Cascade Oxidative C?H Annulation of Thiophenes: Heck-Type Pathway Enables Concise Access to Thienoacenes

Article abstract of DOI:10.1002/anie.202103160

The pursuit of efficient synthetic route to thienoacenes represents an appealing yet challenging task in the fields of both organic synthetic chemistry and organic functional materials. In this work, we disclose a rhodium-catalyzed cascade C?H annulation of phenacyl phosphoniums with (benzo)thiophenes via a Heck-type pathway to provide a new class of planar thienoacenes, which involves the formation of three C_aryl-C_aryl bonds and one C_aryl?O bond in a single operation. The neutral S,O-heteroacenes exhibit superior stability and adopt a herringbone-like packing mode with efficient π–π stacking in the crystals, suggesting their potential in organic semiconducting materials. This work first exemplifies the superiority of cascade oxidative C?H annulation involving a Heck-type pathway in the development of concise access to heteroacenes.

Full text of DOI:10.1002/anie.202103160

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How to cite:
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Heteroacenes
À
Cascade Oxidative C H Annulation of Thiophenes: Heck-Type
Pathway Enables Concise Access to Thienoacenes
Xingyu Chen, Yudong Yang,* Weiguo Han, Quan Huang, Zhenmei Huang, and Jingsong You*
Abstract: The pursuit of efficient synthetic route to thienoa-
cenes represents an appealing yet challenging task in the fields
of both organic synthetic chemistry and organic functional
materials. In this work, we disclose a rhodium-catalyzed
À
cascade C H annulation of phenacyl phosphoniums with
(benzo)thiophenes via a Heck-type pathway to provide a new
class of planar thienoacenes, which involves the formation of
À
three Caryl-Caryl bonds and one Caryl O bond in a single
operation. The neutral S,O-heteroacenes exhibit superior
stability and adopt a herringbone-like packing mode with
efficient p–p stacking in the crystals, suggesting their potential
in organic semiconducting materials. This work first exempli-
À
fies the superiority of cascade oxidative C H annulation
involving a Heck-type pathway in the development of concise
access to heteroacenes.
H
eteroacenes have received much interest due to their rigid
planar conformations and improved air stability compared
with large acene analogues, rendering them attractive materi-
als for organic electronic applications.[1] Additionally, the
optical-electronic properties of heteroacenes could be tuned
by modifying the incorporated heteroatoms and tailoring the
degree of p-extension.[2] In this regard, thienoacene deriva-
tives are considered as one of the most promising skeletons in
the fields of organic field effect transistors (OFETs), organic
photovoltaic cells (OPVs) and organic light-emitting diodes
(OLEDs), owing to the potentially beneficial S-S interaction
on the intermolecular charge transport.[3] Thus, the develop-
ment of concise and efficient methods for the construction of
thienoacenes is highly attractive in both organic synthetic
chemistry and material chemistry. Representative classical
synthetic approaches to thienoacenes mainly rely on transi-
tion metal-catalyzed C-M/C-X coupling reactions between
organometallic reagents and organohalides, and heteroannu-
lation of o-haloethynylbenzene precursors, which typically
suffer from complex/inaccessible substrates and tedious
synthetic routes (Scheme 1a).[1c,4]
Scheme 1. Synthetic approaches toward sulfur-containing heteroa-
cenes.
À
À
strated the chelation-assisted oxidative C H/C H cross-
coupling between two (hetero)arenes/intramolecular cycliza-
tion as an efficient strategy for the construction of fused
heteroaromatics.[5,6] However, the reaction conditions for
oxidative cross-coupling and intramolecular cyclization are
À
À
generally inconsistent, and the intramolecular C H/Z H
(Z = heteroatom) annulations are often quite challenging.
Thus, these synthetic routes usually still require multiple
steps, especially for the synthesis of large heteroacenes
(> heteropentacenes) which involve the formation of two or
more different aromatic rings. More recently, we disclosed an
iridium-catalyzed annulation of (benzo)thiophenes with (het-
ero)aromatic or a,b-unsaturated carboxylic acids for the one-
pot synthesis of thiophene-fused coumarin-type frame-
À
In recent years, transition metal-catalyzed C H activation
has emerged as a powerful tool for the construction of p-
conjugated materials.[5] Our group and others have demon-
[*] X. Chen, Prof. Dr. Y. Yang, W. Han, Q. Huang, Z. Huang,
Prof. Dr. J. You
Key Laboratory of Green Chemistry and Technology of Ministry of
Education, College of Chemistry, Sichuan University
29 Wangjiang Road, Chengdu 610064 (P. R. China)
E-mail: yangyudong@scu.edu.cn
works.[7] Different from the majority of reported C H
À
arylations which typically undergo the electrophilic aromatic
substitution (SEAr) and concerted metalation-deprotonation
(CMD) processes,[5a,8] this reaction proceeded via a relatively
jsyou@scu.edu.cn
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202103160.
rare Heck-type mechanism in C H activation.[7,9] Inspired by
À
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À
this work, we envisioned that the oxidative C H annulation
conveniently removed in the presence of Cs2CO3 in MeOH/
H2O, providing the neutral S,O-heteroacene 4aa (Sche-
me 2b).
To clarify the reaction pathway, the control experiments
were conducted. Firstly, treatment of phenacyl triphenylphos-
phonium salt 1a or 1 f with 1.0 equiv of (benzo)thiophene 2a
under the standard conditions afforded the O-annulation
product 5aa and 5 fa with a E/Z ration of 1:1 in 62% and 53%
yield, respectively (Scheme 3a). Further reaction of 5aa with
2a under the standard conditions provided the desired
reactions starting from two (hetero)arenes via Heck-type
pathway would be advantageous for the construction of
heteroacenes. Herein, we disclose the rhodium-catalyzed
À
cascade C H annulation of phenacyl phosphoniums with
(benzo)thiophenes via a Heck-type pathway, which enables
the rapid assembly of a diverse set of S,O-heteroacenes
(Scheme 1b).
Considering the potential C,O-bifunctionalization ability
of ketone enabled by keto–enol tautomerism,[10] acetophe-
none and benzothiophene were initially selected as the model
substrates for the synthesis of S,O-heteroacenes. However, all
attempts failed to afford the desired product in the presence
of [Cp*RhCl2]2, oxidant and additive. Given that a highly
electrophilic catalytic metal center could be beneficial for
a Heck-type pathway,[9] a series of acetophenone derivatives
with an electron-withdrawing auxiliary at the a-position of
ketone moiety were considered to enhance the electrophilic-
ity of formed cyclometallic intermediate to the thiophene ring
(Scheme 2a). It was found that triphenylphosphonium tri-
fluoroacetate was an effective auxiliary to afford S,O-hetero-
acene phosphonium salt 3aa in 63% yield, which was
confirmed by single-crystal X-ray analysis.[11] Optimization
of the reaction parameters indicated that other catalysts such
as RhCl(PPh3)3, Cp*Rh(OAc)2, [RhCp*(MeCN)3][SbF6]2 and
[Cp*IrCl2]2 were less effective (Supporting Information,
Table S1, entries 2–5). No product was observed in the
absence of metal catalyst and oxidation (Supporting Infor-
mation, Table S1, entries 6 and 7). Except TFE and
1,1,1,3,3,3-hexafluoroisopropanol (HFIP), other solvents
such as toluene, DMF and t-AmylOH did not give any
product (Supporting Information, Table S1, entries 8–11). We
assumed that acidic polyfluorinated alcohols could facilitate
the formation of a more stable electrophilic [RhIII] species.[12]
Pleasantly, the triphenylphosphonium auxiliary could be
Scheme 3. Mechanistic studies. Standard conditions: [RhCp*Cl2]2
(5.0 mol%), CsOAc (4.0 equiv), and Ag2O (4.0 equiv) in TFE (1.0 mL)
at 1208C under N2 for 24 h.
product 3aa in 57% yield (Scheme 3b). These results
À
indicated that the cascade C H annulation might undergo
an O-annulation and C-annulation sequence. In addition, the
intramolecular cyclization of the biaryl 1aa did not occur
under the standard conditions, which could not give the
mono-annulation product 5aa (Scheme 3c). This result sug-
gested that the resulting rhodium species after the arylation
step might be important in the subsequent O-annulation (see
intermediate C in Scheme 4). Furthermore, ESI-HRMS
experiment was performed after running the reaction of 1a
with 2a for 10–20 min under the standard conditions. The
intermedites (B or D: m/z calcd: 751.1665, found: 751.1669; F
or H: m/z calcd: 881.1542, found: 881.1537) were detected,
which were proposed as the adducts of (benzo)thiophene 2a
with rhodacycles A and E, respectively (Scheme 4; Support-
ing Information, Figures S1–S4). These findings suggested
Scheme 2. a) Screening of substituent group X. Reaction conditions:
1 (0.1 mmol), 2a (0.4 mmol), [RhCp*Cl2]2 (5.0 mol%), CsOAc
(4.0 equiv) and Ag2O (4.0 equiv) in 2,2,2-trifluoroethanol (TFE, 1.0 mL)
at 1208C under N2 for 24 h. Isolated yields. b) Removal of the
triphenylphosphonium auxiliary.
À
that the C H activation process of (benzo)thiophene might
involve a Heck-type pathway. Furthermore, the parallel
experiments of 1a and 2a/[C2-D]-2a or 1a and 2a/[C3-D]-
2a gave the kinetic isotope effect (KIE) values of 1.15 and
2
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Scheme 4. Plausible mechanistic pathway.
1.04, respectively (Supporting Information, Part VII), which
cenes 3 could partly undergo dephosphorization to give
À
À
demonstrates that the C H bond cleavage of thiophene is not
their neutral analogues 4 during the cascade C H annulation,
related to the rate-determining step.[13] The competition
reaction between electronically different 1c and 1g with 2a
suggested an electrophilic aromatic substitution process for
a one-pot manipulation was conducted to obviate the
unnecessary purification process and improve the overall
efficiency. As shown in Table 1, a variety of thiophenes 2
could react with phenacyl phosphonium 1 in moderate to high
yields. (Benzo)thiophenes with functional groups, such as
phenyl, methoxy, anisole, carbazolyl, chloro and bromo,
smoothly underwent the annulation (4ab–4ae and 3af–3ai).
However, because of low solubility of the neutral S,O-
heteroacenes with halo-substituent, the triphenylphospho-
nium salts 3af–3ai were used for structural analysis without
further removal of the triphenylphosphonium auxiliary.
(Benzo)thiophenes with the substituents at the different
positions of aryl rings could also participate in this reaction. It
is worthy to note that this transformation could enable a facile
approach to S,O-heteroacenes with different topological
structures (3cj, 4ak and 4al). In addition, 2-phenyl thiophene,
thieno[3,2-b]thiophene and benzoselenophene were also
effective substrates (4am–4ao). Furthermore, phenacyl phos-
phoniums with the substituent on the phenyl ring, such as
phenyl, methoxy and bromo, could also work under the
standard conditions (4ba–4da).
À
the C H activation of triphenylphosphonium trifluoroacetate
(Scheme 3d). Moreover, parallel competition reactions of 1a
or [D5]-1a with 2a gave a KIE value of 1.24 (SI, Part VII).
This result is also consistent with a typical SEAr process, in
which deprotonation is usually fast compared to the forma-
tion of the arenium s-complex.
Based on the above results and previous reports,[8,9,14]
a plausible mechanism is proposed (Scheme 4). Firstly, the
ylidic form of 1a reacts with [RhIIICp*] to give the inter-
mediate A (detected by HR-MS, calcd: 617.1475, found:
617.1476; Supporting Information, Figure S1) through an
electrophilic aromatic substitution process. Subsequently, the
reaction of A with 2a via a carborhodation delivers the
intermediate B, which further undergoes a b-H elimination to
provide the intermediate C. The following intramolecular
attack of the coordinated oxygen atom at C3 position of
(benzo)thiophene moiety affords the intermediate D. Next,
the second b-H elimination takes place to furnish the mono-
annulated product 5aa (obtained by control experiment) and
release [RhI] species, which is reoxidized by Ag2O to generate
the [RhIII], to close the catalytic cycle (Cycle I). Subsequently,
The single crystals of 4aa that qualified for X-ray
crystallographic analysis was obtained by slowly volatilizing
a chlorobenzene solution. As shown in Figure 1, the planar
4aa molecules are packed in a herringbone-like arrangement
of which packing structures are typically adopted by high
performance organic semiconductors such as pentacene.[1b]
Each molecule packs in the layer-by-layer structure with a p-p
distance of approximately 3.50 ꢀ. Weak sulfur-sulfur contacts
are observed with distances ranging from 3.66–3.91 ꢀ.
Nucleus-independent chemical shift (NICS) calculations of
4aa, 4al and 4ak reveal that pyran rings possess weak local
aromaticity with the NICS(1) of À0.7, À0.6, À0.4 ppm,
respectively (Supporting Information, Figure S7). In contrast
À
5aa undergoes a C H activation process to generate the
rhodacycle E (detected by HR-MS, calcd: 747.1352, found:
747.1354; Supporting Information, Figure S3), which then
reacts with 2a via a Heck-type pathway to afford the
intermediate F. The following two b-H eliminations and
intramolecular cyclization deliver the desired product 3aa.
The released [RhI] species is reoxidized to finish the catalytic
cycle (Cycle II). No obvious other regioisomer was detected
À
in this cascade C H annulation reaction. We assumed that the
carboxylate might play an important role in the regioselec-
tivity at the thiophene insertion steps.[7,15]
to a typical C O single bond (1.43 ꢀ), those in 4aa are
significant shorter (1.37 and 1.39 ꢀ), suggesting the lone-pair
electrons of oxygen participate in the p-electron delocaliza-
À
The substrate scope of this cascade reaction was inves-
tigated. Because the triphenylphosphonium S,O-heteroa-
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Table 1: Scope of Rh-catalyzed one-pot synthesis of S,O-heteroacenes.[a]
absorption spectrum was detected when either acid (H2SO4,
CF3COOH) or base (NaOH) was added to the solution of 4aa
and 4al in CH2Cl2 (Supporting Information, Figure S9b).
Thermogravimetric analysis (TGA) indicated that 4aa and
4al could be stable upon heating up to 3548C and 4258C (5%
weight loss), respectively (Supporting Information, Fig-
ure S10). In contrast, significant degradation was observed
for pentacene derivatives within several minutes at room
temperature.[17] The S,O-heteroacene 4 in CH2Cl2 showed
multiple absorptions and blue fluorescence emissions with the
peaks ranging from 376–413 nm and 455–490 nm, respectively
(Supporting Information, Table S5), suggesting S,O-doped
acenes 4 would be potential candidates for organic optoelec-
tronic materials.
In summary, we have developed the rhodium-catalyzed
À
cascade oxidative C H annulation of thiophenes with aryla-
cetyl triphenylphosphonium toward thienoacenes via a Heck-
À
type pathway, in which three Caryl-Caryl bonds and one Caryl
O
bond are constructed in a single operation. S,O-heteroacenes
have superior stability compared with pentacene derivatives
and pack tightly in a herringbone-like arrangement in the
crystals, suggesting their potential in organic semiconducting
materials. This work demonstrates the advantages of Heck-
À
type pathway in the evolution of cascade oxidative C H
annulation as an efficient synthetic route to thienoacenes, and
could open a new door to streamline the synthesis of
heteroacenes.
[a] Reaction conditions: 1) 1 (0.1 mmol), 2 (0.4 mmol), [RhCp*Cl2]2
(5.0 mol%), Cs2CO3 (4.0 equiv), and Ag2O (4.0 equiv) in HFIP (1.0 mL)
at 1208C under N2 for 24 h. 2) Treatment with a solution of Cs2CO3
(4.0 equiv) and MeOH/H2O (9/1) at 1008C under N2 for 12 h. Isolated
yields. [b] Emission maximum in CH2Cl2 (10.0 mM). [c] CsOAc
(4.0 equiv) was used instead of Cs2CO3 in TFE and the removal process
of triphenylphosphonium auxiliary was not conducted.
Acknowledgements
This work was supported by grants from the National NSF of
China (No 22031007 and 21772128).
Conflict of interest
The authors declare no conflict of interest.
À
Keywords: cascade annulation · C H activation ·
Heck-type pathway · heteroacenes · thiophenes
Figure 1. Crystal structures of 4aa. The direction of the lattice vectors,
a, b, c, are determined from the single crystal structures. a) Herring-
bone packing motif. b) Face-to-face molecular array with the closest p-
p distance (dashed green line) and intermolecular sulfur-sulfur con-
tacts (dashed violet and blue line).
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Manuscript received: March 3, 2021
[11] Deposition Numbers 2059422 (for 3aa), and 2059423 (for 4aa)
contain the supplementary crystallographic data for this paper.
Accepted manuscript online: March 23, 2021
Version of record online: && &&, &&&&
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Communications
Heteroacenes
X. Chen, Y. Yang,* W. Han, Q. Huang,
Z. Huang, J. You*
&&&& — &&&&
À
Cascade Oxidative C H Annulation of
Thiophenes: Heck-Type Pathway Enables
Concise Access to Thienoacenes
A rhodium-catalyzed cascade oxidative
is firstly exemplified as a superior process
À
À
C H annulation provides a rapid access
to a new class of planar thienoacenes
with superior stability. Heck-type pathway
enabling multiple C H fusions of heter-
oaromatics for the synthesis of hetero-
acenes.
6
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Angew. Chem. Int. Ed. 2021, 60, 1 – 6
These are not the final page numbers!