Br?nsted acid-catalysed hydroarylation of unactivated alkynes in a fluoroalcohol-hydrocarbon biphasic system: Construction of phenanthrene frameworks

Article abstract of DOI:10.1039/c9cc04152d

Transition metal-free hydroarylation of unactivated alkynes was achieved by combining a Br?nsted acid catalyst and a two-phase solvent system consisting of 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) and cyclohexane. This protocol is applicable to a wide variety of 2-alkynylbiaryls which leads to the synthesis of substituted phenanthrenes via 6-endo-selective ring closure. The biphasic system achieves highly efficient ring closure by appropriate separation of cationic intermediates from neutral compounds. The vinyl carbocation intermediates are stabilised in the HFIP phase, while the substrates and products are distributed in the cyclohexane phase to suppress intermolecular side reactions.

Full text of DOI:10.1039/c9cc04152d

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DOI: 10.1039/C9CC04152D
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Brønsted Acid-Catalysed Hydroarylation of Unactivated Alkynes in
Fluoroalcohol–Hydrocarbon Biphasic System: Construction of
Phenanthrene Frameworks
Received 00th January 20xx,
Accepted 00th January 20xx
Ikko Takahashi, Takeshi Fujita, Noriaki Shoji and Junji Ichikawa*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Transition metal-free hydroarylation of unactivated alkynes was alkynes, which again required installing electron-donating siloxy
achieved by combining a Brønsted acid catalyst and a two-phase and amino substituents (Z), respectively, adjacent to the vinyl
solvent system consisting of 1,1,1,3,3,3-hexafluoropropan-2-ol cation centre (Scheme 1b).9
(HFIP) and cyclohexane. This protocol is applicable to a wide variety
of 2-alkynylbiaryls and leads to the synthesis of substituted
phenanthrenes via 6-endo-selective ring closure. The biphasic
system achieves highly efficient ring closure by appropriate
separation of cationic intermediates from neutral compounds. The
vinylic carbocation intermediates might be stabilised in the HFIP
phase, while the substrates and products are distributed in the
cyclohexane phase to suppress intermolecular side reactions.
Electrophilic activation of alkynes has occupied a significant
position in synthetic organic chemistry, because it allows
carbon–carbon and carbon–heteroatom bond formation at
alkyne moieties.1 Particularly, intramolecular hydroarylation of
alkynes via electrophilic activation has been widely studied as
one of the most effective methods for constructing carbocycles,
including polycyclic aromatic hydrocarbons (PAHs).2
Hydroarylation of 2-alkynylbiaryls has been conducted mostly
via electrophilic activation by transition metal catalysts,3 which
yields phenanthrenes via 6-endo cyclisation or dibenzofulvenes
via 5-exo cyclisation (Scheme 1a).4 To date, however, Brønsted
acid-mediated hydroarylation has shown limited success with
an electron-donating group and/or an excess amount of
acid,3m,5,6 probably because unstable vinyl cation intermediates
are required to be generated via protonation of alkynes.7,8 For
example, Swager and Chalifoux reported Brønsted acid-
mediated hydroarylation of alkynes by using an excess amount
of Brønsted acid and placing aryl groups with electron-donating
alkoxy or amino substituents (Y),5 which stabilised the vinyl
cation intermediates (Scheme 1b). Kozmin and Hsung
succeeded in Brønsted acid-catalysed carbocyclisation of
Division of Chemistry, Pure and Applied Sciences, University of Tsukuba, Tsukuba,
Ibaraki 305-8571, Japan. E-mail: junji@chem.tsukuba.ac.jp
Electronic Supplementary Information (ESI) available: Experimental procedures and
characterisation data of new compounds. See DOI: 10.1039/x0xx00000x
Scheme 1 Cyclisation via electrophilic activation. (a) Transition metal-catalysed
hydroarylation of alkynes. (b) Acid-mediated (or -catalysed) hydroarylation of
alkynes bearing an electron-donating group (c) Acid-catalysed dehydrative
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Journal Name
cycloaromatisation of carbonyl compounds. (d) Acid-catalysed hydroarylation of
alkynes (this work).
9
TsOH·H2O
TsOH·H2O
i-PrOH
60
9
N.D.cView ArNtic.Dle.Oc nline
DOI: 10.1039/C9CC04152D
HFIP–Hexane
(1/2)
10
78
67
85
85
9
Recently, we demonstrated Brønsted acid-catalysed
dehydrative cycloaromatisation of carbonyl compounds and
their analogues by using 1,1,1,3,3,3-hexafluoropropan-2-ol
(HFIP) as a solvent,10 which exhibits a cation-stabilising effect
(Scheme 1c).11,12,13 The key to success in this reaction was to
stabilise oxocarbenium ion intermediates in HFIP. We thus
assumed that HFIP could stabilise even more unstable vinyl
cations in a similar manner and enable Brønsted acid-catalysed
hydroarylation of unactivated alkynes. By careful examination,
the use of cyclohexane as a co-solvent with HFIP, which forms a
biphasic solvent system, was found to be the best system for 6-
endo-selective hydroarylation of 2-alkynylbiaryls (Scheme 1d).
This protocol thus offers one of the most simple and atom-
economical approaches to phenacenes.14
The combination of trifluoromethanesulfonic acid (TfOH) as
a catalyst and HFIP as a solvent,15 which was the most effective
for dehydrative cycloaromatisation of carbonyl compounds,10
was initially tested for intramolecular hydroarylation of 2-
(phenylethynyl)biphenyl (1a) as an unactivated alkyne.
Hydroarylation of 1a proceeded to afford 6-endo cyclisation
product 2a and 5-exo cyclisation product 3a in 53% and 7%
yields, respectively (Table 1, Entry 1). To improve the yield of
2a, several other Brønsted acids, such as methanesulfonic acid
(MsOH), tetrafluoroboric acid (HBF4), 10-camphorsulfonic acid
(CSA), and p-toluenesulfonic acid monohydrate (TsOH·H2O),
were examined as catalysts, affording 2a in 44%54% yields
(Table 1, Entries 2–5). Since cheap and easily handled TsOH·H2O
gave the best result, several solvents were screened in the
presence of a catalytic amount of TsOH·H2O (Table 1, Entries 6–
9). As expected, HFIP was found to be the most effective among
the solvents examined. To suppress side reactions, biphasic
systems consisting of HFIP and aliphatic hydrocarbons, such as
hexane, decalin, and cyclohexane (c-C6H12), were employed in
the hydroarylation (Table 1, Entries 10–12, vide infra).16 When
using c-C6H12 as a co-solvent, 6-endo product 2a was obtained
in 85% yield with high selectivity (Table 1, Entry 12).
Furthermore, this reaction proceeded smoothly even in air
(Table 1, Entry 13).
HFIP–Decalin
(1/2)
11
TsOH·H2O
TsOH·H2O
TsOH·H2O
9
9
9
11
13
11
HFIP–c-C6H12
(1/2)
12
HFIP–c-C6H12
(1/2)
13d
Yield was determined by 1H NMR measurement using CH2Br2 as an internal
standard. b HBF4 was used as an aqueous solution (48 wt%). c N.D. = Not detected.
d Reaction was conducted in air.
a
The optimal conditions obtained above for synthesising 2a
from 1a were then successfully applied to the hydroarylation of
(phenylethynyl)biaryls 1 bearing various substituents (Table 2).
Hydroarylation of (phenylethynyl)biaryls 1b–1e bearing
electron-donating alkyl groups on the nucleophilic aryl group
proceeded smoothly to afford the corresponding substituted
phenanthrenes 2b–2e in high yields. Despite the acidic
conditions, the tert-butyl group was neither removed nor
rearranged through the retro-Friedel–Crafts alkylation (2e).
Hydroxy, fluorine, and chlorine substituents were tolerated in
the
reaction,
providing
phenanthrenes
2f–2h.
(Phenylethynyl)biaryls 1i–1m bearing electron-withdrawing
acetyl, ethoxycarbonyl, cyano, nitro, and trifluoromethyl groups
also underwent successful hydroarylation. It is noteworthy that
cyclisation of substrates 1k–1m bearing strongly electron-
withdrawing groups proceeded in high to excellent yields.
Furthermore, tetracyclic benzenoids such as chrysene 2n
([4]phenacene), naphtho[1,2-b]benzothiophene 2o, and
[4]helicene 2p were synthesised in high yields. Additionally, this
method provided substituted picene 2q ([5]phenacene), which
was expected to serve as an organic semiconductor.17
Installation of electron-donating methyl and methoxy groups on
the benzene ring on the side opposite to the biphenyl group
across the C–C triple bonds accelerated the reaction to afford
the corresponding 9-arylphenanthrenes 2r and 2s, respectively,
in high yields, because the substituents could effectively
stabilise the intermediary vinyl cations. Furthermore,
(arylethynyl)biphenyls 1t and 1u bearing electron-withdrawing
chlorine and bromine substituents also afforded phenanthrenes
2t and 2u in 73% and 71% yields, respectively. These results
suggested that even electron-withdrawing halogen substituents
did not prohibit the formation of vinyl cations adjacent to the
haloaryl groups.
Table 1 Screening of conditions for hydroarylation of 1a
To determine the effect of the biphasic solvent system, the
distribution ratios of substances between fluoroalcohol and
hydrocarbon phases were examined. The distribution ratios of
2-(phenylethynyl)biphenyl (1a), 9-phenylphenanthrene (2a),
and TsOH between HFIP and c-C6H12 were independently
determined by 1H NMR measurement of the c-C6H12 layer (Table
3). As listed in Table 3, 88% of the substrate alkyne 1a was
dissolved in c-C6H12, while 12% of 1a was dissolved in HFIP.
Phenanthrene (2a) and TsOH were completely separated in the
c-C6H12 and HFIP layers, respectively.
Entry Acid
Solvent
HFIP
Time (h) 2a (%)a 3a (%)a
1
2
3
4
5
6
7
8
TfOH
9
53
7
MsOH
HFIP
9
47
7
b
HBF4
HFIP
9
48
9
CSA
HFIP
9
44
5
TsOH·H2O
TsOH·H2O
TsOH·H2O
TsOH·H2O
HFIP
9
54
5
Hexane
CH2Cl2
CH3NO2
60
60
60
N.D.c
trace
1
N.D.c
N.D.c
N.D.c
Based on the observations described above, plausible
behaviours of alkynes 1, phenacenes 2, and TsOH during
2 | J. Name., 2012, 00, 1-3
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hydroarylation in the fluoroalcohol and hydrocarbon biphasic
solvent system are shown in Scheme 2. Alkynes 1 and
phenacenes 2 were mainly dissolved in the c-C6H12 phase, while
TsOH was dissolved in the HFIP phase. A portion of alkynes 1
was protonated by TsOH in the HFIP phase to generate vinyl
cation intermediates A, which settled in the HFIP phase.18
Subsequent intramolecular cationic cyclisation of vinyl cations
A proceeded in a 6-endo fashion in the HFIP phase to afford
phenacenes 2 as major products, which moved from the HFIP
phase into the c-C6H12 phase. Thus, the biphasic system
successfully separated alkynes 1 and phenacenes 2 from the
vinyl cations to prevent undesirable intermolecular reactions.
View Article Online
DOI: 10.1039/C9CC04152D
Scheme 2 Proposed behaviour of substances in HFIP–cyclohexane biphasic
system.
Table 2 Synthesis of PAHs via hydroarylation of 1a
In addition, we successfully recycled the HFIP solution of
TsOH. After the reaction, the c-C6H12 layer containing 2a and 3a
was separated from the HFIP layer containing TsOH, which was
repeatedly reused by adding a new c-C6H12 solution of 1a. When
sequential reactions using the same HFIP layer containing TsOH
were conducted five times with stirring under the same
conditions, the corresponding phenanthrene 2a and fulvene 3a
were obtained in 96% (1st cycle), 97% (2nd cycle), 94% (3rd
cycle), 92% (4th cycle), and 91% (5th cycle) total yields (Table 4).
Thus, the reactivity of the HFIP solution of TsOH·H2O was
confirmed to be maintained over five cycles, which fully
demonstrated the feasibility of this procedure.19
Table 4 Recycling of HFIP solution containing TsOH for sequential hydroarylationa
Total yield of 2a and 3a (%)b
2a/3a ratioc
92/8
1st
96
97
94
92
91
2nd
3rd
4th
5th
92/8
91/9
90/10
90/10
a Reaction conditions (1st cycle): alkyne 1a (0.3 mmol), TsOH·H2O (10 mol%), HFIP
(1.5 mL), and c-C6H12 (3.0 mL) at room temperature for 9 h. Reaction conditions
(2nd–5th cycles): alkyne 1a (0.3 mmol), TsOH in HFIP (ca. 1.5 mL), and c-C6H12 (3.0
mL) at room temperature for 9 h. b Yield was determined by 1H NMR measurement
using CH2Br2 as an internal standard. cRatio was determined by 1H NMR
measurement.
In summary, we have developed an efficient and atom-
economical method for synthesising phenacenes through TsOH-
catalysed intramolecular hydroarylation of unactivated alkynes
with a wide variety of substituents. This reaction requires only
a catalytic amount of Brønsted acid. Therefore, (i) there are no
metal impurities in the phenacene products; (ii) quenching and
workup are dramatically simplified, compared to conventional
methods employing an excess amount of Brønsted acid. The
two-phase c-C6H12/HFIP solvent system promoted the catalytic
reaction via the vinyl cation intermediate and suppressed side
reactions, which enabled recycling of expensive HFIP including
the acid catalyst. Thus, we discovered the excellent potential of
the two-phase solvent system containing HFIP.
a Reaction conditions: alkyne 1 (0.3 mmol), TsOH·H2O (10 mol%), HFIP (1.5 mL), and
c-C6H12 (3.0 mL). Isolated yield. Yield determined by 1H NMR measurement using
CH2Br2 as an internal standard is given in parentheses. b TfOH (10 mol%) was used
instead of TsOH·H2O. c Gel permeation chromatography (GPC) was performed for
purification. d rt, 3 h. e rt, 1 h. f TsOH·H2O (20 mol%), 60 °C, 3 h.
Table 3 Distribution Ratios of 1a, TsOH, and 2a between c-C6H12 and HFIP
1a (%)
88b
12d
TsOHa (%)
N.D.b,c
>99d
2a (%)
>99b
<1d
c-C6H12 layer
HFIP layer
a
b
TsOH was added to the biphasic system as TsOH·H2O. Distribution ratio was
determined by 1H NMR measurement using CH2Br2 as an internal standard. c N.D.
= Not detected. d Distribution ratio in the HFIP layer was calculated based on that
in the c-C6H12 layer.
This work was financially supported by JSPS KAKENHI Grant
Number JP16H04105 (J.I.) in Grant-in-Aid for Scientific Research
(B), JSPS KAKENHI Grant Number JP18H04234 (J.I.) in Precisely
Designed Catalysts with Customized Scaffolding, and JSPS
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S. Parpart, A. Villinger, P. Ehlers and P. Langer, Angew.
A. V. Vasilyev, Russ. Chem. Rev., 2013,D8O2I,: 1108.710–3290/C49. CC04152D
For isolation and structural determination of a vinyl cation,
see: T. Müller, M. Juhasz and C. A. Reed, Angew. Chem., Int.
Ed., 2004, 43, 1543–1546.
For Brønsted acid-catalysed hydroarylation of alkynes
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and S. A. Kozmin, J. Am. Chem. Soc., 2004, 126, 10204–
10205; (b) Y. Zhang, R. P. Hsung, X. Zhang, J. Huang, B. W.
Slafer and A. Davis, Org. Lett., 2005, 7, 1047–1050.
KAKENHI Grant Number JP18K05116 (T.F.) in Grant-in-Aid for
Scientific Research (C). We acknowledge Central Glass Co., Ltd.
for a generous gift of HFIP.
View Article Online
Chem., Int. Ed., 2017, 56, 4575–4578.
7
8
Notes and references
9
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16 For example, mixing HFIP, 2,2,2-trifluoroethanol, hexane,
and perfluoroheptane (v/v/v/v = 1/1/3/3) forms a three-
phase system at 0 °C, whereas the system is transformed
into one phase upon heating to 33 °C. See: J. Rabái, Fluorous
Phase Systems for the Games In Handbook of Fluorous
Chemistry, Eds. J. A. Gladysz, D. P. Curran and I. T. Horváth,
Wiley-VCH, Weinheim, 2004, pp. 574–585. In contrast, the
current system composed of HFIP and cyclohexane provides
two phases at any temperature.
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18 Treatment of diphenylacetylene with an equimolar amount
of TfOH in HFIP at room temperature caused complete
consumption of the acetylene to afford an uncharacterized
product. It provided a peak at 191.2 ppm on 13C NMR
measurement, which is presumably attributed to a positively
charged carbon atom and indicates the formation of a
cation. See also ref. 8.
2
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4
5
Although hydroarylation of 2-alkynylbiaryls via electrophilic
activation by transition metal catalysts mostly proceeds in a
6-endo manner, 5-exo cyclisation sometimes occurs
preferentially according to substrates or reaction conditions.
See refs. 3a and 3b.
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4 | J. Name., 2012, 00, 1-3
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