Rh(III)-Catalyzed Synthesis of N -Unprotected Indoles from Imidamides and Diazo Ketoesters via C-H Activation and C-C/C-N Bond Cleavage

Article abstract of DOI:10.1021/acs.orglett.5b03669

(Chemical Equation). The synthesis of N-unprotected indoles has been realized via Rh(III)-catalyzed C-H activation/annulation of imidamides with α-diazo β-ketoesters. The reaction occurs with the release of an amide coproduct, which originates from both t

Full text of DOI:10.1021/acs.orglett.5b03669

Letter
pubs.acs.org/OrgLett
Rh(III)-Catalyzed Synthesis of N‑Unprotected Indoles from
Imidamides and Diazo Ketoesters via C−H Activation and C−C/C−N
Bond Cleavage
Zisong Qi, Songjie Yu, and Xingwei Li*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China
S
* Supporting Information
ABSTRACT: The synthesis of N-unprotected indoles has been realized via Rh(III)-catalyzed C−H activation/annulation of
imidamides with α-diazo β-ketoesters. The reaction occurs with the release of an amide coproduct, which originates from both
the imidamide and the diazo as a result of CN cleavage of the imidamide and C−C(acyl) cleavage of the diazo. A rhodacyclic
intermediate has been isolated and a plausible mechanism has been proposed.
rguably, indoles are one of the most important heterocycles
redox-neutral conditions to furnish various heterocycles.13 On
Awidely present in natural products, pharmaceuticals, and the other hand, in the rhodium(III)-catalyzed oxidative
agrochemicals.1 Therefore, numerous efficient methodologies to
annulation between imidamides and alkynes, the steric bulk of
the C-phenyl of imidamides has a significant influence on the
access this motif have been developed,2 the majority of which
reaction selectivity.14 As a continuation of our interest in Rh(III)-
catalyzed C−H activation of arenes and construction of
heterocycles,15 we reasoned that changing the C-aryl of
imidamides to a C-alkyl group may favor the selectivity of the
C−H activation at the N-aryl ring, leading to efficient indole
synthesis. We now report a convenient, redox-neutral approach
to synthesize unprotected indoles via Rh(III)-catalyzed C−H
activation of imidamides in the coupling with diazo compounds,
and this reaction occurred with cleavage of C−C and CN
bonds.
involve Fischer-type indole synthesis.3 In addition, metal-
catalyzed cyclization of o-alkynylanilines,4 annulation of ortho-
halogenated anilines with alkynes,5 reductive cyclization of 2-
substituted nitroarenes,6 and other coupling/condensation
cascades are also well-known.7 However, most of these
approaches still suffer from limited availability or high cost of
starting materials.
Recently, transition-metal-catalyzed C−H bond activation has
emerged as an important strategy in organic syntheses,8 and
several indole syntheses via C−H activation have been reported.
Takemoto reported synthesis of indoles via activation of benzylic
C−H bonds and insertion into isocyanides.9 Cross-dehydrogen-
ative coupling reactions have also been employed as useful
synthetic tools for indoles.10 In 2008, Fagnou and co-workers
reported an elegant Rh(III)-catalyzed intermolecular oxidative
annulation between internal alkynes and acetanilides to access N-
protected indoles.11 Afterward, Pd-, Rh-, and Ru-catalyzed indole
syntheses via C−H activation were widely explored under
oxidative or redox-neutral conditions.12 For example, the Glorius
group reported a Rh(III)-catalyzed C−H activation/cyclization
approach to access NH indoles by taking advantage of an
oxidizing directing group.12f Very recently, Wan and co-workers
developed a regioselective synthesis of unsymmetrically 2,3-
disubstituted NH indoles via Rh(III)-catalyzed annulation of
nitrones with symmetrical alkynes.12i Despite these significant
advances, direct access to unsymmetrically 2,3-disubstituted
protic indoles using a C−H activation strategy still needs further
exploration.
We initiated our investigations with the coupling of N-
phenylacetimidamide (1a) with ethyl 2-diazo-3-oxobutanoate
(2a) using [RhCp*Cl2]2 as a catalyst in the presence of CsOAc.
To our delight, indole 3aa was isolated in 56% yield (Table 1,
entry 1). Employment of other bases such as NaOAc and CsOPiv
gave lower yields (entries 2 and 3). Further screening of solvents
revealed that acetone, MeCN, and MeOH were inferior to DCE
(entries 4−6). Gratifyingly, addition of HOAc (0.2 equiv)
resulted in formation of desired product 3aa in 74% yield (entry
7). This encouraged us to further increase the amount of HOAc.
Product 3aa was isolated in 79% yield when the amount of HOAc
was increased to 0.5 equiv (entry 8), and the optimal yield (89%)
was obtained when 1.5 equiv of HOAc was introduced (entries
9). Control experiments revealed that no reaction occurred when
the rhodium catalyst was omitted (entry 11). Thus, the following
conditions were eventually established for subsequent studies:
[RhCp*Cl2]2 (4 mol %), CsOAc (30 mol %), and HOAc (1.5
equiv) in DCE at 80 °C for 12 h.
It has been recently demonstrated that rhodium- and iridium-
catalyzed C−H activation of arenes bearing a protic directing
group in the coupling with diazo compounds occurred under
Received: December 26, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03669
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Letter
a
Table 1. Screening of Reaction Parameters
Scheme 1. Rh(III)-Catalyzed Synthesis of N-Unprotected
a
Indoles
b
entry
additive (equiv)
base
solvent
yield (%)
1
2
3
4
5
6
7
8
9
10
CsOAc
NaOAc
CsOPiv
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
DCE
56
42
27
31
38
<5
74
79
89
88
nd
DCE
DCE
acetone
MeCN
MeOH
DCE
HOAc (0.2)
HOAc (0.5)
HOAc (1.5)
HOAc (2.0)
HOAc (1.5)
DCE
DCE
DCE
c
11
DCE
a
Reaction conditions: 1a (0.2 mmol), 2a (1.5 equiv), [RhCp*Cl2]2 (4
mol %), additive, base (30 mol %), solvent (2 mL), at 80 °C for 12 h.
b
c
Isolated yield. No [RhCp*Cl2]2 was used.
With the optimized reaction conditions in hand, we first
examined the scope and limitations of the diazo substrate in the
coupling with 1a (eq 1 and Scheme 1, 3aa−ae). Surprisingly, the
a
Reaction conditions: 1 (0.3 mmol), 2 (0.45 mmol), [RhCp*Cl2]2 (4
couplings of ethyl 2-diazo-3-oxo-3-phenylpropanoate (2f) and
ethyl 2-diazo-3-oxopentanoate (2g) with 2a all afforded the same
product (3aa) in good yields (eq 1). Furthermore, benzamide
was detected as a coproduct by GC−MS in the coupling of 1a
with 2f, suggesting the relevancy of C−N bond cleavage of
imidamides and C−C(acyl) bond cleavage of diazo com-
pounds.16 Thus, the coupling for symmetrical α-diazo
acetylacetone (2e) and other unsymmetrical diazo compounds
delivered protic indoles in 51−90% yields (3ab−ae). We next
investigated the scope of the imidamide substrate (Scheme 1).
Acetimidamides bearing methyl, tert-butyl, and halo group at the
para position of the N-phenyl ring all underwent smooth
coupling and afforded indoles 3ba−fa in 79−89% yields.
Acetimidamides bearing methyl and halo group at the meta
position also reacted smoothly in good yields (3ga−ja), and the
coupling occurred at the less hindered ortho site. However, C−H
activation occurred exclusively at the more hindered ortho site for
a m-fluoro-substituted imidamide (1j), as elucidated from 1H and
13C NMR spectroscopic analyses of 3ja. Moreover, ortho-
substituted imidamides are also applicable as in the isolation of
products 3ka and 3la in 80% and 66% yields, respectively.
Notably, several imidamides bearing two substitutes (1m−s) in
the phenyl ring have also been examined, affording the
corresponding indoles (3ma−sa) in moderate to good yields
and in high regioselectivity. In addition, the cyclization system
could be extended to other N-phenylalkylimidamides (1ab−ak),
furnishing the desired products in moderate to good yields
(3aba−aka). In some cases, the imidamide was used in excess to
facilitate the isolation of the products 3ada−aga. In contrast to
these efficient couplings, N-phenylbenzimidamide only reacted
with poor efficiency under the standard conditions.
mol %), CsOAc (30 mol %), HOAc (0.3 mmol), DCE (3 mL), at 80
°C for 12 h, isolated yields. The reaction of 1a with ethyl 2-diazo-3-
b
oxo-3-phenylpropanoate (2f) or ethyl 2-diazo-3-oxopentanoate (2g)
afforded 3aa in 83% and 85% isolated yields, respectively. 1 (0.36
mmol) and 2 (0.3 mmol) were used.
c
The synthetic applications of indoles have been well-
documented. For example, 3aa could be converted to 4, an
analogue of the seroonin antagonist ICS 205-903 and to 5 that is
known to exhibit anti-inflammatory and analgesic activities
(Scheme 2).17
Scheme 2. Synthetic Utilities of an Indole Product
To probe the reaction mechanism, several experiments have
been performed. An intermolecular competitive reaction has
been performed using an equimolar mixture of 1a and 1a-d5 in
the coupling with 2a. 1H NMR analysis of the product mixture
revealed a kinetic isotope effect (KIE) of 3.3 at a low conversion,
indicating that the C−H bond cleavage may be involved in the
turnover-limiting step (see the SI). In another experiment, a
stoichiometric reaction between 1a and [RhCp*Cl2]2 was
performed in the presence of NaOAc from which a rhodacyclic
complex (A) was isolated in 71% yield (eq 2 and Figure 1).
B
DOI: 10.1021/acs.orglett.5b03669
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Letter
intramolecular nucleophilic addition. The product 3aa is
eventually formed from E by elimination of one molecule of
AcNH2.
In summary, we have developed a redox-neutral and efficient
synthetic route to access N-unprotected indoles by Rh(III)-
catalyzed C−H activation of readily available imidamides in the
coupling with α-diazo β-ketoesters. This reaction proceeded
under relatively mild conditions with broad substrate scope.
Interestingly, this coupling occurred with the release of a
molecule of amide, which originated from the nitrogen of the
imidamide and the acyl group of the α-diazo β-ketoester as a
result of the CN cleavage of the imidamide and C−C(acyl)
cleavage of the diazo. The coupling occurred via a C−H
activation mechanism with the isolation of a rhodacyclic
intermediate. This method to access unprotected indoles and
the cleavage of C−C bond of diazos may find applications in the
synthesis of complex structures.
ASSOCIATED CONTENT
* Supporting Information
■
Figure 1. ORTEP drawing of the molecular structure of complex A
S
(hydrogen atoms were omitted for clarity).
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03669.
Complex A, which has been fully characterized by NMR, mass
spectrometry, and X-ray crystallography, proved to be an active
catalyst because when it was designated as a catalyst (8 mol %)
for the coupling of 1a with 2a, the product 3aa was isolated in a
comparable yield (82%, eq 3).
Crystallographic data of complex A (CIF)
Experimental procedure, characterization of the products,
1
and copies of the H and 13C NMR spectra of selected
products (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: xwli@dicp.ac.cn.
Notes
On the basis of these observations and literature prece-
dents,12i,13i a plausible mechanism is given in Scheme 3. First, an
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Scheme 3. Proposed Mechanism
■
Financial support from the NSFC (Nos. 21525208 and
21272231) and the dedicated grant for new technology of
methanol conversion from Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, is gratefully acknowledged.
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D
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