Zinc Stabilized Azo-anion Radical in Dehydrogenative Synthesis of N-Heterocycles. An Exclusively Ligand Centered Redox Controlled Approach

Article abstract of DOI:10.1021/acscatal.1c00275

Herein we report an exclusively ligand-centered redox controlled approach for the dehydrogenation of a variety of N-heterocycles using a Zn(II)-stabilized azo-anion radical complex as the catalyst. A simple, easy-to-prepare, and bench-stable Zn(II)-complex (1b) featuring the tridentate arylazo pincer, 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline, in the presence of zinc-dust, undergoes reduction to form the azo-anion radical species [1b]- which efficiently dehydrogenates various saturated N-heterocycles such as 1,2,3,4-tetrahydro-2-methylquinoline, 1,2,3,4-tetrahydro-isoquinoline, indoline, 2-phenyl-2,3-dihydro-1H-benzoimidazole, 2,3-dihydro-2-phenylquinazolin-4(1H)-one, and 1,2,3,4-tetrahydro-2-phenylquinazolines, among others, under air. The catalyst has further been found to be compatible with the cascade synthesis of these N-heterocycles via dehydrogenative coupling of alcohols with other suitable coupling partners under air. Mechanistic investigation reveals that the dehydrogenation reactions proceed via a one-electron hydrogen atom transfer (HAT) pathway where the zinc-stabilized azo-anion radical ligand abstracts the hydrogen atom from the organic substrate(s), and the whole catalytic cycle proceeds via the exclusive involvement of the ligand-centered redox events where the zinc acts only as the template.

Full text of DOI:10.1021/acscatal.1c00275

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Research Article
Zinc Stabilized Azo-anion Radical in Dehydrogenative Synthesis of
N‑Heterocycles. An Exclusively Ligand Centered Redox Controlled
Approach
Siuli Das, Rakesh Mondal, Gargi Chakraborty, Amit Kumar Guin, Abhishek Das, and Nanda D. Paul*
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ABSTRACT: Herein we report an exclusively ligand-centered redox controlled approach for
the dehydrogenation of a variety of N-heterocycles using a Zn(II)-stabilized azo-anion radical
complex as the catalyst. A simple, easy-to-prepare, and bench-stable Zn(II)-complex (1b)
featuring the tridentate arylazo pincer, 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline, in
the presence of zinc-dust, undergoes reduction to form the azo-anion radical species [1b]−
which efficiently dehydrogenates various saturated N-heterocycles such as 1,2,3,4-tetrahydro-2-
methylquinoline, 1,2,3,4-tetrahydro-isoquinoline, indoline, 2-phenyl-2,3-dihydro-1H-benzoimi-
dazole, 2,3-dihydro-2-phenylquinazolin-4(1H)-one, and 1,2,3,4-tetrahydro-2-phenylquinazo-
lines, among others, under air. The catalyst has further been found to be compatible with the
cascade synthesis of these N-heterocycles via dehydrogenative coupling of alcohols with other
suitable coupling partners under air. Mechanistic investigation reveals that the dehydrogen-
ation reactions proceed via a one-electron hydrogen atom transfer (HAT) pathway where the
zinc-stabilized azo-anion radical ligand abstracts the hydrogen atom from the organic
substrate(s), and the whole catalytic cycle proceeds via the exclusive involvement of the ligand-
centered redox events where the zinc acts only as the template.
KEYWORDS: zinc catalyst, azo-anion radical, ligand-centered redox, dehydrogenation, N-heterocycles
ation of N-heterocycles under standard conditions is a
thermodynamically uphill process.7,8 Moreover, the classical
transition-metal-catalyzed approaches for the dehydrogenation
of N-heterocycles proceed via the formation of metal-hydride
intermediates involving energetically demanding two-electron
metal-centered redox events. As a consequence, these reactions
almost always require harsh reaction conditions.8−11 Several
heterogeneous catalysts based on noble metal and transition
metal oxides have been reported for dehydrogenation of N-
heterocycles.9 However, most of these catalytic systems require
high temperature and showed limited functional group
tolerance. On the other hand, only a few homogeneous
catalysts are reported for these reactions (Scheme 1). After the
pioneering work by Yamagushi and Fujita10a on Ir-catalyzed
dehydrogenation of tetrahydroquinoline derivatives, a few
more Ir-catalyzed dehydrogenation reactions have been
reported by Xiao,10b Huang,10c Crabtree,10d Albrecht,10e and
others.10f,g Despite significant advances, the use of expensive
INTRODUCTION
■
Organic radicals are usually highly reactive in the pure state.
However, upon coordination to a transition metal ion, its
stability increases significantly.1 The metal-bound organic
radicals are recognized as one of the key intermediates in
bringing about radical-type reactions in a controlled manner.2
In this regard, the synthesis of new transition-metal complexes
of redox-active ligands has drawn significant attention in recent
years.2−5 This class of ligands other than coordinating to the
metal ions can generate comparatively stable ligand-centered
radicals upon selective oxidation and reduction.3 They can also
act as a reservoir of electrons during chemical transformations
and hence allow the chemical reactions to proceed under mild
conditions avoiding energetically demanding metal-centered
redox couples as well as control the substrate binding affinity
via selective ligand centered redox processes.4,5 Over the past
decade, significant advances have been made in developing
new synthetic strategies utilizing the active participation of
transition metal-bound redox-active scaffolds, and various
chemical transformations were achieved successfully, which
are otherwise difficult or sometimes impossible to accomplish
using classical synthetic routes.2−5
Received: January 20, 2021
Revised: May 26, 2021
Published: June 9, 2021
The catalytic dehydrogenation of N-heterocycles is of
significant importance to access heteroarenes which are often
found as the structural motifs in various natural products and
the pharmaceutical industry.6 The acceptorless dehydrogen-
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Scheme 1. A Brief Survey of Previous Works and Schematic Representation of Our Present Work.10a,b,d,e,11a,b,12b
and relatively scarce iridium catalysts may limit the
applications of these methods. The use of inexpensive, earth-
abundant, and environmentally benign 3d-base metals for these
reactions would be beneficial in terms of sustainability and
scalability. In this regard, the work by Jones and co-workers on
the Fe and Co-catalyzed dehydrogenation of N-heterocycles is
worthy to mention.11 However, the use of an air-sensitive
phosphine-based ligand limits the application of these methods
to some extent. In 2014, Stahl and co-workers reported the
dehydrogenation of N-heterocycles under air using a bifunc-
tional quinone catalyst.12 Therefore, the development of
alternative synthetic strategies for the dehydrogenation of N-
heterocycles under comparatively mild conditions is desirable.
Herein we report a simple strategy for the dehydrogenation
reactions utilizing a metal(zinc)-stabilized azo-anion radical
ligand. A well-defined Zn(II)-complex ([1b]−) featuring a one-
electron reduced 2-((4-chlorophenyl)diazenyl)-1,10-phenan-
throline ligand shows good activity in the dehydrogenation of a
wide variety of N-heterocycles such as 1,2,3,4-tetrahydro-2-
methyl quinoline, 1,2,3,4-tetrahydroisoquinoline, indoline, 2-
phenyl-2,3-dihydro-1H-benzimidazole, 2,3-dihydro-2-phenyl-
quinazolin-4(1H)-one, and 1,2,3,4-tetrahydro-2-phenylquina-
zolines. The catalyst [1b]− has further been found to be
compatible with the cascade synthesis of these N-heterocycles
via dehydrogenative coupling of alcohols with other suitable
coupling partners under air.13 Mechanistic investigation reveals
that the dehydrogenation reactions proceed via a one-electron
hydrogen atom transfer (HAT) pathway where the zinc-
stabilized azo-anion radical ligand abstracts the hydrogen atom
from the organic substrate(s), and the whole catalytic cycle
proceeds via the exclusive involvement of the ligand-centered
redox events where the zinc acts only as the template.
RESULT AND DISCUSSION
■
Synthesis and Characterization of the Catalysts. Our
present work begins with the synthesis of two new well-defined
Zn(II)-complexes 1a and 1b featuring the redox noninnocent
tridentate pincers, 2-(phenyldiazenyl)-1,10-phenanthroline
(L1a) and 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline
(L1b), respectively. The reaction of L1a,1b with an equimolar
amount of ZnCl2 in ethanol at room temperature under air
yielded two new orange-colored complexes [Zn(L1a)Cl2] (1a)
and [Zn(L1b)Cl2] (1b) in 93 and 94% yields, respectively
(Scheme 2). The elemental analysis supports their formula-
1
tions. Both the complexes are diamagnetic. Well-resolved H
NMR spectra were obtained in the range 7.6−9.5 ppm (see
SI). X-ray structure of 1b reveals a penta-coordinate geometry
around the Zn(II)-ion (Figure 1). The pincer ligand, L1b,
coordinates the Zn(II)-center in a tridentate fashion, and two
chlorido ligands occupy the rest of the two coordination sites.
The two N-donor atoms N1(phen) and N4(azo) occupy the
apical positions with an N1−Zn−N4 angle of 143.60(6)° with
a calculated τ value of 0.24.14 The N−N bond distance was
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corresponding electrochemical data are submitted in SI. Zn(II)
being the redox-inactive center, the redox responses are
expected because of the ligand-centered reduction (Figure 2).
The EPR spectrum of the electrogenerated species obtained
after exhaustive electrolysis of 1b at −0.5 V showed a single
Scheme 2. Synthesis of the Zn(II)-Complexes 1a and 1b
line isotropic spectrum with minor hyperfine coupling at giso
=
1.9737, indicating the formation of a ligand-centered radical
upon one-electron reduction (Figure 3).15,16 Isotropic
Figure 3. X-band EPR spectrum of [1b]− in dichloromethane at 77K.
simulation of the experimental spectrum indeed is in
agreement with the ligand-centered EPR spectrum with
hyperfine interaction with 14N (I = 1) nuclei. It is worth
mentioning here that the experimental spectrum (along with
hyperfine) simulation requires consideration of three species:
one with hyperfine for 1 N, another for 2 N with a slightly
different A value, and the third one without nitrogen hyperfine.
This may be because the radical can delocalize into the
aromatic ring of the arylazo scaffold which has been
substantiated by the DFT studies. The experimental and
simulated EPR spectra are displayed in Figure 3, and the
simulation parameters are shown in SI. Notably, these
complexes in the presence of zinc dust and KOtBu also
undergo one-electron reduction to form the azo-anion radical
species [1a/1b]−. We tried to isolate and grow the single
crystals of the one-electron reduced species [1a/b]−. However,
after repeated trials, we could not obtain the single crystals
suitable for X-ray diffraction.
Figure 1. ORTEP view of [ZnL1bCl2] (1b) with ellipsoids at the 50%
probability level. Hydrogen atoms are omitted for clarity.
found to be 1.246(2)Å, indicating the presence of unreduced
azo-chromophore.15,16
The complexes 1a and 1b undergo one single-electron
reversible reduction in the potential range between −0.17 and
−0.19 V, and one irreversible reduction in the potential range
between −1.07 and −1.09 V, in acetonitrile/0.1 M Et4NClO4
using platinum as working electrode, and saturated Ag/AgCl as
the reference electrode (see SI for details). The cyclic
voltammogram of 1b is displayed in Figure 2, and the
Electronic structure elucidation using DFT at the B3LYP
level revealed that the complex, Zn(L1b)Cl2 (1b), is a closed-
shell singlet species. Molecular orbital analysis showed that the
LUMOs are ligand-centered; primarily localized on the azo-
chromophore. On the other hand, the one-electron reduced
complex, [Zn(L1b)Cl2]− ([1b]−), was found to be an open-
shell doublet species with a spin population of +0.003 on the
Zn(II)-center and −0.997 spin population on the azo-aromatic
scaffold (Figure 4).
Catalysis. Catalytic Dehydrogenation of N-Heterocycles.
With the characterization data of 1a and 1b in hand, we
envisaged that a one-electron reduced azo-anion radical ligand
coordinated to Zn(II)-ion in [1a]− and [1b]− would abstract
the hydrogen atom from various organic substrates (N-
heterocycles) leading to efficient dehydrogenation reactions.
To begin with, initially, we studied the monodehydrogenation
reactions where the removal of two hydrogen atoms (one from
N−H and one from C−H) from the saturated N-heterocycles
would produce the desired N-heterocycles. We chose 2,3-
Figure 2. Cyclic voltammogram of 1b in acetonitrile/0.1 M Et4NClO4
using platinum as working electrode, platinum wire as auxiliary
electrode, and saturated Ag/AgCl as reference electrode. Scan rate =
100 mV s−1.
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catalyst (Table 1, entry 12). Among the different bases and
solvents used during dehydrogenation reactions, KOtBu and
toluene, respectively, were found to be most effective (see
Table 1 for details). The catalytic dehydrogenation did not
proceed under an argon atmosphere (Table 1, entry 15). A
quantitative conversion was observed when one equivalent of
2a was subjected to dehydrogenation in the presence of one
equivalent of the catalyst under an argon atmosphere.
However, under a pure oxygen environment, we did not
observe any significant improvement of yield (Table 1, entry
16).
Control experiments revealed that the reaction did not
proceed well in the absence of a base (Table 1, entry 17). In
the presence of only KOtBu, 3a was obtained in 19% yields
(Table 1, entry 18). In the presence of ZnCl2 or L1b, 3a was
obtained in 20 and 19% yields, respectively (Table 1, entries
19, 20). Instead of 1b, when a 1:1 mixture of ZnCl2 and L1b
was used as catalyst, 3a was obtained in 43% yields (Table 1,
entry 21). This slight increase of yield may be due to the in situ
formation of some amount of the catalyst 1b. The reaction did
not proceed at all in the presence of only zinc dust (Table 1,
entry 22). No noticeable increase in yields was observed on
varying the catalyst loading or any other reaction parameters
such as base, solvent, or temperature beyond the optimal
conditions.
Figure 4. (a) LUMO of 1b. (b) Spin density plot of [1b]−.
dihydro-2-phenylquinazolin-4(1H)-one (2a) as the model
substrate to find out the optimal reaction parameters.
Optimization of the reaction conditions reveals that the
dehydrogenation of 2a proceeds best in the presence of 4.0
mol % of 1b, 0.5 equiv of KOtBu, 0.5 equiv of Zn dust under
air at 100 °C (Table 1). Catalyst 1b was found to be more
effective as compared to catalyst 1a. A maximum yield of 96%
was obtained under the optimal conditions using 1b as the
We screened the substrate scope for a variety of multi-
substituted 2,3-dihydro-2-phenylquinazolin-4(1H)-ones. The
present strategy is compatible with various 2,3-dihydro-2-
phenylquinazolin-4(1H)-ones bearing both electron-donating
Table 1. Optimization of the Reaction Conditions for Dehydrogenation of 2,3-Dihydro-2-phenylquinazolin-4(1H)-one
a b c d
,
, ,
(2a)
b
entry
catalyst (mol %)
solvent
base
temperature (°C)
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1b (7 mol %)
1b (7 mol %)
1b (7 mol %)
1b (7 mol %)
1b (7 mol %)
1b (7 mol %)
1b (7 mol %)
1b (7 mol %)
1b (7 mol %)
1b (7 mol %)
1b (7 mol %)
1b (4 mol %)
1b (3 mol %)
1a (4 mol %)
1b (5 mol %)
1b (4 mol %)
1b (4 mol %)
-
toluene
toluene
xylene
THF
ACN
KOtBu (0.7 equiv)
KOtBu (0.7 equiv)
KOtBu (0.7 equiv)
KOtBu (0.7 equiv)
KOtBu (0.7 equiv)
KOtBu (0.7 equiv)
NaOtBu (0.7 equiv)
KOH (0.7 equiv)
NaOH (0.7 equiv)
K3PO4(0.5 equiv)
KOtBu (0.5 equiv)
KOtBu (0.5 equiv)
KOtBu (0.5 equiv)
KOtBu (0.5 equiv)
KOtBu (0.5 equiv)
KOtBu (0.5 equiv)
-
120
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
96
96
95
73
trace
trace
94
92
91
32
96
96
94
92
trace
96
trace
19
20
19
43
NR
ethanol
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
c
15
d
16
17
18
19
20
21
22
KOtBu (0.5 equiv)
KOtBu (0.5 equiv)
KOtBu (0.5 equiv)
KOtBu (0.5 equiv)
KOtBu (0.5 equiv)
ZnCl2 (5 mol %)
L1a,b (4.0 mol %)
ZnCl2+L1b (1:1) (5 mol %)
Zn-dust
a
b
c
d
Stoichiometry: 2a (1.0 mmol), Zn-dust (0.5 equiv). Isolated yields after column chromatography. Under argon atmosphere. Under pure
oxygen atmosphere.
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Figure 5. Dehydrogenation of various saturated N-heterocycles using 1b as catalyst.
and -withdrawing substituents at the different positions of the
phenyl ring. The corresponding substituted 2-phenylquinazo-
lin-4(1H)-ones (3a−f) were obtained in moderate to good
isolated yields (Figure 5, entries 1−6). Five-membered N-
heterocycles, such as indoline (4), and 2-phenyl-2,3-dihydro-
1H-benzoimidazole (6), also undergo dehydrogenation to
produce 5 and 7 in 71 and 89% yields, respectively (Figure 5,
entries 7 and 8).
We also studied the double dehydrogenation reactions with
a few chosen N-heterocycles. Dehydrogenation of 1,2,3,4-
tetrahydro-2-phenylquinazoline (8a) proceeds smoothly under
the optimized conditions used during monodehydrogenation
reactions (Figure 5, entry 9); only slightly higher catalyst
loading 5.0 mol % is required. Different substituted 1,2,3,4-
tetrahydro-2-phenylquinazolines (8b−f) undergo dehydrogen-
ation to afford the respective 2-phenylquinazolines (9b−f) in
moderate to good isolated yields (Figure 5, entries 10−14).
More difficult substrates such as 1,2,3,4-tetrahydro-2-methyl-
quinoline (10) and 1,2,3,4-tetrahydro-isoquinoline (12) were
also found to be compatible for dehydrogenation to yield 2-
methylquinoline (11a) and isoquinoline (13) in 51 and 55%
isolated yields, respectively, under the optimal conditions with
a slightly higher catalyst loading of 10.0 mol % (Figure 5,
entries 15 and 16). It is worth mentioning that with low-
yielding reactions other than the desired products, we observe
unreacted starting materials during chromatographic purifica-
tion. However, we did not notice any side products.
One-Pot Cascade Synthesis of N-Heterocycles via
Dehydrogenative Coupling. Dehydrogenative coupling of
alcohols with suitable coupling partners offers an attractive
eco-friendly approach for the construction of various N-
heterocycles from cheap and earth-abundant raw materi-
als.13,17−20 Since our Zn-catalyst, 1b, showed promising results
during the dehydrogenation of preformed saturated N-
heterocycles, we decided to check the viability of the one-
pot cascade synthesis of a few selected N-heterocycles such as
2-arylquinazolin-4(3H)-ones,18 2-arylquinazolines,19 and qui-
nolines20 via dehydrogenative functionalization of alcohols. A
wide variety of 2-arylquinazolin-4(1H)-ones and 2-arylquina-
zolines were prepared via dehydrogenative coupling of alcohols
with 2-aminobenzamides and 2-aminobenzylamines, respec-
tively. However, quinolines were prepared via dehydrogenative
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Table 2. Substrate Scope for the Dehydrogenative Coupling of Benzyl Alcohols and 2-Aminobenzamides to Various
a,b,c
Substituted Quinazolin-4(3H)-ones
a
b
Stoichiometry: 2-aminobenzamide (1.0 mmol); benzyl alcohol (1.1 mmol); Zn dust (0.5 equiv), KOtBu (0.5 equiv). Isolated yields after column
c
chromatography. Under air.
Table 3. Substrate Scope for the Dehydrogenative Coupling of Benzyl Alcohols and 2-Aminobenzylamines to Various
a,b,c
Substituted Quinazolines
a
b
Stoichiometry: 2-aminobenzylamine (1.0 mmol); benzyl alcohol (1.1 mmol); Zn dust (0.5 equiv), KOtBu (0.5 equiv). Isolated yields after
c
column chromatography. Under air.
coupling of alcohols with ketones bearing an active methylene
group.
yields of 3a and 9a obtained under the optimal conditions are
96 and 66%, respectively (Table S5, entry 8). Additionally, the
dehydrogenative coupling of benzyl alcohol and ketone
proceeds at 90 °C to produce a maximum yield of 96% of 2-
phenylquinoline (11b) in just 10 h with 1.0 mol % catalyst
(1b) loading in the presence of 0.3 equiv. KOtBu and 0.4 equiv
of Zn dust under air (Table S5, entry 3, see SI for details).
Substrate scope was explored to check the versatility of the
1b-catalyzed dehydrogenative coupling reactions. The catalyst
Various reaction conditions were screened to get the optimal
conditions for these dehydrogenative coupling reactions
(Table S5, entries 1−19). Best results for 2-phenylquinazo-
lin-4(3H)-one (3a) and 2-phenylquinazoline (9a) were
obtained on carrying out the reactions in toluene at 100 °C
for 16 h in the presence of 0.5 equiv of Zn dust, 0.5 equiv.
KOtBu and 5.0 mol % 1b under air. The maximum isolated
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Table 4. Substrate Scope for the Dehydrogenative Coupling of Ketones and 2-Aminobenzylalcohols to Various Substituted
a,b,c
Quinolines
a
b
Stoichiometry: acetophenone (1.1 mmol); 2-aminobenzyl alcohol (1.0 mmol); Zn dust (0.4 equiv), KOtBu (0.3 equiv). Isolated yields after
c
column chromatography. Under air.
Scheme 3. Control Experiments to Probe the Active Involvement of [1b]− during Dehydrogenation and Dehydrogenative
a
Coupling Reactions
a
All the above reactions were performed under air.
1b is well tolerant toward a wide variety of substituted benzyl
alcohols and ketones bearing electron-donating, -withdrawing,
and heteroaryl functionalities. Irrespective of the position
(-ortho, -meta, or -para) of the electron-donating substituents
such as -Me, - OMe, on the phenyl ring of benzyl alcohols, the
desired quinazolin-4(3H)-ones, 3b, 3g, 3j, 3k, 3l, and 3n were
isolated in 68−95% yields (Table 2, entries 2, 3, 9, 11, 12, and
14). While the respective quinazolines 9b, 9c, 9i, 9j, and 9l
were obtained in 59−67% isolated yields (Table 3, entries 2, 3,
8, 10, 12). Electron-withdrawing halogens also survived during
the catalytic reaction to produce the corresponding quinazolin-
4(3H)-ones, 3c, 3d, 3f, 3h, 3m, 3o in 55−85% (Table 2,
entries 4−6, 10, 13, 15) and quinazolines 9d, 9f, 9g, 9h, and 9k
in 51−65% isolated yields (Table 3, entries 4−6, 9, 11).
Reactions also proceed well in the presence of −CF3 and
−NO2 groups to produce the corresponding quinazolin-
4(3H)-ones, 3e and 3i, in 55 and 93% yields, respectively
(Table 2, entries 7 and 8). Under identical conditions, 2-(4-
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Scheme 4. (1)Trapping of (Ketyl)-Radical-TEMPO Adduct during [1b]− Catalyzed Dehydrogenation of 2a and 15a and
TEMPO Bound [1b]− by HRMS. (2) Dehydrogenation of Cyclobutanol
nitrophenyl)quinazoline, 9e was obtained in 50% yield (Table
3, entry 7). With heteroaryl alcohols, the corresponding
quinazolin-4(3H)-ones, 3p, 3q, and quinazoline 9m were
isolated in 55, 81, and 60% isolated yields, respectively (Table
2, entries 16 and 17, Table 3, entry 13). Reaction with pentan-
1-ol (15r) produced the corresponding quinazolin-4(3H)-one,
3r, in 34% isolated yield (Table 2, entry 18) while cyclopropyl
methanol afforded the corresponding quinazoline, 9n in 65%
isolated yield (Table 3, entry 14). Reactions also proceed well
with a variety of 2-aminobenzamides and 2-aminobenzyl-
amines containing both electron-donating and -withdrawing
groups. The corresponding quinazolin-4(3H)-ones (3s−3v,
Table 2, entries 19−22) and quinazolines (9o, 9p, Table 3,
entries 15, 16) were obtained in moderate to good isolated
yields.
quinolines in 95 and 83% isolated yields, respectively (Table 4,
entries 2, 3). Reactions proceed efficiently with electron-
withdrawing halogens as well as with strong electron-
withdrawing groups like −CF3 to produce the corresponding
quinolines (11e-11h) in 68−96% yields (Table 4, entries 4−
7). Heteroaryl and nonmethyl ketones were also found to be
compatible under the optimal reaction conditions yielding the
respective quinolines 11l, 11m, and 11n in 76, 63, and 84%
yields, respectively (Table 4, entries 11−13). Quinolines were
also obtained in satisfactory yield with substituted 2-amino-
benzyl alcohol (Table 4, entry 14).
Mechanistic Investigation. We performed a set of control
experiments to understand and unveil the reaction mechanism.
The catalyst 1b undergoes reduction in the presence of Zn-
dust to form the one-electron reduced Zn(II)-stabilized azo-
anion radical species [1b]− (see above). To probe the
involvement of [1b]−, the substrates 2a and 15a were
subjected to dehydrogenation using the preformed [1b]− as
Substrate scope was also explored for the synthesis of
quinolines. Acetophenones having electron-donating −Me and
−OMe groups at para-position produced the corresponding
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the catalyst. As expected, the dehydrogenation reactions
proceed smoothly under the optimized conditions, and the
corresponding dehydrogenated products were isolated in 97
and 95% yields, respectively (Scheme 3). [1b]− was further
found to be equally efficient during the synthesis of 2-phenyl
quinazolin-4(3H)-one (3a), quinazoline (9a), and quinoline
(11b) via dehydrogenative coupling reactions (Scheme 3).
Once the active involvement of the Zn(II)-stabilized azo-
anion radical species [1b]− is affirmed in the above
dehydrogenative reactions, we performed the dehydrogenation
of 2a and 15a under the optimized conditions in the presence
of a radical inhibitor like TEMPO. The dehydrogenation
reactions did not proceed at all in the presence of TEMPO;
instead, investigation of the reaction mixture using HRMS
indicates the formation of adducts like 2a−TEMPO (20),
1b2a (21), and 1b15a−TEMPO (22) (Scheme 4, (1a), (1b);
see SI for details). The preformed [1b]− also reacts with
TEMPO. HRMS analysis of the reaction mixture of TEMPO
and in situ formed [1b]− showed an intense peak at 597.1011
amu, indicating the formation of [1bH−TEMPO] (23) adduct
(Scheme 4, (1c)). To further substantiate the involvement of
organic radical and to differentiate between two-electron
hydride transfer and one-electron hydrogen atom transfer
(HAT) pathways, the radical-clock substrate cyclobutanol was
subjected to dehydrogenation under the optimal conditions.
The formation of multiple ring cleavage products is indeed in
accordance with the one-electron hydrogen atom transfer
(HAT) pathway involving the ketyl radical intermediate
(Scheme 4, (2), and SI Figure 125).21 A kinetic isotope effect
(KIE) experiment was also carried out to have a better
understanding of the [1b]− catalyzed dehydrogenation
reactions (Scheme 5). Under the optimized conditions,
dehydrogenation of 15a-D exhibited a KIE value of ∼14,
which is in agreement with similar HAT processes involving
ligand-centered radicals.4g,22
Figure 6. IR spectra of reaction mixture showing N−H and N−D
stretching.
under both argon atmosphere and air, when dehydrogenation
of 1-phenyl ethanol (15e) was carried out separately in the
presence of 19e, no hydrogenated product of the correspond-
ing aldehyde (19e) was obtained (see SI Scheme S1). Instead
spectroscopic investigation of the reaction mixture reveals the
formation of H2O2 (Figure 7).15,16 We also have quantified the
Scheme 5. Kinetic Isotope Effect Study
Figure 7. Detection of H2O2. Absorption spectral changes during
−
formation of I3 in prescence of H2O2.
In catalyst 1b, since Zn(II) is redox inactive while the aryl-
azo scaffold is redox-active and undergoes reduction to form
azo-anion radical, we performed a few control experiments to
investigate the involvement of the aryl-azo scaffold. Stoichio-
metric reaction was carried out using 1b, and 1-phenyl ethanol
(15e) under the optimal conditions in the presence of argon.
IR spectroscopic analysis of the reaction mixture revealed the
N−H stretching at 3030 and 3063 cm−1 (Figure 6; see SI for
details), indicating the active participation of the azo-
chromophore during the dehydrogenation of alcohols.15,16,22,23
N−N single bond stretching was observed at 1300 cm−1.23
Deuterium labeling experiment using deuterated 1-phenyl
ethanol showed the N−D stretching at 2084 and 2120 cm−1,
which further confirms the involvement of the coordinated
aryl-azo scaffold (Figure 6).
amount of H2O2 formed during the dehydrogenation of benzyl
alcohol (15a). After 2 h of the reaction, under the optimized
conditions, 0.69 equiv of H2O2 was obtained, while after the
completion of the reaction after 5 h, we obtained 0.83 equiv of
H2O2 from the reaction mixture. Compared with the yield of
benzaldehyde after 5 h, it seems that the H2O2 formed during
the reaction decomposes under our experimental conditions
(see SI). Next, 2a was subjected to dehydrogenation in the
presence of H2O2 to check the background reaction. Under the
optimized conditions in the presence of 1.0 equiv of H2O2, 3a
was obtained in 18% yield, which is identical to the yield
obtained in the presence of only KOtBu (Table 1, entry 18),
indicating the noninvolvement of H2O2 during the dehydro-
genation reactions.
To investigate the fate of the abstracted hydrogen atom, an
intermolecular hydrogen transfer reaction was performed with
4-methoxy benzaldehyde (19e). Maintaining a closed system
To get a deeper insight into the reaction mechanism, kinetic
studies were carried out using 1-phenyl ethanol (15e) as the
model substrate.16 Under pseudo-first-order conditions, kobs
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Figure 8. Pseudo-first-order kinetic analysis (with respect to catalyst) for the dehydrogenation of 1-phenyl ethanol at room temperature. (a)
Change in absorbance with catalyst concentration variation. (b) Rate constant calculation. (c) Plot of kobs vs change in catalyst concentration.
Figure 9. Pseudo-first-order kinetic analysis (with respect to substrate) for the dehydrogenation of 1-phenyl ethanol at room temperature. (a)
Change in absorbance with substrate concentration variation. (b) Rate constant calculation. (c) Plot of kobs vs change in substrate concentration.
showed a linear dependence on both catalyst and substrate
concentration (Figures 8 and 9). A linear increase in the rate
constant (kobs) was observed upon increasing the catalyst
loading over a range of 0.02−0.25 mol % with respect to the
substrate (15e). A similar, linear rise of kobs was also noticed
when the concentration of the substrate (15e) was increased
over the range of 5−100 equiv with respect to the catalyst
(1b). Thus, the rate equation for the dehydrogenation of 1-
phenyl ethanol (15e) can be expressed as rate = k[1b][1-
phenylethanol]. The nonzero intercepts observed in the kinetic
plots may be attributed to the KOtBu-catalyzed dehydrogen-
ation of 15e. Under the optimized conditions, in the presence
of 0.1 equiv of base we observed ∼5% of conversion of 15e to
18a. The kinetic data also corroborates the fact that the
catalyst and the substrate are bound together in the rate-
determining step.
On the basis of the results obtained from the above control
experiments and available literature, a plausible mechanism is
proposed in Scheme 6. In the presence of zinc dust, the catalyst
1b undergoes one-electron reduction to form the active
catalyst [1b]− (A). Deprotonated alcohols or the saturated N-
heterocycles then bind the active catalyst [1b]− to form the
intermediate B. The Zn(II)-stabilized azo-anion radical, in the
next step, abstracts a hydrogen atom from the α-carbon atom
of the O-coordinated alcohols or the N-coordinated hetero-
cycles resulting in the ketyl-type radical anion intermediate C.
Under aerobic conditions, intermediate C undergoes a rapid
one-electron transfer, and the desired aldehyde/N-heterocycle
is produced, leaving behind the intermediate D. In the absence
of experimental evidence, the aerial oxidation of C via the
formation of superoxide anion is intuitive.22,23 In fact,
oxidation of C by an equivalent of B′ can also not be ruled
out. A second molecule of deprotonated alcohol or saturated
N-heterocycle then coordinates the intermediate D to form the
intermediate E. It is believed that the monoanionic azo-
chromophore in D abstracts the hydrogen during deprotona-
tion of the second molecule of the alcohol or saturated N-
heterocycle.16,22 Finally, aerial oxidation of E generates H2O2
and forms B′, which on one-electron reduction in the presence
of the zinc-dust completes the catalytic cycle. Notably, the
electrochemically generated two-electron reduced hydrazo-
species in the presence of alcohol produces H2O2 upon
exposure to air (see SI).
The formation of quinazolin-4(3H)-ones and quinazolines
via dehydrogenative coupling reactions is believed to proceed
via the base-mediated condensation of 2-aminobenzamides and
2-aminobenzylamines with the in situ formed aldehydes to
form the corresponding cyclic aminals which upon further
[1b]− catalyzed dehydrogenation produce the desired products
(Scheme 6). On the other hand, the in situ-formed aldehydes
undergo base-mediated cross-aldol type condensation with the
ketones to form α, β-unsaturated ketones, which upon further
cyclization affords the desired quinolines (Scheme 6).
CONCLUSION
■
In summary, our present work represents an exclusively ligand-
centered redox-controlled approach for the dehydrogenation of
a wide variety of N-heterocycles under air. Utilizing a Zn(II)-
stabilized azo-anion radical complex as the catalyst, dehydro-
genation reactions were achieved via one-electron HAT
pathway where all the electron transfer events occur at the
aryl-azo scaffold and the Zn(II) acts as a template. Overall, the
catalysts used herein are bench-stable, easy-to-prepare, cheap,
and show good efficiency during dehydrogenation and
dehydrogenative coupling reactions yielding a variety of N-
heterocycles in moderate to good isolated yields. The
successful development of similar approaches utilizing the
metal-stabilized organic radicals is expected to open up many
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Scheme 6. Plausible Mechanistic Pathway
new chemical transformations, which usually require harsh
reaction conditions or sometimes difficult to achieve following
the classical synthetic routes.
JEOL JES-FA200 spectrometer. Gouy balance (Sherwood
Scientific, Cambridge, U.K.) was used for room-temperature
magnetic moment measurements. Unless otherwise men-
tioned, all the catalytic reactions were performed under air.
Caution! Perchlorates have to be handled with care and
EXPERIMENTAL SECTION
■
appropriate safety precautions.
General Information. Toluene, xylene, and tetrahydrofur-
an (THF) used in the reactions were purified via distillation
over sodium/benzophenone, maintaining an argon atmos-
phere, and were stored over 4 Å molecular sieves. All the other
chemicals were commercially available and were used without
further purification. Analytical TLC was done using Merck 60
F254 silica gel plate of 0.25 mm thickness, and column
chromatography was performed with Merck 60 silica gel of
60−120 mesh. Bruker DPX-300 (300 MHz), Bruker DPX-400
(400 MHz), and Bruker DPX-500 (500 MHz) spectrometers
were used for recording 1H and 13C NMR spectra. PerkinElmer
240C elemental analyzer was used for collecting micro-
analytical data (C, H, N). Q-TOF mass spectrometer (serial
no. YA 263) was used for HRMS analysis. A PC-controlled
AUT.MAC204 electrochemistry system was used for all
electrochemical measurements. Cyclic voltammetry experi-
ments were performed under a nitrogen atmosphere using an
Ag/AgCl reference electrode, with a Pt-disk working electrode
and a Pt-wire auxiliary electrode, in acetonitrile containing
supporting electrolyte, 0.1 M [Et4N]ClO4, respectively. A Pt-
wire-gauge was used as the working electrode during
coulometry. EPR spectra in the X band were recorded with a
16
Synthesis of Azo-aromatic Ligands L1a,1b
.
First, 1.0 g
of 2-amino-1,10-phenanthroline (5.122 mmol) was taken in a
250 mL round-bottom flask. Next, 1.0 equiv of nitrosobenzene
was dissolved in 20 mL of toluene and 10.0 equiv of NaOH
was dissolved in distilled water separately. Then the NaOH
solution was added to the round-bottom flask containing 2-
amino-1,10-phenanthroline followed by the addition of
nitrosobenzene solution in toluene. The round-bottom flask
was then fitted with a water condenser and allowed to stir at 80
°C for 12 h. After the completion of the reaction, the toluene
layer of the reaction mixture was separated and concentrated
under a vacuum. The pure ligand was isolated via silica gel
column chromatography using toluene/dichloromethane
mixture as eluent.
Synthesis of Dichloro-2-(phenyldiazenyl)-1,10-phe-
nanthroline-Zn(II) Complex (1a). In an oven-dried round-
bottom flask, 100 mg (0.314 mmol) of the ligand, L1a, was
taken, and 10.0 mL of ethanol was added. To it, 42.8 mg
(0.314 mmol) of anhydrous ZnCl2 was added. An instanta-
neous formation of orange color precipitate was observed. The
reaction mixture was stirred for 4 h at room temperature. Once
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the reaction was complete, the whole reaction mixture was
filtered, and the precipitate obtained was purified by fractional
crystallization with dichloromethane/hexane (10:1) solvent
mixture. Its yield and characterization data are as follows: Yield
93%. Color: Orange. UV/vis: λmax/nm(ε, M−1cm−1), 231-
(12,628), 278(7,114), 329(11,708), 390(6,349). IR (KBr
pure form by column chromatography (silica gel). Eluent:
petroleum ether/ethyl acetate (20:1).
General Procedure for One-Pot Cascade Synthesis of
Quinolines. A mixture of catalyst 1b (1.0 mol %), KOtBu (0.3
equiv), Zn dust (0.4 equiv), ketone (1.1 mmol), and 2-
aminobenzylalcohol (1.0 mmol) was added to a 50.0 mL
round-bottom flask. To it, 5.0 mL of toluene was added. The
round-bottom flask containing the reaction mixture was then
fitted with a water condenser and placed in an oil bath
preheated at 90 °C. The reaction was continued for 10 h under
air. After completion of the reaction, the resulting mixture was
concentrated under a vacuum. The product was isolated in
pure form by column chromatography (silica gel). Eluent:
petroleum ether/ethyl acetate (20:1).
1
cm−1): 1620 (ν, C = N), 1420 (ν, N = N). H NMR (400
MHz, CDCl3+1drop CD3OD): δ (ppm) = 9.34 (dd, J = 4.6,
1.6 Hz, 1H), 8.94 (d, J = 8.4 Hz, 1H), 8.73−8.69 (m, 3H),
8.58 (dd, J = 8.2, 1.2 Hz, 1H), 8.59−8.56 (m, 1H), 8.12 (s,
1H), 8.05−8.02 (m, 1H), 7.71−7.65 (m, 3H). Elemental
Analysis: C18H12Cl2N4Zn: calcd: C, 51.40; H, 2.88; N, 13.32.
Found: C, 51.32; H, 3.00; N, 13.43.
Synthesis of Dichloro-2-((4-chlorophenyl)diazenyl)-
1,10-phenanthroline-Zn(II) Complex (1b). Catalyst 1b
was prepared following the same procedure as catalyst 1a. Its
yield and characterization data are as follows: Yield 94%.
Color: Orange. UV/vis: λmax/nm(ε, M−1cm−1), 238(10,988),
337(4,985), 396(9,128). IR (KBr cm−1): 1580 (ν, C = N),
1420 (ν, N = N). 1H NMR (400 MHz, CDCl3+1drop
CD3OD): δ (ppm) = 9.30 (d, J = 3.6 Hz, 1H), 8.94 (d, J = 8.0
Hz, 1H), 8.70 (d, J = 8.4 Hz, 1H), 8.67 (d, J = 8.8 Hz, 2H),
8.59 (d, J = 8.0 Hz, 1H), 8.12 (s, 2H), 8.05−8.02 (m, 1H),
7.63 (d, J = 8.8 Hz, 2H). Elemental Analysis: C18H11Cl3N4Zn:
calcd: C, 47.51; H, 2.44; N, 12.31. Found: C, 47.40; H, 2.56;
N, 12.44.
General Procedure for Catalytic Dehydrogenative
Synthesis of N-Heterocycles. A mixture of catalyst 1b,
KOtBu, Zn dust, and the respective N-heterocycles were added
to a 50.0 mL round-bottom flask in optimum amounts. To it,
5.0 mL of toluene was added. The round-bottom flask
containing the reaction mixture was then fitted with a water
condenser and placed in an oil bath preheated at the required
optimum temperature. The reaction was continued for the
specified time for respective N-heterocycles under air. After the
completion of the reaction, the resulting mixture was
concentrated under a vacuum. The dehydrogenated N-
heterocycles were isolated in pure form by column
chromatography (silica gel). Eluent: petroleum ether/ethyl
acetate.
General Procedure for One-Pot Cascade Synthesis of
Quinazolin-4(3H)-ones. A mixture of catalyst 1b (4.0 mol
%), KOtBu (0.5 equiv), Zn dust (0.5 equiv), alcohol (1.1
mmol), and 2-aminobenzamide (1.0 mmol) were added to a
50.0 mL round-bottom flask. To it, 5.0 mL of toluene was
added. The round-bottom flask containing the reaction
mixture was then fitted with a water condenser and placed in
an oil bath preheated at 100 °C. The reaction was continued
for 16 h under air. After the completion of the reaction, the
resulting mixture was concentrated under a vacuum. The
product was isolated in pure form by column chromatography
(silica gel). Eluent: petroleum ether/ethyl acetate (3:1).
General Procedure for One-Pot Cascade Synthesis of
Quinazolines. A mixture of catalyst 1b (5.0 mol %), KOtBu
(0.5 equiv), Zn dust (0.5 equiv), alcohol (1.1 mmol), and 2-
aminobenzylamine (1.0 mmol) was added to a 50.0 mL round-
bottom flask. To it, 5.0 mL of toluene was added. The round-
bottom flask containing the reaction mixture was then fitted
with a water condenser and placed in an oil bath preheated at
100 °C. The reaction was continued for 16 h under air. After
the completion of the reaction, the resulting mixture was
concentrated under a vacuum. The product was isolated in
Detection of Hydrogen Peroxide during the Catalytic
Reactions.16 Production of H2O2 during catalytic dehydro-
genation reactions was detected spectrophotometrically. The
−
gradual formation of characteristic absorption band for I3 at
350 nm was monitored. 1-Phenylethanol (1.0 mmol), KOtBu
(0.1 mmol), Zn-dust (0.5 equiv), and 2.0 mol % of catalyst 1b
were added in a 50 mL round-bottom flask containing a stir
bar. To it, 5.0 mL of dry toluene was added and stirred at room
temperature for 4 h. Then 5.0 mL of distilled water was added
to the reaction mixture, and the resultant solution was
extracted three times with dichloromethane. To stop further
dehydrogenation of alcohol, the separated aqueous layer thus
obtained was then acidified with H2SO4 to pH 2. A 10% KI
solution and a few drops of 3.0% ammonium molybdate
solution were added to it. The hydrogen peroxide generated
during the catalytic cycle oxidizes I− to I2, which reacts with
excess I− to form I3 according to the following chemical
−
reactions:
(i) H2O2 + 2I− + 2H+
→ 2H2O + I2; (ii) I2(aq) + I−
→ I3−
Procedure for Kinetic Studies. To a 20.0 mL round-
bottom flask, required amounts of 1-phenyl ethanol (15e),
catalyst (1b), KOtBu (0.1 equiv), and zinc dust (0.5 equiv)
were added under air. To this mixture, 8.0 mL of dry toluene
was added, and the reaction mixture was stirred at room
temperature under air. A 0.1 mL aliquot of the reaction
mixture was taken from the round-bottom flask after a certain
time interval, and the spectral changes were monitored at 290
nm with appropriate dilution.
X-ray Crystallography. We obtained single crystals of 1b,
suitable for X-ray diffraction, via slow evaporation of its
solution in dichloromethane-hexane (10:1) solvent mixture.
Single-crystal X-ray diffraction data of 1b were collected with
monochromated Mo Kα radiation (λ = 0.71073 Å) on a
Bruker SMART Apex II diffractometer equipped with a CCD
area detector. Data reduction was performed with SAINT-NT
software package.24 SADABS program was employed to apply
multiscan absorption correction to all intensity data.25
A
combination of direct methods with subsequent difference
Fourier syntheses were used to solve the crystal structure, and
it was refined by full-matrix least-squares on F2 using the
SHELX-2013 suite.26 An anisotropic treatment was given to
the non-hydrogen atoms in all the cases. The crystal data,
along with the refinement details, are shown in Table S1.
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Computational Details. Gaussian 09 program was used to
perform all the DFT calculations.27 B3LYP hybrid functional
(G09/B3LYP) was used.28 No geometric constraints were
imposed during geometry optimizations. For Zn, the quasi-
relativistic effective core pseudo potential proposed by Hay
and Wadt, LANL2DZ pseudo potential,29 and the correspond-
ing optimized set of basis function has been employed. For
carbon, hydrogen, nitrogen, and chlorine 6-31G* basis set28
was used.
discussion during EPR simulation. We sincerely thank Dr. C.
Bhattacharya and Sangeeta Ghosh for coulometry experiment.
REFERENCES
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c00275.
■
sı
ORTEP, FMOs, characterization data of all the
1
synthesized compounds, copies of H and 13C NMR
spectral data (PDF)
X-ray data for 1 (CIF)
AUTHOR INFORMATION
Corresponding Author
Nanda D. Paul − Department of Chemistry, Indian Institute of
Engineering Science and Technology, Howrah 711103,
India; orcid.org/0000-0002-8872-1413;
■
Email: ndpaul@gmail.com, ndpaul2014@chem.iiests.ac.in
Authors
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H.; de Bruin, B. Ligands that Store & Release Electrons during
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N. P.; Epping, R. F. J.; van Vliet, K. M.; Lankelma, M.; van den
Heuvel, E. J.; Heijtbrink, N.; Broersen, R.; de Bruin, B. Single-
Electron Elementary Steps in Homogeneous Organometallic
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Siuli Das − Department of Chemistry, Indian Institute of
Engineering Science and Technology, Howrah 711103, India
Rakesh Mondal − Department of Chemistry, Indian Institute
of Engineering Science and Technology, Howrah 711103,
India
Gargi Chakraborty − Department of Chemistry, Indian
Institute of Engineering Science and Technology, Howrah
711103, India
Amit Kumar Guin − Department of Chemistry, Indian
Institute of Engineering Science and Technology, Howrah
711103, India
Abhishek Das − School of Chemical Sciences, Indian
Association for the Cultivation of Science, Jadavpur, Kolkata
700032, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c00275
Author Contributions
S.D. did all the catalysis work. R.M., G.C., and A.K.G.
participated during mechanistic investigations and NMR
studies. A.D. did the EPR simulation. N.D.P. supervised and
coordinated the project. N.D.P. and S.D. contributed during
manuscript writing.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The research was supported by DST, Govt. of India (Project
CRG/2019/001737) and Department of Science & Technol-
ogy and Biotechnology, Govt. of West Bengal (Project ST/P/
S&T/15G-13/2019). S.D., G.C., and A.K.G. thank UGC, and
R.M. thanks CSIR for fellowship support. Financial assistance
from IIESTS is acknowledged. DST-sponsored SAIF, IIESTS
is acknowledged for providing SCXRD, HRMS, and NMR
facilities. We sincerely thank Prof. T. K. Paine for helpful
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