Gold(III)-Catalyzed Site-Selective and Divergent Synthesis of 2-Aminopyrroles and Quinoline-Based Polyazaheterocycles

Article abstract of DOI:10.1002/anie.201810369

A facile, site-selective, and divergent approach to construct 2-aminopyrroles and quinoline-fused polyazaheterocycles enabled by a simple gold(III) catalyst from ynamides and anthranils under mild reaction conditions is described. This one-pot strategy uses readily available starting materials, proceeds in a highly step- and atom-economical manner, with broad substrate scope and scale-up potential. The key element for success in this tandem reaction is a catalyst-directed preferred quenching of the in situ generated gold carbene intermediates by a nucleophilic benzyl/2-furylmethyl moiety on the ynamides as an alternative to the known C?H annulation leading to indoles.

Full text of DOI:10.1002/anie.201810369

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Accepted Article
Title: Gold(III)-Catalyzed Site-Selective and Divergent Synthesis of 2-
Aminopyrroles and Quinoline-Based Polyazaheterocycles
Authors: Zhongyi Zeng, Hongming Jin, Matthias Rudolph, Frank
Rominger, and A. Stephen K. Hashmi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201810369
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Angewandte Chemie International Edition
10.1002/anie.201810369
Gold(III)-Catalyzed Site-Selective and Divergent Synthesis of 2-Aminopyrroles
and Quinoline-Based Polyazaheterocycles
Zhongyi Zeng,+,a Hongming Jin, Matthias Rudolph, Frank Rominger, and A.
+,a
a
a
Stephen K. Hashmi*,
a,b
aOrganisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im
Neuenheimer Feld 270, 69120 Heidelberg, Germany
b
Chemistry Department, Faculty of Science, King Abdulaziz University (KAU), 21589
Jeddah, Saudi Arabia
E-mail: hashmi@hashmi.de
Homepage: http://www.hashmi.de
+
[
] These authors contributed equally.
Abstract: We herein describe a facile, site-selective and divergent approach to
construct 2-aminopyrroles and quinoline-fused polyazaheterocycles enabled by a
simple gold(III) catalyst from ynamides and anthranils under mild reaction conditions.
This one-pot strategy uses readily available starting materials, proceeds in a highly step-
and atom-economical manner, with broad substrate scope and scale-up potential. The
key element for success in this tandem reaction is a catalyst-directed preferred
quenching of the in-situ generated gold carbene intermediates by a nucleophilic
benzyl/2-furylmethyl moiety on the ynamides as an alternative to the known C–H
annulation leading to indoles.
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The efficient and rapid synthesis of aza-heterocycles is of high significance for
[
1]
[2]
[3]
synthetic and medicinal chemistry and material science. 2-Aminopyrroles as well
[
4]
as quinoline-fused poly-aza-heterocycles, such as pyrrolo[2,3-b]quinoline and
dihydrodibenzo[b,f][1,8]naphthyridine derivatives,
[
5,6f]
are fundamental motifs in
numerous pharmaceuticals, natural products, and organic functional materials. Several
synthetic routes towards these quinoline-fused polyazaheterocycles have been
[
6]
developed. However, most methods involve multiple-step syntheses from the relevant
[6]
quinoline precursors, and suffer from low functional group tolerance. More important,
the installation of a propenal side chain to the parent pyrrole ring is also a challenge,
making it difficult to construct the pyrrolo[2,3-b]quinolines with such a valuable
grip.[
6a-d]
To this end, the development of a novel, general, and environmentally benign
method towards these polyazaheterocycles in a selective and divergent fashion from
easily accessible starting materials is still highly desired.
Recent rapid advances in gold carbene-promoted reactions have provided efficient and
[
7]
powerful strategies to access complex and diverse carbon- and azaheterocycles. In
this context, gold-catalyzed intra-/intermolecular nitrene transfers of azide moieties
onto carbon-carbon triple bonds were used to prepare different azaheterocycles,
[8a-c]
[8d-h]
[8i]
[8j-m]
[8n-o]
including pyrroles,
indoles,
imidazoles, (iso)quinolines,
and others.
The azide moieties were generally introduced from sodium azide, a toxic and
potentially explosive reagent. Intermolecular protocols to access α-imino gold carbene
intermediates from non-toxic, safe and easily available nitrenoid precursors, are more
practical and flexible. In line with this principle, various nitrenoid equivalents have
been realized for intermolecular nitrene-transfer processes to ynamides, generating
[
9]
highly electrophilic gold carbene intermediates A. Typically, a nucleophilic
functionality tethered on the nitrene-transfer reagent traps the gold carbene, leading to
[
9]
formal [3+2] cycloadducts (Scheme 1, top); or a new C–C double bond is formed via
[
9a,9o]
an 1,2-hydride shift if the α-hydrogens exist (middle).
The site-selective trapping
1
by the R substituent at the ynamide is more challenging, as the competing nucleophilic
site Z can induce undesired side reactions. Recently, we disclosed a gold(I)-catalyzed
synthesis of 7-acylindoles via the C–H annulation between alkynes and anthranils by
a
[
9n]
exploiting the potential binucleophility of anthranils (Scheme 2, path a).
We
envisioned that the in-situ generated α-imino gold carbene species A1 could be
b
a
quenched by the C–H bond of N-benzyl ynamides as alternative to the C–H insertion
[9n]
known from the indole synthesis.
Finally, the desired quinoline-embedded
polyazaheterocycles would be formed via an nucleophilic addition of the enamin to the
aldehyde and a subsequent elimination from intermediate B1. In this context a related
approach using N-aryl ynamides as competing nucleophile, that was reported during
[
10]
the preparation of the manuscript must be mentioned here.
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Angewandte Chemie International Edition
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Nucleophilic nitrenoid precursors include:
repesents nucleophilic site
R
R'
O
R
O
O
O
Het
N
N
N
N
X
N
N
N
N
N
N
O
N
X = O, NR(H)
R2
Au]
Y
Z
GP
Y
N
N
R1
[3+2] annulation
reaction with Z
[
Refs [9b-n, 9p-t]
GP
N
R1
R2
Y
N
Z
R2
1
,2-hydride shift
N
Z
GP
reaction with R2
Refs [9a, 9o]
N
GP
1
N
R
R
[Au]
R1
main challenge:
nucleophilic competition with Z
Z = C, O, N
This
work
A
reaction with R1
Scheme 1. Different site-selective reaction patterns of α-amino gold carbene
intermediates.
Scheme 2. Site-selective C–H functionalization of the gold carbene intermediates. path
a: our previous work; path b: present new initial design.
To evaluate the feasibility, N-benzyl ynamide 1a and anthranil 2a were initially chosen
as the model substrates (Table 1). After screening, a “standard condition” (5 mol%
o
o
KAuBr in 1,2-DCE at –20 C for 2 h; then heating to 40 C for 10 h) was developed,
4
which delivered product 3a in 50% isolated yield accompanied by a small amount of
the undesired indole product 4a (Table 1, entry 1). Other tested gold(III) catalysts gave
a similar ratio of 3a and 4a (entries 2–5). Switching to a cationic gold(I) complex
favored the formation of indole product, the unactivated catalyst showed an unselective
reaction (entries 6–7). The neutral ligand-free AuCl also worked in a selective manner,
affording 3a in 43% yield (about 5:1 3a:4a; entry 8). PhCF as reaction medium did not
3
improve the efficiency (entry 9). The control experiment in the absence of catalyst
showed no conversion (entry 10).
Table 1: Representative examples from the optimization of the reaction conditions.[a]
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Angewandte Chemie International Edition
10.1002/anie.201810369
entr deviation from “standard”
yield of
yield of
4a
[
b]
[b]
y
conditions
3a
4%
5
1
None
9
(50%)
2
3
4
5
6
7
8
9
NaAuBr
HAuBr
AuBr as catalyst
PicAuCl as catalyst
IPrAuCl/AgNTf as catalyst
4
as catalyst
as catalyst
50
49
45
47
14
15
17
14
37
Trace
8
4
3
2
2
27
IPrAuCl as catalyst
10 mol % AuCl as catalyst
trace
43
48
-
PhCF
3
instead of 1,2-DCE
12
-
10
no KAuBr
4
[a] “Standard” conditions: a solution of 1a (0.1 mmol) in 1,2-DCE
(
0.5 mL) was added over 5 mins to a mixture of 2a (0.15 mmol),
o
KAuBr
4
(5 mol%) in 1,2-DCE (0.5 mL) at –20 C and after 2 h the
o
1
reaction mixture was heated to 40 C for 10 h. [b] Measured by H
NMR with 1,3,5-trimethoxybenzene as the internal standard. Yield
of isolated product given in parentheses.
Under the optimized conditions, a diverse set of N-benzyl ynamides with different
anthranils were first tested (Table 2). Ynamides bearing different protecting groups (Ms,
Ts, Bs, Ns, SO Ph) on nitrogen reacted quite well with anthranil 2a, giving 3a–e in
2
satisfying yields. A scale-up (1 mmol) synthesis of 3e provided a slightly lower yield.
A tolyl group in 3f delivered 58% yield. Various substituted anthranils were then
2
examined. Besides a phenyl group (3g), methyl or hydrogen at the R position afforded
the desired products 3h or 3i in 48% and 62% yields, respectively. A less-polar carbon-
[9n]
carbon triple bond, that was also reactive in our previous work, remained intact. The
obtained product 3j emitted violet-blue fluorescence (Φ = 0.21), somewhat
F
showcasing the importance of the compatibility with such a triple bond.Some other
functional groups including chloride (3k), bromide (3l), ester (3m) and acetal (3n) were
well tolerated, thus allowing further derivatization. If an aryl group was introduced at
the C6 position of anthranils, 3o was obtained in 60% yield. For the structural
assignments of these products, single crystal X-ray structure analyses of 3b and 3e were
[
10]
conducted.
Table 2: Substrate scope of N-benzyl ynamides and anthranils.[a]
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Angewandte Chemie International Edition
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Electron-rich heteroaromatic systems tethered to the ynamide also can serve as an
alternative nucleophilic site to trap the gold carbene intermediate were also tested.
Under the above optimum condition, it was found that the treatment of N-furanylmethyl
ynamide 5a and anthranil 2a proceeded well. However, the above expected tetracyclic
N-heterocycles derived by C3–H insertion into the gold carbene were not obtained;
instead pyrrole (E)-6a was isolated in 69% yield. Despite being thermodynamically less
stable than its (E)-isomer, the isolation of (Z)-6a was also viable by just omitting the
subsequent warming, which suggests the latter was likely to be generated first via a ring
opening of furan by the regioselective attack of the C2-position to the gold carbene
(
Scheme 3). The primary product can be further transformed to the thermodynamically
stable (E)-isomer 6a or to tricyclic azaheterocycles (E)-7a via a Friedel-Crafts-type
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Angewandte Chemie International Edition
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cyclization driven by heating or/and a Lewis acid.
Scheme 3. Proposed mechanism for the formation of 2-aminopyrroles and related
products.
This synthetic strategy opens the route to 2-aminopyrroles 6 with a propenal side chain
at C4 position. At this position such a versatile handle is not easy to be introduced,
[
12]
which distinguishes this method for pyrrole synthesis. Besides, a myriad of potential
derivatizations for the enal moiety could be anticipated, such as cycloaddition and some
[
13]
textbook transformations (oxidation, reduction and Michael addition). With this in
mind, various 2-aminopyrroles 6 were first investigated from anthranils and N-
furanylmethyl ynamide 5a at the temperature shown in Table 3. Aromatic (6b),
2
aliphatic (6c and 6d) and electronically neutral substituents (6f–i) as R provided the
desired products in moderate to good yields. Again, a carbon-carbon triple bond could
be pre-installed on the anthranil substrate (6e). Other functional groups such as chloride
(
6a), methoxy (6d and 6f), ester (6g), acetal (6h) were tolerated well. While internal N-
benzyl ynamides were unsuccessful for the synthesis of 3, the corresponding internal
N-furanylmethyl derivative was converted into trisubstituted pyrrole 6j. At a slightly
high reaction temperature, some pyrrole-fused compounds 7 were also accessible in
3
5–59% yield in a one-pot process. A 45% yield of 7a was isolated on an 1 mmol scale
[
11]
and its constitution was verified by single crystal X-ray diffraction.
Table 3: Scope of the reaction between N-furanylmethyl ynamides and anthranils.[a]
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Angewandte Chemie International Edition
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In conclusion, an atom-economical, site-selective, divergent assembly of valuable
and versatile 2-aminopyrroles and quinoline-fused polyazaheterocycles has been
achieved. This protocol was enabled by a simple gold(III) catalyst leading to an
preferable quenching of the gold carbene intermediate by a nucleophilic functionality
on the ynamides, complementary to the C–H annulation known from the indole
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Angewandte Chemie International Edition
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synthesis. This will also open up a new window of opportunity for challenging
intramolecular site-selective C–H functionalization. Unlike known synthetic methods,
our strategy features a one-step operation, easily accessible starting materials, good
functional-group tolerance, and scale-up potential accompanied by a broad substrate
scope. Further studies on the application of anthranils for the synthesis of (π-extended)
N-doped polycyclic aromatics are ongoing in our lab.
Acknowledgements
Z. Z. is grateful to the Guangzhou Elite Scholarship Council for a Ph.D. sponsorship,
H. J for Ph.D. fellowships from the CSC (China Scholarship Council).
Keywords: aminopyrroles • quinoline-embedded polyazaheterocycles • α-imino gold
carbenes • anthranils • site-selective and divergent synthesis
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