Binary-Acid Catalysis with Sc(OTf)3/TfOH in the Alkenylation of Arenes with Alkynes

Article abstract of DOI:10.1021/acs.orglett.1c02065

Herein we report binary acid Sc(OTf)3/TfOH-catalyzed alkenylation of arenes with alkynes. In this system, the high-energy vinyl carbocations with activated and weakly coordinating trifluoromethanesulfonate anions by Lewis acid Sc(III) can undergo facile F

Full text of DOI:10.1021/acs.orglett.1c02065

pubs.acs.org/OrgLett
Letter
Binary-Acid Catalysis with Sc(OTf)3/TfOH in the Alkenylation of
Arenes with Alkynes
Wenxuan Zhang, Jiaying Sun, Linrui Xiang, Wen Si, Ran Song, Daoshan Yang, and Jian Lv*
Cite This: Org. Lett. 2021, 23, 5998−6003
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ABSTRACT: Herein we report binary acid Sc(OTf)3/TfOH-catalyzed alkenylation of arenes with alkynes. In this system, the high-
energy vinyl carbocations with activated and weakly coordinating trifluoromethanesulfonate anions by Lewis acid Sc(III) can
undergo facile Friedel−Crafts reactions with arenes to give the desired adducts in up to 90% yield and with high Z-selectivity.
CF3CO2H could promote the hydroarylation reaction of
he alkenylation of arenes with alkynes is one of the most
Tstraightforward strategies to build versatile structural phenylacetylene with toluene via carbocations I-b, but with
lower regioselectivity.10b Recently, Nelson has developed
weakly coordinating anions (WCA) salts and thioureas
catalysis for the functionalization of unactivated C−H bonds
via vinyl carbocations II (Scheme 1, B).12
motifs of polysubstituted olefins.1 A number of transition-
metal catalysts have been reported to promote the alkenylation
of (hetero)arenes with alkynes.2−9 In addition, the Friedel−
Crafts reaction of arenes with in situ generated vinyl
carbocations I-a (Scheme 1, A), which is generally difficult
to achieve and requires in combined acids (e.g., HF-SbF5,
TfOH-SbF5) as dual super acids and solvents, has appeared as
a complementary yet appealing strategy for alkenylation
reaction.10a,11 Kitamura found that stoichiometric amounts of
In fact, 2-alkynyphenols are also one of the most attractive
precursors for in situ generated vinylidene-quinone methides
(VQM).13 Recently, Tan developed chiral phosphoric acid
catalysis for the asymmetric hydroarylation reaction of
arylacetylenes with 2-naphthols.13a Previously, we developed
enantioselective cationic functionalizations catalyzed by
asymmetric binary acids (ABC) endowed with bi/multi-
activation site (e.g., proton and metal center).14 According
to Yamamoto’s combined acid principles, acidity/electro-
philicity of Lewis acid, Brønsted acid, or both could be
enhanced to activate inert substrates.15 In fact, it was found
that anionic ligands could stabilize the developing carbocations
from simple olefins through anion-pair effect in binary acid
systems.14a We describe herein an alkenylation reaction of
arenes with 2-alkynyphenols that is mediated by Sc(OTf)3/
TfOH (Scheme 1, C). In this scenario, binary acids enhanced
electrophilicity and acidity may facilitate the nucleophilic
attack of 2-alkynyphenols to form vinyl carbocations III with a
Scheme 1. Alkenylation Reaction of Arenes
Received: June 21, 2021
Published: July 8, 2021
https://doi.org/10.1021/acs.orglett.1c02065
© 2021 American Chemical Society
Org. Lett. 2021, 23, 5998−6003
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Letter
weakly coordinating Sc(III)-based trifluoromethanesulfonate
anion (A−), which undergo subsequent Friedel−Crafts
reaction to afford final products.
Alkenylation of toluene 2a with 1-(phenylethynyl)-
naphthalene-2-ol 1a was chosen as a model reaction to test
the catalytic activity (Table 1). We were able to identify a
The catalytic scope of the obtained optimal binary acid
Sc(OTf)3/TfOH was next explored in the alkenylation of
toluene 2a with alkynes 1 (Scheme 2). First, different
substituent groups on the phenyl ring 1a−m were investigated.
Either electron-withdrawing groups (F, Cl, Br, and CF3) or
electron-donating groups (Me, tBu, MeO, and Ph) at different
a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Substrate Scope of Alkenylation of Toluene 2a
a
with Alkynes 1
b
entry
variation from standard condition
yield (%)
1
2
3
standard conditions
no TfOH
no TfOH, 80 °C
76
nr
c
32
38
trace
59
17
72
69
55
53
81
61
83
4
5
6
7
8
9
10
11
12
13
(PhO)2PO2H instead of TfOH
CF3CO2H
MsOH
TsOH
Ni(OTf)2 instead of Sc(OTf)3
Cu(OTf)2
In(OTf)3
no Sc(OTf)3
5 mol % Sc(OTf)3
30 mol % TfOH and 5 mol % Sc(OTf)3
5 mol % Sc(OTf)3 and 5 mol % NaBArF
c
14
a
Standard reaction condition: 1a (0.1 mmol), Sc(OTf)3 (10 mol %),
and TfOH (20 mol %) at room temperature in PhCH3 2a (1.0 mL)
b
c
for 30 min. Isolated yield, Z/E value of product 3aa (>19:1). 12 h.
nr = no reaction.
viable complex composed of Sc(OTf)3 (10 mol %) and TfOH
(20 mol %) at room temperature. Under optimized conditions,
the desired adduct 3aa can be isolated in 76% yield (Z/E >
19:1) (entry 1). Interestingly, no reaction occurred in the
absence of TfOH at room temperature (entry 2), but a
reaction temperature 80 °C was found to give the adduct 3aa
in 32% yield (entry 3). However, the other Brønsted acids,
such as (PhO)2PO2H, CF3CO2H, MsOH, and TsOH, were
found to be inferior to TfOH (entries 4−7 vs entry 1). We
hypothesized that trifluoromethanesulfonate anion by Lewis
acid Sc(III) would help stabilize the developing carbocations
through an ion-pair effect (like III) and suppress further
decomposition of alkyne 1a and product 3aa. In addition,
screening of various Lewis acids was carried out with TfOH
(20 mol %) at room temperature. Other Lewis acids such as
Ni(II), Cu(II), and In(III), or no Lewis acid, were less
effective under these conditions (<72% yield, entries 8−11 vs
entry 1). The impact of the ratio of Sc(OTf)3 and TfOH was
also examined. The activity was improved by increasing the
ratio of TfOH/Sc(OTf)3 from 2:1 to 4:1 (entry 12 vs entry 1);
however, further increasing the ratio to 6:1 did not lead to
further improvement in activity (entry 13 vs entry 12).
Interestingly, it was found that an enhanced cationic nature of
the scandium center (by the addition of NaBArF instead of
OTf− salt; BArF = [3,5-(CF3)2C6H3]B) is essential for
reactivity of Sc(III) Lewis acid in the absence of TfOH, but
it still required much longer reaction times (12 h) than the
binary-acid system (entry 14 vs entry 12).
a
Reaction condition: 1 (0.2 mmol), Sc(OTf)3 (5 mol %), and TfOH
(20 mol %) at room temperature in PhCH3 2a (2.0 mL) for 0.5 h.
Isolated yield for 3, Z/E ratio of 3 in parentheses determined by H
NMR. At room temperature for 2 h.
1
b
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positions on the phenyl ring were tolerated well, giving the
desired adducts 3aa−3la in moderate to high yields and with
high Z-selectivity. In addition, a dimethyl substituted group 1m
could also react with toluene 2a, giving much lower yield
(47%, Z/E >20:1). Moreover, the reaction could work well
with naphthyl- and heteroaromatic substituted o-alkynylnaph-
thols 1n and 1o, affording the desired products 3na and 3oa in
40−60% yields and with high Z-selectivity (up to 18:1).
Furthermore, we found that a substrate 1p with a substituent
(Me) on the naphthalene ring was also tolerated in this
catalytic system, affording the adduct 3pa in 65% yield (Z/E >
20:1). Satisfyingly, the reaction of alkyl substituted ortho-
alkynylnaphthol 1q also worked well, giving the product 3qa in
62% yield (Z/E > 20:1). Significantly, O-protected alkynes 1r−
s also reacted with 2a under the optimal conditions with
excellent Z-selectivity, albeit in lower yields (70% yield for 3ra,
53% yield for 3sa), thus indicating that the formation of VQM
was not crucial for this transformation. Surprisingly, 1-
(phenylethynyl)naphthalene 1t was also tolerated, affording
the product 3ta in 47% yield (Z/E = 10:1) and high
regioselectivity (p/o > 99:1) like other alkynes containing the
naphtholic OH. Although we still have no clear experimental
evidence now, we think that this result may be caused by the
steric hindrance of naphthalene ring shielding the ortho-attack
of toluene. Next, a series of substituted 2-(phenylethynyl)-
phenols 1u−w were reacted with toluene 2a under optimized
reaction conditions. Compared with 2-(phenylethynyl)phenol
1u, the reaction of arylalkynes bearing substitution on either
the benzene ring or the phenol moiety gave much better yields
(69% yield for 3va and 71% yield for 3wa). The relative
configuration of 3sa was assigned by X-ray diffraction analysis,
and other adducts with toluene 2a can therefore be assigned by
analogy.16
Scheme 3. Substrate Scope of Alkenylation of Arenes 2 with
Alkynes 1
a
To further expand the substrate scope, we next tested other
simple arenes (Scheme 3). m- and p-xylene 2c,d worked well,
providing much better yields than their ortho-substituted
counterpart 2b. Gratifyingly, anisole could react with 1-
(phenylethynyl)-naphthalen-2-ol 1a, giving the product 3ae in
83% yield. Different substituents of anisole were also tested,
and it was found that an electron-donating group (CH3) was
preferred to an electron-withdrawing group (F, Cl, and Br). In
the presence of Et3N, treatment of 3fh and 3fi with acetyl
chloride in DCM at 0 °C led to the desired product O-
protected α-alkynynaphthalen-2-ols 4 and 5 in 87% and 85%
yields, respectively (Scheme 4, A). The relative configuration
of 3af, 4, and 5 was assigned as Z-selective olefins by X-ray
diffraction analysis, and other adducts with arenes 2b−i can
therefore be assigned by analogy.16 In addition, 4-
(phenylethynyl)phenol 1x could also reacted with anisole to
give the adduct 3xe in 78% yield and with lower Z/E value
(1:1, 3xe vs 3ae).17 These results show that the ortho-hydroxyl
group may not only increase the nucleophilicity of the alkyne
via resonance donation but also tune the developing anion by
an ortho-directing group (Scheme 3, TS-1 vs TS-2). To our
surprise, when 1-methoxy-4-methylbenzene 2j was used, the
reactions of 1x and 1y gave the products 3xj and 3yj with
excellent Z/E value (>20:1), respectively, due to the effect of
ortho-steric hindrance on the arenes (Scheme 3, TS-3 vs TS-
4).
a
Reaction condition: 1 (0.2 mmol), Sc(OTf)3 (5 mol %), and TfOH
(20 mol %) at room temperature in Ar1H 2 (2.0 mL) for 0.5 h.
Isolated yield for 3, Z/E ratio of 3 in parentheses determined by H
NMR. 50 °C for 3 h. At room temperature for 12 h.
1
b
c
Scheme 4. Gram-Scale Reaction and Transformation of
Trisubstituted Olefins 3
The developed alkenylation could be performed on a gram
scale. Under the standard conditions, trisubstituted olefin 3aa
was obtained in 72% yield (Z/E = 19:1, Scheme 4, B). Next,
the olefins 3aa or 3ae could also be easily converted into the
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corresponding acetate 6 and triflates 7 or 10, respectively.
Epoxidation18 of olefin 6 were catalyzed by Rh2(OAc)4 to give
the corresponding product 8 in 57% yield. In addition,
palladium catalyzed-coupling reactions13c between the corre-
sponding triflates and diphenyl phosphine oxide or aniline
furnished 9 and 11 in 65% and 71% yields, respectively.
To gain insights into the mechanism of this methodology,
several control experiments were carried out. From our studies,
it was found that TfOH was actually a viable catalyst for the
model reaction; however, the combination of TfOH with
Sc(OTf)3 could further improve the catalysis with better yield
(Table 1, entry 11 vs 12), although the substrate 1a is
completely consumed almost at the same time under two
different conditions. The difference in reactivity between
TfOH alone and Sc(OTf)3/TfOH, however, was more
significant when the substrate did not contain the phenolic
OH found in the model substrate 1a (or another EDG). As
such, when 1t was treated with TfOH (20 mol %), alkenylation
was only observed in <5% yield (Scheme 5, A). This result
these cases (Figure S1), which might be resulting from the
failure of in situ redundant vinyl carbocation to be captured by
toluene in time. Furthermore, 5 mol % of cationic Lewis acid
Sc(OTf)2(BArF) could also promote the alkenylation of
toluene 2a with O-protected alkyne 1r to give adduct 3ra in
78% yield in the absence of TfOH for 12 h; however, only
trace product 3ta was observed when 1-(phenylethynyl)-
naphthalene 1t was used (Scheme 5, D), thus suggesting that
alkynes bearing an electron-donating group at 2-position on
the naphthyl ring may be more easily activated to form a vinyl
carbocation V that has a weakly coordinating anion, such as
−
BArF−, and Sc(OTf)4 through π-coordination (IV) by
Scandium(III).19 It is noteworthy that more electrophilicity
of Sc(OTf)3/TfOH is crucial to generate vinyl carbocations
from neutral alkynes.
The characteristic absorption of carbocation can be clearly
observed by UV−visible spectra.20 Thus, the UV−vis spec-
troscopy of 1a has been examined in CH2Cl2 in the presence of
different acids (Figure S2), respectively, showing peak
absorption at 303−355 nm, which tails above 400 nm (ε400
= 3882, ε450 = 2243 for the mixture of 1a, and Sc(OTf)3/
TfOH). Upon in situ mixing 1a and the binary acids, the
colorless solution became red (Figure S2, inset). In order to
exclude the new absorption at λ > 400 nm due to presence of
VQM, the UV−vis experiments with O-protected alkyne 1r
were also conducted (Figure S3), giving similar results as the
use of 1a. These observations suggest that Sc(OTf)3/TfOH
should more effectively generate vinyl carbocation than either
TfOH or Sc(OTf)2(BArF).
Scheme 5. Control Experiments
In summary, we have developed Sc(III)/TfOH-catalyzed
alkenylation of arenes with alkynes via the vinyl cation,
affording trisubstituted olefins 3 in high yields and with high Z-
selectivity. Salient features of the alkenylation include binary-
acid catalysis in carbocation transformation. Further studies on
the challenging enantioselective synthesis of axial chirality by
asymmetric binary-acid-catalyzed alkenylation of arene with
alkynes are underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c02065.
Experiment procedures, and characterization data for
compounds (PDF)
Accession Codes
CCDC 2040079, 2040082, 2040089, and 2040509 contain 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 Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
shows the importance of the binary acid system to efficient
generation of vinyl carbocation species, particularly in less
activated systems. In addition, we found that Sc(OTf)3 or the
binary acid catalyzed reaction of vinyl triflate 12 with arene 2j
could not work (Scheme 5, B), excluding the possibility of a
vinyl triflate as an active intermediate in this reaction. When
isolated adduct 3aa was treated under binary acid catalytic
conditions, 3aa can be recovered quantitatively, while the
recovery is only 80% of 3aa in the absence of Sc(OTf)3
(Scheme 5, C), thus suggesting product 3aa should be more
stable under the standard condition. Moreover, when only 10
equiv of 2a was used in the binary-acid system, it was found
that a survey of different solvents, such as CHCl3, MeCN, and
THF, resulted in poor yields (Table S5, entries 1−3), and no
product 3aa was obtained when using 1 equiv of TfOH alone
in toluene (entry 4). Unknown byproducts were observed in
AUTHOR INFORMATION
Corresponding Author
■
Jian Lv − Key Laboratory of Optic-electric Sensing and
Analytic Chemistry for Life Science, MOE, College of
Chemistry and Molecular Engineering, Qingdao University of
Science & Technology, Qingdao 266042, China;
orcid.org/0000-0001-7641-5411; Email: lvjian@
iccas.ac.cn
6001
https://doi.org/10.1021/acs.orglett.1c02065
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Authors
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Letter
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Wenxuan Zhang − Key Laboratory of Optic-electric Sensing
and Analytic Chemistry for Life Science, MOE, College of
Chemistry and Molecular Engineering, Qingdao University of
Science & Technology, Qingdao 266042, China
Jiaying Sun − Key Laboratory of Optic-electric Sensing and
Analytic Chemistry for Life Science, MOE, College of
Chemistry and Molecular Engineering, Qingdao University of
Science & Technology, Qingdao 266042, China
Linrui Xiang − Key Laboratory of Optic-electric Sensing and
Analytic Chemistry for Life Science, MOE, College of
Chemistry and Molecular Engineering, Qingdao University of
Science & Technology, Qingdao 266042, China
Wen Si − Key Laboratory of Optic-electric Sensing and
Analytic Chemistry for Life Science, MOE, College of
Chemistry and Molecular Engineering, Qingdao University of
Science & Technology, Qingdao 266042, China
Ran Song − Key Laboratory of Optic-electric Sensing and
Analytic Chemistry for Life Science, MOE, College of
Chemistry and Molecular Engineering, Qingdao University of
Science & Technology, Qingdao 266042, China
Daoshan Yang − Key Laboratory of Optic-electric Sensing and
Analytic Chemistry for Life Science, MOE, College of
Chemistry and Molecular Engineering, Qingdao University of
Science & Technology, Qingdao 266042, China
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c02065
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Taishan Scholars Construction Projects of
Shandong (tsqn201812075), the Natural Science Foundation
of Shandong Province for Distinguished Yong Scholars
(ZR2020JQ07), Youth Innovation Science and Technology
Plan of Colleges and Universities in Shandong Province
(2019KJC003), the Natural Science Foundation of China (No.
21702115, 21801007), the Natural Science Foundation of
Shandong Province, China (No. ZR2019PB020,
ZR2017BB028, ZR2016JL012), and Qingdao University of
Science & Technology for financial support.
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ring indicates that they all demonstrate reliable chemical shift
differences for the Z and E isomers. The alkene hydrogen for the E
isomer is always located around 6.8−7.0 ppm. The resonance for the
Z isomer alkene hydrogen can overlap with other signals and appear
over 7.0 ppm, whereas the alkene hydrogen of adduct 3 was located
around 7.31−7.59 ppm. According to these results of X-ray diffraction
1
and H NMR data, adducts 3 were determined as Z-olefins.
(17) The Z/E mixture of olefins 3xe could also be easily converted
into the corresponding methyl ether 13 (Supporting Information),
which is a known compound; see: Wang, T.; Hu, Y.; Zhang, S. A
Novel and Efficient Method for the Olefination of Carbonyl
Compounds with Grignard Reagents in the Presence of Diethyl
Phosphite. Org. Biomol. Chem. 2010, 8, 2312−2315.
(18) Shabashov, D.; Doyle, M. P. Rhodium Acetate-Catalyzed
Aerobic Mukaiyama Epoxidative of Alkenes. Tetrahedron 2013, 69,
10009−10013.
(19) For a selected example of π-coordination of alkyne by
Scandium(III) salts, see: Mandal, S.; Verma, P. K.; Geetharani, K.
Lewis Acid Catalysis: Regioselective Hydroboration of Alkynes and
Alkenes Promoted by Scandium Triflate. Chem. Commun. 2018, 54,
13690−13693.
(20) Some selected examples of UV−visible spectra of carbocation,
̈
see: (a) Byrne, P. A.; Kobayashi, S.; Wurthwein, E.-U.; Ammer, J.;
Mayr, H. Why Are Vinyl Cations Sluggish Electrophiles? J. Am. Chem.
Soc. 2017, 139, 1499−1511. (b) Lv, J.; Zhang, Q.; Zhong, X.; Luo, S.
Asymmetric Latent Carbocation Catalysis with Chiral Trityl
Phosphate. J. Am. Chem. Soc. 2015, 137, 15576−15583. (c) Cozens,
F. L.; Kanagasabapathy, V. M.; McClelland, R. A.; Steenken, S.
Lifetimes and UV-visible Absorption Spectra of Benzyl, Phenethyl,
and Cumyl Carbocations and Corresponding Vinyl Cations. A Laser
Flash Photolysis Study. Can. J. Chem. 1999, 77, 2069−2082.
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Org. Lett. 2021, 23, 5998−6003