Gold(I)-Catalyzed Cascade Cyclization Reactions of Allenynes for the Synthesis of Fused Cyclopropanes and Acenaphthenes

Article abstract of DOI:10.1002/anie.201903384

A gold-catalyzed reaction of phenylene-tethered allenynes with benzofurans gave 1-(naphth-1-yl)cyclopropa[b]benzofuran derivatives, whereas the reaction of 1-allenyl-2-ethynyl-3-methylbenzene derivatives in the absence of benzofurans gave acenaphthenes in good yields. These results can be rationalized by nucleophilic attack of the alkyne moiety on an activated allene to form a vinyl cation intermediate.

Full text of DOI:10.1002/anie.201903384

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Accepted Article
Title: Gold(I)-Catalyzed Cascade Cyclization Reactions of Allenynes for
the Synthesis of Fused Cyclopropanes and Acenaphthenes
Authors: Takaya Ikeuchi, Shinsuke Inuki, Shinya Oishi, and Hiroaki
Ohno
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903384
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10.1002/anie.201903384
Angewandte Chemie International Edition
COMMUNICATION
Gold(I)-Catalyzed Cascade Cyclization Reactions of Allenynes for
the Synthesis of Fused Cyclopropanes and Acenaphthenes
Takaya Ikeuchi, Shinsuke Inuki, Shinya Oishi, Hiroaki Ohno*
Abstract: Gold-catalyzed reaction of phenylene-tethered allenynes
with benzofuran gave 1-(naphth-1-yl)cyclopropa[b]benzofuran
acenaphtenes 4 without forming 2 or 3. In this paper, we report
gold-catalyzed cyclizations of allenynes to form naphthalene-
fused cyclopropanes 2 and acenaphthenes 4, depending on the
substrate structure.
a
derivatives, whereas the reaction of 1-allenyl-2-ethynyl-3-
methylbenzene derivatives in the absence of benzofuran gave
acenaphthenes in good yields. These results can be rationalized by
nucleophilic attack of the alkyne moiety to an activated allene to form
a vinyl cation intermediate.
(A) Allylic cation formation and nucleophilic attack/deprotonation
R1
Ph
Ph
[Au+]
R2
( )n
or
or
•
R3
Gold catalysis has emerged as a powerful tool for electrophilic
activation of carbon–carbon multiple bonds to promote
nucleophilic reactions under mild conditions.[1] Allenes are well
known as versatile building blocks for the construction of cyclic
compounds by gold-catalyzed reactions.[2] Intramolecular
reaction of allenes with a heteronucleophile such as nitrogen,
oxygen, or sulfur derivatives are highly useful for the synthesis
of a variety of heterocyclic compounds, including dihydropyrroles,
dihydrofurans, and dihydrothiophenes. Allenenes undergo
various types of carbocyclizations including [2 + 2]-,[3] [2 + 3]-,[4]
and [4 + 3]-type reactions,[5] proceeding through formation of
alkyl cation species. However, the gold-catalyzed cyclization of
allenynes is relatively undeveloped. Two reaction modes are
possible for gold-catalyzed intramolecular carbocyclization of
allenynes: (1) the allene functions as a nucleophile to form an
allylic cation species (Scheme 1A)[6] and (2) the alkyne functions
as a nucleophile to form a vinyl cation species (Scheme 1B).[7] In
both cases, the reactions are terminated by nucleophilic attack
or deprotonation. It should be noted that a gold(I)-catalyzed
cascade cyclization of allenynes by contiguous formation of
carbon–carbon bonds via vinyl cation formation has not yet been
achieved.[8]
HO
(n = 0)
Me
(n = 1)
Ph
(n = 0 or 1)
(n = 1)
(B) Vinyl cation formation and nucleophilic attack
Ph
O
H2O
Ph
R1
R2
Ph
Me
Ph
Me
[Au+]
[Au]
( )n
X
(n = 2)
Ph
•
R3
Me
Me
O
Me
Me
R4
(n = 2 or 0)
H2O
Ph
nBu
nBu
(n = 0)
[Au]
(C) This work: vinyl cation formation and carbon
–
carbon bond formation
X
R2
R1
R2
R2
R1
[Au+]
•
[Au]
(R1 = H)
1
3
A
1' (R1 = Me)
X
R2
X
R2
Recently,
we
investigated
gold-catalyzed
cascade
R1 = Me
R1 = H
cyclizations of substrates bearing several alkynes.[9] As a part of
an ongoing program directed toward the development of efficient
carbocyclization reactions, we designed a carbocyclization of
allenynes that terminates with a carbon–carbon bond formation.
Our approach is shown in Scheme 1C. Activation of allenyne 1
would generate vinyl cation intermediate A[10] via nucleophilic
attack of the alkyne moiety on the activated allene. Intermediate
A would then be trapped by a heteroarene such as benzofuran
to give naphthalene derivative 2 or 3. During the course of this
study, we found that the 3-methyl congener 1’ underwent a
second intramolecular carbon–carbon bond formation to give
4
2
Scheme 1. Related research and this work
We chose allenyne 1a as the model substrate and
investigated the reaction with benzofuran 5a as the nucleophile
(Table 1). The reaction of 1a and 5a with
5 mol %
JohnPhosAuCl/AgNTf2 in dichloromethane (DCM) at room
temperature gave fused cyclopropane 2aa bearing a naphthyl
group in 47% yield as the sole stereoisomer (entry 1). The NOE
analysis of 2aa revealed that the naphthyl group is located at the
convex face of the fused cyclopropane (see the Supporting
information). Among the catalysts examined (entries 2–12),
BrettPhosAuNTf2 provided the highest yield of 2aa (63%, entry
12). Alternative solvents, such as toluene, MeCN, THF, and
iPrOH were not effective, nor was the addition of HFIP or iPrOH
as a proton source (Table S1 in the Supporting information). As
a result of further investigations into the substrate concentration
and reaction temperature (Table S1), the reaction proceeded
[*]
T. Ikeuchi, Dr. S. Inuki, Prof. Dr. S. Oishi, Prof. Dr. H. Ohno
Graduate School of Pharmaceutical Sciences
Kyoto University
Sakyo-ku, Kyoto 606-8501 (Japan)
Fax: (+81) 75-753-4570
E-mail: hohno@pharm.kyoto-u.ac.jp
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201903384.
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10.1002/anie.201903384
Angewandte Chemie International Edition
COMMUNICATION
most effectively at 0.5 M and 0 °C (entry 13) to give 2aa in 78%
isolated yield on a 1.0 mmol scale.
nucleophile to provide the corresponding cyclopropane-fused
indoles 2ad (74%) and 2ae (79%), respectively. Unfortunately,
the use of benzothiophene in the reaction gave 2af in poor yield
(21%).[12] Next, the scope of the allenynes 1 was examined.
Substrates 1b, 1f and 1j bearing a cyano group on the
phenylene tether only produced, if any, a small amount of the
desired products (2ba, 2fa, and 2ja),[13] while allenynes 1e and
1i substituted with an electron-withdrawing chloro substituent
were smoothly transformed to the fused cyclopropanes 2ea and
2ia in high yields (93–94%). Electron-donating substituents on
the phenylene tether (Me and OMe) at the m- or p-position to the
alkyne were tolerated in the reaction, although a small decrease
in yields was observed in some cases (2ca, 2da, 2ga and 2ha;
54–81%). Additionally, when using allenynes bearing a substi-
tuent (p-OMe, p-Cl, or o-Me) on the phenyl group at the alkyne
terminus, the desired products (2ka–2ma) were obtained in
good yields. When a methyl group was present at the o-position
of the alkyne, change of reaction mode was observed (no
formation of cyclopropane 2’aa but of acenaphthene, vide infra).
Table 1. Optimization of the reaction conditions for cyclopropanation
O
H
Ph
H
O
Ph
catalyst (5 mol %)
+
•
DCM (0.2 M)
rt
5a
(2 equiv)
1a
2aa
Entry
Catalyst[a]
Time [h]
Yield
[%][b]
Recov.
[%][b]
1
JohnPhosAuCl/AgNTf2
CyJohnPhosAuCl/AgNTf2
IPrAuCl/AgNTf2
2
3
47
42
37
29
6
–
2
–
3
2
–
4
SPhosAuCl/AgNTf2
PPh3AuCl/AgNTf2
24
24
1
19
74
–
5
Table 2. Scope of the cyclopropanation reaction[a]
6
BisPhePhosAuCl/AgNTf2
XPhosAuCl/AgNTf2
BrettPhosAuCl/AgNTf2
BrettPhosAuCl/AgSbF6
BrettPhosAuCl/AgOTf
BrettPhosAu(MeCN)SbF6
BrettPhosAuNTf2
42
56
60
61
13
60
63
7
2
–
X
Y
H
Ar
[BrettPhosAuNTf2]
8
1
–
X
(5 mol %)
Ar
H
+
R
DCM (0.5 M)
0 °C
•
9
1
–
Y
R
1
5
10
11
12
13[c]
24
2
27
–
2
(2 equiv)
O
X
O
Y
1
–
H
H
H
R
H
H
Ph
Ph
H
Ph
BrettPhosAuNTf2
9
74 (78)
–
[a] The ligand structures are shown below. [b] Determined by 1H NMR
analysis. Yield in parenthesis is the isolated yield on a 1.0 mmol scale. [c]
Reaction was conducted at 0.5 M concentration at 0 °C.
[d]
2'aa:
2ba
2aa: Y = H
2ab:Y = Br (55%)
(78%)
2ad:X = NBoc (74%)[c]
2ae: X = NTs (79%)
R = Me (0%)
: R = CN (NR)
[c,e]
[b]
2ac
2af
: Y = OMe (<67%)
: X = S
(21%)
iPr
iPr
O
H
O
O
H
R'
PR2
PCy2
OMe
H
N
N
MeO
H
H
H
Ph
Ph
R
iPr iPr
JohnPhos (R = tBu)
CyJohnPhos (R = Cy)
IPr
SPhos
R
2ca:
2da:
2ea
2ga: R = OMe (66%)
2ha R = Me (81%)
2ka: R' = p-OMe (91%)
2la: R' = p-Cl (85%)
2ma R = o-Me (61%)
R = OMe (54%)
R = Me (76%)
OMe
:
: R = Cl (<94%)[b,c]
: R = CN (0%)[c,f]
: R = Cl
(<93%)[b,c]
: '
2ia
2ja: R = CN (<8%)[b,f]
2fa
MeO
PCy2
PCy2
PCy
2
iPr
iPr
iPr
iPr
iPrO
OiPr
[a] Isolated yields. [b] Containing small amounts of impurities. [c] The
reaction was conducted at room temperature. [d] Acenaphthene 4a was
produced in 85% yield (vide infra). [e] No reaction. [f] A complex mixture
of unidentified products was observed.
iPr
iPr
XPhos
BisPhePhos
BrettPhos
With the optimized conditions in hand (Table 1, entry 13), we
investigated the scope of the cyclopropanation reaction (Table
2). The use of benzofuran with an electron-withdrawing (Br) or -
donating group (OMe) at the 5-position resulted in formation of
the desired products 2ab and 2ac in moderate yields (55% and
67%, respectively). The structure of 2ab was confirmed by X-ray
crystallography (see the Supporting information).[11] Indoles
protected by a Boc or Ts group also worked well as the
Our mechanistic proposal for the cyclopropane formation is
shown in Scheme 2. The allene moiety of 1a is activated by a
cationic gold catalyst to give gold complex 1aꞏAu+, followed by
intramolecular nucleophilic attack by the alkyne to form a vinyl
cation intermediate A.[10] Nucleophilic attack of benzofuran 5a on
the cationic carbon then leads to the formation of adduct B, if the
C2 attack of benzofuran is favored over C3. Finally,
cyclopropane formation accompanying aromatization and
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10.1002/anie.201903384
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COMMUNICATION
protodeauration results in the formation of the fused
cyclopropane 2aa. Exclusive production of the stereoisomer
where the naphthalene ring is present at the convex face of the
fused ring can be rationalized as shown Scheme 3. The (Z)-
isomer of the intermediate B has two possible conformations for
cyclopropanation, (Z)-B1 and (Z)-B2. Conformation (Z)-B2
would lead to 2aa’, which was not observed in any cases
presumably owing to steric repulsion between the benzene ring
of the benzofuran and the hydrogens of the dihydronaphthalene.
Therefore, the cyclopropanation selectively proceeds through
the favorable conformation (Z)-B1 resulting in exclusive
formation of 2aa. Similarly, the (E)-isomer of the intermediate B
would stereoselectively provide the same isomer 2aa for steric
reasons (see Scheme S1 in the Supporting information).
allenyne 1’e.[13] The position of the methyl group on the terminal
benzene ring did not affect the reaction significantly, and
produced the desired products 4c, 4g and 4h in good yields
(82–92%). Alkyl groups at the alkyne terminus (substituent R2)
were well tolerated to produce acenaphthenes 4i–4k. When
using ethyl-substituted allenyne 1’l, the reaction proceeded
without stereoselectivity to give 4l as a mixture of diastereomers
(trans:cis = 52:48).
Table 3. Scope of the acenaphthene formation[a]
R2
R1
R1
R2
BrettPhosAuNTf2
(2 mol%)
DCM, rt or
•
1,2-DCE, 80 °C
1'
4
4a
Ph
Ph
O
R
nBu
Bn
5a
•
•
Ph
[Au+]
1aꞏAu+
1a
4i
4j
[Au]
(53%)
( )3OH
(71%)
Ph
A
O
Me
4a R = H
(96%)
:
O
H
4b
4c
:R = p-OMe (82%)
: R = p-Me
:
Ph
aromatization
&
(82%)
(61%)
H
Ph
4d R = p-Cl
H
(0%)[b]
protodeauration
4e
: R = p-CN
4f
: R = p-CO2Me (57%)
4l
4g R = m-Me
(92%)
(89%)
4k (75%)[c]
(72%)
:
[Au]
4h
2aa
Scheme 2. Proposed reaction mechanism
:R = o-Me
trans:cis = 52:48[d]
B3
[a] Isolated yields. [b] A complex mixture of unidentified products was
obtained. [c] Using silylated substrate and TBAF treatment after
completion. [d] Determined by 1H NMR analysis.
H
H
O
O
O
H
O
Ph
Ph
Ph
Ph
Me
Ph
H
Me
Ph
H
H
H
H
Ph
H
H
•
•
[Au+]
[Au]
[Au]
1'aꞏAu+
H
[Au+]
1'a
2aa
B1
B2
(Z)-
2aa'
(Z)-
[Au]
Scheme 3. Rationalization of the observed stereoselectivity
A'
Ph
As described above, we found that the allenyne 1’a bearing
a methyl group at the o-position to the alkynyl group gave
acenaphthene 4a without forming a fused cyclopropane. The
structure of 4a was confirmed by X-ray crystallography (Table
3).[11] Considering this interesting reaction which involved
C(sp3)–H bond functionalization of a benzylic methyl group to
form a carbon–carbon bond, we next focused our attention to
the acenaphthene formation. Screening of gold catalysts in DCM
at room temperature revealed that BrettPhosAuNTf2 also
exhibited the highest activity for this reaction (Table S2 in the
Supporting information). Using the optimized conditions, the
reaction scope was briefly investigated (Table 3). Allenynes 1’
bearing an electron-donating group (OMe, Me), halogen, or
ester group at the p-position of the terminal benzene ring
reacted smoothly to afford the corresponding acenaphthenes
4b–4d, and 4f. Similar to the cyclopropanation reaction, no
acenaphthene was produced using the cyano-substituted
aromatization
&
H
Ph
1,5-H shift
protodeauration
H
4a
[Au]
B'
Scheme 4. Proposed reaction mechanism
A plausible reaction mechanism is shown in Scheme 4.
Coordination of a cationic gold complex to the allene moiety of
1’a gives complex 1'aꞏAu+. Nucleophilic cyclization from the
alkyne forms vinyl cation intermediate A’, similar to the
cyclopropanation reaction.[10] The neighboring methyl group at
the o-position facilitates the subsequent 1,5–H shift to produce a
benzyl cation intermediate B’.[14,15] Finally, aromatization and
protodeauration results in the formation of 4a. This proposal is
supported by a deuterium labelling experiment (Scheme 5).
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10.1002/anie.201903384
Angewandte Chemie International Edition
COMMUNICATION
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Reaction of the deuterated substrate 1’a-d3 (95%-d) under the
standard conditions gave the corresponding acenaphthene 4a-
d3 bearing deuterium atoms at the 1-and 2-positions without a
loss of deuterium content.
[4]
[5]
a) J. H. Lee, F. D. Toste, Angew. Chem. 2007, 119, 930–932; Angew.
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(ca. 95%-d)
(95%-d)
CD3
Ph
(ca.
D
D
Ph
95%-d
)
D
2
1
BrettPhosAuNTf2
(2 mol%)
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•
DCM (0.2 M)
rt
1'a-d3
4a-d
3 (72%)
Scheme 5. Isotopic labeling experiment
[6]
a) G.-Y. Lin, C.-Y. Yang, R.-S. Liu, J. Org. Chem. 2007, 72, 6753–
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In conclusion, we have developed
a gold(I)-catalyzed
cyclization of allenynes that terminates with a carbon–carbon
bond formation for the construction of naphthalene-substituted
fused cyclopropanes and acenaphthenes. These results can be
explained by the formation of a vinyl cation intermediate via
intramolecular nucleophilic attack of an alkyne on the activated
allene, followed by cyclopropanation or 1,5–H shift. Studies
directed towards further determination of the reaction
mechanism as well as application to π-conjugated molecules are
currently underway in our laboratory.
[7]
[8]
C.-Y. Yang, G.-Y. Lin, H.-Y. Liao, S. Datta, R.-S. Liu, J. Org. Chem.
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Acknowledgements
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This work was supported by the JSPS KAKENHI (Grant
Numbers JP15KT0061, JP17H03971, and JP18H04408), Basis
for Supporting Innovative Drug Discovery and Life Science
Research (BINDS, JP18am0101092j0001) from AMED, Japan,
and the Hoansha Foundation.
Keywords: gold catalysis • allene • cyclopropanation • C-H
[10] For examples of gold-catalyzed reactions via vinyl cation intermediates,
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[11] CCDC 1900611 (2ab) and 1900610 (4a) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre.
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[14] For a 1,5–H shift via vinyl cation intermediate, see: a) U. Biermann, R.
Koch, J. O. Metzger, Angew. Chem. 2006, 118, 3146–3150; Angew.
Chem. Int. Ed. 2006, 45, 3076–3079; b) T. Jin, M. Himuro, Y.
Yamamoto, J. Am. Chem. Soc. 2010, 132, 5590–5591; c) F. Zhang, S.
Das, A. J. Walkinshaw, A. Casitas, M. Taylor, M. G. Suero, M. J. Gaunt,
J. Am. Chem. Soc. 2014, 136, 8851–8854; d) Z. Zheng, L. Zhang, Org.
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Halla, K. Wrobel, C. R. Solorio-Alvarado, R. Ortiz Alvarado, B.
Yahuaca-Juárez, Org. Biomol. Chem. 2018, 16, 7330–7335.
[3]
a) M. R. Luzung, P. Mauleón, F. D. Toste, J. Am. Chem. Soc. 2007,
129, 12402–12403; b) H. Teller, S. Flügge, R. Goddard, A. Fürstner,
Angew. Chem. 2010, 122, 1993–1997; Angew. Chem. Int. Ed. 2010, 49,
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[15] C-H insertion mechanism would be another possible reaction pathway,
see ref. 14d.
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10.1002/anie.201903384
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
R2
R1
R2
R1
One golden stone, two birds:
Reaction of phenylene-tethered
allenynes with a benzofuran gave 1-
(naphth-1-yl)cyclopropa[b]benzo-
furan derivatives, whereas the
reaction of 1-allenyl-2-ethynyl-3-
methybenzene derivative in the
absence of benzofuran gave
acenaphthenes, both via vinylcation
intermediates.
Takaya Ikeuchi, Shinsuke Inuki,
Shinya Oishi, Hiroaki Ohno*
[Au+]
•
vinylcation
formation
[Au]
Page No. – Page No.
Gold(I)-Catalyzed Cascade
Cyclization Reactions of Allenynes
for the Synthesis of Fused
Cyclopropanes and
X
X
R2
R2
R1 = H
cyclopropanation
R1 = Me
C(sp3)-H
Acenaphthenes
functionalization
This article is protected by copyright. All rights reserved.