Catalytic Synthesis of Substituted Indoles and Quinolines from the Dehydrative C-H Coupling of Arylamines with 1,2- and 1,3-Diols

Article abstract of DOI:10.1021/acs.organomet.6b00273

The cationic ruthenium-hydride complex catalyzes the dehydrative C-H coupling reaction of arylamines with 1,2-diols to form the indole products. The analogous coupling of arylamines with 1,3-diols afforded the substituted quinolines. The catalytic method directly forms these coupling products in a highly regioselective manner without generating any toxic byproducts.

Full text of DOI:10.1021/acs.organomet.6b00273

Article
pubs.acs.org/Organometallics
Catalytic Synthesis of Substituted Indoles and Quinolines from the
Dehydrative C−H Coupling of Arylamines with 1,2- and 1,3-Diols
Hanbin Lee and Chae S. Yi*
Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881, United States
S
* Supporting Information
ABSTRACT: The cationic ruthenium-hydride complex catalyzes
the dehydrative C−H coupling reaction of arylamines with 1,2-
diols to form the indole products. The analogous coupling of
arylamines with 1,3-diols afforded the substituted quinolines. The
catalytic method directly forms these coupling products in a highly
regioselective manner without generating any toxic byproducts.
group tolerance, and relatively harsh reaction conditions (>170
°C).
INTRODUCTION
■
To promote synthetic efficiency and to reduce environmental
pollutants resulting from the formation of byproducts,
concerted research efforts have been directed to devise
transition metal catalyzed C−H coupling methods for the
synthesis of biologically active nitrogen heterocyclic com-
pounds.1 Catalytic C−H coupling methods have been shown to
provide step-efficient routes to indole and related nitrogen
heterocyclic compounds, while alleviating inherent problems
associated with the classical methods such as requiring
prefunctionalized substrates and wasteful byproduct formation.2
Most notably, a number of oxidative C−H coupling methods of
arylamines with unsaturated hydrocarbon substrates have been
successfully developed for the synthesis of indole and quinoline
compounds,3 and very recently, the analogous C−H coupling
reactions have been achieved by using earth-abundant Co and
Ni catalysts.4 Since Watanabe’s seminal report on the
ruthenium-catalyzed annulation of aniline with diols,5 the
dehydrative C−H coupling methods of arylamines with diols
have emerged as a sustainable green catalytic protocol for the
synthesis of nitrogen heterocycles. A number of groups
employed Ru catalytic systems to effect the intermolecular
couplings of anilines with diols and glycols in combination with
trialkylammonium salts to synthesize indole and quinoline
derivatives.6 Larock’s Pd-catalyzed coupling and annulation
methods have been successfully utilized to synthesize a variety
of substituted indole products.7 Using the IrCl3/BINAP catalyst
system, Ishii achieved a highly regioselective coupling of
naphthylamines with 1,2- and 1,3-diols to give indole and
quinoline derivatives.8 Recently, a number of atom-economical
C−H cross-coupling methods have been used to promote
intramolecular annulation of N-arylimines and enamines.9 So
far, the synthetic utility of these catalytic coupling methods has
been greatly limited because of the requirement of an excess
amount of prefunctionalized nitrogen substrates, difficulty in
controlling regioselectivity as well as the lack of functional
We recently reported that the well-defined cationic
ruthenium-hydride complex [(C6H6)(PCy3)(CO)RuH]+BF4
−
(1) is a highly effective catalyst for the dehydrative C−H
coupling reaction of phenol with diols to afford benzofuran
derivatives.10 We reasoned that the analogous coupling of
arylamines with diols might lead to the formation of indoles.
Herein, we disclose an effective catalytic synthesis of substituted
indoles and quinolines from the dehydrative coupling reactions
of arylamines with diols. The catalytic method employs readily
available amine and diol substrates and gives the coupling
products without using any reactive reagents or forming any
wasteful byproducts.
RESULTS AND DISCUSSION
■
Catalyst Screening and Optimization Studies. We
initially screened suitability of the coupling reaction between
aniline and 1,2-diols by using the Ru catalyst 1. The treatment
of aniline (0.5 mmol) with 1-phenyl-1,2-ethanediol (0.75
mmol) in the presence of 1 (3 mol %) and cyclopentene
(1.5 mmol) in 1,4-dioxane (2 mL) was heated at 110 °C for 14
h (eq 1). The product 3a was cleanly formed, albeit with a low
conversion (ca. 20%), as analyzed by both GC and NMR
methods.
To attain optimized reaction conditions, we next screened
both Ru catalysts and additive effects for the coupling reaction
(Table 1). The cationic Ru−H complex, either in isolated form
of the complex 1 or in situ generated from the tetranuclear Ru
complex [(PCy3)(CO)RuH]4(μ-O)(μ-OH)2 (2), was found to
be the most effective among the screened ruthenium catalysts
(entries 2, 5).11 We also found that the addition of a catalytic
amount of HBF4·OEt2 to the Ru catalyst 1 led to a dramatic
improvement in the product yield (entry 2), although the
Received: April 6, 2016
© XXXX American Chemical Society
A
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Table 2. Dehydrative Coupling of Arylamines with 1,2-
a
Diols
Table 1. Catalyst Screening for the Dehydrative Coupling of
a
Aniline with 1-Phenyl-1,2-ethanediol
b
entry
catalyst
additive
yield (%)
[RuH(C6H6)(CO)(PCy3)]+BF4 (1)
19
83
0
−
1
[RuH(C6H6)(CO)(PCy3)]+BF4 (1)
HBF4·OEt2
−
2
3
[RuH(CO)(PCy3)]4(O)(OH)2 (2)
[RuH(CO)(PCy3)]4(O)(OH)2 (2)
[RuH(CO)(PCy3)]4(O)(OH)2 (2)
RuHCl(CO)(PCy3)2
RuHCl(CO)(PCy3)2
RuCl2(PPh3)3
4
NH4PF6
18
96
0
5
HBF4·OEt2
6
7
HBF4·OEt2
HBF4·OEt2
HBF4·OEt2
NH4PF6
23
<3
42
0
8
9
RuCl2(PPh3)3
10
11
12
13
14
15
16
17
18
19
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
Ru3(CO)12
0
0
Ru3(CO)12
0
RuH2(CO)(PPh3)3
0
[RuH(CO)(CH3CN)2(PCy3)2]+BF4
0
−
RuCl3·3H2O
0
[Ru(COD)Cl2]x
HBF4·OEt2
<3
0
Cy3PH+BF4
−
HBF4·OEt2
0
a
Reaction conditions: aniline (0.5 mmol), 1-phenyl-1,2-ethanediol
(0.75 mmol), catalyst (3 mol % Ru), additive (7 mol %), cyclopentene
a
b
Reaction conditions: amine (1.0 mmol), 1,2-diol (1.5 mol), 2 (0.75
mol %), cyclopentene (3.0 mmol), HBF4·OEt2 (7 mol %), 1,4-dioxane
(1.5 mmol), 1,4-dioxane (2 mL), 110 °C, 14 h. The product yield was
determined by 1H NMR using hexamethylbenzene as an internal
standard.
b
1
(3 mL), 14−16 h. The product yield was determined by H NMR.
12−15). The coupling reaction of 1-naphthylamine with 1,2-
indandiol and 1,2-cyclooctanediol gave polycyclic indole
products 3p and 3q, respectively (entries 16, 17). The catalytic
method achieves regioselective synthesis of indoles from the
direct coupling of arylamines with 1,2-diols without employing
any reactive reagents or forming harmful byproducts.
We next explored the coupling of substituted anilines with 1-
phenyl-1,2-ethanediol to further illustrate its synthetic
versatility (Table 3). Mono- and disubstituted anilines readily
reacted with the diol substrate to give the indole products in
good to excellent yields. Anilines with an electron-releasing
group were found to promote the coupling reaction in yielding
the indole products 3r and 3s. As indicated in the coupling with
3-chloroaniline, anilines with an electron-withdrawing group
generally gave lower product yield than ones with an electron-
donating group. Sterically demanding 2-substituted anilines and
4-aminoindan also gave the corresponding indole products 3u,
3v, and 3y, respectively. In all cases, the indole products were
formed predictively in a highly regioselective fashion.
promotional effect of HBF4·OEt2 is not entirely clear at this
point. Both 1,4-dioxane and chlorobenzene were found to be
suitable solvents for the coupling reaction.
Reaction Scope. We explored the scope of the coupling
reaction of anilines with 1,2-diol substrates (Table 2). Both
aliphatic and aryl-substituted 1,2-diols readily reacted with
aniline to give the regioselective 2-substituted indole products
3a−c (entries 1−3). The secondary diol 2,3-butanediol gave
the 2,3-disubstituted indole product 3d (entry 4), but with a
considerably lower yield than the primary−secondary diols.
The coupling with unsymmetric secondary 1,2-diols generally
gave a mixture of indole regioisomers. For the coupling of 3-
methoxyaniline with 1,2-diols, 6-methoxyindole products 3e
and 3f were obtained predominantly with a trace amount of 4-
methoxyindole products (entries 5, 6), while the reaction with
cyclic 1,2-diols smoothly formed the tricyclic indole products
3g−i (entries 7−9). The coupling of electron-rich 3,4,5-
trimethoxyaniline with 1,2-diols also gave the indole products
3j and 3k (entries 11, 12). The coupling reaction of 1-
naphthylamine with 1,2-diols predictively led to the formation
of polycyclic indole derivatives 3l−q (entries 12−17). Both
primary−secondary and secondary−secondary 1,2-diols af-
forded the corresponding 2-substituted and 2,3-disubstituted
1H-benzo[g]indoles 3l−o in moderate to good yields (entries
We have been able to extend the scope of the catalytic
method to the coupling between arylamines and 1,3-diols to
produce the quinoline derivatives (Table 4). The coupling
reaction of mono- and disubstituted anilines with 1,3-diols
directly afforded the quinoline products 4. Generally, a higher
temperature was needed to attain an optimal conversion and
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the quinoline product 4i and the N-alkylated aniline product 5i.
The analogous treatment of 1-naphthylamine with 1,3-diols
formed the corresponding polycyclic quinoline derivatives 4k−
n.
Table 3. Dehydrative Coupling of Anilines with 1-Phenyl-
1,2-ethanediol
a
Mechanistic Studies. We performed a deuterium labeling
experiment to probe the H/D exchange pattern on the coupling
products and to discern its mechanistic implications.
Deuterium-labeled C6D5NH2 (0.5 mmol) with 1-phenyl-1,2-
ethanediol (0.75 mmol) in the presence of 2 (0.75 mol %),
HBF4·OEt2 (7 mol %), and cyclopentene (3 equiv) in 1,4-
dioxane was heated at 110 °C for 14 h (eq 2). The deuterium
content of the isolated product 3a-d showed a selective
deuterium incorporation to both ortho (42% D) and para (54%
D) arene positions as analyzed by 1H and 2H NMR (Figure S1,
SI). The observed H/D exchange pattern suggests a rapid and
reversible ortho- and para-C−H bond activation process that is
mediated by the Ru catalyst. Such a rapid and reversible ortho-
arene C−H activation process has been commonly observed in
chelate-assisted C−H coupling reactions.12 We previously
observed a similar H/D exchange pattern on the para-arene
C−H position from the Ru-catalyzed coupling reaction of
arylamines with terminal alkynes, and we attributed the result
to an arene C−H metalation process mediated by an
electrophilic Ru catalyst.13
a
Reaction conditions: aniline (1.0 mmol), 1-phenyl-1,2-ethanediol
(1.5 mol), 2 (0.75 mol %), HBF4·OEt2 (7 mol %), cyclopentene (3.0
mmol), 1,4-dioxane (3 mL), 14−16 h. Combined yield of two
regioisomers.
b
Table 4. Dehydrative Coupling of Arylamines with 1,3-
Diols
a
We conducted a series of control experiments to distinguish
possible reaction pathways. We reasoned that the observed
regioselectivity of the indole products could be explained via an
initial dehydrogenation of diol substrate and the formation of
an imine intermediate. To test this hypothesis, the reaction of
3-methoxyaniline with benzoin was carried out in the absence
of the hydrogen scavenger (cyclopentene) under otherwise
similar reaction conditions, which formed the indole product
3aa in 82% yield (eq 3). In support of the initial formation of α-
a
Reaction conditions: amine (1.0 mmol), 1,3-diol (1.5 mol), 2 (0.75
mol %), HBF4·OEt2 (7 mol %), cyclopentene (3.0 mmol), 1,4-dioxane
(3 mL), 130−150 °C, 14 h.
selectivity in forming the quinoline products (130−150 °C). In
most cases, a significant amount of the byproducts N-
alkylanilines 5 was formed, but both quinoline and N-
alkylaniline products 4 and 5 were readily separated by silica
gel column chromatography. The byproduct 5 is likely resulted
from the dehydrative hydrogenolysis of the alcoholic group
over the requisite ortho-C−H coupling and annulation of the
diol substrate.
The N-alkylaniline 5 was formed as the major product with
electron-deficient 3- and 4-chloroanilines, apparently from the
preferential alcohol hydrogenolysis over the ortho-C−H
coupling path. In contrast, anilines with an electron-donating
group reacted efficiently with 1,3-diols to form the quinoline
products 4g and 4h with a trace amount of the byproducts 5.
The coupling with secondary 1,3-diols was sluggish, leading to a
complex mixture of coupling products, but the coupling of 3,5-
dimethoxyaniline with 2,4-pentanediol led to a 1:1 mixture of
hydroxyketone, 2-hydroxyacetophenone was cleanly formed
when 1-phenyl-1,2-ethanediol was treated with 2 and cyclo-
pentene (24% conversion in 3 h) (eq S2, SI). A number of late
transition metal catalysts have been found to mediate
chemoselective oxidation of secondary alcohols over the
primary ones.14
In a separate experiment, the coupling reaction of aniline
with n-butanol selectively formed N-butylaniline in 70% yield
(eq 4). In this case, N-alkylation product was formed
preferentially over the ortho-alkylated aniline product. This
result is in sharp contrast to the previously reported coupling
reaction of phenol with alcohols, where the ortho-alkylphenol
product was exclusively formed.10 These experimental results
support a reaction sequence involving the initial dehydrogen-
C
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Lab, Department of Chemistry and Biochemistry, University of
Wisconsin−Milwaukee (Milwaukee, WI, USA). Elemental analyses
were performed at the Midwest Microlab (Indianapolis, IN, USA).
General Procedure for the Catalytic Synthesis of Indole and
Quinoline Products. In a glovebox, complex 2 (13 mg, 0.75 mol %)
and HBF4·OEt2 (12 mg, 7 mol %) were dissolved in 1,4-dioxane (1
mL) in a 25 mL Schlenk tube equipped with a Teflon stopcock and a
magnetic stirring bar. The resulting mixture was stirred for 5 to 10 min
until the solution turned to a pale green color. In an alternative
procedure, complex 1 (17 mg, 3 mol %) and HBF4·OEt2 (12 mg, 7
mol %) were dissolved in 1,4-dioxane (1 mL). An arylamine (1.0
mmol), a diol (1.5 mmol), cyclopentene (204 mg, 3 equiv), and 1,4-
dioxane (2 mL) were added to the reaction tube. After the tube was
sealed, it was brought out of the glovebox and was stirred in an oil bath
set at 110−130 °C (130−150 °C for the quinoline products) for 14 h.
The reaction tube was taken out of the oil bath and was cooled to
room temperature. After the tube was open to air, the solution was
filtered through a short silica gel column by eluting with CH2Cl2 (10
mL), and the filtrate was analyzed by GC-MS. Analytically pure
product was isolated by a simple column chromatography on silica gel
(280−400 mesh, hexanes/EtOAc).
ation of diol to α-hydroxy ketone and the subsequent ortho-
arene C−H activation and annulation processes.
On the basis of these results, we present a plausible
mechanistic hypothesis for the coupling reaction of aniline
with a 1,2-diol (Scheme 1). As observed in the control
Scheme 1. Plausible Mechanistic Pathway for the
Dehydrative Coupling of Aniline with a 1,2-Diol
ASSOCIATED CONTENT
■
experiments, we propose the initial dehydrogenation of diol
substrate followed by the dehydrative coupling of aniline with
the resulting α-hydroxy ketone, which would lead to the
formation of an α-hydroxyimine intermediate product 6. The
subsequent ortho-arene C−H metalation followed by the
dehydrative C−O bond cleavage and the reductive annulation
steps would form the indole product 3, in a similar fashion to
the dehydrative C−H insertion reactions of phenols.10 The
requisite ortho-metalation and dehydration steps would be
promoted by an electrophilic Ru-hydroxo species 7. We
recently showed that Ru-hydroxo complexes are a key
intermediate species in ketone hydrogenolysis reaction.15 Late
metal-hydroxo and -phenoxo complexes have also been found
to mediate a number of C−O cleavage reactions.16 In addition,
the formation of N-alkylated product 5 can be readily explained
by invoking a competitive hydrogenolysis pathway especially in
the case for the coupling of an electron-poor aniline with a 1,3-
diol substrate. Our mechanistic hypothesis not only can explain
the observed regioselectivity pattern on the indole product 3
but also is supported by both control experiments as well as the
literature precedents on the alcohol dehydrogenation reac-
tion.14,17
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00273.
Experimental procedures and NMR spectra of organic
products (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: chae.yi@marquette.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support from the
National Science Foundation (CHE-1358439) and National
Institute of Health General Medical Sciences (R15
GM109273).
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■
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■
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