Synthesis of 2-Aminoindoles through Gold-Catalyzed C-H Annulations of Sulfilimines with N-Arylynamides

Article abstract of DOI:10.1021/acs.orglett.9b01501

A new gold-catalyzed C-H annulation of sulfilimines with N-phenylynamides is presented. As key intermediates of this operationally simple reaction, the in situ generated α-imino gold carbenes insert into the ortho C-H bonds of the phenyl groups to afford 2-aminoindoles bearing a variety of substitution patterns in high selectivities. This reaction offers a facile approach to biologically important 2-aminoindoles by using inexpensive and readily available starting materials.

Full text of DOI:10.1021/acs.orglett.9b01501

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of 2‑Aminoindoles through Gold-Catalyzed C−H
Annulations of Sulfilimines with N‑Arylynamides
Xianhai Tian,† Lina Song,† Matthias Rudolph,† Frank Rominger,† and A. Stephen K. Hashmi*,†,‡
†Institut fu
̈
r Organische Chemie, Heidelberg University, Im Neuenheimer Feld 270, Heidelberg 69120, Germany
‡Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
S
* Supporting Information
ABSTRACT: A new gold-catalyzed C−H annulation of sulfilimines
with N-phenylynamides is presented. As key intermediates of this
operationally simple reaction, the in situ generated α-imino gold
carbenes insert into the ortho C−H bonds of the phenyl groups to
afford 2-aminoindoles bearing a variety of substitution patterns in high
selectivities. This reaction offers a facile approach to biologically
important 2-aminoindoles by using inexpensive and readily available
starting materials.
base-promoted [4 + 1] cycloaddition of o-aminobenzyl
he indole core, an extraordinarily important structural
Tcomponent, is present in many natural products and
synthetic medicinal compounds.1 2-Aminoindoles are among
the most important of these substrates in pharmaceutical
chemistry.2,3 Beyond the diverse bioactivities shown by
prominent examples (Figure 1), the 2-aminoindole unit can
act as a planar aromatic structure and interact with a protein
target by virtue of the two adjacent hydrogen-bond donors.4
Consequently, the development of methodologies toward 2-
aminoindoles is highly desirable. Traditional methods include
palladium-catalyzed aminations of 2-haloindoles,5 cross-cou-
pling cascade reactions,6 multistep reactions,7 and a recent
chlorides with isocyanides.8 These protocols, however, require
strong bases, high temperatures, harsh substrates, or give
specialized products bearing only special substituents at the 3-
positions of the indole cores. Recently, gold-catalyzed
conversion of ynamides offers new alternatives to highly
regioselective routes to these compounds. Skrydstrup et al.
synthesized 2-aminoindoles through a gold(I)-catalyzed hydro-
amination of ynamides with 2-iodoanilines and a subsequent
palladium-catalyzed cross-coupling reaction (Figure 1).9 Ye’s
group10 developed a gold-catalyzed reaction of the potentially
explosive benzyl azides and N-phenylynamides for the
synthesis of 2-aminoindoles. Hashmi et al. prepared 2-
aminoindoles from anthranils and ynamides with the
restriction of an acyl group at the 7-position of the indole
framework.11a Given these advances and drawbacks, a general,
facile method using safe and readily accessible substrates is still
challenging and of great importance.
Gold catalysis12 involving alkyne activation and subsequent
oxygen,13 carbene,14 as well as nitrene transfer15 reactions
represents one of the most concise and promising complexity-
generating strategies. Aza-heterocycles with labile N−X bonds
(X = C, N, O) including 2H-arizines,16 pyrido[1,2-b]-
indazole,s17 and isoxazole derivatives18 as gold nitrene transfer
reagents facilitate the syntheses of heterocycles (Scheme 1A),
whereas the ring-opening in situ generated α-imino gold
carbenes are mostly trapped by a limited number of
functionalities, mainly originating from the nitrene transfer
reagents. By comparison, ylides and ylide-like reagents such as
azides,19 pyridium aza-ylides,20 sulfonium ylides,14 and
sulfilimines21 have propelled the rapid development of efficient
transformations because of the diversity of trapping function-
alities on the anions (Scheme 1B). While conjugated ylides
Figure 1. Representative bioactive 2-aminoindoles and syntheses of 2-
aminoindoles by means of gold catalysis.
Received: April 29, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01501
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
We began our investigation by employing N-sulfonylsulfili-
mines 1 as a nitrene source. As shown in Scheme 2, under the
Scheme 1. Gold-catalyzed reactions involving carbene
intermediates
a,b
Scheme 2. Reaction Scope
a
Reaction conditions: 1 (0.3 mmol, 1.5 equiv), 2 (0.2 mmol, 1.0
always undergo [3 + 2] dipolar annulations, with non-
conjugated groups on the anions the generated gold carbenes
can be intramolecularly trapped by the functional groups
within the alkynes. An intramolecular C−H insertion reaction
for the synthesis of bicyclic systems can be expected by
introduction of an aromatic group to the alkyne moiety. Unlike
oxidative cyclization22 reactions using N-oxides, with the main
drawback of overoxidation to form a diketone product
(Scheme 1C), gold-catalyzed annulations of N-arylynamides
and benzyl azides efficiently delivered 2-aminoindoles.10
Azides, however, are potentially hazardous and poisonous.
Moreover, nitrogen as a leaving group creates the danger of
explosive decomposition in the scale-up of reactions, which
limits the synthetic applicability of this method. To this end, a
readily available and safe reagent with a mild leaving group is
highly desirable. We recently reported [3 + 2] annulations of
sulfilimines with ynamides for the syntheses of 2-amino-
indoles,21a 4-aminoimidazoles,21a fused 2-aminoimidazoles,21b
and 4-aminooxazles21c by installing different functional groups
(aryls, heteroaryls, iminyls, and acyls) on the nitrogen anions
of sulfilimines (Scheme 1D, path a). We now envisioned a new
synthetic approach to 2-aminoindoles (Scheme 1D, path b).
The generated α-imino gold carbenes insert into the ortho C−
H bonds of the preinstalled aryl groups, providing N,N′-
disubstituted 2-aminoindoles. As nonexplosive waste, the
released sulfides showed no poisoning of the gold catalyst.
equiv), PicAuCl2 (3.8 mg, 5 mol %), toluene (2.0 mL, 0.1 M).
Isolated yields.
b
same reaction conditions (PicAuCl2, 80 °C, toluene) as in our
previous reports,21 N-methylsulfonyl-protected sulfilimine 1a
reacted efficiently with N-phenylynamide 2a and the desired
product 3aa (single-crystal X-ray structure analysis, see Figure
2) was obtained in 82% yield. This reaction was compatible
Figure 2. Solid-state molecular structure of 3aa.
B
DOI: 10.1021/acs.orglett.9b01501
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
with N-phenylsulfonyl-substituted sulfilimine 1b, giving indole
3ba in 50% yield. Ynamides 2b,c with other protecting groups
(Ms, SO2Ph) on the nitrogen atoms were also converted to
products 3ab,ac in good yield. With either electron-rich or
electron-deficient aryl groups on the nitrogen atoms, ynamides
2d−f were suitable substrates to undergo the desired
transformation. 3-Methylphenyl ynamide 2g afforded a mixture
of 4-methylindole 3ag and 6-methylindole 3ag′ with a ratio of
2.1:1 in 84% combined yield (Scheme 3). This C−H
Scheme 4. Synthesis of 2-Aminoindoles 3 or 4 through
Competitive C−H Annulations
Scheme 3. Test of a 3-Methylphenyl Ynamide 2g and a
a
Millimole-Scale Reaction
a
b
Isolated yields. The ratio of 3ag/3ag′ was determined by 1H NMR.
annulation reaction even tolerates an electron-rich dimethox-
yphenyl ynamide, providing 3ah in 87% yield. When R2 was
varied from an electron-poor aryl group (either a phenyl halide
or a trifluoromethylbenzene) to an electron-rich group (p-
methylbenzene) within the ynamides, the reactions proceeded
smoothly and the corresponding products 3ai−ao were
prepared in 49−89% yield. The high-quality success with a
thiophene-containing ynamide is notable. Unfortunately, an
alkyl ynamide reacted sluggishly and unselectively, and the
target indole 3aq was not obtained. Furthermore, the reaction
of 3ab (Scheme 3) could conveniently be conducted on a
mmole scale.
a
Reaction conditions: 1 (0.3 mmol, 1.5 equiv), 2 (0.2 mmol, 1.0
equiv), PicAuCl2 (3.8 mg, 5 mol %), and toluene (2.0 mL, 0.1 M).
a
Isolated yields of products 3 and 4.
The reaction scope was further extended to N-arylsulfili-
mines (Scheme 4). The α-imino gold carbenes from such
ylides could lead to two types of indole products by
competitive C−H insertions. N-Phenyl ylides 1c,d and N-
phenyl ynamides with a methoxy group at the meta-position
were examined. Gratifyingly, the treatment of S,S-dimethylsul-
filimine 1c with the 3-methoxyphenyl-substituted ynamide 2r
delivered N-phenyl-1-tosyl-1H-indol-2-amines 3cr in 87% yield
through a regioselective C−H annulation (Scheme 4). A 3,5-
dimethoxyphenylynamide 2h also showed excellent efficiency
for the preparation of product 3ch. Clearly, the gold carbenes
prefer to insert into the ortho-C−H bonds of the more
electron-rich phenyl group. By reacting with triphenylsulfili-
mine 1d, 3-methoxyphenylynamide 2r gave divergent products
4dr (73% yield) and 3dr (25% yield), while 3,5-dimethox-
yphenyl-derived ynamide 2h afforded a single product 3dh in
75% yield. Notably, the reaction between sulfilimine 1d and 4-
methoxyphenylynamide 2e yielded 4de as a single product.
These results demonstrate that the different electronic
properties of the two nitrogen atoms (the imino nitrogen
atom and the amide nitrogen atom) also have a significant
influence on the C−H insertion processes.
group makes this method a practical and safely scalable
alternative for synthesizing these biologically important
heterocyclic compounds.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01501.
Experimental procedures and compound characteriza-
tion (PDF)
Accession Codes
CCDC 1861977 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
In conclusion, we have described a new gold(III)-catalyzed
C−H annulation of sulfilimines and N-phenylynamides for the
synthesis of 2-aminoindoles. The reaction involves α-imino
gold carbene intermediates. The current report features readily
available substrates, mild conditions, and good substrate
tolerance. Dimethyl (or diphenyl) sulfide as a safe leaving
■
Corresponding Author
*E-mail: hashmi@hashmi.de.
ORCID
A. Stephen K. Hashmi: 0000-0002-6720-8602
C
DOI: 10.1021/acs.orglett.9b01501
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
X.T. and L.S. are grateful to the CSC (China Scholarship
Council) for a PhD fellowship.
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