Tandem gold(III)-catalyzed amination-intramolecular hydroamination reactions of 1-En-4-yn-3-ols with sulfonamides: Efficient approach to highly substituted pyrroles

Article abstract of DOI:10.1002/adsc.200700452

A simple, convenient and efficient synthetic approach to highly substituted pyrroles has been developed by utilizing a gold(III)-catalyzed tandem amination-intramolecular hydroamination reaction. The first examples of gold-catalyzed, selective amination of 1-en-4-yn-3-ols have also been disclosed.

Full text of DOI:10.1002/adsc.200700452

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DOI: 10.1002/adsc.200700452
Tandem Gold(III)-Catalyzed Amination-Intramolecular
A
Hydroamination Reactions of 1-En-4-yn-3-ols with Sulfonamides:
Efficient Approach to Highly Substituted Pyrroles
Xing-Zhong Shu,a Xue-Yuan Liu,a Hui-Quan Xiao,a Ke-Gong Ji,a Li-Na Guo,a
and Yong-Min Lianga,b,*
a
State Key Laboratory of Applied Organic Chemistry, Lanzhou UniVersity, Lanzhou 730000, Peopleꢀs Republic of China
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science,
b
Lanzhou 730000, Peopleꢀs Republic of China
Phone: (+86)-931-891-2593; fax: (+86)-931-891-2582; e-mail: liangym@lzu.edu.cn
Received: September 13, 2007; Revised: November 22, 2007; Published online: January 4, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A simple, convenient and efficient syn-
thetic approach to highly substituted pyrroles has
been developed by utilizing a goldACHTRE(UNG III)-catalyzed
substituted pyrroles have been reported.[10] This
strongly motivated us to extend our research into this
area. In our ongoing efforts to develop new method-
tandem amination-intramolecular hydroamination
reaction. The first examples of gold-catalyzed, se-
lective amination of 1-en-4-yn-3-ols have also been
disclosed.
ologies for the synthesis of heterocycles promoted by
[11]
transition metalcataylsts,
we envisioned that 1-en-
4-yn-3-ols 1, readily available from alkynes and a,b-
unsaturated ketones, might undergo the following
process (Scheme 1), leading to highly substituted pyr-
roles. First, being activated by gold catalysts,[12] prop-
argylic alcohols 1 should undergo an amination pro-
cess with excellent regiochemical selectivity to afford
enynamines 2.[13] Then compounds 2 could undergo
intramolecular hydroamination to afford products II
followed by facile isomerization resulting in the pyr-
Keywords: amination; 1-en-4-yn-3-ols; gold; hydro-
amination; pyrroles
The pyrrole core is an important heteroaromatic roles 3. Herein, we report a convenient synthetic ap-
system found in a diverse array of structures, includ- proach to highly substituted pyrroles by utilizing a
[1]
ing those of many naturalproducts,
biologically goldACHTRE(UGN III)-catalyzed tandem amination-hydroamination
active agents,[2] and components in polymers.[3] The process. In this simple two-step, one-pot reaction,
construction of this important class of heterocycle readily available propargylic alcohols 1 and sulfon-
typically relies on reactions which date from the 19th
ACHTREaUNG mides are used as starting materials to produce pyr-
century, such as the Paal–Knorr cyclization[4] and the role products 3 with high diversity.
Hantzsch synthesis.[5] These classic condensation reac-
We started by treating propargylic alcohol 1a
tions can be limited in their efficiency, functional (0.3 mmol) and TsNH2 (15 equivs.) with 5 mol% of
group compatibility, regiospecificity, and substituent HAuCl4·4H2O in CH3CN (2 mL) and, gratifyingly, the
diversity. Although more recent progress in pyrrole desired product 3a was formed in 21% yield after 8 h,
syntheses including catalytic multicomponent cou- along with uncyclized enynamine 2a (37% yield;
pling approaches[6] and transition metal-based strat- Table 1, entry 1). To our delight, on increasing the
egies[7] looks attractive, a general approach by con- amount of catalyst to 20%, an excellent yield (81%)
verting commercially available or readily accessible of 3a was obtained and no intermediate 2a could be
materials in one step to highly substituted pyrroles is isolated (Table 1, entry 3). With other gold catalysts,
still lacking.
such as NaAuCl4·2H2O, AuCl3, AuCland Au(I) sys-
Gold catalysts have recently been shown to be su- tems generated from silver salts, no superior results
perior reagents in activating alkynes and alkenes to- were obtained (Table 1, entries 4–8). PtCl2 and protic
wards nucleophilic attack,[8] especially in the field of acid such as chloroacetic acid have also been applied
hydroamination[9] which offers an attractive alterna- to the reaction under identicalconditions, but no re-
À
tive means for C N bond formation. However, only action occurred (Table 1, entries 9 and 10). Other sol-
few examples of the gold-catalyzed synthesis of highly vents like CH2Cl2, toluene and THF produced lower
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243

Xing-Zhong Shu et al.
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Scheme 1. Proposed gold
ACHTRE(UNG III)-catalyzed synthesis of substituted pyrroles.
Table 1. Optimization of reaction conditions.[a]
Entry
Catalyst (mol%)
Solvent
Yield [%][b]
2a
3a
1
2
3
4
5
6
7
8
HAuCl4·4H2O (5)
HAuCl4·4H2O (10)
HAuCl4·4H2O (20)
NaAuCl4·2H2O (20)
AuCl3 (20)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
37
15
0
23
0
37
0
0
0
0
0
0
21
46
81
42
51
trace
42
37
0
0
0
24
8
AuCl(20)
CH CN
3
Au
A
CH3CN
CH3CN
CH3CN
CH3CN
THF
4
Au(PPh3)Cl(20)/AgSbF (20)
E
6
9[c]
10[d]
11
12
13
PtCl2 (20)
ClCH2CO2H (20)
HAuCl4·4H2O (20)
HAuCl4·4H2O (20)
HAuCl4·4H2O (20)
CH2Cl2
toluene
0
[a]
Reactions were conducted with 0.3 mmolof 1a, 15 equivs. of TsNH2 (13–14 equivs. of TsNH2 could be recovered after the
reaction in most cases) in 2 mL of solvent at 308C for 8 h.
Isolated yield.
93% of 1a was recovered.
90% of 1a was recovered.
[b]
[c]
[d]
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Tandem GoldACHTREUNG(III)-Catalyzed Amination-Intramolecular Hydroamination Reactions
COMMUNICATIONS
Table 2. GoldACHTREUNG
Entry
Time [h]
Yield [%][b]
1
2
3
4
5
6
7
3a, R=Ph
8
48
8
7
9
81
40
64
75
70
75
74
3b, R=p-CH3OC6H4
3c, R=p-CH3C6H4
3d, R=p-CH3COC6H4
3e, R=p-BrC6H4
3f, R=n-C5H11
2
1
3g, R=H
8
9
10
11
12
3h, R=2-thienyl5
3i, R=p-ClC6H4
3j, R=p-CH3OC6H4
3k, R=p-CH3C6H4
3l, R=m-CH3C6H4
54
7
8
7
4.5
83
25
46
64
13
14
10
78
74
8
15
16
1
96
1
76
89
17[c]
0.5
[a]
Reactions were conducted with 0.3 mmolof 1, 15 equivs. of TsNH2 (13-14 equivs. of TsNH2 could be recovered after the
reaction in most cases), 20 mol% of HAuCl4·4H2O in 2 mL of CH3CN at 308C.
Isolated yield.
[b]
[c]
The Z isomer was always the most stable product.[14]
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Xing-Zhong Shu et al.
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yields of the desired products (Table 1, entries 11–13). sponding products were decreased (entries 10–12).[16]
Thus, the use of HAuCl4·4H2O (20 mol%) in CH3CN Propargylic alcohols with larger ring size, such as a
at 308C was found to be the most efficient and used seven-membered substrate, reacted smoothly to yield
as the standard conditions.
3m in 78% yield (entry 13). Substitution on the ring
With these optimalconditions in hand, we exam-
was also tolerated (entry 14) (Figure 1).[17] Although,
ined the scope of this reaction, as shown in Table 2. 1o–1q could efficiently undergo the formal amination
Compared with previous conclusion[15] that arylacety- in excellent yields (up to 96%), the following intra-
lenes were not effective for the gold
ACHTRE(UNG III)-catalyzed in- molecular hydroamination could not proceed even
tramolecular hydroamination, various aryl substitu- after 24 h under reflux (entries 15–17).
ents on the alkyne, including phenyl ,electron-rich,
Interestingly, with the use of 1r as the substrate,
and electron-poor ones, were compatible with this re- bis(pyrrol-2-yl)arylene 3o was obtained in 61% yield
action, and generally good yields of the corresponding after 8 h (Scheme 2). Such bis(pyrrol-2-yl)arylenes
pyrroles were obtained (entries 1–5). When aliphatic could be used to provide electroactive poly-
and terminalakl ynes were used, the reaction proceed-
[bis(pyrrol-2-yl)arylene] polymers[18] with polypyrrole-
ACHTREUNG
ed faster, furnishing the corresponding products in like electrochromic properties that are useful for bio-
[19]
high yields within 2 h (entries 6 and 7). Alkynes with medicalappilcations.
heteroaromatic groups such as 2-thienylaslo proceed-
Then we investigated various sulfonamides, amides,
ed smoothly (entry 8). Substitution on the allyl and amines under these optimized conditions. Besides
moiety was tolerated. With an electron-withdrawing p-methylphenylsulfonamide, phenylsulfonamide could
aryl group, propargylic alcohol 1i afforded the desired also be employed as an efficient nucleophile (Table 3,
product 3i in 83% yield (entry 9). However, when entry 1). However, with the use of p-bromophenylsulfo-
electron-rich ones were used, the yields of corre- namide or alkylsulfonamides, such as methylsul-
Table 3. Gold
ACHTRE(UNG III)-catalyzed synthesis of substituted pyrroles
3 by cyclization of 1a with sulfonamides.[a]
Entry
R
3 (Yield [%])[b]
1
2
3
4
C6H5
3p (62)
3a (81)
0
p-CH3C6H4
p-BrC6H4
CH3
3q (34)
[a]
Reactions were conducted with 0.3 mmolof
1a, 15
equivs. of RSO2NH2 (13–14 equivs. of RSO2NH2 could
be recovered after the reaction in most cases), 20 mol%
of HAuCl4·4H2O in 2 mL of CH3CN at 308C.
Isolated yield.
[b]
Figure 1. X-ray structure of 3n.
Scheme 2. One pot synthesis of bis(pyrrol-2-yl)arylene 3o.
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Tandem GoldACHTREUNG(III)-Catalyzed Amination-Intramolecular Hydroamination Reactions
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added 24.7 mg (20 mol%) of HAuCl4·4H2O under air at
308C. When the reaction was considered complete as deter-
mined by TLC analysis, the reaction mixture was diluted
with diethylether (20 mL) and evaporated under reduced
pressure. The residue was purified by chromatography on
silica gel to afford the corresponding products.
2a: Compound 2a was prepared according to the above
method, but employing 10 mol% of HAuCl4·4H2O to afford
2a as a solid; yield: 73%; mp: 184–1868C; 1H NMR
(300 MHz, CDCl3): d=7.84–7.81 (d, J=8.1 Hz, 2H), 7.44–
7.17 (m, 12H), 5.93–5.91 (d, J=8.1 Hz, 1H), 5.64–5.61 (d,
J=8.1 Hz, 1H), 2.34 (s, 3H), 2.19–2.13 (m, 1H), 2.04–1.87
(m, 2H), 1.73–1.62 (m, 1H), 1.46–1.29 (m, 4H); 13C NMR
(75 MHz, CDCl3): d=143.2, 142.2, 139.2, 137.2, 131.5, 129.2,
128.5, 128.3, 128.1, 127.5, 127.4, 126.2, 123.3, 118.8, 93.3,
89.0, 60.0, 30.0, 23.9, 21.8, 21.6, 21.4; IR (KBr): n=3268,
2928, 2858, 1597, 1492, 1449, 1328, 1159 cmÀ1; anal. calcd.
for C28H27NO2S: C 76.16, H 6.16, N 3.17; found: C 76.23, H
6.19, N, 3.01.
Scheme 3. Two-step synthesis of pyrrole 3a.
ACHTREUNGfonamide, the reactions gave no or low yields of the de-
sired products (Table 3, entries 3 and 4). Amides and
amines did not work as nucleophiles for this reaction.
In order to confirm the mechanism shown in
Scheme 1 for the one-pot reaction, we next conducted
the reactions, as shown in Scheme 3. After the reac-
tion was run with 10 mol% of HAuCl4·4H2O for 1 h,
an intermediate 2a was isolated in 73% yield along
with 18% of pyrrole 3a. Compound 2a could subse-
quently be converted to 3a in high yield (up to 86%)
under the same conditions. These results strongly sup-
port our proposed two-step mechanism. Compared
with two-step reactions, one-pot procedure with low
amounts of gold catalysts are usually problematic.
This may be due to the noble character of gold, which
is easily deactivated by reduction.[21] The choice of N-
nucleophiles might be crucial, they must exert enough
nucleophilicity in each step of the reaction but should
not deactivate the gold catalysts.[22] Thus sulfonamides
with low Lewis basicity should be the appropriate
choice as a dualnucel ophiel for this process. Further-
more, to compete with the rapid decomposition of the
substrate, the use of an excess amount of sulfona-
mides was also needed.
3a: The reaction mixture was chromatographed using 40:1
hexanes/EtOAc to afford the indicated compound as a
1
solid; yield: 107.2 mg (81%); mp: 118.5–119.58C; H NMR
(300 MHz, CDCl3): d=7.33–7.17 (m, 10H), 7.04–6.97 (m,
4H), 4.30 (s, 2H), 2.47–2.42 (m, 2H), 2.31 (s, 3H), 2.27–2.23
(m, 2H), 1.67–1.63 (m, 2H), 1.60–1.55 (m, 2H); 13C NMR
(75 MHz, CDCl3): d=143.7, 139.9, 136.3, 132.4, 131.2, 131.1,
130.0, 129.0, 128.4, 128.2, 127.3, 127.1, 126.7, 125.8, 125.5,
125.0, 32.1, 23.3, 23.2, 22.4, 22.1, 21.5; IR (KBr): n=3028,
2930, 2858, 1712, 1598, 1492, 1446, 1367, 1171 cmÀ1; anal.
calcd for C28H27NO2S: C 76.16, H 6.16, N, 3.17; found: C
76.21, H 6.11, N 3.12.
Acknowledgements
We thank the NSF (NSF-20621091, NSF-20732002) and the
“Hundred Scientist Program” from the Chinese Academy of
Sciences for financial support.
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In summary, we have developed a simple, conven-
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tuted pyrroles utilizing a goldCAHTRE(UNG III)-catalyzed tandem
amination-intramolecular hydroamination reaction
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[17] The molecular structure of the corresponding product
3n was determined by X-ray crystallography (Figure 1).
X-ray data for compound 3n: C29H29NO2S, MW =
455.59, T=273(2) K, l=0.71073 ꢂ, monoclinic space
group, P21/c, a=8.6485(10) ꢂ, b=32.699(3) ꢂ, c=
9.0126(12) ꢂ, a=908, b=109.287(2)8, g=908, V=
2405.7(5) ꢂ3, Z=4, 1cald =1.258 mgmÀ3, m=0.161 mmÀ1,
FACHTREUNG
AHCTREUNG
collected 12226, refinement method, full-matrix least-
squares on F2, goodness-of-fit on F2 1.015, final R indi-
ces [I>2s(I)] R1 =0.0437, wR2 =0.1278, R indices (all
date) R1 =0.0489, wR2 =0.1318, largest diff. peak and
hole 0.207 and À0.355 eꢂÀ3. Supplementary crystallo-
graphic data have been deposited at the Cambridge
Crystallographic Data Centre: CCDC 660656.
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[22] Strong N-nucleophiles (or N-nucleophiles with strong
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Adv. Synth. Catal. 2008, 350, 243 – 248