Sequential alkylation/gold-catalyzed annulation reactions of anilines with propargylic bromide derivatives

Article abstract of DOI:10.1016/j.tetlet.2011.07.115

Preliminary results of sequential alkylation/gold-catalyzed annulation reactions of anilines with propargylic bromide derivatives to provide quinoline scaffolds are described. The efficiency of NaAuCl₄·2H ₂O in the sequential procedu

Full text of DOI:10.1016/j.tetlet.2011.07.115

Tetrahedron Letters 52 (2011) 5145–5148
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Sequential alkylation/gold-catalyzed annulation reactions of anilines
with propargylic bromide derivatives
Maria Alfonsi a, Antonio Arcadi a,b, , Marco Chiarini c, Fabio Marinelli a
⇑
a Dipartimento di Chimica, Ingegneria Chimica e Materiali-Università di L’Aquila-Via Vetoio, 67010 L’Aquila, Italy
b Unità di Ricerca AQ2, Consorzio Interuniversitario La Chimica per l’Ambiente, Via della Libertà 5/12, 30175 Marghera (VE), Italy
c Department of Food Science, University of Teramo, Via Carlo Lerici 1, Mosciano Sant’Angelo, I-64023 Teramo, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 June 2011
Revised 6 July 2011
Accepted 26 July 2011
Available online 3 August 2011
Preliminary results of sequential alkylation/gold-catalyzed annulation reactions of anilines with propar-
gylic bromide derivatives to provide quinoline scaffolds are described. The efficiency of NaAuCl4ꢀ2H2O in
the sequential procedure was investigated and compared with different transition metal salts and Au(I)
complexes. Selectivity of the reaction of aniline derivatives with propargylic bromides has also been
investigated.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Quinolines
N-Propargylation
Intramolecular hydroarylation
Gold catalysis
Sequential reactions
Heterocyclic compounds, particularly some fused heterocycles,
play great relevance in pharmaceutical and agrochemical fields.1
Although a variety of synthetic methods for the synthesis of these
important frameworks have been developed, most of them involve
their preparation through a stepwise process. In recent years, one-
pot synthetic sequences have been recognized as more sustainable
approaches to target molecules by minimizing the number of steps
and the amount of time, labor, and waste.
ently site-iodinated relevant heterocyclic frames was achieved by
judicious ligand tuning in gold(I)-catalyzed IMHA of simple teth-
ered iodoalkynyl and arene partners.10 An efficient synthesis of
exo-methylene tetrahydroquinolines and dihydroquinolines was
developed from readily accessible N-aminophenyl propargyl malo-
nates.11 The hydroarylation process, which could be performed
with a low loading of [XPhosAu(NCMe)]SbF6 in nitromethane at
100 °C (1 mol %), proved to be rapid and general allowing the pres-
ence of a plethora of functional groups on the aromatic ring. Fur-
thermore, the dihydroquinolines were shown to easily rearrange
to functionalized indoles under photochemical conditions. Analo-
gously the formation of 4-exo-methylene-1,2-dihydrocinnolines
by [XPhosAu(NCMe)]SbF6 was allowed by IMHA of N-propargyl-
N0-arylhydrazinecarboxylic acid methyl ester.12
Interestingly, iron complex Fe(OTf)3 has been, also, proven to be
an effective catalyst for IMHA of aryl-substituted alkynes to form
1,2-dihydroquinolines and phenanthrenes in good to high yields.13
Moreover, 3H-pyrano[3,2-f]quinoline-3-one, 4-methyl-4,7-phen-
anthrolin-3(4H)-one, and 1,3-dimethylpyrido[3,2-d]pyrimidine-
2,4(1H,3H)-dione derivatives have been synthesized by hitherto
unreported silver-catalyzed 6-endo-dig mode of cycloisomerization
from various N-propargylated heterocyclic compounds.14
Gold catalysis represents an important and powerful tool for the
promotion of one-pot synthesis of fused heterocycles from readily
available starting materials.2
The cyclization of substituted propargylic aryl ethers3 or pro-
pagylic anilines4 by electrophiles to 3,4-disubstituted 2H-benzopy-
rans or substituted quinolines has been examined. The cyclization
of aryl-substituted alkynes via intramolecular hydroarylation has
proven to be an efficient method for the construction of carbo-
cycles5 and heterocycles.6 In particular, azaanthraquinone assem-
bly from N-propargylamino quinone via a Au(I)-catalyzed7 or
iodine- induced 6-endo-dig electrophilic cyclization8 has been
investigated. A number of important heterocyclic motifs including
2H-chromenes, coumarins, benzofurans, and dihydroquinolines
were formed expeditiously from readily accessible starting materi-
als through protocols involving Au(I)-catalyzed intramolecular
hydroarylation (IMHA) of terminal alkynes.9 The access to differ-
The preparation of the N-propargylated intermediates usually
involves the N-alkylation reaction of aniline derivatives with prop-
argyl bromides.
Interestingly, the Rh(I)-catalyzed amino-Claisen rearrangement
of N-propargylanilines led to the formation of indole derivatives
which have been also obtained through a one-pot procedure by
⇑
Corresponding author. Tel.: +39 0862 433774; fax: +39 0862 434202.
E-mail address: antonio.arcadi@univaq.it (A. Arcadi).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.07.115

5146
M. Alfonsi et al. / Tetrahedron Letters 52 (2011) 5145–5148
the rhodium-catalyzed reaction of N-alkylaniline with propargyl
bromide in the presence of K2CO3 in hexafluoroisopropyl alcohol.15
By contrast, the one-pot synthesis of quinolines which should oc-
cur via the divergent 6-endo annulation pathway has not been
yet reported.
Herein, we report our preliminary results of sequential alkyl-
ation/gold-catalyzed annulation reactions of anilines 1 with prop-
argylic bromide derivatives 2 directed toward the synthesis of
quinoline scaffolds 4 (Scheme 1).
We selected the reaction of 2,4-dimethoxyaniline 1a with prop-
argyl bromide 2a under the presence of a gold catalyst for initial
studies (Scheme 2).
R1
Some of our results of the screening of the reaction conditions
are summarized in Table 1. The recent discovery of the high effi-
ciency of the stable and commercially available Au(I) complex A
in the IMHA of alkynes9 prompted us to investigate its effective-
ness in our sequential process. The reaction of an excess of 1a with
propargyl bromide 2a (1a:2a:A = 3:1:7%) in toluene at 100 °C (Ta-
ble 1, entry 1) gave after 24 h the corresponding quinoline deriva-
tive 4a, but only in 28% yield (the N-alkylated derivative
intermediate 2,4-dimethoxy-N-(propen-2-yn-1-yl)-aniline 3a was
isolated in 48% yield). The formation of 4a can be considered to oc-
cur via the gold-catalyzed annulation/aromatization reaction of 3a
generated in situ from the alkylation reaction of 1a with 2a.
When ethanol was employed as the solvent, the yield of 4a in-
creased to 48% (Table 1, entry 2), but the formation of pyrrole 6a
was observed as a side-product of the reaction. Very likely, the
in situ generated nitrogen-tethered 1,6-diynes 5a underwent un-
der the reaction conditions a gold(I)-catalyzed cycloisomerization
to provide pyrrole 6a.16
While the catalyst was switched to NaAuCl4ꢀ2H2O, different
reaction conditions were screened. In toluene as solvent at
100 °C only the formation of the N-alkylation and the N,N-dialky-
lated derivatives was observed (Table 1, entry 3). Again ethanol re-
sulted in a better solvent for the sequential process. It should be
mentioned that an equal ratio of 1a and 2a failed to give satisfac-
tory results, but, by adding an organic or inorganic base to a 1:1 or
1:1.5 mixture of 1a and 2a, respectively, the subsequent treatment
with NaAuCl4ꢀ2H2O significantly improved the yield of 4a (Table 1,
entries 4 and 5). Under these latter conditions the N,N-dialkynyl
derivatives 5a (Table 1, entry 4) and pyrrole 6a (Table 1, entry 5)
were isolated as by-products. Quinoline 4a was isolated in 64%
yield as the only reaction product when the reaction of an excess
of 1a with 2a was carried out in ethanol at 70 °C for 24 h in the
presence of NaAuCl4ꢀ2H2O as the catalyst (7 mol %) (Table 1, entry
R'
GF
Fg
[Au]
+
N
R
NH2
R
Br
1
2
4
R'
GF
N
R
H
3
Scheme 1. Sequential synthesis of quinolines.
O
O
Catalyst
+
N
NH2
Br
O
O
4a
1a
2a
O
O
O
N
N
O
N
O
O
H
5a
3a
6a
Scheme 2. Transition-metal catalyzed reaction of 1a with 2a.
Table 1
Different conditions screeneda
Entry
Catalyst (mol %)
3a Yield (%)
4a Yield (%)
5a Yield (%)
6a Yield (%)
1b
48
28
—
—
2c
A (7%)
—
74
—
—
—
68
46
—
—
—
48
—
13
—
—
8
—
—
3b
NaAuCl4ꢀ2H2O (7%)
NaAuCl4ꢀ2H2O (5%)
NaAuCl4ꢀ2H2O (7%)
NaAuCl4ꢀ2H2O (7%)
AgOTf (7%)
10
12
—
—
—
18
—
—
—
—
4d
58
38
64
10
7
38
41
41
28
41
24
24
5e
6c
7c
8c
FeCl3ꢀ6H2O (7%)
9c
CuBr (7%)
—
—
—
—
—
—
10c
11c
12c
13c
14c
15c
CuCl (7%)
Cu(NO3)2ꢀ6H2O (7%)
CuSO4ꢀ5H2O (7%)
Cu(OTf)2 (7%)
Cu(OAc)2 (7%)
CuCl2ꢀ5H2O (7%)
—
—
18
22
—
—
a
b
c
Yields refer to single non optimized runs, and are given for pure isolated products.
Reaction was carried in toluene at 100 °C for 24 h by using the following molar ratios: [1a]:[2a] = 3:1.
Reaction was carried in ethanol at 70 °C for 24 h by using the following molar ratios: [1a]:[2a] = 3:1.
Reaction was carried in ethanol at 70 °C for 24 h by using the following conditions: [1a]:[2a]:[DBU] = 1:1:1.
Reaction was carried in ethanol at 70 °C for 24 h by using the following conditions: [1a]:[2a]:[K2CO3] = 1:1.5:1.5.
d
e

M. Alfonsi et al. / Tetrahedron Letters 52 (2011) 5145–5148
5147
Table 2
ðꢁ CÞ Time
Different conditions screened for the N-alkylation reaction of the aniline 1a with propargyl bromide 2a 1a þ 2a Temperature
ðhÞ 3a þ 5a
ƒƒƒƒƒƒƒƒƒƒƒƒƒƒ!
Solvent; Base
Entry
Molar Ratio[1a]:[2a]:base
Base
Solvent
Temperature (°C)/time (h)
3aa (yield %)
5aa (yield %)
1
2
3
4
5
[3.0]:[1.0]:[—]
[1.0]:[1.0]:[1.0]
[2.3]:[1.0]:[1.0]
[1.0]:[1.0]:[2.0]
[1.0]:[1.5]:[1.5]
—
Toluene
Ethanol
Ethanol
Ethanol
Ethanol
80/2.0
70/6.5
70/6.5
70/4.0
70/4.0
73
57
66
70
90
10
12
10
—
DBU
DBU
K2CO3
K2CO3
—
a
Yields refer to single non optimized runs, and are given for pure isolated products.
Table 3
ꢁ
Different conditions screened for the gold-catalyzed IMHA reaction of the 2,4-dimethoxy-N-(propen-2-yn-1-yl)-aniline 3a 3a Temperatureð CÞTimeðhÞ 4a
ƒƒƒƒƒƒƒƒƒƒƒ!
Solvent; Catalyst
Entry
Catalyst
Solvent
Temperature (°C)/Time (h)
4aa (yield %)
1
2
3
4
A (5%)
A (5%)
Ethanol
DCE
Ethanol
HOAc
70/6.0
70/6.0
70/23
76
74
65
NaAuCl4ꢀ2H2O (5%)
Ph3PAuCl (10%)/AgOTf (10%)
100/1.0
Quantitative
a
Yields refer to single non optimized runs, and are given for pure isolated products.
Table 4
Scope of the sequential process
6). For a further optimization of the reaction conditions we have,
also, used various metal catalysts. Among the catalysts used,
AgOTf14 and FeCl3ꢀ6H2O17 gave very poor yield of quinoline 3a (Ta-
ble 1, entries 7 and 8). Better results salts were observed with copper
salts18 (Table 1, entries 9–15), but gold catalysts19 proved to be the
most effective in promoting the sequential process to quinoline 4a.
Control experiments indicated the influence of the reaction con-
ditions in the N-alkylation step (Table 2) as well as in the IMHA
step (Table 3). We failed to avoid the N,N-dialkylative process by
using DBU20 as the base in ethanol (Table 2, entries 2 and 3). In
the absence of the gold catalyst, the formation of the side-product
5a can be suppressed by the use of K2CO3 even in the presence of
an excess of propargyl bromide 2a which leads to a better yield of
intermediate 3a (Table 2, entries 2 and 3).
Entry
1
1
2
Catalyst (mol %)
4a,b (yield %)
1a
2a
NaAuCl4ꢀ2H2O
(7%)
4a (64)
2
1a
2a
A (7%)
4a (48)
OCH3
OCH3
NaAuCl4ꢀ2H2O
3
4
2a
2a
(7%)
NH2
N
OCH3
1b
1b
OCH3
4b (60)
A (7%)
4b (43)c
Br
H3CO
The gold-catalyzed IMHA step deserves further comment. Au(I)-
catalyzed IMHA reactions of terminal alkynes have been previously
investigated.7,9,12 In toluene as the solvent the gold(I)-catalyzed
IMHA allowed to isolate quinoline 4a in 28% yield (Table 1, entry
1). According to the literature21 Au(III) is not effective in promoting
the IMHA in toluene (Table 1, entry 3). Remarkably, in ethanol as the
solvent, both the Au(I) catalyst A and NaAuCl4ꢀ2H2O accomplished
the formation of 4a in comparable yield (Table 3, entry 1 vs 3). Inter-
estingly catalyst A was as effective in the more eco-friendly ethanol
as in the halogenated 1,2-dichlorethane (DCE) (Table 2, entry 3).
Beneficial effect of the ethanol as the reaction medium for gold-cat-
alyzed annulation reactions has been highlighted.22 According to
previous results7 the IMHA reaction of 3a in HOAc as the solvent
completed in just 1 h, but the use of this latter solvent is incompat-
ible with the sequential N-alkylation/IMHA process. Furthermore,
the gold catalysis seems to play a relevant role for the transforma-
tion of the in situ generated 2,4-dimethoxy-N,N-bis(propen-2-yn-
1-yl)aniline 5a into the corresponding pyrrole derivative 6a.
In order to explore the scope of the sequential gold-catalyzed
approach to quinolines 3, anilines 1a–b were reacted with the
alkynyl bromides 2a–d (Table 4).23
NaAuCl4ꢀ2H2O
5
6
1a
(7%)
N
2b
2b
OCH3
4c (67)
OCH3
NaAuCl4ꢀ2H2O
1b
(7%)
N
OCH3
4d (60)
H3CO
NaAuCl4ꢀ2H2O
N
OCH3
7
8
1a
1a
Br
2c
(7%)
4e (62)
H3CO
NaAuCl4ꢀ2H2O
N
OCH3
(7%)
Br
2d
4f (20)
The use of terminal alkyne 2a, of the internal alkyne 1-bromo-
pent-2-yne 2b and the branched derivative 3-bromobut-1-yne 2c
is compatible with this protocol. The 4-bromobut-1-yne derivative
2d should lead to the corresponding N-butynyl aniline derivative
intermediates which via, in this case, a 6-exo-cyclization to exo-
methylenetetrahydroquinoline derivatives might undergo isomer-
ization to the corresponding 4-methylquinolines.
The cyclization of N-butynyl tosylaniline has not been previously
observed to occur under gold catalysis, while the N-(but-3-yn-1-yl)
N-methylaniline gave the 1-methyl-4-methylidene-1,2,3,4-terahy-
a
Yields refer to single non optimized runs, and are given for pure isolated
products.
b
The reactions were carried out at 70 °C in ethanol using the following molar
ratios: [1]:[2] = 3:1.
c
Compound 6b was isolated in 12% yield.
droquinoline in 11% yield when the IMHA reaction was conducted
in nitromethane at 100 °C with [XPhosAu(NCMe)]SbF6 as catalyst.11
An encouraging 20% yield of 4f was observed under our sequential
conditions.

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M. Alfonsi et al. / Tetrahedron Letters 52 (2011) 5145–5148
16. Zhang, D.-H.; Yao, L.-F.; Wei, Y.; Shi, M. Angew. Chem., Int. Ed. 2011, 50, 2583–
In summary, our preliminary results show the feasibility of the
2587. 1H NMR and 13C NMR of compounds 6a: 1H NMR (CDCl3): d = 2.1 (s, 6H),
3.8 (s, 3H), 3.8 (s, 3H), 6.5 (dd, J = 8.6 Hz, J = 2.6 Hz, 1H), 6.6 (d, J = 2.6 Hz, 1H),
6.6 (s, 2H), 7.1 (d, J = 8.6 Hz, 1H), and 13C NMR (CDCl3): d = 10.1, 55.5, 55.7, 99.7,
104.0, 118.6, 119.8, 124.2, 126.1, 153.7, 158.7.
one-pot/one-flask synthesis of functionalized quinolines from
commercial available anilines and propargylic bromide derivatives.
The formation of the quinolines proceeds through the sequential
N-propargylation of aniline derivative followed by regioselective
6-endo-dig transition metal-catalyzed intramolecular hydroaryla-
tion of the N-propargylamine intermediate and aromatization
reaction. NaAuCl4ꢀ2H2O represents a suitable catalyst for the
sequential procedure which occurs under mild reaction conditions
in the eco-friendly ethanol as the reaction medium. The catalytic
efficiency of both Au(III) and Au(I) catalysts in the sequential pro-
cess has been investigated and compared with different transition
metal salts.
17. Zotto, C. D.; Wehbe, J.; Virieux, A.; Campagne, J.-M. Synlett 2008, 2033–2035.
18. Bhilare, S. V.; Darvatkar, N. B.; Deorukhkar, A. R.; Raut, D. G.; Trivedi, G. K.;
Salunkhe, M. M. Tetrahedron Lett. 2009, 50, 893–896.
19. Abbiati, G.; Arcadi, A.; Bianchi, G.; Di Giuseppe, S.; Marinelli, F.; Rossi, E. J. J. Org.
Chem. 2003, 68, 6959–6966.
20. Arcadi, A.; Cerichelli, G.; Chiarini, M.; Di Giuseppe, S.; Marinelli, F. Tetrahedron
Lett. 2000, 41, 9195–9198.
21. Pastine, S. J.; Youn, S. W. D.; Sames, D. Org. Lett. 2003, 5, 1055–1058.
22. Arcadi, A.; Bianchi, G.; Marinelli, F. Synthesis 2004, 610–618.
23. General procedure for the alkylation/gold-catalyzed annulation reactions of
anilines with propargylic bromide derivatives: to
a solution of aniline 1
(2.0 mmol) in absolute ethanol (2 mL) were added propargylic bromide 2
(0.66 mmol) and NaAuCl4ꢀ2H2O (0.05 mmol). The resulting mixture was heated
at 70 °C for 24 h. The reaction was monitored by TLC. After cooling, the solvent
was concentrated under reduced pressure. The residue was dissolved in ethyl
acetate and extracted three times with a saturated solution of NaHCO3. The
combined aqueous extracts were extracted three times with ethyl acetate. The
combined organic extracts were dried (Na2SO4) and evaporated under reduced
pressure. The residue was purified by flash chromatography (silica gel, n-
hexanes–ethyl acetate mixtures) to give quinoline 4. 1H NMR and 13C NMR of
compounds 4. Compound 4a: 1H NMR (CDCl3): d = 3.84 (s, 3H), 3.99 (s, 3H),
6.57 (d, J = 2.5 Hz, 1H), 6.65 (d, J = 2.5 Hz, 1H), 7.30 (dd, J = 8.4 Hz, J = 4.2 Hz,
1H), 7.93 (dd, J = 8.4 Hz, J = 1.6 Hz, 1H), 8.70 (ddd, J = 4.2 Hz, J = 1.6 Hz,
J = 0.55 Hz, 1H). 13C NMR (CDCl3): d = 55.3, 55.8, 96.6, 101.0, 121.9, 129.8,
134.5, 136.7, 146.5, 156.1, 158.1. Compound 4b: 1H NMR (CDCl3): d = 3.95 (s,
3H), 4.03 (s, 3H), 6.75 (d, J = 8.5 Hz, 1H), 6.93 (d, J = 8.5 Hz, 1H), 7.43 (dd,
J = 8.5 Hz, J = 4.3 Hz, 1H), 8.55 (dd, J = 8.5 Hz, J = 1.7 Hz, 1H), 8.95 (dd, J = 4.3 Hz,
J = 1.7 Hz, 1H). 13C NMR (CDCl3): d = 55.7, 55.9, 103.6, 106.8, 120.8, 121.6,
130.9, 140.2, 148.5, 149.2, 149.4. Compound 4c: 1H NMR (CDCl3): d = 1.37 (t,
J = 7.42 Hz, 3H), 3.00 (q, J = 7.42 Hz, 2H), 3.92 (s, 3H), 4.04 (s, 3H), 6.70 (d,
J = 2.47 Hz, 1H), 6.78 (d, J = 2.20 Hz, 1H), 7.22 (d, J = 4.12 Hz, 1H), 8.67 (d,
J = 4.40 Hz, 1H). 13C NMR (CDCl3): d = 13.2, 25.2, 55.2, 55.9, 92.8, 100.3, 120.4,
128.8, 136.5, 146.4, 147.8, 156.7, 157.8. Compound 4d: 1H NMR (CDCl3):
d = 1.27 (t, J = 7.3 Hz, 3H), 3.27 (q, J = 7.4 Hz, 2H), 3.88 (s, 3H), 4.02 (s, 3H), 6.74
(d, J = 8.7 Hz, 1H), 6.90 (d, J = 8.7 Hz, 1H), 7.18 (d, J = 4.4 Hz, 1H), 8.77 (d,
J = 4.5 Hz, 1H). 13C NMR (CDCl3): d = 15.5, 29.8, 55.3, 55.8, 104.3, 106.2, 120.7,
122.1, 141.3, 148.9, 149.5, 150.4, 151.4. Compound 4e: 1H NMR (CDCl3):
d = 2.74 (s, 3H), 3.88 (s, 3H), 4.03 (s, 3H), 6.61 (d, J = 2.5 Hz, 1H), 6.69 (d,
J = 2.5 Hz, 1H), 7.24 (d, J = 8.4 Hz, 1H), 7.88 (d, J = 8.4 Hz, 1H). 13C NMR (CDCl3):
d = 25.2, 55.3, 55.9, 96.7, 100.9, 122.8, 127.9, 135.0, 136.1, 155.3, 155.5, 157.3.
Compound 4f: 1H NMR (CDCl3): d = 2.63 (s, 3H), 3.94 (s, 3H), 4.05 (s, 3H), 6.72
(d, J = 2.5 Hz, 1H), 6.74 (d, J = 2.5 Hz, 1H), 7.23 (d, J = 4.4 Hz, 1H), 8.63 (d,
J = 4.4 Hz, 1H). 13C NMR (CDCl3): d = 19.3, 55.4, 56.0, 93.2, 100.6, 122.9, 129.7,
130.9, 142.5, 146.3, 156.7, 158.0.
Acknowledgments
The authors thank the Ministero dell’Università, dell’Istruzione
e della Ricerca (MIUR), Roma and the Università degli Studi di
L’Aquila for support of this work.
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