Ir(III)-Catalyzed room-temperature synthesis of multisubstituted benzofurans initiated by C-H activation of -aryloxy ketones

Article abstract of DOI:10.1055/s-0030-1260981

Cyclodehydration of various -aryloxy ketones proceeded to give various multisubstituted benzofurans by using an Ir(III) catalyst, which was prepared from [Cp IrCl AgSbF and Cu(OAc) The use of the cationic iridium complex with a carboxylate salt realized t

Full text of DOI:10.1055/s-0030-1260981

LETTER
2075
Ir(III)-Catalyzed Room-Temperature Synthesis of Multisubstituted Benzo-
furans Initiated by C–H Activation of a-Aryloxy Ketones
S
ynthesis of Mu
a
ltisubstitut
k
ed
B
enzofur
a
ans nori Shibata,*a Yu-ki Hashimoto,a Maiko Otsuka,a Kyoji Tsuchikama,a Kohei Endob
a
Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, Okubo, Shinjuku,
Tokyo, 169-8555, Japan
Fax +81(3)52868098; E-mail: tshibata@waseda.jp
Waseda Institute for Advanced Study, Shinjuku, Tokyo, 169-8050, Japan
b
Received 9 May 2011
ring apparently decreased the reactivity, and most of sub-
strates remained (Scheme 2).
Abstract: Cyclodehydration of various a-aryloxy ketones proceed-
ed to give various multisubstituted benzofurans by using an Ir(III)
catalyst, which was prepared from [Cp*IrCl2]2, AgSbF6, and
Cu(OAc)2. The use of the cationic iridium complex with a carbox-
ylate salt realized the efficient transformation at ambient tempera-
ture.
Ac
Ac
R3
[Ir(cod)2]BARF (5 mol%)
rac-BINAP (5 mol%)
R2
R2
R3
PhCl, 135 °C, 24 h
O
O
R1
R1
O
Key words: C–H activation, benzofurans, cyclodehydration, iridi-
um, room temperature
BARF: tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
Scheme 1 Ir(I)-catalyzed cyclodehydration of a-aryloxy ketones
Direct C–H bond functionalization is an atom-economical
transformation, because it can omit the pre-activation step
of the substrate and, as a result, the formation of byprod-
uct(s) derived from the activating group. In the last de-
cade, various fascinating transformations initiated by
transition-metal-catalyzed C–H bond activation have
been reported.1 But the bond energy of the C–H bond is
intrinsically large, and high reaction temperature is gener-
ally required, which sometimes limits the scope of syn-
thetic application. Therefore, the development of efficient
catalysts for the C–H bond activation, which can operate
under milder reaction conditions, is strongly desired.2
Ac
Ac
4
Me
[Ir(cod)2]BARF (5 mol%)
rac-BINAP (5 mol%)
H(D)
O
5
PhCl, 135 °C, 24 h
KIE = ca.1.1
Me
6
O
R1
R1
7
O
R1 = H: 94%
5-Cl: 9%
6-Cl: 28%
7-Cl: 15%
Scheme 2 Effect of substituents (R1) on the benzene ring in the pre-
vious catalysis
Judging from these results, we assumed that C–H bond
cleavage by electrophilic metalation is more probable
than oxidative addition (Scheme 3)6 and anticipated that a
more electrophilic complex could accelerate the reaction.7
In these years, we have focused on the development of
cationic iridium(I) catalyzed synthesis initiated by C–H
bond activation and realized carbonyl- or amide-directed
C–H bond cleavage for the reaction with alkynes and al-
kenes, respectively.3 We further reported the synthesis of
benzofurans initiated by C–H bond activation of a-3-
acetylphenyloxy ketones along with intramolecular 1,2-
additon and dehydration (Scheme 1).4,5 This regioselec-
tive transformation provided various 4-acetyl benzofurans
without the formation of 6-acetyl benzofurans, but the
high reaction temperature of 135 °C was required. We
herein report an iridium(III)-catalyzed synthesis, which
realized a reaction at room temperature and widened the
substrate scope.
Me
O
Me
O
Ir
H
O
Ir+
H
+
Me
Me
OH
O
R1
R1
O
O
O
Me
O
Me
O
Me
Me
Ir
O
2
intramolecular
1,2-addition
– H2O
O
R1
R1
In the course of mechanistic study of the above transfor-
mation,4 we found that the value of the KIE (kinetic iso-
tope effect) was approximately 1.1, and that introduction
of the electron-withdrawing chloro group on the benzene
Scheme 3 Possible mechanism including electrophilic metalation
We chose a-aryloxy ketone 1a as a model substrate and
submitted it to the reaction using the dicationic iridium
species Cp*Ir2+ (Cp*: pentamethylcyclopentadienyl),
which was in situ prepared from [Cp*IrCl2]2 (10 mol% Ir)
and AgSbF6 (20 mol%),8 as a catalyst in 1,2-dichloro-
ethane (DCE, Table 1, entry 1). We were pleased to find
SYNLETT 2011, No. 14, pp 2075–2079
x
x
.x
x
.2
0
1
1
Advanced online publication: 03.08.2011
DOI: 10.1055/s-0030-1260981; Art ID: U04111ST
© Georg Thieme Verlag Stuttgart · New York

2076
T. Shibata et al.
LETTER
that the cyclodehydration proceeded even at room temper-
ature to give benzofuran 2a yet in low yield due to low
conversion. In order to improve the yield, we screened a
catalytic amount of additives: when sodium acetate was
added, the yield dramatically increased to 66%, which in-
cludes benzofuranol 3a (Table 1, entry 2).9 More bulky
sodium pivalate achieved the yield of 85% without the
formation of benzofuranol 3a (Table 1, entry 3). When
potassium pivalate was used, the yield was further im-
proved (Table 1, entry 4), and, in the case of cesium piv-
alate, benzofuranol 3a was a major product (Table 1,
entry 5).10 The heavy metal acetates were also efficient
additives to give benzofuran 2a as a sole product, and cop-
per acetate realized the excellent yield of 96% (Table 1,
entries 6 and 7).11 Between potassium pivalate and copper
acetate, the latter gave better results under the conditions
of lower catalyst loading (Table 1, entries 8 and 9). When
[Cp*RhCl2]2 was used in place of [Cp*IrCl2]2, the reac-
tion sluggishly proceeded to give product 2a in much low-
er yield (Table 1, entry 10).
[Cp*IrCl2]2 (5 mol% Ir)
Cu(OAc)2 (5 mol%)
or
[Cp*IrCl2]2 (10 mol% Ir)
AgOAc (20 mol%)
1a
n.r.
DCE, 5–8 h
room temperature
Scheme 4 Preparation of Ir(III) catalyst without AgSbF6
Me
O
Me
O
O
Ir
I+r
+
Me
Me
O
Cp*Ir(OAc)
1a
H
O
+
Me
Ir = Cp*Ir
– AcOH
O
O
O
Scheme 5 Proposed mechanism of hydrogen abstraction by acetate
ked cationic species along with stable counteranion
(SbF6) was important,12 therefore, we now assume that
monocationic iridium acetate is an efficient catalyst13 and
that acetate facilitates hydrogen abstraction,14 which real-
ized the reaction even at room temperature (Scheme 5).
Table 1 Screening of Various Carboxylates in the Ir(III)-Catalyzed
Cyclodehydration at Room Temperature
We next investigated the substrate scope under the reac-
tion conditions of entry 9 in Table 1 (Table 2). All reac-
tions proceeded with perfect regioselectivity to give 4-
acetylbenzofurans or -furanols.15 When bulky ketones
such as phenyl and tert-butyl ketones were submitted, the
reaction smoothly proceeded to give benzofurans 2b and
2c along with the formation of benzofuranols 3b and 3c
(Table 2, entries 1 and 2). 2,3,4-Trisubstituted benzo-
furans 2d and 2e were also obtained in high yields as sole
products, respectively (Table 2, entries 3 and 4). Cyclic
ketone 1f was converted into tricyclic compound 2f in
quantitative yield (Table 2, entry 5). When a methoxy
group was installed on the benzene ring, the correspond-
ing regioisomers were obtained in high to excellent yields
(Table 2, entries 6–8). Notably, the regioselective reaction
of ketone 1h, which has acetyl and methoxy groups at the
meta position, proceeded to give 4-acetylbenzofuran 2h as
a sole product, without the formation of 6-acetylbenzofu-
ran. These results mean that the C–H bond adjacent to the
acetyl group, not the methoxy group, was selectively
cleaved, which supports the directing effect of the acetyl
group. Elevated temperature of 60 °C was required, but it
is noteworthy that chloro-substituted aryloxy ketones 1j,
1k, and 1l were also transformed into benzofurans 2j, 2k,
and 2l in high yield (Table 2, entries 9–11). A more elec-
tron-withdrawing fluoro group could be also introduced
into the benzene ring of benzofurans (Table 2, entry 12).
Ac
Me
Ac
[Cp*IrCl2]2 (10 mol% Ir)
O
AgSbF6 (20 mol%)
2a
additive (10 mol%)
Me
+
O
DCE
r.t.
Ac
O
Me
O
1a
OH
3a
Entry Additive Time (h) Yield of 2a (%) Yield of 3a (%)
1
none
8
7
n.d.a
21
2
NaOAc
NaOPiv
KOPiv
CsOPiv
AgOAc
Cu(OAc)2
KOPiv
8
45
85
93
28
87
96
85
93
7
3
8
n.d.a
3
4
8
5
8
51
6
8
n.d.a
n.d.a
n.d.a
n.d.a
n.d.a
7
8
8b
9b
24
Cu(OAc)2 20
10b,c Cu(OAc)2 20
a Not detected.
Other than the acetyl group, the acetylamino group could
also operate as an efficient directing group, and 4-ami-
nobenzofuran 5 was obtained in excellent yield at room
temperature (Scheme 6).
b The amount of catalyst was halved: [Cp*IrCl2]2 (5 mol% Ir), AgSbF6
(10 mol%), additive (5 mol%).
c [Cp*RhCl2]2 was used in place of [Cp*IrCl2]2.
NHAc
NHAc
[Cp*IrCl2]2 (5 mol% Ir)
AgSbF6 (10 mol%)
Cu(OAc)2 (5 mol%)
Me
The combination of [Cp*IrCl2]2 and Cu(OAc)2 without
AgSbF6 showed almost no catalytic activity. Moreover,
when AgOAc was used in place of AgSbF6 for the forma-
tion of cationic Ir species, almost no reaction proceeded
(Scheme 4). These results mean that the existence of na-
Me
DCE, 24 h
r.t.
O
O
O
4
5 98%
Scheme 6 Amide-directed synthesis of benzofuran 5
Synlett 2011, No. 14, 2075–2079 © Thieme Stuttgart · New York

LETTER
Synthesis of Multisubstituted Benzofurans
2077
Table 2 Substrate Scope of Ir(III)-Catalyzed Cyclodehydration
Ac
R3
Ac
R3
Ac
[Cp*IrCl2]2 (5 mol% Ir)
R2
OH
AgSbF6 (10 mol%)
Cu(OAc)2 (5 mol%)
R3
R2
R2
+
O
R1
DCE, 8 h
r.t.
O
O
O
R1
R1
1
3
2
Entry
Substrate 1
Yield of 2 and 3 (%)
Ac
Ac
Ph
O
Ac
Ph
OH
1
2
3
Ph
O
O
O
3b 18
2b 65
1b
1c
1d
1e
1f
Ac
Ac
Ac
Ac
t-Bu
OH
Ac
t-Bu
t-Bu
O
O
O
O
O
O
3c 33
2c 51
Ac
Me
Me
Me
Me
O
O
2d 97
Ac
Me
p-anisyl
Me
4
5
p-anisyl
O
O
2e 87
Ac
Ac
O
O
O
2f >99
Ac
Ac
Me
2
3
5
Me
6
O
O
4
MeO
MeO
7
O
6
7
8
1g 2-OMe
1h 3-OMe
1i 4-OMe
2g 5-OMe, 91
2h 6-OMe, >99
2i 7-OMe, 91
Ac
Ac
Me
2
3
5
Me
6
O
O
Cl
4
7
Cl
O
9
1j 2-Cl
1k 3-Cl
1l 4-Cl
2j 5-Cl, 87
2k 6-Cl, 89
2l 7-C,l 89
10a
11a
Synlett 2011, No. 14, 2075–2079 © Thieme Stuttgart · New York

2078
T. Shibata et al.
LETTER
Table 2 Substrate Scope of Ir(III)-Catalyzed Cyclodehydration (continued)
Ac
R3
O
Ac
R3
Ac
[Cp*IrCl2]2 (5 mol% Ir)
R2
OH
R2
AgSbF6 (10 mol%)
Cu(OAc)2 (5 mol%)
R3
R2
+
O
R1
DCE, 8 h
r.t.
O
O
R1
R1
1
3
2
Entry
Substrate 1
Yield of 2 and 3 (%)
Ac
Ac
Me
12a
Me
F
O
O
F
O
2m 77
1m
a The reaction was examined at 60 °C.
4-Acetyl-6-chloro-3-methylbenzofuran (2k)
In conclusion, we developed Ir(III)-catalyzed cyclodehy-
dration of a-aryloxy ketones at room temperature. The
catalyst prepared from [Cp*IrCl2]2, AgSbF6, and
Cu(OAc)2 realized the mild reaction conditions, and vari-
ous multisubstituted benzofurans were obtained in high to
excellent yield with perfect regioselectivity. The precise
mechanistic study of the present catalysis and its applica-
tion for other reactions are under way in our laboratory.
White solid; mp 50–51 °C. 1H NMR (400 MHz, CDCl3): d = 2.27
(s, 3 H), 2.66 (s, 3 H), 7.48 (s, 1 H), 7.57 (s, 1 H), 7.61 (s, 1 H). 13
C
NMR (100 MHz, CDCl3): d = 11.3, 28.8, 115.4, 117.0, 124.1, 125.0,
128.9, 134.1, 144.8, 156.7, 198.5. IR (KBr disk): 1691, 1107, 721
cm–1. HRMS (FAB+): m/z calcd for C11H10ClO2: 209.0369 [M + H];
found: 209.0369.
1-(5-Acetyl-2-chlorophenoxy)propan-2-one (1l)
White solid; mp 74–75 °C. 1H NMR (400 MHz, CDCl3): d = 2.37
(s, 3 H), 2.58 (s, 3 H), 4.66 (s, 2 H), 7.41 (d, J = 2.3 Hz, 1 H), 7.51
(s, 1 H), 7.52 (d, J = 2.3 Hz, 1 H). 13C NMR (100 MHz, CDCl3):
d = 26.5, 26.8, 73.4, 111.8, 122.8, 128.7, 130.6, 136.8, 153.6, 196.5,
204.0. IR (KBr disk): 1720, 1670, 1219, 827 cm–1. HRMS (FAB+):
m/z calcd for C11H12ClO3: 227.0475 [M + H]; found: 227.0468.
In a Schlenk tube, [Cp*IrCl2]2 (2.1 mg, 2.5 mmol) and AgSbF6 (3.6
mg, 11 mmol) were placed under an atmosphere of argon and ace-
tone (0.5 mL) was added. Silver salt was precipitated, and the ace-
tone solution was transferred into another Schlenk tube via a
syringe filter for the removal of the precipitate. After acetone was
excluded under reduced pressure, and the Schlenk tube was back-
filled with argon (×3), a DCE solution (0.3 mL) of substrate 1 (0.1
mmol) and Cu(OAc)2 (1.0 mg, 6 mmol) was added. The reaction
mixture was stirred at r.t. for 8 h, then the crude products were pu-
rified by preparative TLC to give analytically pure benzofuran 2.
4-Acetyl-7-chloro-3-methylbenzofuran (2l)
White solid; mp 89–90 °C. 1H NMR (400 MHz, CDCl3): d = 2.31
(s, 3 H), 2.67 (s, 3 H), 7.33 (d, J = 12.2 Hz, 1 H), 7.56 (d, J = 12.2
Hz, 1 H), 7.56 (s, 1 H). 13C NMR (100 MHz, CDCl3): d = 11.5, 28.7,
118.1, 121.2, 123.4, 124.7, 127.9, 132.4, 144.9, 152.3, 198.7. IR
(KBr disk): 1684, 1099, 804 cm–1. HRMS (FAB+): m/z calcd for
C11H9ClO2: 208.0291 [M]; found: 208.0277.
1-(3-Acetyl-4-chlorophenoxy)propan-2-one (1j)
Pale yellow solid (mp 29–30 °C). 1H NMR (400 MHz, CDCl3): d =
2.28 (s, 3 H), 2.66 (s, 3 H), 4.58 (s, 2 H), 6.94 (dd, J = 3.2, 8.8 Hz,
1 H), 7.05 (d, J = 3.2 Hz, 1 H), 7.34 (d, J = 8.8 Hz, 1 H). 13C NMR
(100 MHz, CDCl3): d = 26.5, 30.7, 73.1, 115.0, 118.6, 123.8, 131.8,
139.9, 156.4, 199.9, 204.0. IR (KBr disk): 1718, 1668, 1178, 889
cm–1. HRMS (FAB+): m/z calcd for C11H12ClO3: 227.0475 [M + H];
found: 227.0468.
1-(3-Acetyl-5-fluorophenoxy)propan-2-one (1m)
Colorless oil. 1H NMR (400 MHz, CDCl3): d = 2.30 (s, 3 H), 2.58
(s, 3 H), 4.62 (s, 2 H), 6.81–6.85 (m, 1 H), 7.26–7.29 (m, 2 H). 13
C
NMR (100 MHz, CDCl3): d = 26.5, 26.6, 73.1, 107.2 (d, J = 25.6
Hz, 1 C), 108.8 (d, J = 22.3 Hz, 1 C), 109.6 (d, J = 3.3 Hz, 1 C),
139.7 (d, J = 8.2 Hz, 1 C), 159.1 (d, J = 9.1 Hz, 1 C), 163.4 (d,
J = 247.9 Hz, 1 C), 196.2 (d, J = 2.5 Hz, 1 C), 203.5. IR (neat):
1689, 1593, 1144, 858 cm–1. HRMS (FAB+): m/z calcd for
C11H11FO3: 211.0770 [M]; found: 211.0731.
4-Acetyl-5-chloro-3-methylbenzofuran (2j)
Colorless oil. 1H NMR (400 MHz, CDCl3): d = 2.08 (s, 3 H), 2.68
(s, 3 H), 7.25 (d, J = 8.8 Hz, 1 H), 7.40 (d, J = 8.8 Hz, 1 H), 7.45 (s,
1 H). 13C NMR (100 MHz, CDCl3): d = 9.1, 32.3, 113.2, 115.0,
122.9, 125.1, 125.9, 133.7, 144.0, 154.2, 202.2. IR (neat): 1707,
1234, 797 cm–1. HRMS (FAB+): m/z calcd for C11H10ClO2:
209.0369 [M + H]; found: 209.0405.
4-Acetyl-6-fluorobenzofuran (2m)
White solid; mp 43–44 °C. 1H NMR (500 MHz, CDCl3): d = 2.56
(s, 3 H), 2.65 (s, 3 H), 7.32 (d, JH = 2.3 Hz, JF = 7.9 Hz, 1 H), 7.34
(d, JH = 2.3 Hz, JF = 9.9 Hz, 1 H), 7.48 (s, 1 H). 13C NMR (100
MHz, CDCl3): d = 11.3, 28.8, 98.3, 102.8 (d, J = 20.9 Hz, 1 C),
111.7 (d, J = 24.8 Hz, 1 C), 116.8, 122.7, 133.8 (d, J = 7.1 Hz, 1 C),
144.7 (d, J = 4.1 Hz, 1 C), 159.3 (d, J = 243.0 Hz, 1 C), 198.6. IR
(KBr disk): 1712, 1274, 764 cm–1. HRMS (FAB+): m/z calcd for
C11H9FO2: 192.0587 [M]; found: 192.0607.
1-(3-Acetyl-5-chlorophenoxy)propan-2-one (1k)
White solid; mp 92–93 °C. 1H NMR (400 MHz, CDCl3): d = 2.29
(s, 3 H), 2.57 (s, 3 H), 4.62 (s, 2 H), 7.11 (s, 1 H), 7.34 (s, 1 H), 7.55
(s, 1 H). 13C NMR (100 MHz, CDCl3): d = 26.5, 26.6, 72.9, 111.9,
119.8, 122.1, 135.5, 139.4, 158.5, 196.2, 203.5. IR (KBr disk):
1716, 1683, 1068, 750 cm–1. HRMS (FAB+): m/z calcd for
C11H11ClO3: 226.0397 [M]; found: 226.0393.
Synlett 2011, No. 14, 2075–2079 © Thieme Stuttgart · New York

LETTER
Synthesis of Multisubstituted Benzofurans
2079
(6) We already excluded a Friedel–Crafts mechanism by
Acknowledgment
competitive experiment of 4-(3-acetylphenyl)butan-2-one
(1a) and 4-(3-methoxyphenyl)butan-2-one, see Supporting
Information of ref. 4 in detail.
This research was supported by Grant-in-Aid for Scientific Re-
search on Innovative Areas, ‘Molecular Activation Directed toward
Straightforward Synthesis’, MEXT, Japan, and by a Waseda Uni-
versity Grant for Special Research Projects.
(7) Dicationic Pd complex as an efficient catalyst for C–H bond
activation: Nishikata, T.; Abela, A. R.; Huang, S.; Lipshutz,
B. H. J. Am. Chem. Soc. 2010, 132, 4978.
(8) Preparation and characterization of dicationic Cp*Ir
complex: (a) White, C.; Thompson, S. J.; Maitlis, P. M.
J. Organomet. Chem. 1977, 127, 415. (b) Amouri, H.;
Guyard-Duhayon, C.; Vaissermann, J.; Rager, M. N. Inorg.
Chem. 2002, 41, 1397.
(9) Base suppressed dehydration from benzofuranols to
benzofurans: (a) Liu, G.; Lu, X. J. Am. Chem. Soc. 2006,
128, 16504. (b) Liu, G.; Lu, X. Tetrahedron 2008, 64, 7324.
(10) The effect of inorganic salts for the ratio of benzofuran 2a
and benzofuranol 3a is unclear.
(11) In recent years, a stoichiometric or more amounts of
Cu(OAc)2 was often used as an oxidant in C–H activation
using Rh catalyst prepared from [Cp*RhCl2]2 and AgSbF6:
(a) Stuart, D. R.; Bertrand-Laperle, M.; Burgess, K. M. N.;
Fagnou, K. J. Am. Chem. Soc. 2008, 130, 16474.
(b) Guinond, N.; Fagnou, K. J. Am. Chem. Soc. 2009, 131,
12050. (c) Rakshit, S.; Patureau, F. W.; Glorius, F. J. Am.
Chem. Soc. 2010, 132, 9585. (d) Patureau, F. W.; Glorius, F.
J. Am. Chem. Soc. 2010, 132, 9982. (e) Stuart, D. R.;
Alsabeh, P.; Kuhn, M.; Fagnou, K. J. Am. Chem. Soc. 2010,
132, 18326. (f) Patureau, F. W.; Besset, T.; Glorius, F.
Angew. Chem. Int. Ed. 2011, 50, 1064. (g) Tsai, A. S.;
Brasse, M.; Bergman, R. G.; Ellman, J. A. Org. Lett. 2011,
13, 540.
(12) Other silver salts, such as AgOTf and AgBF4, gave poorer
results than AgSbF6.
(13) Iridium dicarboxylate complex, which was prepared from
[Cp*IrCl2]2 (5 mol% Ir), AgSbF6 (10 mol%), and KOPiv (10
mol%), had almost no catalytic activity.
(14) (a) Davies, D. L.; Al-Duaji, O.; Fawcett, J.; Giardiello, M.;
Hilton, S. T.; Russell, D. R. Dalton Trans. 2003, 4132.
(b) Davies, D. L.; Donald, S. M. A.; Al-Duaij, O.;
Macgregor, S. A.; Polleth, M. J. Am. Chem. Soc. 2006, 128,
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Chem. Soc. 2008, 130, 12414. (d)Li, L.; Brennessel, W. W.;
Jones, W. D. Organometallics 2009, 28, 3492.
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Synlett 2011, No. 14, 2075–2079 © Thieme Stuttgart · New York

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