Chemoselectivity control: Gold(I)-catalyzed synthesis of 6,7-dihydrobenzofuran-4(5H)-ones and benzofurans from 1-(alkynyl)-7- oxabicyclo[4.1.0]heptan-2-ones

Article abstract of DOI:10.1002/chem.201301698

New and chemoselective gold(I)-catalyzed transformations of 1-(arylethynyl)-7-oxabicyclo[4.1.0]- heptan-2-ones were developed. Two completely different products - 6,7-dihydrobenzofuran-4(5H)-ones and benzofurans - could be obtained from the same starting material. The selectivity is determined by the ligand of the gold catalyst: triphenylphosphine delivers 6,7-dihydrobenzofuran-4(5H)-ones, and 1,3-bis(diisopropylphenyl)imidazol-2- ylidene leads to benzofurans. Eleven examples of each case are provided. The mechanistic suggestions for the pathways to both product types are supported by isotope labeling experiments. Complete chemoselectivity control of gold(I)-catalyzed transformations of 1-(arylethynyl)-7-oxabicyclo[4.1.0]heptan- 2-ones, which can be achieved by changing the ligand of the gold catalyst, allows two different products to be obtained from the same starting material, namely, 6,7-dihydrobenzofuran-4(5H)-ones and benzofurans (see scheme), both of which are potent building blocks for synthetic chemistry. IPr: 1,3-bis(diisopropylphenyl)imidazol-2-ylidene. Copyright

Full text of DOI:10.1002/chem.201301698

DOI: 10.1002/chem.201301698
Chemoselectivity Control: Gold(I)-Catalyzed Synthesis of 6,7-
Dihydrobenzofuran-4(5H)-ones and Benzofurans from
1-(Alkynyl)-7-oxabicyclo AHCTNUGTERNNUN[G 4.1.0]heptan-2-ones
[
a]
[a]
[a]
[a]
[a]
Tao Wang, Shuai Shi, Mie Højer Vilhelmsen, Tuo Zhang, Matthias Rudolph,
[
a]
[a, b]
Frank Rominger, and A. Stephen K. Hashmi*
Abstract: New and chemoselective
gold(I)-catalyzed transformations of
is determined by the ligand of the gold
catalyst: triphenylphosphine delivers
6,7-dihydrobenzofuran-4(5H)-ones, and
1,3-bis(diisopropylphenyl) AHCTUNGTERNNUNiG midazol-2-
ylidene leads to benzofurans. Eleven
examples of each case are provided.
The mechanistic suggestions for the
pathways to both product types are
supported by isotope labeling experi-
ments.
1-(arylethynyl)-7-oxabicyclo AHCTUNGTERNNUN[G 4.1.0]-
heptan-2-ones were developed. Two
completely different products—6,7-di-
hydrobenzofuran-4(5H)-ones and ben-
zofurans—could be obtained from the
same starting material. The selectivity
Keywords: annulation · chemose-
lectivity · gold · ligand effects · syn-
thetic methods
Introduction
ACHTUNGTRENNUNG[ 4.1.0]heptane substrates to afford mono-C4-substituted 4,5-
[4]
dihydrobenzo[b]furans [Scheme 1, Eq. (1)]. In this reac-
tion, a carbon–carbon triple bond was attacked by the
oxygen atom of a carbonyl group. Zhang et al. and others
reported on the CÀC bond cleavage of epoxide motifs
It is well known that reaction pathways can be redirected by
changing reaction parameters such as temperature, solvent,
and catalyst. In transition-metal-catalyzed organic transfor-
mations, ligands play an important role in the control of
chemoselectivity, regioselectivity, and enantioselectivity. In
the past decades, gold-catalyzed transformations have
become an useful tool in organic synthesis, and enormous
[5]
[Scheme 1, Eq. (2)]. In these reactions, a carbonyl group
and an epoxide moiety are installed in the same starting ma-
terial. In analogy to the reactions with a cyclopropane unit,
the carbonyl oxygen atom attacks the triple bond and then
affords different products. We envisioned that the reaction
pathway might be redirected by changing the reaction con-
ditions, for example, by ligand variations. As shown in
Scheme 1, by controlling the initial nucleophilic attack it
should be possible to get two different products from one
starting material. Attack of the carbonyl oxygen atom on
the triple bond in the presence of a nucleophile could deliv-
[1]
progress has been made in this field. Considering its great
importance for organic synthesis, it is essential to study the
influence of ligands in gold catalysis. In 2008, Toste et al. re-
viewed ligand effects in homogeneous gold catalysis and the
[2]
development of ligands in gold catalysis. As part of our ef-
[
3]
forts in gold catalysis, we herein document gold-catalyzed
chemoselective reactions which enable the preparation of
different products from the same starting materials by varia-
tion of the ligands.
Recently, Sarpong and co-workers reported on
a
W(CO) ·THF-catalyzed cycloisomerization of bicyclo-
5
[
a] T. Wang, S. Shi, Dr. M. H. Vilhelmsen, T. Zhang, Dr. M. Rudolph,
Dr. F. Rominger, Prof. Dr. A. S. K. Hashmi
+
Organisch-Chemisches Institut
Ruprecht-Karls-Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax : (+49)6221-54-4205
E-mail: hashmi@hashmi.de
[
b] Prof. Dr. A. S. K. Hashmi
Chemistry Department, Faculty of Science
King Abdulaziz University
Jeddah 21589 (Saudi Arabia)
+
[
] Crystallographic investigation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301698.
Scheme 1. Previous work and our hypothesis.
12512
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 12512 – 12516

FULL PAPER
[
a,b]
er product 2, a valuable pre-
cursor for a subsequent iso-
merization reaction to give
Table 1. Optimization of reaction conditions.
substituted
benzofurans
[Scheme 1, Eq. (3)]. Initial
attack of the epoxide oxygen
atom would afford 6,7-dihy-
drobenzofuran-4(5H)-ones,
[
c]
[c]
[c]
[c]
Entry
Catalyst (5 mol%)
CH
3
OH
Solvent
Acid
(2 equiv)
t
1a
3a
4a
5a
A
H
U
T
N
U
G
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
[%]
[%]
[%]
[%]
which, like benzofurans, are
important motifs in natural
products and building blocks
for the pharmaceutical chemis-
try (for representative exam-
1
2
3
4
5
6
7
8
9
AuCl
AuCl
3
3
–
CH
CH
CH
CH
CH
CH
CH
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
2
2
2
2
2
2
–
–
–
–
–
–
–
–
–
–
–
–
–
120
30
5
20
30
600
600
20
20
10
10
10
120
120
5
95
–
–
–
–
–
–
–
–
–
–
–
95
92
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2.0
2.0
2.0
2.0
–
44
60
45
39
<5
49
65
60
83
86
85
–
trace
16
17
20
–
A
H
U
G
R
N
U
G
3
3
3
3
3
3
3
3
3
3
P)AuCl/AgOTf
P)AuCl/AgNTf
A
H
U
G
R
N
N
2
A
H
U
G
R
N
U
G
P)AuCl/AgSbF
P)AuCl/AgOTf
P)AuCl/AgOTf
P)AuCl/AgOTf
P)AuCl/AgOTf
P)AuCl/AgOTf
P)AuCl/AgOTf
P)AuCl/AgOTf
6
[6,7]
ples, see Scheme 2).
A
H
N
R
N
N
[
d]
A
H
U
G
R
N
U
G
2.0
2.0
2.0
2.0
0.2
–
–
A
H
U
G
E
N
N
toluene
THF
trace
trace
–
A
T
G
R
N
G
Results and Discussion
10
11
12
13
14
15
A
H
U
T
N
U
G
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
3
3
3
3
3
2
2
2
2
2
2
CN
CN
CN
CN
CN
A
T
N
R
N
N
–
–
–
–
78
63
65
A
H
U
T
N
U
G
We started our study by treat-
AgOTf
AgOTf
–
ing
oxabicyclo
1-(phenylethynyl)-7-
ACHTUNGTRENNUNG[ 4.1.0]heptan-2-one
0.2
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
–
–
–
–
–
–
–
–
A
H
U
G
R
N
U
G
Cl
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
2
–
–
–
(
1a) with AuCl in CH Cl at
3 2 2
16
17
A
H
U
G
R
N
N
(IPr)AuCl/AgNTf
2
5
5
room temperature (Table 1,
entry 1). However, after 2 h no
conversion could be detected.
Methanol was then added and
A
H
U
G
R
N
N
(IPr)AuCl/AgSbF
6
–
[
[
f]
f]
[g]
18
19
20
21
A
H
U
G
E
U
G
PTSA
MsOH
5+10
5+10
5+10
5+10
30
–
–
–
–
50
–
n.d.
n.d.
n.d.
n.d.
51
trace
68
20
–
[g]
A
T
N
T
E
N
N
[
e]
[f]
[f]
[g]
A
H
U
G
R
N
N
HCl
HCl
–
[e]
[g]
A
H
N
R
N
N
toluene
6,7-dihydrobenzofuran-4(5H)-
22
A
H
N
T
E
N
G
CH CN
–
–
30
3
[
e]
[f]
[g]
23
A
H
U
G
R
N
U
G
CHCl
3
HCl
5+10
n.d.
77
one (3a) was obtained in mod-
erate yield (Table 1, entry 2).
[a] For more reaction conditions tested, see Supporting Information. [b] The reactions were carried out on a
.4 mmol scale in 2 mL of solvent. [c] Yield of isolated product. [d] 2.0 equiv of H O were added instead of
MeOH. [e] 37% HCl aq. [f] Reaction time for the second step. [g] Not detected.
0
2
Next we examined (Ph P)AuCl
3
in combination with different
silver salts as halide scavenger
(
Table 1, entries 3–5). Among these (Ph P)AuCl in combina-
as ligand, the desired change in chemoselectivity was ach-
ieved, and 4a was mainly obtained as product (Table 1,
entry 15; compounds 4 were always obtained as 1:1 mixtures
of diastereomers). Like in the case of the phosphine ligands,
AgOTf was the most efficient silver salt (Table 1, entries 15–
17). Product 4a could be easily transformed into the more
valuable benzofuran 5a in the present of an acid in a one-
pot procedure. Further investigations indicated that HCl
3
tion with AgOTf gave the best result (Table 1, entry 3). The
reaction yielded traces of 3a without MeOH (Table 1,
entry 6) and proceeded smoothly to afford 3a in moderate
yield on replacing MeOH with H O. However, water as ad-
ditive resulted in a much longer reaction time (Table 1,
2
entry 7). Further studies indicated that CH CN was the best
3
solvent (Table 1, entry 10) and that MeOH was not essential
for this coordinating solvent (Table 1, entry 12). Control ex-
periments with a silver salt in the absence of a gold catalyst
showed no conversion no matter whether MeOH was pres-
ent or not (Table 1, entries 13 and 14). When PPh was re-
placed by 1,3-bis(diisopropylphenyl)imidazol-2-ylidene (IPr)
was the most suitable acid (Table 1, entry 20) and CHCl the
3
best solvent for this transformation (Table 1, entry 23).
With the optimized conditions in hand, we first examined
the scope of the reaction for the synthesis of 6,7-dihydro-
benzofuran-4(5H)-ones. However, when the reactions were
carried out without MeOH, complete conversion was not
detected after 4 h (Scheme 3, 3b and 3c). Thus, 0.2 equiva-
lents of MeOH were used in addition to the standard condi-
tions. As shown in Scheme 3, the substituents at the acety-
lenic terminus could be aryl or alkyl groups. Aromatic sub-
stituents usually afforded the corresponding products in
good yields. Substrate 1d, bearing a strongly electron donat-
ing group, was an exception. It delivered only a moderate
yield of product accompanied by a long reaction time
[8]
3
(
Scheme 3, 3d). When the aromatic alkynes were replaced
by alkyl alkynes, the corresponding products were only ob-
tained in moderate yields (Scheme 3, 3j and 3k).
Scheme 2. Representative examples of the importance of the addressed
structures: tyrolobibenzyls and the synthesis of angelicins.
Chem. Eur. J. 2013, 19, 12512 – 12516
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12513

A. S. K. Hashmi et al.
[
a]
Scheme 4. Scope of the one-pot benzofuran synthesis. Reaction time
for the first/second step.
Scheme 3. Scope of the reaction for the synthesis of 6,7-dihydrobenzofur-
an-4(5H)-ones. The reactions were carried out without MeOH for 4 h
[
a]
at room temperature.
The one-pot transformation into benzofurans was also ex-
amined (Scheme 4). In most cases, substrates with aryl-sub-
stituted alkyne groups showed high selectivity and afforded
the desired benzofurans in moderate to good yields. In ac-
cordance with the above results, a lower yield and incom-
plete conversion were observed with an electron-donating
methoxyl group at the arene (Scheme 4, 5d). Substrates
bearing a naphthyl group or alkyl groups also showed low
selectivity and partly decomposed under standard conditions
(
Scheme 4, 5i–5k).
To obtain an unambiguous structural assignment for a
compound 4, 4 f was oxidized to ketone 6, a solid that al-
lowed us to grow single crystals suitable for an X-ray dif-
fraction analysis. The solid-state molecular structure of 6 is
Figure 1. a) Transformation of 4 f into 6 and b) molecular structure of 6
in the solid state.
[9]
depicted in Figure 1.
plausible pathways are reasonable (Scheme 6). On path-
To investigate the mechanism of the transformations, deu-
terated substrate 1 f’ was prepared and converted under the
two reaction conditions; 3 f’ was obtained in 80% yield and
with complete preservation of the labeling in the position a
to the carbonyl group [Scheme 5, Eq. (1)]. Treatment of 1 f’
with (IPr)AuOTf and three equivalents of MeOH in CHCl3
for 10 min afforded 4 f’ in 83% yield, in which part of the
deuterium had migrated to the hydroxyl group and to the 2-
position of the furan [Scheme 5, Eq. (2)]. Compound 4 f’
was subsequently treated with HCl, and deuterated benzo-
furan 5 f’ was obtained in 90% yield [Scheme 5, Eq. (3)]. By
way I, Ph P acts as the ligand. The epoxide coordinates to
3
the catalyst and is attacked by MeOH, which affords inter-
mediate A and/or intermediate B. After cyclization and sub-
sequent elimination, the final product 3 is obtained. The
second pathway can be addressed by changing the ligand
from Ph P to IPr. On this pathway, the triple bond coordi-
3
nates to the activated catalyst but, instead of the epoxide
opening, the carbonyl oxygen atom acts as a nucleophile
which generates intermediate C. A subsequent 1,2-hydride
shift then affords vinyl gold intermediate D. In the presence
of MeOH, the catalyst is regenerated and, at the same time,
intermediate E is formed, which subsequently undergoes re-
1
8
treating 1a with H2 O in the present of (PPh )AuOTf, 3a
3
1
8
[11]
was obtained with 40% O labeling [Scheme 5, Eq. (4)].
According to the results of the labeling experiments to-
arrangement to compound 4. After aromatization by elim-
ination of MeOH and H O, benzofuran 5 is obtained as
2
[10]
gether with related transformation in the literature, two
final product.
12514
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ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 12512 – 12516

Synthesis of 6,7-Dihydrobenzofuran-4(5H)-ones and Benzofurans
FULL PAPER
to coordinating to the softer
triple bond. This mode leads to
furan product 3a (Table 2, en-
tries 1 and 2), while soft acids
(
electron-donating
mainly afford product 4a
which is transformed into 5a
ligands)
(
under the reaction condition)
by activating the triple bond
through
p activation rather
than epoxide opening (Table 2,
entry 3). 2) All of the P-coordi-
nated gold catalysts, including
that with a secondary phos-
[13]
phine oxide, tend to be less
alkynophilic, and therefore ep-
oxide opening is the initial re-
action step, which delivers
mainly furan product 3a
(
Table 2, entries 4–11), while
C-coordinated gold catalysts
tend to activate the triple bond
as initial reaction step, which
leads to 4a as main product
1
8
Scheme 5. Deuterium labeling experiments. D/H ratio was determined by NMR analysis and O by HRMS
analysis.
(
Table 2, entries 12–16). 3) In
line with the above observations, among the P-coordinated
gold catalysts, electron-deficient ones show high selectivity
for the furan synthesis (Table 2, entries 8, 10, and 11), while
electron-rich ones deliver only a mixture of the two com-
pounds (Table 2, entries 4–7). The only exception is the tris(-
pentafluorophenyl)phosphine (L6)-ligated gold catalyst,
which gave a mixture of 3a and 4a in a ratio of 1/0.8 despite
its electron-withdrawing nature (Table 2, entry 9). This
might be rationalized by the special effect of fluoride. The
ratio of 3a to 4a (which was transformed into 5a under the
reaction conditions) was dramatically increased by addition
of catalytic amounts of p-toluenesulfonic acid in combina-
tion with the IPr ligand (Table 2, entry 16). The added
Brønsted acid can activate the epoxide, which changes the
3
a/4a ratio significantly. This further indicates that, in the
absence of an additionally added acid, (IPr)AuOTf tends to
active the triple bond in preference to the epoxide.
Conclusion
We have developed new chemoselective gold(I)-catalyzed
transformations
of
1-(arylethynyl)-7-oxabicyclo ACHTUNGTRENNUNG[ 4.1.0]-
heptan-2-ones. Two different products can be obtained from
the same starting material, namely, 6,7-dihydrobenzofuran-
Scheme 6. Plausible pathways for the transformations.
4
(5H)-ones and benzofurans, both of which are potent build-
[12]
To gain insight into the ligand properties, different li-
gands were then examined. The ratios of 3a to 4a in de-
pendence on the ligands are summarized in Table 2. Several
trends can be readily identified: 1) electron-withdrawing li-
gands decrease the electron density at the metal center, and
the thus-formed harder Lewis acids prefer epoxide opening
ing blocks for synthetic chemistry. In this intramolecular
competition the two potential nucleophiles are the carbonyl
and epoxide oxygen atoms. Changing the ligand of the gold
catalyst enables complete control of the selectivity, which is
an important aspect for selectivity control in homogeneous
gold catalysis.
Chem. Eur. J. 2013, 19, 12512 – 12516
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12515

A. S. K. Hashmi et al.
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[
3
[
a]
[b]
Entry Catalyst (5 mol%)
Conversion NMR
3a/4a
[
b]
[%]
yield
3a+4a)
(
[
b]
[%]
[c]
1
AuCl
3
(5 mol%)/
100
75
>1/0.01
5
0, 5762–5765; d) A. S. K. Hashmi, A. M. Schuster, F. Rominger,
AgOTf (15 mol%)
AuCl/AgOTf
AuCl
P)AuCl/AgOTf
Cy PAuCl/AgOTf
L3AuCl/AgOTf
L4AuCl/AgOTf
L5AuCl/AgOTf
L6AuCl/AgOTf
L7AuCl/AgOTf
L8AuCl/AgOTf
L9AuCl/AgOTf
L10AuCl/AgOTf
(IPr)AuCl/AgOTf
(IMes)AuCl/AgOTf
(IPr)AuCl/AgOTf/
[
c]
Angew. Chem. 2009, 121, 8396–8398; Angew. Chem. Int. Ed. 2009,
48, 8247–8249; e) A. S. K. Hashmi, M. Rudolph, J. Huck, W. Frey, J.
Bats, M. Hamzi c´ , Angew. Chem. 2009, 121, 5962–5966; Angew.
Chem. Int. Ed. 2009, 48, 5848–5852.
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
60
30
30
25
91
88
88
76
95
73
80
70
10
75
90
78
88
1/0.08
[
e]
1/1.9
1/0.3
1/0.3
1/0.8
1/0.4
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(Ph
3
100
100
100
100
100
100
100
100
3
[
[
4] E. L. Fisher, S. M. Wilkerson-Hill, R. Sarpong, J. Am. Chem. Soc.
2012, 134, 9946–9949.
5] a) T. Wang, J. Zhang, Chem. Eur. J. 2011, 17, 86–90; b) T. Wang, C.-
H. Wang, J. Zhang, Chem. Commun. 2011, 47, 5578–5580; c) F. Hu,
J. Yan, M. Cheng, Y. Hu, Chem. Asian J. 2013, 8, 482–487.
[
c]
1/0.04
1/0.8
1/0.01
>1/0.01
1/3.9
1/2
[
[
c]
c]
0
1
2
3
4
5
6
[
f]
[c]
c]
[6] For isolation of tyrolobibenzyls and investigations on their biological
activity, see: a) C. Zidorn, E.-P. Ellmerer-Müller, H. Stuppner, Helv.
Chim. Acta 2000, 83, 2920–2925; b) C. Zidorn, R. Spitaler, E.-P. Ell-
merer-Müller, N. B. Perry, C. Gerhꢀuser, H. Stuppner, Z. Natur-
forsch. C 2002, 57, 614–619.
[
40
[
c]
100
100
100
100
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
<1/40
1/3
[
g]
[c]
ACHTUNGTRENNUNG
[
e]
[7] For the application of 6,7-dihydrobenzofuran-4(5H)-ones in synthe-
sis of angelicins, see: T. Kitamura, K. Otsubo, J. Org. Chem. 2012,
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
1/6.4
[
d]
PTSA
7
7, 2978–2982.
8] Silver salts can catalyze the reaction with or without MeOH in
CH Cl with lower yields. See Supporting Information for more re-
[
a] Ligands L3–L10.
[
2
2
action conditions examined.
[
9] CCDC-936447 (6) contains the supplementary crystallographic data
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[
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2
2
2
1
Acknowledgements
T.W., S.S., and T.Z. are grateful for fellowships from China Scholarship
Council (CSC). M.H.V. acknowledges the Danish Council for Independ-
ent Research, Natural Sciences for funding. The authors thank Umicore
AG & Co. KG for the generous donation of gold salts.
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