenes is more challenging, not to mention
the DHR of selenophenes, which has not been well-explored.
Herein we report the first palladium-catalyzed C2 direct cross-
dehydrogenative alkenylation of selenophenes with a variety of
vinyl olefins.
a
yield
(%)
entry
catalyst
oxidant
1
2
3
4
5
6
7
8
Pd(PPh3)4
Pd2(dba)3
Pd(PhCN)2Cl2 Ag2CO3
Pd(PPh3)2Cl2
Pd(OAc)2
Pd(OAc)2
Ag2CO3
Ag2CO3
15
3
25
11
70
69
0
5
3
5
31
32
37
72
71
68
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Cu(OAc)2
Co(OAc)2
b
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
9
10
11
12
13
14
15
Ag2O
AgOAc
AgOCOCF3
Ag2CO3 + PivOH (30 mol %)
Ag2CO3 + PivOH (60 mol %)
Ag2CO3 + PPh3 + PivOH (30 mol %)
c
16
a
Conditions: 1 (0.5 mmol, 1 equiv), a (1.2 equiv), oxidant (2 equiv),
b
c
catalyst (5 mol %), DMF (1 mL). 10 mol % Pd(OAc)2. 20 mol %
PPh3 was added.
Increasing the amount of Pd(OAc)2 to 10 mol % gave a
similar yield of 69%. The reaction completely failed when no
As an initial attempt, bare selenophene was reacted with
electron-deficient methyl acrylate to afford 2-olefinated 1a.
The optimization of conditions is shown in Table 1. As shown
in entries 1−4, using common catalysts such as Pd(PPh3)4,
Pd2(dba)3, Pd(PhCN)2Cl2, and Pd(PPh3)2Cl2 in the presence
of Ag2CO3 as an oxidant in N,N-dimethylformamide (DMF) at
100 °C for 24 h resulted in only low yields of 1a. Fortunately,
when 5 mol % Pd(OAc)2 was used as the catalyst, the yield of
1a was dramatically improved to 70% (entry 5).
Received: February 8, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00506
© XXXX American Chemical Society
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palladium catalyst was added (entry 7). The optimization of
the conditions by changing the solvent, temperature, and
reaction time is shown in Table S1. Using other oxidants,
including Cu(OAc)2 and Co(OAc)2 (entry 8 and 9), or no
oxidant (entry 10) resulted in poor yields of less than 5%,
indicating that both Pd(OAc)2 and Ag2CO3 are crucial for the
reaction to proceed. Combination of Pd(OAc)2 with other
silver salts such as Ag2O, AgOAc, and AgOCOCF3 gave lower
yields of 31−37% (entries 11−13). Furthermore, with 30 mol
% pivalic acid as an additive, the yield was improved to 72%
(entry 14). Introducing PPh3 (20 mol %) as a ligand slightly
decreased the yield to 68%. The substrate scope of alkenes for
the direct mono-olefination of selenophene was explored using
the optimized conditions (Table 1, entry 14).
Scheme 2. Direct Olefination of Mono- and Disubstituted
Selenophenes with Methyl Acrylate
a b
,
The DHR of selenophene can be successfully achieved with
various alkenes having ester, keto, aldehyde, sulfone, and amide
substituents (Scheme 1). Ester-substituted olefins a−f with
a
Conditions: 2−9 (0.5 mmol, 1 equiv), a (1.2 equiv), Ag2CO3 (2
Scheme 1. Direct Olefination of Selenophene (1) with
equiv), Pd(OAc)2 (5 mol %), PivOH (30 mol %), DMF (1 mL).
a b
,
Carbonyl-Substituted Olefins
b
Isolated yields are shown.
brominated selenophene substrates 7−9 were also tested.
Similarly, the regioselective olefination at C5 rather than the
debrominative Heck-type reaction was observed, generating
7a, 8a, and 9a in moderate yields. The selectivity allows the
intact bromo group to be further functionalized by a metal-
catalyzed cross-coupling reaction for π extension.
In addition to the carbonyl-substituted olefin substrates, we
also exploited phenyl-substituted olefins as coupling partners
(Scheme 3). In some cases, Pd2(dba)3 was used to improve the
Scheme 3. Direct Olefination of Selenophene (1) with
a c
,
Phenyl-Substituted Olefins
a
Conditions: 1 (0.5 mmol, 1 equiv), a−l (1.2 equiv), Ag2CO3 (2
equiv), Pd(OAc)2 (5 mol %), PivOH (30 mol %), DMF (1 mL).
b
Isolated yields are shown.
different carbon chains furnished the corresponding regiose-
lective 2-olefinated selenophene products in good yields (53−
72%). Acrolein (g), methyl vinyl ketone (h), and methyl vinyl
sulfone (i) with increasing electron deficiency afforded the 2-
olefinated selenophene products 1g (71%), 1h (60%), and 1i
(63%), respectively. Furthermore, olefins j−l bearing more
electron-rich amide substituents also yielded the respective
products 1j−l in higher yields (75−83%). For clarification of
the oxidative DHR, product 1k was characterized by single-
crystal X-ray diffraction analysis (Figure S1 and Table S2).
The substrate scope was further extended to mono- and
disubstituted selenophenes (Scheme 2). Reaction of methyl
acrylate with 2-formylselenophene (2) and 2-cyanoseleno-
phene (3) having electron-withdrawing groups at the 2-
position furnished the 5-olefinated products 2a (70%) and 3a
(42%), respectively. Electron-rich 2,2′-biselenophene (4) and
benzo[b]selenophene (5) also led to the corresponding
products 4a and 5a in 61% and 56% yield. It was found that
2-bromoselenophene gave the 5-olefinated product 6a,
indicating that the DHR is more favorable than the traditional
Heck-type reaction under the reaction conditions. Other
a
Conditions: 1 (0.5 mmol, 1 equiv), m−w (1.2 equiv), Ag2CO3 (2
b
equiv), Pd(OAc)2 (5 mol %), PivOH (30 mol %), DMF (1 mL). 5
mol % Pd2(dba)3 was used as the catalyst. Isolated yields are shown.
c
reaction yield. The reaction of selenophene with styrene
furnished the 2-olefinated product 1m in 40% yield. 4-
Methylstyrene, 4-methoxystyrene, and 3,4-dimethoxystyrene
with electron-donating substituents at the para position
resulted in lower yields of 28%, 33%, and 31%, respectively.
However, 4-nitrostyrene and 4-fluorostyrene with electron-
B
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withdrawing substituents at the para position slightly improved
the yield to over 40%. 4-Chlorostyrene and 4-bromostyrene
afforded the corresponding products 1q and 1r in moderate
yields of 39% and 32%, respectively. 1-Vinylnaphthalene and 4-
vinylbiphenyl also successfully generated the corresponding
products 1u and 1v. Selenophene can be olefinated with
electron-rich N,N-diphenyl-4-vinylaniline (w) to afford donor-
π-bridged conjugated material 1w.
Scheme 5. Proposed Mechanism for the Direct Olefinations
of Selenophene
Symmetrical 2,5-diolefinated selenophenes can also be
obtained when an excess amount (3 equiv) of the
corresponding olefins is employed with increased catalyst
amount and reaction time (Scheme 4). The four examples 1aa,
a
b
Scheme 4. Symmetrical and Unsymmetrical Direct
c
Diolefinations of Selenophene
be reoxidized to Pd(II) in the presence of Ag2CO3 and
continue the catalytic cycle. Evidence for the reduction of
Ag(I) to Ag(0) was confirmed by the observation of a silver
mirror after completion of the reaction. Subsequently, the
second catalytic cycle generates the symmetrical 2,5-diolefi-
nated product.
Utilizing the sequential C2 and C5 olefination of
selenophene can facilitate the synthesis of a new class of
unsymmetrical D−π−A-type or symmetrical D−π−D-type
conjugated materials using divinyleneselenophene as the π
bridge and the triphenylamino group as the donor. Thus, a
series of 2,5-diolefinated selenophenes 1dw, 1kw, 1mw, 1pw,
1sw, and 1ww were synthesized in 24−38% yield by reacting
mono-olefinated 1d, 1k, 1m, 1p, 1s, and 1w with w (Scheme
6). These new selenophene-based materials can be used as
hole-transporting or light-emitting materials for organic
optoelectronic applications.
Scheme 6. Second Direct Olefination of Selenophenes with
a b
,
N,N-Diphenyl-4-vinylaniline (w)
a
Conditions: 1 (0.5 mmol, 1 equiv), olefin a, c, m, p (3 equiv),
Ag2CO3 (3 equiv), Pd(OAc)2 (10 mol %), PivOH (60 mol %), DMF
b
(1 mL). Conditions: 1k (0.5 mmol, 1 equiv), olefin a, b, i, m (1.2
equiv), Ag2CO3 (2 equiv), Pd(OAc)2 (5 mol %), PivOH (30 mol %),
c
DMF (1 mL). Isolated yields are shown.
1cc, 1mm, and 1pp were generated in good yields (51−63%)
from the reaction of selenophene with a, c, m, and p,
respectively. Likewise, it was envisaged that 2-olefinated
selenophenes could undergo a subsequent regioselective direct
olefination at C5 in a similar manner. We selected 1k as a
model substrate for the second olefination at C5. Accordingly,
the diolefinated products 1ka and 1kb were synthesized from
the corresponding ester-substituted olefins a and b in high
yields (85% and 81%, respectively). The reactions of 1k with
methylsulfonyl-substituted olefin (i) and styrene (m) afforded
the products 1ki (56%) and 1km (42%).
a
Conditions: w (1.2 equiv), olefin 1d, 1k, 1m, 1p, 1s, 1w (1 equiv),
Pd(OAc)2 (10 mol %), Ag2CO3 (2 equiv), PivOH (30 mol %), 24 h,
100 °C. Isolated yields are shown.
b
A plausible catalytic cycle is shown in Scheme 5.9 The
reaction of selenophene and Pd(OAc)2 via a concerted
metalation−deprotonation (CMD) process furnishes inter-
mediate A, and subsequent elimination of acetic acid generates
Pd−aryl species B. Afterward, the olefin coordinates to the
palladium to afford C, which undergoes a 1,2-migratory
insertion to form a C−C bond (D). The desired product and
Pd(0) are then generated by β-hydride elimination. Pd(0) can
We further evaluated the scalability of the DHR method.
Gram-scale selenophene 1 was reacted with olefin k to form
product 1k, which was further coupled with a to generate the
unsymmetrical diolefinated compound 1ka (Scheme 7). Both
reactions efficiently afforded the target products in good yields
(77% and 80%, respectively) on a large scale.
C
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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, U.K.; fax: +44 1223 336033.
Scheme 7. Gram-Scale Synthesis of Mono- and Diolefinated
Selenophenes
AUTHOR INFORMATION
■
Corresponding Author
Yen-Ju Cheng − Department of Applied Chemistry and Center
for Emergent Functional Matter Science, National Chiao Tung
University, Hsinchu, Taiwan, ROC; orcid.org/0000-0003-
0780-4557; Email: yjcheng@mail.nctu.edu.tw
The UV absorption and emission spectra of 1dw, 1kw, 1mw,
1pw, 1sw, and 1ww were measured in dichloromethane to
investigate the photophysical properties of these 2,5-diolefi-
nated selenophenes containing a triphenylamino group (Figure
S2 and Table S3). The absorption λmax of these materials was
distributed in the range of 426−470 nm. The fluorescence
color of the unsymmetrical 2,5-diolefinated selenophenes
under irradiation at 365 nm can be fine-tuned by using
different electron-accepting substituents (Figure 1). Notably,
the emission of 1sw was greatly quenched in the presence of
the strong electron-withdrawing nitro group.10
Authors
Shi-Yen Chen − Department of Applied Chemistry, National
Chiao Tung University, Hsinchu, Taiwan, ROC
Santosh K. Sahoo − Department of Applied Chemistry,
National Chiao Tung University, Hsinchu, Taiwan, ROC
Ching-Li Huang − Department of Applied Chemistry, National
Chiao Tung University, Hsinchu, Taiwan, ROC
Tung-Hsien Chan − Department of Applied Chemistry,
National Chiao Tung University, Hsinchu, Taiwan, ROC
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00506
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work was supported by the Ministry of Science and
Technology, (MOST 107-2628-M-009-003-MY3 and MOST
107-3017-F-009-003), and the Ministry of Education, Taiwan
(Sprout Project: Center for Emergent Functional Matter
Science of National Chiao Tung University).
Figure 1. Fluorescence of 1dw, 1kw, 1mw, 1pw, 1sw, and 1ww under
UV light (365 nm) in CH2Cl2 solution (10−5 M).
REFERENCES
■
In summary, we have demonstrated an efficient palladium-
catalyzed direct dehydrogenative C2 alkenylation of seleno-
phenes with a broad olefinic substrate scope and high
functional group tolerance. Carbonyl-based olefins with
aldehyde, keto, ester, sulfone, and amide substituents and
styrene derivatives with electron-rich or electron-deficient
substituents at the para position can be regioselectively
installed at the 2-position of selenophene. The 2-olefinated
selenophenes can further undergo a second alkenylation at C5
to produce a new class of 2,5-diolefinated symmetrical D−π−
D and unsymmetrical D−π−A semiconducting materials that
are promising for various organic optoelectronic applications.
This strategy provides a facile protocol to rapidly extend the π
conjugation of selenophene.
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