Extending the Chemistry of Hexamethylenetetramine in Ruthenium-Catalyzed Amine Oxidation

Article abstract of DOI:10.1021/acs.organomet.9b00399

A very efficient, highly atom economical, and environmentally benign oxidation of primary and secondary amines using an in situ catalyst system generated from commercially available ruthenium(II) benzene dichloride dimer and hexamethylenetetramine has been demonstrated. Mechanistic studies revealed that hexamethylenetetramine acted as a source of hydride to generate the active ruthenium hydride catalyst and amine oxidation involves a dehydrogenative pathway. In comparison to reported catalyst systems for the dehydrogenative oxidation of amines, this synthetic protocol makes use of a simple ruthenium precursor and a cheaper additive; it is very selective, leading to the exclusive formation of nitrile/imine compounds. Further, it releases hydrogen as the only side product, suggesting the potential application of the developed catalyst system in hydrogen storage.

Full text of DOI:10.1021/acs.organomet.9b00399

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Extending the Chemistry of Hexamethylenetetramine in Ruthenium-
Catalyzed Amine Oxidation
Muthukumar Kannan and Senthilkumar Muthaiah*
National Institute of Technology Kurukshetra, Kurukshetra 136119, Haryana, India
S
* Supporting Information
ABSTRACT: A very efficient, highly atom economical, and
environmentally benign oxidation of primary and secondary
amines using an in situ catalyst system generated from
commercially available ruthenium(II) benzene dichloride
dimer and hexamethylenetetramine has been demonstrated.
Mechanistic studies revealed that hexamethylenetetramine
acted as a source of hydride to generate the active ruthenium
hydride catalyst and amine oxidation involves a dehydrogenative pathway. In comparison to reported catalyst systems for the
dehydrogenative oxidation of amines, this synthetic protocol makes use of a simple ruthenium precursor and a cheaper additive;
it is very selective, leading to the exclusive formation of nitrile/imine compounds. Further, it releases hydrogen as the only side
product, suggesting the potential application of the developed catalyst system in hydrogen storage.
Though this method produces nitriles and imines in excellent
yields, sometimes it suffers with selectivity issues and is known
to produce an amide as the side product.11a
INTRODUCTION
■
Nitrile and imine groups are considered as very important
functional groups because they act as key building blocks for
the synthesis of several organic compounds such as carboxylic
acids, amines, amides, and heterocyclic compounds and several
industrially important products such as pharmaceuticals, dyes,
pesticides, polymers, etc.1,2 The development of efficient,
selective, cost-effective, and environmentally friendly catalyst
systems for the synthesis of nitrile and imine compounds has
always been one of the biggest pursuits of the scientific
community.3,4 Conventional methods available for synthesiz-
ing nitrile compounds include the ammoxidation method,5a
the Sandmeyer type reaction,5b,c the Rosenmund−von Braun
reaction,5d dehydration of aldoximes and amides,6 and metal-
catalyzed cyanation.7 Nitriles are also being synthesized from
alcohols, aldehydes, and azides.8 On the other hand, the
traditional synthesis of imines involves the oxidation of
secondary amines using sacrificial amounts of oxidizing agents
such as IBX, DDQ, MnO2, sulfur, HgO-I2, and many others.9
Many metal-based heterogeneous and homogeneous catalyst
systems are also known to catalyze the oxidation of secondary
amines.10 Though a plethora of methods are available for the
syntheses of both imine and nitrile compounds from a variety
of starting materials, most of the methods suffer from at least
one of the following drawbacks: (a) use of toxic reagents, (b)
poor atom economy, (c) harsh reaction conditions, and (d)
poor selectivity.3−10
An alternative approach that employs transition-metal
complexes for the dehydrogenative oxidation of amines with
the evolution of hydrogen as the sole byproduct has been
found to be an effective methodology. Observing the
dehydrogenative oxidation of amines in the absence of a
hydrogen acceptor is considered to be superior in comparison
to aerobic oxidation and other traditional methods due to high
atom economy and greener nature. Notable examples of
ruthenium complexes that are known to catalyze acceptorless
dehydrogenative oxidation of secondary amines to imines are
RuH2(CO)(PPh3)3 and Shvo’s catalyst systems reported by
Hong and co-workers and a Ru(II)-NNC pincer complex used
by Wang et al.12 In comparison to the dehydrogenation of
secondary amines, reports on the double dehydrogenation of
primary amines to produce nitriles are very scarce. To the best
of our knowledge, there is only one report available on
ruthenium-catalyzed base-free, acceptorless double dehydro-
genation of primary amines to afford nitriles using an NNN-
Ru(II) hydride system reported by Szymczak and co-workers
(Figure 1).13a,b Recently, Bera and co-workers reported an
efficient ruthenium pyrazole/KOBut system for the accept-
orless dehydrogenation of primary amines into nitriles (Figure
1).13c
The role of additives in transition-metal-catalyzed organic
transformations is very crucial in deciding the overall catalytic
activity and selectivity of the catalyst. It is recognized that
additives in catalysis often play vital roles in activating the
substrate and/or in generating/regenerating the active catalyst.
Transition-metal-catalyzed oxidation of amines to form
nitriles and imines has been considered as one of the most
suitable synthetic methods and is known to follow two
different pathways. The synthetic methodology which involves
aerobic oxidation of amines works in the presence of a
transition metal in tandem with an oxygen molecule or some
other oxygen source and produces water as the side product.11
Received: June 14, 2019
© XXXX American Chemical Society
A
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Table 1. Optimization of Reaction Conditions
[Ru]
(mol
%)
HMTA
temp time yield (%)
a
entry [Ru]
(mol %)
solvent
(°C)
(h)
(TON)
19
1
2
3
4
1
3
1
3
4
5
5
5
5
5
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
110
110
110
110
110
110
110
110
110
110
110
110
64
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
6
22
96 (19)
70
5
5
5
5
2.5
4
3
2
1
0.5
2
2
2
2
5
43
6
7
8
9
1
1
1
1
1
1
1
1
1
1
1
5
4
3
2
1
0.5
2
2
2
46
94(24)
90(30)
96(48)
74(74)
45(90)
9
Figure 1. Ruthenium-catalyzed amine dehydrogenation and examples
of ruthenium complexes used for double-dehydrogenative oxidation
of primary amines.
10
11
12
13
14
15
16
17
The nature of the additives used in the literature varies from
inorganic acids and bases to metal salts, and their amount
ranges from catalytic to stoichiometric quantities.14 Apart from
the above examples, several nitrogen-based compounds,
including hexamethylenetetramine (HMTA), have also been
used as additives in the literature.14d HMTA and its derivatives
are being used in chemical synthesis not only because of their
solubility in water and in a variety of organic polar solvents but
also for their simple operation, mild conditions, and environ-
mental friendliness.15 HMTA is a well-known methylating
agent and is used in the synthesis of several industrially
important products, including Bakelite.15,16 In a seminal report
Fu and his co-workers have reported the use of HMTA as both
a ligand and a reducing agent in copper-catalyzed AGET-ATR
batch emulsion polymerization.16c Recently, Qiang and co-
workers have utilized HMTA as a single source of carbon and
nitrogen for the synthesis of N-doped graphenes.17 Notwith-
standing its potential application as a reducing and methylating
agent, HMTA has rarely been used in catalysis15e,16,17 and to
the best of our knowledge HMTA and its derivatives have not
been used as the source of a hydride group.
DCM
40
80
110
110
1
40
29
toluene
toluene
toluene
2
2
2
12
57
a
GC yield using dodecane as internal standard and average of at least
two runs.
was obtained as the product in excellent and moderate yields,
respectively (entries 3−5). In order to test the role of
ruthenium in the formation of 5a, we performed the reaction
under identical conditions: however, the reaction was carried
out in the absence of any ruthenium compound, which did not
yield the product 5a (entry 6). After identifying compound 1
as the suitable precatalyst, we optimized the ratio between 1
and 2 and found that a 1:1 equiv ratio was ideal for
dehydrogenation of 4a, as a further change in ratio resulted in a
diminished yield of 5a (entry 7). In view of decreasing the
catalyst loading, several reactions were tried with 4, 3, 2, 1, and
0.5 mol % of 1 and 2, and 2 mol % was found to be suitable for
obtaining 5a in excellent yield (entries 8−12); however, the
highest turnover number of 90 was obtained when 0.5 mol %
of 1 and 2 was used (entry 12). The reaction solvent,
temperature, and time were also optimized, and it was found
that toluene reflux conditions for 24 h were ideal for double
dehydrogenation of 4a to form the product 5a in excellent
yield (entries 13−17).
Having the optimized reaction conditions in hand, we
expanded the substrate scope of our catalyst system for the
double-dehydrogenative oxidation of several primary amines
into nitriles, as summarized in Table 2. As a beginning, the
electronic and steric effects of various substituents on
benzylamine were explored. The presence of electron-donating
groups such as methyl (4b) and methoxy (4c) groups on the
para position of benzylamine afforded the nitrile products 5b,c,
respectively, in excellent yields (entries 2 and 3). On
substitution of the para position with electron-withdrawing
groups such as chloro (4d), fluoro (4e), and nitro (4f) groups,
the product yields decreased slightly (entries 4−6). To check
the chemoselective nature of our catalyst system, double
dehydrogenation of 4-aminobenzylamine (4g) having both
H2N−CR2 and H2N−CH2− groups (entry 7) was conducted.
It was found that under the given experimental conditions our
catalyst system yielded 4-aminobenzonitrile (5g) as the only
Herein, we report a very efficient and greener process for the
synthesis of nitriles and imines using an in situ catalyst system
developed from commercially available ruthenium(II) benzene
dichloride dimer (1) and hexamethylenetetramine (2) (Figure
1). We also gathered several pieces of experimental evidence to
strongly indicate that this work represents the first example of
the use of HMTA in catalysis as an additive and hydride donor.
RESULTS AND DISCUSSION
■
Currently, our group is involved in developing water-soluble
ruthenium catalysts using the 1,3,5-phosphaadamantane
(PTA) ligand.18 We have observed that the inadvertent
presence of HMTA (2) as an impurity in PTA played a role
in ruthenium-catalyzed oxidation reactions, which prompted us
to employ HMTA (2) in the field of catalysis. We have chosen
ruthenium-catalyzed conversion of benzylamine (4a) to
benzonitrile (5a) as a model reaction, and several reactions
were tried to optimize the reaction conditions, as summarized
in Table 1. Primarily, 5 mol % of ruthenium(II) benzene
dichloride dimer (1) and ruthenium(II) p-cymene dichloride
dimer (3) were tested in the absence of any additive under
open conditions to nitrogen, using toluene as the solvent at
110 °C for 24 h, which afforded 5a in poor yields (Table 1,
entries 1 and 2). When compounds 1, 3, and RuCl3·nH2O (4)
were tested in the presence of HMTA (2), benzonitrile (5a)
B
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a
aminoethanol (4m) did not result in the formation of a nitrile
product, leaving only unreacted starting material (entry 13).
Encouraged by results obtained from the double dehydro-
genation of primary amines, we also tested the activity of the
developed catalyst system for the oxidation of secondary
amines and nitrogen-containing heterocyclic compounds.
Dehydrogenation of several secondary amines including acyclic
and cyclic amines (azaheterocycles) was tested, which afforded
the corresponding dehydrogenated products, i.e. imines and
enamines, in moderate to excellent yields, as depicted in Table
3. It has already been proved in the literature that inclusion of
Table 2. Dehydrogenative Oxidation of Amines to Nitriles
a
Table 3. Dehydrogenative Oxidation of Secondary Amines
a
Reaction conditions: reactions were carried out with amine substrate
(0.5 mmol), [Ru(benzene)Cl2]2 (2 mol %), and HMTA (2 mol %) in
b
a
toluene (1 mL) under reflux. Isolated yields and average of at least
Reaction conditions: reactions were carried out with amine substrate
two runs.
(0.5 mmol), [Ru(benzene)Cl2]2 (2 mol %), and HMTA (2 mol %) in
toluene (1 mL) under reflux. Isolated yields and average of at least
b
two runs of the reaction.
product, suggesting not only the highly chemoselective nature
of the catalyst system but also the advantage of using a
dehydrogenative oxidation method over an aerobic oxidation
method. The presence of ortho substituents such as methyl
(4h) and chloro (4i) groups resulted in decreased yields of the
products 5h,i, respectively (entries 8 and 9), suggesting that
the developed catalyst system is sensitive to steric effects. In
addition to arylamines a few alkylamines were also tested, and
all of them gave the corresponding nitrile products in very
good yields (entries 10−12). An attempt to oxidize 2-
nitrogen into a cyclic system facilitates the dehydrogenation
process by decreasing the endothermicity of the reaction and
thus these species are also considered as potential hydrogen
storage materials/liquid hydrogen storage media.19
Dehydrogenation of dibenzylamine (4m) and N-benzylani-
line (4n) resulted in very good yields of their corresponding
imine products 5m,n, respectively (Table 3, entries 1 and 2).
Dehydrogenation of 1,2,3,4-tetrahydroquinoline (4o) and
1,2,3,4-tetrahydroisoquinoline (4p) proceeded to yield quino-
C
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line (5o) and isoquinoline (5p), respectively, in very good
yields (entries 3 and 4). Interestingly, both 1,2,3,4-tetrahy-
droquinoline and 1,2,3,4-tetrahydroisoquinoline resulted in
exclusive formation of fully dehydrogenated products without
forming any partially dehydrogenated products, which proves
the high effeciency and selectivity of our catalyst system.
Dehydrogenation of indoline (4q) was tested and resulted in
the formation of indole (5q) in excellent yield (entry 5).
Following this, 2-methylindoline (4r) was tested and resulted
in the dehydrogenated product 2-methyl-1H-indole (5r) in
excellent yield, which suggests that the catalyst system is not
sensitive to steric effects (entry 6). Dehydrogenation of 5-
methoxyindoline (4s) using our catalyst system also resulted in
a very good yield of the dehydrogenated product 5s (entry 7).
In order to identify the reaction pathway, the oxidation of
benzylamine (4a) was carried out in the presence of
cyclohexene as a hydrogen acceptor. In a typical closed-vessel
reaction, benzylamine and cyclohexene in a 1:10 equiv ratio in
dry and degassed toluene in the presence of 1 (2 mol %) and 2
(2 mol %) were heated at 115 °C for 24 h (Scheme 1). This
HMTA ligand, two hydride signals were also observed at
−1.33 and −2.42 ppm (Figure 2B). Continuing the heating
further led to the elimination of both hydrogen and benzene
from the active catalyst (Figure 2C). At high temperature free
HMTA (2) as well as metal-coordinated HMTA complexes
were reported to decompose to afford products such as NH3,
H2, HCN, CH4, and many others.20a,d,e It is also evident from
the literature that the hydrogen byproduct which is formed
from decomposition of 2 may act as the potential source of a
hydride group.20b,c The observation of a hydride peak in the
NMR spectra indicated that HMTA (2) can act as a source of
a hydride group. Further, to support our claim of HMTA
acting as a hydride donor, we performed another NMR study
by heating a mixture of RuCl3·nH2O and HMTA (2) in
DMSO-d6, which also resulted in a set of hydride peaks at
−0.23 and −0.75 ppm (Figure S21).
In order to get an insight into the mechanism, the in situ 1H
NMR spectra of a mixture of 1 and 2 (Figure 2B) was
compared with those of arene ruthenium hydride complexes
available in the literature. Suss-Fink and co-workers reported a
̈
series of cationic trinuclear ruthenium hydride complexes
obtained from the reaction of 1 with hydrogen in the presence
of reagents such as NaClO4, NaBF4, NaCl, etc.21 It was
observed that the chemical shift values of hydride peaks
obtained from our catalyst system were considerably shifted
downfield in comparison to the hydride peak of −13.35 ppm
reported for the cationic trinuclear species [Ru(η6-C6H6)(μ-
Cl)(μ3-O)(μ-H)2]+, which was obtained from a reaction of 1,
H2, and NaClO4.21b Further, the observed chemical shift values
were found to be in good agreement with the values obtained
for mononuclear Ru hydride species formed in situ from a
reaction of RuCl3·nH2O and 2 (Figure S21). These
observations revealed that the skeletal formula of the active
catalyst might be [Ru(H)2(C6H6)], and further investigations
are underway to identify the exact structure and mechanism for
its formation.
Scheme 1. Oxidation of Benzylamine (4a) in the Presence of
Cyclohexene
resulted in the formation of cyclohexane in 19% yield (64% of
conversion yield), which suggested that the reaction involved
hydrogen evolution and followed a dehydrogenative pathway.
To investigate the role of HMTA (2) and the nature of the
active catalyst, a series of NMR studies were carried out, as
shown in Figure 2. An NMR-tube reaction of a solution of 1
and 2 in a 1:1 equiv ratio in DMSO-d6 at room temperature
showed major peaks corresponding to unreacted starting
materials 1 and 2 in addition to slight formation of HMTA-
coordinated ruthenium species at 4.46, 4.88, and 5.82 ppm
(Figure 2A). When the reaction mixture was heated at 90 °C
for 1 h in the NMR tube, in addition to the coordinated
On the basis of the above experimental studies and literature
reports13a,22 the following mechanism is proposed for the
double dehydrogenation of amines (Figure 3). According to
the proposed mechanism, the active catalyst [Ru(benzene)-
(H)2] (A) generated from the reaction between 1 and 2
undergoes oxidative addition with an amine, followed by
hydrogen elimination to form Ru−H intermediate B.
Compound B undergoes β-hydrogen transfer to form imine-
coordinated ruthenium complex C. The formation of an
1
Figure 2. H NMR evidence for the formation of ruthenium hydride
intermediate.
Figure 3. Mechanism involving double dehydrogenation of amine.
D
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4-Fluorobenzonitrile (5e).23b Colorless oil. Yield: 0.2002 g, 83%.
The desired pure product was obtained after short-column
chromatography (hexane/ethyl acetate). 1H NMR (CDCl3, 300
MHz): δ 7.85 (d, J = 9 Hz, 2 H), 7.37 (d, J = 9 Hz, 2 H) ppm.
4-Nitrobenzonitrile (5f).23c White solid. Yield: 0.1863 g, 77%. The
desired pure product was obtained after short column chromatog-
ruthenium imine complex as an intermediate during the course
of reaction was confirmed with the help of H NMR (Figure
1
S20) and was in agreement with a literature report.22 The
formation of a ruthenium imine complex as an intermediate
during the double dehydrogenation of amine is different from
that of the ruthenium imide complex reported by Szymczak
and co-workers.13a Intermediate C undergoes β-hydrogen
transfer with the elimination of hydrogen to the form nitrile-
coordinated intermediate D. Compound D further undergoes
reductive elimination to eliminate the nitrile product and
regenerate the active catalyst A.
1
raphy, hexane/ethyl acetate). H NMR (CDCl3, 300 MHz): δ 8.18
(d, J = 9 Hz, 2 H), 7.52 (d, J = 9 Hz, 2H) ppm.
4-Aminobenzonitrile (5g).23a Pale yellow solid. Yield: 0.1023 g,
42%. The desired pure product was obtained after short column
chromatography (hexane/ethyl acetate). 1H NMR (CDCl3, 300
MHz): δ= 7.42 (d, J = 9 Hz, 2 H), 6.64 (d, J = 6 Hz, 2H), 4.16(s, 2H)
ppm.
In conclusion, we have developed a very efficient in situ
catalyst system using commercially available ruthenium(II)
benzene dichloride dimer complex and hexamethylenetetr-
amine for the acceptorless dehydrogenation of both primary
and secondary amines. Mechanistic studies revealed the
formation of a [Ru(benzene)(H)2] fragment as the catalyti-
cally active species, while the oxidation of amines proceeded
through a dehydrogenative pathway with the evolution of
hydrogen gas. We have also experimentally shown the role of
hexamethylenetetramine as a hydride donor in the above
process.
2-Methylbenzonitrile (5h).23d Colorless oil. Yield: 0.1521 g, 63%.
The desired pure product was obtained after short column
chromatography (hexane/ethyl acetate). 1H NMR (CDCl3, 300
MHz): δ= 7.56−7.58 (m, 1H), 7.46−7.48 (m, 1H), 7.28−7.32 (m,
2H), 2.53 (s, 3H) ppm.
2-Chlorobenzonitrile (5i).23e Colorless oil. Yield: 0.1421g, 58%.
The desired pure product was obtained after short-column
chromatography (hexane/ethyl acetate). 1H NMR (CDCl3, 300
MHz): δ= 8.33−8.35 (m, 1 H), 7.58−7.63 (m, 1H), 7.26−7.29 (m,
1H), 7.16−7.20 (m, 1H) ppm.
2-Phenylacetonitrile (5j).23h Colorless liquid. Yield: 0.2002 g, 82%.
The desired pure product was obtained after short-column
chromatography (hexane/ethyl acetate).1H NMR (CDCl3, 300
MHz): δ= 7.32−7.40 (m, 5H), 3.70 (s, 2H) ppm.
EXPERIMENTAL SECTION
1-Heptanenitrile (5k).23g Colorless liquid. Yield: 0.1832 g,
72%.The desired pure product was obtained after short-column
chromatography (hexane/ethyl acetate).1H NMR (CDCl3, 300
MHz): δ= 2.34 (t, J = 6 Hz, 2H), 1.66 (q, J = 6 Hz, 2H), 1.45−
1.57 (m, 2H), 1.32 (bs, 4H), 0.90 (t, J = 3 Hz, 3H) ppm.
Octanenitrile (5l).23g Colorless liquid. Yield: 0.1913 g, 79%. The
desired pure product was obtained after short-column chromatog-
■
General Information. All procedures were carried out under
purified nitrogen environment using standard Schlenk apparatus. All
of the apparatus were oven-dried prior to use. An airless procedure
was followed for all syntheses irrespective of the air stability of the
final compounds. All of the solvents were used after a drying and
degassing process. Other chemicals were purchased from commercial
suppliers and used as received without further purification.
General Procedure for Dehydrogenation of Amine. [Ru-
(benzene)Cl2]2 (2 mol %), HMTA (2 mol %), amine, and dry toluene
(1.0 mL) were placed in a Schlenk tube. The reaction mixture was
stirred under open conditions to nitrogen and refluxed for 24 h. After
completion of the reaction all of the toluene was evaporated under
vacuum; the nitrile product was isolated from the crude mixture by
column chromatography using hexane/EtOAc as eluent. The
formation of the product was confirmed by comparing the 1H
NMR data with literature reports.
1
raphy (hexane/ethyl acetate). H NMR (CDCl3, 300 MHz): δ 2.34
(t, J = 6 Hz, 2H), 1.66(p, J = 6 Hz, 2H), 1.45 (m, 2H), 1.30(bs, 6H),
0.89 (t, J = 3 Hz, 3H) ppm.
N-Benzylidene-1-phenylmethanamine (5m).23f Colorless liquid.
Yield: 0.1954 g, 79%. The desired pure product was obtained after
short-column chromatography (hexane/ethyl acetate). 1H NMR
(CDCl3, 300 MHz): δ 8.44 (s, 1H), 7.85−7.87 (m, 1H), 7.48 (bs,
2H), 7.47 (bs, 2H), 7.42 (bs, 2H), 7.40−7.41 (m, 2H), 7.32−7.35
(m, 1H), 4.88 (s, 2H) ppm.
N-Benzylideneaniline (5n).23f Colorless liquid. Yield: 0.1804g,
73%. The desired pure product was obtained after short-column
chromatography (hexane/ethyl acetate). 1H NMR (CDCl3, 300
MHz): δ 8.44 (s, 1H), 7.89 (t, J = 3 Hz, 2H), 7.45−7.47 (m, 2H),
7.36−7.39 (m, 1H), 7.23−7.25 (m, 2H), 7.21−7.22 (m, 2H), 7.19−
7.20 (m, 1H), ppm.
Procedure for Dehydrogenation of Benzylamine in the
Presence of Cyclochexene. In a 50 mL closed-vessel reactor were
placed [Ru(benzene)Cl2]2 (2 mol %), HMTA (2 mol %),
benzylamine (0.05 mL, 0.5 mmol), cyclohexene (0.4 mL, 5 mmol),
and dry toluene (0.6 mL). The resulting mixture was heated at 110 °C
for 24 h. After completion of the reaction, the solution was cooled to
room temperature and the yields were determined using gas
chromatography.
Quinoline (5o).24 Colorless liquid. Yield: 0.2036 g, 84%. The
desired pure product was obtained after short-column chromatog-
1
raphy (hexane/ethyl acetate). H NMR (CDCl3, 300 MHz): δ 8.92
Benzonitrile (5a).23a Colorless oil. Yield: 0.2196 g, 91%. The
desired pure product was obtained after short silica gel column
(t, J = 6 Hz, 1 H), 8.14−8.17 (m, 1H), 8.10−8.13 (m, 1H), 7.80−
7.83 (m, 1 H), 7.69−7.74 (m, 1 H), 7.51−7.57 (m, 1H), 7.37−7.41
(m, 1H) ppm.
1
chromatography (hexane/ethyl acetate). H NMR: δ 7.57−7.59 (m,
Isoquinoline (5p).24 Yellow liquid. Yield: 0.2088 g, 85%. The
desired pure product was obtained after short-column chromatog-
3H) and 7.41−7.44 (m, 2H) ppm.
4-Methylbenzonitrile (5b).23d Colorless oil. Yield: 0.2096 g, 90%.
The desired pure product was obtained after short-column
chromatography (hexane/ethyl acetate). 1H NMR (CDCl3, 300
MHz): δ 7.50 (d, J = 9 Hz, 2 H), 7.24 (d, J = 9 Hz, 2 H), 2.39 (s, 3
H) ppm.
1
raphy (hexane/ethyl acetate). H NMR (CDCl3, 300 MHz): δ 9.24
(s, 1H), 8.51 (d, J = 6 Hz, 1 H), 7.94 (d, J = 12 Hz, 1 H), 7.79 (d, J =
6 Hz, 1 H), 7.68−7.70 (m, 1 H), 7.61−7.64 (m, 1 H).7.55−7.58 (m,
1H) ppm.
4-Methoxybenzonitrile (5c):23a Colorless oil. Yield: 0.2253 g, 93%.
The desired pure product was obtained after short-column
chromatography (hexane/ethyl acetate). 1H NMR (CDCl3, 300
MHz): δ 7.37(d, J = 9 Hz, 2H), 6.78(d, J = 9 Hz, 2H), 3.79 (s, 3H)
ppm.
Indole (5q).24 White solid. Yield: 0.2137 g, 87%. The desired pure
product was obtained after short-column chromatography (hexane/
1
ethyl acetate). H NMR (CDCl3, 300 MHz): δ 8.12 (s, 1 H), 7.21−
7.24 (m, 5H), 6.59−6.60 (m, 1 H).
2-Methyl-1H-indole (5r).24 Yellow solid. Yield: 0.2183 g, 89%. The
desired pure product was obtained after short-column chromatog-
4-Chlorobenzonitrile (5d).23c White solid. Yield: 0.2065 g, 85%.
The desired pure product was obtained after short-column
chromatography (hexane/ethyl acetate). 1H NMR (CDCl3, 300
MHz): δ 7.63 (d, J = 9 Hz, 2 H), 7.52 (d, J = 9 Hz, 2 H) ppm.
1
raphy (hexane/ethyl acetate). H NMR (CDCl3, 300 MHz): δ 7.87
(s, 1 H), 7.62−7.60 (m, 1 H), 7.38−7.35 (m, 1 H), 7.14−7.22 (m, 2
H), 6.98−6.99 (m, 1 H), 2.36 (s, 3 H) ppm.
E
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5-Methoxyindole (5s).24 White solid. Yield: 0.2120 g, 86%. The
153. (b) Sandmeyer, T. Ueberfu
̈
hrung der drei Amidobenzoesauren
in die Phtalsauren Ber. Ber. Dtsch. Chem. Ges. 1885, 18, 1496−1500.
(c) Sandmeyer, T. Ueberfuhrung der drei Nitraniline in die
̈
̈
desired pure product was obtained after short-column chromatog-
̈
1
raphy (hexane/ethyl acetate). H NMR (CDCl3, 300 MHz): δ 8.08
̈
(s, 1 H), 7.28 (d, J = 9 Hz, 1 H), 7.15−7.19 (m, 2H), 6.93 (m, 1H),
6.52 (s, 1 H), 3.89 (s, 3 H) ppm.
̈
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Nitrobenzoesauren Ber. Ber. Dtsch. Chem. Ges. 1885, 18, 1492−
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00399.
■
S
NMR spectra of oxidation products (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: msenthil@nitkkr.ac.in. Tel.: (+91)-1744-233314.
■
̈
52, 7523−7526. (b) Falk, A.; Goderz, A.−L.; Schmalz, H.−G.
ORCID
Enantioselective Nickel-Catalyzed Hydrocyanation of Vinylarenes
Using Chiral Phosphine−Phosphite Ligands and TMS-CN as a
Source of HCN. Angew. Chem., Int. Ed. 2013, 52, 1576−1580.
(c) Velcicky, J.; Soicke, A.; Steiner, R.; Schmalz, H.−G. Palladium-
Catalyzed Cyanomethylation of Aryl Halides through Domino Suzuki
Coupling−Isoxazole Fragmentation. J. Am. Chem. Soc. 2011, 133,
6948−6951.
Senthilkumar Muthaiah: 0000-0002-7073-6916
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the financial support from Science and
Engineering Research Board of India in the form of a Start-Up
Research Grant (Young Scientists, no. 58/FT/C5-092/2014).
We thank Prof. Dragoslav Vidovic for his useful suggestions in
writing this manuscript.
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DEDICATION
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This work is dedicated to Professor Anil J. Elias on the
occasion of his 58th birthday.
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