Metal-Free Approach to Biaryls from Phenols and Aryl Sulfoxides by Temporarily Sulfur-Tethered Regioselective C-H/C-H Coupling

Article abstract of DOI:10.1021/jacs.6b10278

We have developed metal-free regiocontrolled dehydrogenative C-H/C-H cross-coupling of aryl sulfoxides with phenols by means of trifluoroacetic anhydride. Because the reaction would proceed through an interrupted Pummerer reaction followed by sulfonium-tethered [3,3]-sigmatropic rearrangement, the C-H/C-H coupling takes place exclusively between the ortho positions of both substrates. Various functional groups including carbonyl, halo, siloxy, and even boryl moieties are compatible. The biaryl products naturally possess hydroxy and sulfanyl groups, which allows the products to be useful synthetic intermediates, as evidenced by the syntheses of π-expanded heteroarenes such as unprecedented 7,12-dioxa[8]helicene.

Full text of DOI:10.1021/jacs.6b10278

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Communication
Metal-Free Approach to Biaryls from Phenols and Aryl Sulfoxides
by Temporarily Sulfur-Tethered Regioselective C-H/C-H Coupling
Tomoyuki Yanagi, Shinya Otsuka, Yuko Kasuga, Keisuke Fujimoto,
Kei Murakami, Keisuke Nogi, Hideki Yorimitsu, and Atsuhiro Osuka
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b10278 • Publication Date (Web): 30 Oct 2016
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Metal-Free Approach to Biaryls from Phenols and Aryl Sulfoxides
by Temporarily Sulfur-Tethered Regioselective C–H/C–H Coupling
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Tomoyuki Yanagi, Shinya Otsuka, Yuko Kasuga, Keisuke Fujimoto, Kei Murakami, Keisuke Nogi,
Hideki Yorimitsu,* and Atsuhiro Osuka
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyoꢀku, Kyoto 606ꢀ8502, Japan.
Supporting Information Placeholder
arenes with 2,5ꢀ or 2,6ꢀsubstituted 1,4ꢀbenzoquinone anaꢀ
ABSTRACT: We have developed metalꢀfree regiocontrolled
dehydrogenative C–H/C–H crossꢀcoupling of aryl sulfoxides
with phenols by means of trifluoroacetic anhydride. Since the
logues,14 a restricted range of arenes could engage in these
transformations.
reaction would proceed through an interrupted Pummerer reꢀ
action followed by sulfoniumꢀtethered [3,3]ꢀsigmatropic rearꢀ
rangement, the C–H/C–H coupling takes place exclusively
between the ortho positions of both substrates. Various funcꢀ
tional groups including carbonyl, halo, siloxy, and even boryl
moieties are compatible. The biaryl products naturally possess
hydroxy and sulfanyl groups, which allows the products to be
useful synthetic intermediates, as evidenced by the syntheses
of πꢀexpanded heteroarenes such as unprecedented 7,12ꢀ
dioxa[8]helicene.
Scheme 1.
Pummerer reaction
C–H/C–H coupling based on extended
Biaryl motifs represent a privileged substructure in bioactive
compounds and organic functional materials. Therefore, a
number of synthetic approaches to biaryl skeletons have been
exploited. Over the past 40 years, transition metalꢀcatalyzed
crossꢀcoupling reactions of aryl halides with aryl organometalꢀ
lic reagents have occupied a central place in biaryl synthesis.1
For the last decade, direct aromatic C–H arylation has been
emerging as a gameꢀchanging means to connect two aromatic
rings.2 The most straightforward and ideal method would be
dehydrogenative C–H/C–H coupling reaction starting from
two individual arenes. Since the pioneering discovery by
Fagnou in 2007,3 dehydrogenative C–H/C–H crossꢀcoupling
reactions have been actively investigated by utilizing transiꢀ
tion metal catalysts such as copperꢀcatalyzed asymmetric
BINOL synthesis.4,5 However, catalyst loadings to achieve
efficient C–H/C–H coupling were usually high and most of
transition metal complexes are expensive. In addition, heavy
metal contaminations of products can be a fatal problem in
research on bioactive agents6 as well as organic electronic
devices.7 Consequently, it has been desired to establish new
methodologies to realize efficient metalꢀfree dehydrogenative
C–H/C–H crossꢀcoupling.4d,8 To this end, radicalꢀmediated
processes promoted by hypervalent iodine reagents9 or anodic
oxidation10 have been developed. However, these strategies
incur poor regioselectivity, mediocre functional group compatꢀ
ibility, and/or employment of large excess amounts of radicalꢀ
accepting arenes. Although several regioselective reactions
have been reported including baseꢀmediated arylation of hetꢀ
eroaromatic Nꢀoxides,11 Bartoliꢀtype reaction of 2ꢀ
halonitroarenes with arylmagnesium reagents,12 chiral
Brønsted acidꢀcatalyzed atropselective synthesis of 2,2'ꢀ
diaminoꢀ1,1'ꢀbinaphthalenes through benzidine rearrangeꢀ
ment,13 and Brønsted acidꢀmediated reaction of hydroxy
Recently, extended Pummerer reactions of unsaturated sulꢀ
foxides have been attracting increasing attention15 since they
offer characteristic transformations utilizing the unique reacꢀ
tivity of sulfur. Some extended Pummerer reactions result in
fascinating C(sp2)–H functionalization at the β position of the
sulfinyl group.16 Indeed, we reported the synthesis of benzofuꢀ
rans by connecting phenols with peculiar vinyl sulfoxides,
ketene dithioacetal monoxides (KDM), at each of their β
C(sp2)–H bonds (Scheme 1a).17 A plausible mechanism inꢀ
cludes 1) the activation of the sulfoxide with trifluoroacetic
anhydride, 2) capture of the resulting sulfonium with phenol,
3) sulfoniumꢀaccelerated [3,3]ꢀsigmatropic rearrangement18
with concomitant loss of the aromaticity of phenol, and 4)
overall aromatization. During the course of the reaction, the
cationic sulfonium moiety acts as a directing group to realize
regioselective C–C bond formation.
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Table 1. Regiocontrolled dehydrogenative cross-coupling of aryl sulfoxides 1 with phenols 2
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b
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aPerformed at –40 ˚C for 2 h. Combined yield of regioisomers. Isomeric ratios are in parentheses. Performed on 19 mmol scale. Perꢀ
e
1
f
g
formed at 40 ˚C. Yield was determined by H NMR analysis. Performed on 5.0 mmol scale. Reaction conditions: 1q (0.20 mmol), 2ꢀ
naphthol (2t, 3.5 equiv), (CF3CO)2O (3.0 equiv), CH2Cl2 (0.1 M), 25 ˚C, 1 h.
We envisioned that this extended Pummerer strategy with
phenols as substrates would be applicable to the synthesis of
biaryls. Our working hypothesis is depicted in Scheme 1b.
Activation of the aryl sulfoxide 1 followed by nucleophilic
substitution at the cationic sulfur center with phenol 2 would
form intermediate B. Given that [3,3]ꢀsigmatropic rearrangeꢀ
ment would occur with the loss of the aromaticity of both aryl
sulfonium and phenol, regiocontrolled C–C bond formation
should result by a temporarily sulfoniumꢀtethered intramolecuꢀ
lar process. Finally, rearomatization of C furnishes biaryl
product 3. Biaryl 3 has synthetically useful hydroxy and sulꢀ
fanyl moieties, further transformation being highly expected.19
Treatment of 2ꢀ(methylsulfinyl)ꢀ1ꢀtosylindole (1a) and
phenol (2a) with trifluoroacetic anhydride in CH2Cl2 at 25 ˚C
afforded an 89% yield of 3ꢀ(2ꢀhydroxyphenyl)ꢀ2ꢀ
methylsulfanylꢀ1ꢀtosylindole (3aa) as the sole product (Table
1). As expected, the C–C bond formation took place
exclusively between the ortho (or β) positions of 1a and 2a.
Phenols having an electronꢀdonating or ꢀwithdrawing group at
the 4ꢀposition reacted with 1a to provide the corresponding
biaryls 3ac–aj in good yields. Carbonyl, halogen, and
pinacolatoboryl moieties were well tolerated under the
reaction conditions to yield 3ah–am. Arylation of 3ꢀ
methoxyphenol (2n) or 3ꢀisopropylphenol (2o) with 1a
occurred exclusively at the less hindered 6ꢀposition yielding
3an or 3ao as the single product, while that of 3ꢀbromophenol
(2p) gave a mixture of regioisomers 3ap and 3ap' with
moderate selectivity favoring sterically less demanding 3ap.
This reaction also accommodates 2ꢀsubstituted phenols to
afford the corresponding sterically congested 2,6ꢀdisubstituted
phenols 3aq and 3ar efficiently. Naphthols 2s and 2t were
also converted to the corresponding arylated products 3as and
3at, respectively. Notably, 2ꢀnaphthol (2t) reacted at the C1
position exclusively due to the stronger doubleꢀbond character
of the C1–C2 bond of the naphthalene core than that of the
C2–C3 bond. These orthoꢀselectivities clearly indicate the
reactions proceeded via the sulfoniumꢀtethered process shown
in Scheme 1b.
This biaryl synthesis was also applicable to a wide range of
aryl
sulfoxides.
Indeed,
2ꢀmethylsulfinylꢀsubstituted
naphthalene 1b, benzofuran 1c, and benzothiophene 1d
afforded the desired products in good to excellent yields.
Instead of the methylsulfinyl moiety, an aromatic pꢀ
tolylsulfinyl moiety served as an effective directing group. In
the reaction of unsymmetrical diaryl sulfoxide 1e, the
potentially reactive pꢀtolyl group remained intact, and
regioselective arylation occurred at the benzothiophene unit of
the weaker aromaticity to yield 3ea. C–C bond formation
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proceeded at the 2ꢀposition when 3ꢀ(methylsulfinyl)indole 1f
immediately in 5 s (Scheme 2b). Naturally, treatment of 4 with
catalytic trifluoroacetic acid gave 3dv.22
1
was employed. Monocyclic pyrrolyl and thienyl sulfoxides 1g
and 1h also reacted at their 3ꢀpositions with perfect
regioselectivity. Not only heteroaromatic sulfoxides but also
phenyl sulfoxides 1i–l, 1o, and 1p participated in the
arylation.20 The triisopropylsilyl ether in 1k survived under the
reaction conditions as well as improved the regioselectivity of
the arylation due to the steric hindrance (3it and 3jt vs. 3kt).
Preferably, the phenyl groups of the sulfoxides should be
electronꢀrich as parent methyl phenyl sulfoxide (1l) afforded
the product 3lt in 26% yield. We assume that the electronꢀ
donating group(s) at the 3ꢀ (and 5ꢀ) position(s) would stabilize
the corresponding cationic intermediate C in Scheme 1b to
facilitate the [3,3]ꢀsigmatropic rearrangement of B to C.
Indeed, 4ꢀmethoxyphenyl methyl sulfoxide (1m), which
stabilizes not C but B, afforded a trace amount of product 3mt
due to the elevated activation energy barrier for the pathway
from B to C. Unfortunately, 4ꢀtrifluoromethylphenyl methyl
sulfoxide (1n) afforded the product 3nt in very low yield. The
coupling reaction of 2,7ꢀdi(dodecylsulfinyl)naphthalene (1q)
with 2ꢀnaphthol (2t) occurred at the peri positions to afford
1,8ꢀdinaphthylated naphthalene 3qt regioselectively in 71%
yield. The formation of sterically congested 1,2,7,8ꢀsubstituted
3qt highlights the efficiency, robustness, and potentiality of
the present methodology (vide infra, specifically in Scheme
3d). The reactions could be conducted on larger scales, as
demonstrated in the synthesis of 3ca and 3ou, which clearly
show the robustness of our system.
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Scheme 3. Derivatizations of biaryl compounds 3
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Scheme 2. Mechanistic investigations
After the conversion of the phenolic hydroxy group into a
triflate, global reduction by means of a hydrosilane/palladium
catalyst system23 gave 5ca (Scheme 3a), traceless biaryl couꢀ
pling24 hence being achieved. Instead of the reductive removal
of the tethering units, we could take advantage of these funcꢀ
tional groups for derivatizations of 3 into πꢀexpanded hetꢀ
eroarenes. After removal of the propionate moiety on 3pu
under basic conditions,25 pꢀtoluenesulfonic acidꢀmediated inꢀ
tramolecular condensation26 furnished benzonaphthothiophene
6pu in 73% yield over two steps (Scheme 3b). The sulfanyl
moiety of 3 could be easily oxidized to a sulfonyl moiety by
mꢀCPBA. As the methylsulfonyl moiety is a good leaving
group, intramolecular SNAr reactions resulted in the forꢀ
mations of highly fused dibenzofuran analogues 7ca, 7da, and
7ou in good yields (Scheme 3c). This SNArꢀbased cyclization
was also applicable to the synthesis of a polycyclic and nonꢀ
planar screwꢀshaped helicene analogue (Scheme 3d).27 After
oxidation of 3qt, intramolecular SNAr cyclization culminated
in a rapid access to unprecedented 7,12ꢀdioxa[8]helicene 8qt
in 68% overall yield.
To further verify our mechanistic proposal in Scheme 1b, the
reaction of 2,6ꢀdimethylphenol (2v) was performed. If this
reaction proceeded through a FriedelꢀCrafts pathway, 4ꢀ
arylatedꢀ2,6ꢀdimethylphenol 3dv' should be the conceivable
major product. However, C–C bond formation proceeded at
the 3ꢀposition of 2v preferentially to furnish 3dv as the major
product, along with 3dv' as the minor isomer (Scheme 2a). On
the basis of this result, we are tempted to propose a
mechanism for the metaꢀselective coupling. According to the
scenario shown in Scheme 1b, cyclohexadienone intermediate
In summary, we have developed metalꢀfree, highly regioseꢀ
lective dehydrogenative C–H/C–H crossꢀcoupling of aryl sulꢀ
foxides with phenols by means of trifluoroacetic anhydride.
The regiocontrolled C(sp2)–C(sp2) bond formation has been
realized by the mechanism involving a sequence of interrupted
Pummerer reaction and [3,3]ꢀsigmatropic rearrangement diꢀ
rected by a temporary and removable tether between the sulfiꢀ
nyl group and the phenolic hydroxy group. Understandably,
4
would be generated via an interrupted Pummerer
reaction/[3,3]ꢀsigmatropic rearrangement pathway. With the
aid of trifluoroacetic acid generated in situ, 4 would then
undergo protonationꢀinduced 1,2ꢀshift21 of the benzothiophene
unit. Notably, intermediate 4 was indeed isolated in 58% yield
when the coupling reaction was quenched by methanolic KOH
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Zhou, J.; Xu, C.; Sun, H.; Kürti, L.; Xu, Q.ꢀL. J. Am. Chem. Soc.
2016, 138, 5202.
the products are useful synthetic intermediates, as clearly
demonstrated by the synthesis of πꢀexpanded heteroaromatic
molecules including 7,12ꢀdioxa[8]helicene. Further expansion
of the scope of substrates and creation of more interesting
aromatic cores as well as asymmetric biaryl synthesis through
this strategy are now in progress.28
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ASSOCIATED CONTENT
Supporting Information
Experimental procedures, Xꢀray crystallographic analysis, photoꢀ
physical property, and spectral data. This material is available free
of charge via the Internet at http://pubs.acs.org.
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(15) Reviews about transformations based on extended Pummerer
reactions: (a) Bur, S. K.; Padwa, A. Chem. Rev. 2004, 104, 2401. (b)
Feldman, K. S. Tetrahedron 2006, 62, 5003. (c) Smith, L. H. S.;
Coote, S. C.; Sneddon, H. F.; Procter, D. J. Angew. Chem. Int. Ed.
2010, 49, 5832. (d) Pulis, A. P.; Procter, D. J. Angew. Chem. Int. Ed.
2016, 55, 9842. (e) Shafir, A. Tetrahedron Lett. 2016, 57, 2673.
(16) (a) Akai, S.; Kawashita, N.; Satoh, H.; Wada, Y.; Kakiguchi,
K.; Kuriwaki, I.; Kita, Y. Org. Lett. 2004, 6, 3793. (b) Akai, S.; Kaꢀ
washita, N.; Wada, Y.; Satoh, H.; Alinejad, A. H.; Kakiguchi, K.;
Kuriwaki, I.; Kita, Y. Tetrahedron Lett. 2006, 47, 1881. (c) Huang,
X.; Maulide, N. J. Am. Chem. Soc. 2011, 133, 8510. (d) Huang, X.;
Patil, M.; Farès, C.; Thiel, W.; Maulide, N. J. Am. Chem. Soc. 2013,
135, 7312. (e) FernándezꢀSalas, J. A.; Eberhart, A. J.; Procter, D. J. J.
Am. Chem. Soc. 2016, 138, 790.
(17) (a) Kobatake, T.; Fujino, D.; Yoshida, S.; Yorimitsu, H.;
Oshima, K. J. Am. Chem. Soc. 2010, 132, 11838. (b) Ookubo, Y.;
Wakamiya, A.; Yorimitsu, H.; Osuka, A. Chem. Eur. J. 2012, 18,
12690. (c) Murakami, K.; Yorimitsu, H.; Osuka, A. Angew. Chem.
Int. Ed. 2014, 53, 7510.
(18) A review of chargeꢀaccelerated [3,3]ꢀsigmatropic rearrangeꢀ
ment: Huang, X.; Klimczyk, S.; Maulide, N. Synthesis 2012, 44, 175.
(19) Although 2ꢀhydroxyꢀ5ꢀmethylphenyl phenyl sulfoxide was reꢀ
ported to react with phenols to form biaryls through Pummerer proꢀ
cess, this sulfoxide is the only applicable substrate. Moreover, the
reactions inevitably yielded mixtures of orthoꢀ and paraꢀregioisomers
in regard with the phenol units since the transformation would proꢀ
ceed through FriedelꢀCrafts arylation of phenols with the reactive
thionium cation. See: Jung, M. E.; Kim, C.; von dem Bussche, L. J.
Org. Chem. 1994, 59, 3248.
(20) When paraꢀunsubstituted 1o reacted with 1ꢀnaphthol (2s), not
only the desired product 3os but the biaryl 3os' derived from [5,5]ꢀ
sigmatropic rearrangement was obtained in a ratio of 1.7:1. See Supꢀ
porting Information for more details.
(21) A similar 1,2ꢀshift was observed in the reaction of 2,6ꢀ
dimethyl(diacetoxyiodo)benzene. See: Ochiai, M.; Ito, T.; Takaoka,
Y.; Masaki, Y. J. Am. Chem. Soc. 1991, 113, 1319.
(22) Although the mechanism for the formation of 3dv' from 4 is
still unclear, simple thermal [1,3]ꢀaryl shift can be excluded because
no rearrangement of 4 took place even at 130 °C in the absence of
trifluoroacetic acid.
(23) Matsumura, T.; Niwa, T.; Nakada, M. Tetrahedron Lett. 2012,
53, 4313.
AUTHOR INFORMATION
Corresponding Author
*yori@kuchem.kyotoꢀu.ac.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by JSPS KAKENHI Grant Numbers
JP23655037,
JP24685007,
JP25107002,
JP16H01019,
JP16H01149, and JP16H04109 as well as by ACTꢀC, JST. H.Y.
thanks Japan Association for Chemical Innovation, Tokuyama
Science Foundation, and The Naito Foundation for financial supꢀ
port. S.O. and K.F. acknowledge JSPS Predoctoral Fellowship.
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yan, R.; Matcha, K.; Antonchick, A. P. Chem. Eur. J. 2015, 21, 14678.
(9) (a) Dohi, T.; Ito, M.; Morimoto, K.; Iwata, M.; Kita, Y. Angew.
Chem. Int. Ed. 2008, 47, 1301. (b) Kita, Y.; Dohi, T. Chem. Rec.
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(10) (a) Kirste, A.; Schnakenburg, G.; Stecker, F.; Fischer, A.;
Waldvogel, S. R. Angew. Chem. Int. Ed. 2010, 49, 971. (b) Kirste, A.;
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(28) As a preliminary experiment, the coupling reaction of enantiꢀ
omerically pure 1b (>99% ee) with 2ꢀnaphthol (2t) was performed.
However, the corresponding binaphthyl 3bt was obtained with 9% ee
even at –40 °C. See Supporting Information for more details.
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