Gold-catalyzed cycloisomerization reactions of 2-tosylamino-phenylprop-1- yn-3-ols as a versatile approach for indole synthesis

Article abstract of DOI:10.1002/anie.201000341

(Chemical equation presented) A great combination: A putative indolyl-substituted vinyl gold species generated in situ from AuCl/AgOTf catalyzed the title transformation. Under these reaction conditions, indenyl-fused and 2,3-disubstituted indole derivati

Full text of DOI:10.1002/anie.201000341

Angewandte
Chemie
DOI: 10.1002/anie.201000341
Homogeneous Catalysis
Gold-Catalyzed Cycloisomerization Reactions of 2-Tosylamino-
phenylprop-1-yn-3-ols as a Versatile Approach for Indole Synthesis**
Prasath Kothandaraman, Weidong Rao, Shi Jia Foo, and Philip Wai Hong Chan*
Dedicated to Professor Koichi Narasaka
Indole derivatives are important cyclic structures in organic
synthesis because of their ability to serve as building blocks
for functional materials and as key components in a myriad of
bioactive compounds.[1] For this reason, the establishment of
new efficient synthetic methods to prepare this class of N-
heterocycles, with selective control of substitution patterns,
from readily available and simple
tive indolyl-substituted vinyl gold species B[8] generated in
situ would then be susceptible to protodeauration and
intramolecular Friedel–Crafts alkylation[9] when R1 = Ar
and afford the indenyl-fused indole 2 (route 1 in Scheme 1).
On the other hand, we envisaged that a more reactive primary
vinyl gold species formed when R1 = H would be prone to a
substrates continues to be actively
pursued.[1,2]
Recently, an increasing number
of studies have highlighted the util-
ity of unsaturated alcohols as sub-
strates in gold-catalyzed carbon–
nitrogen bond-formation strat-
egies.[3–6] For example, our research
group has recently reported an effi-
cient and convenient synthetic route
to 1,2-dihydroquinolines involving
intramolecular allylic amination of
2-tosylaminophenylprop-1-en-3-ols
catalyzed by AuCl3 with AgSbF6 as
a co-catalyst.[5a] As part of an ongo-
ing program to develop this type of
synthetic approach for carbon–
nitrogen bond formation,[4–6] we
reasoned that readily available 2-
tosylaminophenylprop-1-yn-3-ol
derivatives 1 might also undergo
gold-mediated intramolecular cycli-
zation but in a 5-exo-dig manner
Scheme 1. Design of a vinyl gold approach for indole synthesis. Nu=nucleophile, Ts=4-toluenesul-
fonyl.
given the exceptional alkynophilic-
ity of the metal catalyst
(Scheme 1).[3,7] The resultant puta-
more rapid protodeauration/1,3-allylic alcohol isomerization
process (1,3-AAI)[10] and provide the (1H-indol-2-yl)metha-
[*] P. Kothandaraman, W. Rao, S. J. Foo, Prof. Dr. P. W. H. Chan
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University, Singapore 637371 (Singapore)
Fax: (+65)6791-1961
nol product 3 (route 2a in Scheme 1). In the presence of a
strong nucleophile, preferential SN2’ substitution would be
expected to give the 2-alkyl-1H-indole adduct 4 (route 2b in
Scheme 1). Similarly, we reasoned for reactions in which R1 =
CHR2R3, a more facile protodeauration and dehydration step
would proceed and deliver 2-vinyl-1H-indoles of the type 5
(route 3 in Scheme 1). Herein, we disclose the details of this
versatile and efficient gold-catalyzed method for the chemo-
selective synthesis of indenyl-fused and 2,3-disubstituted
indoles that makes use of a putative vinyl gold species
formed in situ from cycloisomerization of 2-tosylaminophen-
ylprop-1-yn-3-ols.[11] Although metal-catalyzed 5-endo-dig
cycloisomerization of o-alkynylanilines have been extensively
E-mail: waihong@ntu.edu.sg
Homepage: http://www3.ntu.edu.sg/home/waihong/
[**] This work was supported by a University Research Committee Grant
(RG55/06) and a Supplementary Equipment Purchase Grant
(RG134/06) from Nanyang Technological University, and by Science
and Engineering Research Council Grant (092 101 0053) from
A*STAR, Singapore. Helpful suggestions made by the referees are
also gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000341.
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Communications
explored methods in indole synthesis,[2f,11] to our knowledge
the analogous 5-exo-dig process and dehydrative cyclizations
of substituted propargylic alcohols of the type 1 have
remained sparse.[2c,f,h]
side product. More importantly, the contrasting catalytic
activities observed for the reactions mediated by AgOTf and
TfOH, respectively, provided evidence that the cationic AuI
complex is the active species (see Table S1 in the Supporting
Information).
Initially, we chose to focus our attentions on the cyclo-
isomerization/Friedel–Crafts alkylation of 1a by a variety of
Lewis and Brønsted acid catalysts to establish the reaction
conditions (Table 1 and Table S1 in the Supporting Informa-
tion). This initial study revealed that treatment of 1a with
5 mol% of AuCl and 5 mol% of AgOTf in toluene at reflux
gave the indenyl-fused indole 2a in 74% yield along with the
side product 6[12,13] in 25% yield (Table 1, entry 1). The indole
To define the scope of the present procedure, we next
turned our attention to the reactions of a variety of 1-phenyl-
1-(2-(tosylamino)phenyl)prop-2-yn-1-ol derivatives (Table 2).
These experiments showed that with AuCl/AgOTf as the
catalyst, starting alcohols 1b–k and 1m efficiently underwent
the tandem cycloisomerization/Friedel–Crafts alkylation pro-
cess and gave the corresponding products 2b–k and 2m in
good to excellent yields (Table 2, entries 1–10 and 12). The
structure of 2 f was also confirmed by X-ray crystal structure
analysis.[14] Under the standard reaction conditions, o,p-
dimethylaniline alcohol 1l was the only example that gave a
mixture of decomposition products based on TLC and
1H NMR analysis of the crude mixture (Table 2, entry 11).
Having established the reaction conditions that gave
indenyl-fused indoles, we next examined the scope of this new
methodology for the synthesis of 2,3-disubstituted indoles
(Table 3). As mentioned earlier, we anticipated such control
of product chemoselectivity could be achieved by varying the
nature of the R1 group on the vinyl gold moiety in species B
shown in Scheme 1. With this in mind, we first tested the
reaction of the terminal acetylenic alcohols 1n and 1o with
5 mol% of AuCl and AgOTf under the standard conditions
and found 3b and 3c could be obtained as the sole product in
77 and 86% yield, respectively (Table 3, entries 1 and 2).
Under similar conditions, repeating the reaction of 1n in the
1
product was confirmed by H NMR analysis (see the Sup-
porting Information for NMR analysis) and X-ray crystal
structure analysis of two closely related products.[14,15] Our
studies subsequently showed that the introduction of HMPA
resulted in an increase in product yield and formation of 3a as
a side product in 6% yield (Table 1, entry 2). By using CaSO4
to remove water from the reaction and in combination with
HMPA, formation of this latter byproduct could be sup-
pressed to give 2a as the sole product in 94% yield (Table 1,
entry 3). In contrast, an inspection of entries 4–10 in Table 1
shows that repeating the reaction with DMI or DMAP in
place of HMPA or in other solvents was markedly less
effective. Similarly, a survey of other Lewis and Brønsted acid
catalysts did not provide improved results (Table 1,
entries 11–24 and Table S1 in the Supporting Information).
In these latter reactions, a number of the conditions examined
also lead to either the recovery of the starting alcohol in yields
of up to 64% or gave the Z-indolin-3-ol 7[16] as an additional
Table 1: Optimization of the reaction conditions.[a]
Entry
Catalyst
Solvent
Yield [%][b]
Entry
Catalyst
Solvent
Yield [%][b]
2a
3a
6
7
2a
3a
6
7
1
AuCl/AgOTf
AuCl/AgOTf
AuCl/AgOTf
AuCl/AgOTf
AuCl/AgOTf
AuCl/AgOTf
AuCl/AgOTf
AuCl/AgOTf
AuCl/AgOTf
AuCl/AgOTf
AuCl/AgSbF6
AuCl/AgPF6
toluene
toluene
toluene
toluene
toluene
(CH2Cl)2
THF
74
87
94
20
–
–
6
–
–
–
–
–
–
–
25
–
–
67
87
40
–
80
55
–
–
–
–
–
–
–
–
–
–
–
–
–
13
14
15
16
17
18
19
20
21
22
23
24
AuCl3/AgOTf
[Au(PPh3)]Cl/AgOTf
[AuL1]Cl/AgOTf
[AuL2]Cl/AgOTf
[AuL3]Cl/AgOTf
[AuL4]Cl/AgOTf
AuCl
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
61
32[g]
38[g]
5
12
3
42
32
17[g]
14
12[g]
8
24
11
26
–
10
–
20
24
12
–
–
–
–
–
76
72
–
–
–
–
–
–
–
–
52
–
–
–
–
–
2[c]
3[d]
4[e]
5[f]
6
44
–
–
[e]
7
8
9
10
11
12
MeCN
AuCl3
1,4-dioxane
DMSO
toluene
toluene
25
–
[Au(PPh3)]Cl
[Au(PPh3)]NTf2
[AuL3MeCN]SbF6
[AuL4]NTf2
[e]
–
32
–
47
–
–
65
38
–
–
7
57
–
[a] All reactions were performed with 0.2 mmol of 1a in the presence of 5 mol% of catalyst at reflux for 17 hours. [b] Yield of isolated product.
[c] Reaction performed with 20 mol% of HMPA for 2 hours. [d] Reaction performed with 20 mol% of HMPA and CaSO4 (35 mg) for 2 hours.
[e] Reaction performed with 20 mol% of DMI and CaSO4 (35 mg) for 2 hours. [f] Reaction performed with 20 mol% of DMAP and CaSO4 (35 mg) for
2 hours. [g] Recovery of 1a in 18–64% yield. DMAP=4-dimethylaminopyridine, DMI=1,3-dimethyl-2-imidazolidinone, DMSO=dimethyl sulfoxide,
HMPA=hexamethylphosphoramide, L1 =tris(4-trifluoromethylphenyl)phosphine; L2 =1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; L3 =(1,1’-
biphenyl-2-yl)-di-tert-butylphosphine; L4 =2-dicyclohexyl(2’,4’,6’-triisopropylbiphenyl)phosphine, Tf=trifluoromethanesulfonyl, THF=tetrahydro-
furan.
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Table 2: Tandem cycloisomerization/Friedel–Crafts alkylation of 1b–m catalyzed by AuCl/AgOTf.[a]
presence of MeOH or 2,6-dimethyl-
phenol gave the corresponding 2-
alkyl-1H-indoles 4a and 4b in 82
and 76% yield, respectively
(Table 3, entries 3 and 4). Likewise,
reactions of 1p–t with a pendant
alkyl group on the acetylenic
carbon centre proceeded well and
furnished the corresponding 2-
vinyl-1H-indoles 5a–e in excellent
yields (Table 3, entries 5–9). In
these latter reactions, the structure
of 5e was also confirmed by X-ray
crystal structure analysis[15] and no
other side products were detected
by TLC and 1H NMR analysis of
the crude mixtures.
Entry
Substrate
Product
Yield
[%][b]
1
2
3
1b, R=Me
1c, R=Cl
1d, R=Br
2b
2c
2d
91
86
87
2e +
4
1e, R1 =H, R2 =Me
82[c]
2e’, R1 =Me, R2 =H
5
6
7
8
1 f, R=p-FC6H5
2 f
2g
2h
2i
94
91
46
76
1g, R=p-ClC6H5
1h, R=p-OMeC6H5
1i, R=p-MeC6H5
Although the isolation of the Z-
indolin-3-ol 7 furnished from reac-
tions of 1a catalyzed by [AuL2]Cl/
AgOTf or [AuPh3P]NTf2 outlined
in entries 16 and 20 in Table 1 was
fortuitous, the result argues in favor
of the mechanism put forward in
Scheme 1. This argument was fur-
ther corroborated by the observa-
tion that when a solution of 7 in
toluene was treated with 5 mol% of
AuCl and AgOTf under the condi-
tions shown in Scheme 2, the
expected indenyl-fused indole 2a
was obtained as the sole product in
9
10
11
1j, R1 =Br, R2 =H
1k, R1 =Cl, R2 =H
1l, R1 =R2 =Me
2j
2k
2l
90
86
–
[d]
12
1m
2m
82
[a] All reactions were performed with 0.2 mmol of 1 in the presence of 5 mol% of AuCl and AgOTf,
20 mol% of HMPA and CaSO4 (35 mg) in toluene at reflux for 2 hours. [b] Yield of isolated product.
[c] Isolated as an inseparable mixture of regioisomers in a 2:1 ratio. [d] Mixture of unknown side
products obtained based on 1H NMR analysis of the crude mixture.
98% yield. The role of the gold catalyst in facilitating the
dehydroxylation step could also be shown by repeating the
reaction in the absence of the metal catalyst, which gave 3a
along with recovery of the starting indolin-3-ol in 10 and 90%
yield, respectively.
Table 3: Synthesis of (1H-indol-2-yl)methanols 3, 2-alkyl-1H-indoles 4,
and 2-vinyl-1H-indoles 5 catalyzed by AuCl/AgOTf.[a]
Entry
Substrate
Product
Yield
[%][b]
1
2
1n, R=H
1o, R=Me
3b
3c
77
86
3[c]
1n
1n
4a
4b
82
76
Scheme 2. Gold-catalyzed intramolecular Friedel–Crafts alkylation of 7.
4[d]
Even though the above results provide evidence for the
mechanism outlined in Scheme 1, other possible pathways
were considered but then discounted based on the following
control experiments. As gold carbene species are often
proposed in alkyne cycloisomerization reactions mediated
by metal catalyst, the reactions of 1n with Ph2SO and styrene
catalyzed by AuCl/AgOTf under the standard conditions
were first examined.[17] Analysis of the crude mixtures of these
reactions by 1H NMR spectroscopy reveals no adducts
resulting from trapping of such a putative species by Ph2SO
or styrene. This test afforded 3b as the sole product in 72–
5
6
1p, R=H
1q, R=nPr
5a
5b
90
80
7
8
9
1r, n=1
1s, n=2
1t, n=3
5c
5d
5e
86
86
87
[a] All reactions were performed with 0.2 mmol of 1 in the presence of
5 mol% of AuCl and AgOTf, 20 mol% of HMPA and CaSO4 (35 mg) in
toluene at reflux for 2 hours. [b] Yield of isolated product. [c] Reaction
conducted in the presence of 8 equivalents of MeOH. [d] Reaction
conducted in the presence of 8 equivalents of 2,6-dimethylphenol.
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4621

Communications
75% yield in both experiments, and led us to surmise that an
Keywords: gold · homogeneous catalysis · indoles ·
tandem reactions · unsaturated alcohols
.
alternative mechanism for the present reaction involving
formation of a gold carbene intermediate was unlikely. Our
findings that show the near quantitative recovery of starting
materials for the reactions of 3a in the presence or absence of
2,6-dimethylphenol as well as that for 6 exposed to 5 mol% of
AuCl/AgOTf under the standard conditions led us to also rule
out the possibility of indole formation proceeding via
intermediates of the type 3 or 6 (depicted in Table 1).
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The beneficial effect of added HMPA led us to initially
consider its role as that of a ligand that moderates the
reactivity of the Au catalyst through coordination to the metal
center in a similar manner to that reported for lanthanide
salts.[18] Indeed, this speculation was supported by 31P NMR
measurements of an equimolar sample of AuCl/AgOTf +
HMPA that shows the phosphorus resonance to shift down-
field by d = 3.3 ppm relative to the respective signal in free
HMPA (d = 23.5 ppm). Further analysis of this mixture by
ESI high-resolution mass spectrometry also reveals the
presence of a peak at m/z 526 that could be assigned to
[AuCl/AgOTf + HMPA + H]+. This evidence is comparable
to the data reported for lanthanide HMPA complexes.[18]
However, a marked downfield shift from d = 9.41 to 10.1
and 10.41 ppm of the NH proton signal in the respective
1H NMR spectra of 1a and 1a + AuCl/AgOTf, presumably
resulting from hydrogen bonding, upon addition of HMPA
implies it could also play a dual role in promoting the
cycloisomerization process. Our earlier findings that show a
lower product yield obtained for the reaction of 1a catalyzed
AuCl/AgOTf in the absence of HMPA (Table 1, entry 1)
would certainly suggest that such interactions are a possibility.
In summary, an efficient gold-catalyzed synthetic route to
indenyl-fused and 2,3-disubstituted indoles from a common
vinyl gold species generated in situ from easily accessible
propargylic alcohol substrates under mild conditions has been
reported. Uniquely, the reactions were found to only proceed
efficiently and with complete control of product chemo-
selectivity for a wide variety of starting alcohols in the
presence of the combined gold and silver catalyst system.
Further exploration of the mechanism, scope, and synthetic
applications of the present reaction are currently underway.
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Experimental Section
A solution of AuCl (11 mmol), AgOTf (11 mmol), and CaSO4 (35 mg)
in toluene (1.5 mL) was stirred under Ar at room temperature for
10 min. Upon completion, HMPA (20 mol%) was added followed by
slow dropwise addition of a solution of 1 (0.22 mmol) in toluene
(1.5 mL; note: 8 equiv of the nucleophile was also added at this point
for reactions of 1n described in entries 3 and 4 in Table 3). The
reaction mixture was then stirred at reflux for 2 h. The solvent was
removed under reduced pressure and the reside was purified by flash
column chromatography on silica gel (eluent: n-hexane/EtOAc
(20:1)) and gave the indole product.
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Received: January 20, 2010
Revised: April 12, 2010
Published online: May 17, 2010
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Angewandte
Chemie
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Information for the ORTEP drawing.
[14] CCDC 749107 (2 f) contains the supplementary crystallographic
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ccdc.cam.ac.uk/data_request/cif. Please see the Supporting
Information for the ORTEP drawing.
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