Tert -Butyl nitrite (TBN) as the N atom source for the synthesis of substituted cinnolines with 2-vinylanilines and a relevant mechanism was studied

Article abstract of DOI:10.1039/c7ob01553d

A green method to synthesize cinnolines by 6π electrocyclic reaction with alkenyl amines and TBN has been developed. TBN plays a dual role both as a nitrogen atom source and an oxidant in this procedure. Relevant mechanism experiments reveal that the reaction proceeds through electrocyclic reaction and with diazo hydroxide as a key intermediate.

Full text of DOI:10.1039/c7ob01553d

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tert-Butyl Nitrite (TBN) as the N atom source for the Synthesis of
Substituted Cinnolines with 2-Vinylanilines and Relevant
Mechanism was Studied
Received 00th January 20xx,
Accepted 00th January 20xx
XiaoBo Pang,a LianBiao Zhao,b DaGang Zhou,a Ping Yong He,c ZhenYu An,a Ji Xiang Ni,a and RuLong
DOI: 10.1039/x0xx00000x
Yan*a
www.rsc.org/
A green method to synthesize cinnolines by 6π electrocyclic
reaction with alkenyl amines and TBN has been developed. TBN
plays a dual role both as the nitrogen atom source and oxidant in
this procedure. Relevant mechanism experiments reveal the
reaction proceeds through electrocyclic reaction and with diazo
hydroxide as a key intermediate.
Substituted cinnolines are important and versatile N-
heterocycles. These compounds have received considerable
interest owing to their anticancer, fungicidal and anti-
inflammatory activities, as well as luminescent and optical
properties.1 The substituted cinnolines are conventionally
synthesized by three well-described approaches with
arenediazonium salt, arylhydrazone, and arylhydrazine as
precursors.2 Particular examples are the Widman-Stoermer
reaction and Borsche-Herbert reaction, of which cinnolines are
synthesized from the cyclization of phenyldiazonium ion with
substituted ortho-alkynes and ortho-aminoacetophenones.3
These strategies, however, rely on the use of well-defined
substrates with N=N bond pre-introduced, suffering from
harsh conditions and unsafe operations. In 2016, the Wu's
group reported a straightforward strategy for the preparation
of cinnolines via multicomponent coupling cyclization of
arynes, tosylhydrazines, and α-bromo ketones.4 One of
practical strategies to overcome these limitations is to develop
the direct intermolecular cyclization reaction from simple
nitrogen-introducing substrates.
Scheme 1. Typical reactions for TBN and AgNO2
years. Besides being used as an inexpensive, mild and readily
accessible oxidant, TBN is also a reliable source of NO and NO2
radicals. So the research in this area has been rapidly
developed recently.5 In this context, the synthesis of
quinoxaline N-oxides have been developed by Jiao's group
with TBN as a NO source (Scheme 1).6 The Liang and co-
workers reported an iron-catalyzed method for the synthesis
of disubstituted isoxazoles from homopropargylic alcohol, TBN
and H2O (Scheme 1).7 In contrast to these research and
traditional N sources,8 the utilization of TBN as a N source has
been rarely exploited for the construction of N-heterocycles.9
Thus, the development of efficient methods for the synthesis
of heterocyclic compounds by employing TBN as N source
following the principles of green chemistry is highly desirable.
The development of new method for the utilization of tert-
butyl nitrite (TBN) has been a hot research topic in recent
a.State Key Laboratory of Applied Organic Chemistry, Key laboratory of Nonferrous
Metal Chemistry and Resources Utilization of Gansu Province, Department of
Chemistry, Lanzhou University, Lanzhou, Gansu, China. E-mail: yanrl@lzu.edu.cn
b.College of Chemical Engineering, Northwest University for Nationalities, Lanzhou,
Gansu, China.
c Zhejiang Huayi Pharmaceutical Co., Ltd, China.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Table 1. Optimization of reaction conditions a
Table 2. Scope of 2-vinylanilines a
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DOI: 10.1039/C7OB01553D
Entry
Solvent
Yield(%)b
1
MeNO2
THF
80
50
62
70
74
59
41
87
77
56
2
3
1,4-diox
DMF
4
5
DMSO
NMP
6
7
PhMe
EtOH
EtOH
EtOH
8
9c
10d
a Reaction conditions: 1a (0.3 mmol), TBN (0.6 mmol),
solvent (2 mL), 120 oC. b Yields of isolated products. c
100 oC. d 80 oC. Entry in bold highlights the optimized
reaction conditions, and the reaction time was
monitored by TLC.
Inspired by our previous studies in dealing with the synthesis
of heterocyclic compounds,10 we reported here a [5+1]
cyclization reaction for the synthesis of cinnolines with TBN as
N1 unit. The reaction proceeds efficiently in ethanol without
using any catalyst and additive.
a Reaction conditions: 1 (0.3 mmol), TBN (0.6 mmol), EtOH (2 mL),
120 oC.
yl)aniline 1w were successfully tolerated in this reaction,
providing 3u and 3w in satisfactory yields, respectively.
The reaction was initially investigated with 2-(1-
phenylvinyl)aniline (1a) and TBN. The desired product 4-
phenylcinnoline (3a) was obtained in 80% yield when the
Table 3. The synthesis of substituted cinnolines a
o
reaction proceeded in MeNO2 at 120 C after 5 h. Encouraged
by this result, several strategies were performed to optimize
the reaction conditions. We firstly evaluated the effect of
solvents, and the reaction in ethanol proved to be the most
efficient, giving 3a in 87% yield (Table 1, entry 8). The
screening of other reaction parameters such as reaction
temperature, reaction atmosphere and reaction time did not
give a better result. Thus the conditions in Table 1 of entry 8
were used as the standard conditions.
With the optimized reaction conditions in hand, the scope
and generality of this reaction were investigated as illustrated
in Table 2. 2-Vinylanilines with either electron-donating or
withdrawing R1 groups reacted smoothly with TBN, giving
desired substituted cinnolines in high yields. Generally, the
reaction was slightly affected by the nature of R1 groups of 2-
vinylanilines, excepting the reaction of halogenated substrates
provided products 3m and 3o in moderate yields.
Based on the successful synthesis of cinnolines from 2-
alkenyl anilines, more challenging substrates were then
employed and the results are illustrated in Table 3. 2-Alkenyl
anilines with different R1 and R2 substituents readily reacted
with TBN, and generated the desired substituted cinnolines in
a Reaction conditions: 1 (0.3 mmol), TBN (0.6 mmol), EtOH (2 mL),
moderate
to
good
yields
Pleasingly,
2-(1-
120 oC.
phenylvinyl)naphthalen-1-amine 1u and 2-(prop-1-en-2-
2 | J. Name., 2012, 00, 1-3
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Table 4. The synthesis of substituted fused heterocyclic
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DOI: 10.1039/C7OB01553D
compounds a
a Reaction conditions: 4 (0.3 mmol), TBN (0.6 mmol), EtOH (2 mL),
Scheme 2. The mechanism study of this reaction
120 oC.
spectra are listed in Figure 1. The experiment was performed
with substrate 1a and TBN in DMSO-d6 at room temperature,
since this reaction proceed rapidly under this conditions.
When the reaction proceeded for 5 min, the signal of 1a
corresponding to the NH2 protons disappeared. Meanwhile,
the desired product 3a was detected during the process. These
results indicated that the reaction was occurred on the NH2
However, when (E)-2-styrylaniline 1x was employed under the
optimized conditions, only traces of corresponding product
was detected.
Based on the success of ortho-alkenyl anilines in this
reaction and the similar reactivity in many kinds of C(sp2)−H
transformations, we further investigated the scope of the
group and ruled out the possibility of the addition of
·NO
substituted heterocyclic compounds
Fortunately, anilines with several ortho-heteroarenes, such as,
pyrrole (4a), indole (4b 4d) and pyrrolo[1,2-a]pyridine (4c),
could generate the desired products in moderate to excellent
yields (5a 5d) under optimized conditions.
4 as substrate (Table 4).
and vinyl group of substrate 1a (Scheme 2). Secondly, the
HRMS (High Resolution Mass Spectrometry) measurement was
also performed after the reaction of substrate 1a with TBN in
DMSO for 5 min. To our delight, a peak at m/z = 225.1022
corresponding to C14H13N2O [M+H]+ was observed. This result
was very important and it mean that the reaction did not
undergo the pathway of cationic addition like Widman-
Stoermer reaction. Based on the results of NMR and HRMS,
the IM2 should be the intermediate for this reaction (Scheme
2).
,
-
To further understand the mechanism of this reaction,
control experiments were then performed. In the presence of
TEMPO (2,2,6,6,-tetramethylpiperidyl-1-oxyl), substrate 1a
reacted smoothly and gave 3a in 84% yield. The ESR (Electron-
Spin Resonance) detection, combined with the use of a spin-
trapping reagent such as MNP (2-methyl-2-nitrosopropane),
were carried out. however, no useful radical intermediates and
radical signals were obtained (see supporting information
Scheme S1 and Figure S1). Moreover, the computational study
of this reaction was further performed, and the results of Path
I
are summarized in Figure 2 (those of Path II are described in
the Supporting Information). All the calculations were
operated with the Gaussian09 suite of programs.11 The
reactants, transition states, intermediates and product were
fully optimized at the M06-2X/6-31G(d, p) level of theory. The
integral equation formalism polarized continuum model
(IEFPCM) was applied to simulate the solvation effect of EtOH.
The computational results displayed that the reactant 1a
Figure 1. 1H NMR analysis of this reaction
interacts with
2 to lose one tert-butanol molecule via
transition state TS1 with an energy barrier of 29.41 kcal/mol.
The intermediate IM1 subsequently underwent intramolecular
rearrangement to yield the intermediate IM2 with an energy
barrier of 30.44 kcal/mol. In the third step, the ring-closing
reaction of IM2 gave an intermediate IM3 with the energy
We next focused on elucidating the mechanism of this
1
transformation. Firstly, in situ H NMR (1H Nuclear Magnetic
Resonance) experiments were carried out to monitor the
evolution of the groups during the process, and the selected
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Figure 2. The total potential energy surface for the conversion from substrates 1a and 2 to product 3a
（2）
N2
（2）
C1
barrier of 31.66 kcal/mol . Finally, with the assistance of one
H2O molecule, the intermediate IM3 lost one H2O molecule to
produce the final product 3a via transition state TS4 with a
21.75 kcal/mol barrier energy. The potential energy surface
revealed that the exothermal releasing of the overall reaction
is about ca.59.15 kcal/mol.
f
 -0.040
f
 0.012
(intermediate IM2) to
and
(intermediate
IM1), indicating that the electrocyclic reaction of IM2 is much easier
than that of IM1 (Scheme 2). Subsequent cyclization of
intermediate IM2 gives the intermediate IM3 via 6π electrocyclic
reaction. Finally, the desired product 3a is formed through
dehydration process.
Conclusions
In conclusion, we have developed a green method to
synthesize substituted cinnolines from 2-vinylanilines and TBN
via tandem oxidation/cyclization reactions. During the
synthesis of cinnolines, TBN acts as a dual role both as the N
source and an oxidant. 2-Vinylanilines with different functional
groups proceed well in this reaction and the desired cinnolines
are obtained in moderate to good yields. The mechanism study
reveals that the IM2 should be the intermediate and the
reaction proceeds through 6π electrocyclization.
Scheme 3. Proposed Mechanism
Based on above results, a possible mechanism is proposed in
Scheme 3. The nitro radical might be firstly generated through the
decomposition of TBN. The reaction of substrate 1a with nitro
radical will produce an intermediate IM1,12 which will equilibrate to
intermediate IM2. The Fukui function can be used to predict the
activity of atoms. The computational results show that the value of
Acknowledgements
This work was supported by National Natural Science
Foundation of China (21672086, 21462035), Gansu Province
Science Foundation for Youths (1606RJYA260) and the
Fundamental Research Funds for the Central Universities
(lzujbky-2016-39).
（2）
N2
（2）
C1
f
 0.241
f
 -0.225
C1 and N2 atoms decreased from
and
Notes and references
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