Rapid, Operationally Simple, and Metal-free NBS Mediated One-pot Synthesis of 1,2-Naphthoquinone from 2-Naphthol

Article abstract of DOI:10.1002/adsc.201701312

A metal-free, one-pot synthesis of 1,2-naphthoquinone was accomplished from 2-naphthol by utilizing economically cheap NBS under open air conditions. Initial formation of 1,1-dibromonaphthalen-2-one and subsequent transformation afforded the 1,2-naphthoquinone. This oxidation was completed within 30 min and had broad substrate scope. Moreover, this system tolerated heterocyclic systems and was also applicable to 1,3-dicarbonyl systems. This practical approach with short reaction times, a simple workup, and insensitivity to moisture could override the usage of expensive and hazardous oxidizing and metal reagents. (Figure presented.).

Full text of DOI:10.1002/adsc.201701312

Title: Rapid, operationally simple, and metal-free NBS mediated one-
pot synthesis of 1,2-naphthoquinone from 2-naphthol
Authors: Jaeuk Sim, Hyeju Jo, Mayavan Viji, Minho Choi, Jin-Ah Jung,
Heesoon Lee, and Jae Kyung Jung
This manuscript has been accepted after peer review and appears as an
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201701312
Link to VoR: http://dx.doi.org/10.1002/adsc.201701312

10.1002/adsc.201701312
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Rapid, operationally simple, and metal-free NBS mediated one-
pot synthesis of 1,2-naphthoquinone from 2-naphthol
Jaeuk Sim,a Hyeju Jo,a Mayavan Viji,a Minho Choi,a Jin-Ah Jung,a Heesoon Lee,a and
Jae-Kyung Junga*
a
College of Pharmacy and Medicinal Research Center (MRC), Chungbuk National University, Cheongju 28160,
Republic of Korea. Tel.: +82-43-261-2635; fax: +82-43-268-2732. E-mail: orgjkjung@chungbuk.ac.kr (J.-K. Jung)
Received:((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
activities (Figure 1). In addition to these, the 1,2-NQ
core plays key roles as a catalyst in oxidative
transformation reactions[3a] and as a highly reactive
intermediate for the construction of heterocyclic
compounds.[3b]
Abstract. A metal-free, one-pot synthesis of 1,2-
naphthoquinone was accomplished from 2-naphthol by
utilizing economically cheap NBS under open air
conditions. Initial formation of 1,1-dibromonaphthalen-
2-one and subsequent transformation afforded the 1,2-
naphthoquinone. This oxidation was completed within
30 min and had broad substrate scope. Moreover, this
system tolerated heterocyclic systems and was also
applicable to 1,3-dicarbonyl systems. This practical
approach with short reaction times, a simple workup,
and insensitivity to moisture could override the usage of
expensive and hazardous oxidizing and metal reagents.
Thus, it is not surprising that several elegant works
have been developed for the synthesis of this
privileged scaffold.[4a] 1,2-NQs were usually prepared
from 1- and 2-naphthols through the oxidative
(dearomatization) reaction.[4b-g] Most of the common
approaches included using excess hypervalent iodine
reagents (e.g. IBX,[4h-j] DMP,[4k-l] and PIFA[4m]),
TBHP,[5a] oxone[5b] or the expensive Fremy’s salt.[5c]
Recently, Oh et al., reported that copper-catalyzed
synthesis of 1,2-NQ under aerobic oxidation required
16 h to complete.[6] However, the aforementioned
methodologies suffer from some drawbacks,
including high costs of the reagent, elevated
temperatures, and/or longer reaction times (Scheme
1). As a result, development of the novel method
using mild, readily available, and cheaper reagent is
highly desirable.
Keywords: Naphthoquinone(s), Metal-free, Oxidation,
NBS, Vicinal tricarbonyls, Chemoselective
Introduction
Naphthoquinones, one of many valuable scaffolds
distributed in plants, fungi, etc., have received wide
attention and a large scope of application in organic
and medicinal chemistry. They were found to exhibit
a wide range of biological activities including
antibacterial, antiviral, trypanocidal, and antifungal
activities.[1]
Figure 1: 1,2-NQ containing natural products
Scheme 1: Previous work and our approach on synthesis of
1,2-NQ
1,2-Naphthoquinone (1,2-NQ), in particular, has
received much attention due to its high affinity to
form covalent bonds with protein and DNA.[2] There
are several natural products that contain 1,2-NQ as a
core structure and display valuable biological
Oxidative dearomatization reactions are widely
utilized in organic synthesis since several natural
1
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10.1002/adsc.201701312
Advanced Synthesis & Catalysis
products contain unaromatized rings in their core
structure.[7] As such, N-bromo succinimide (NBS)
represents a more user-friendly oxidative, and non-
toxic reagent widely utilized in laboratories and
industries.[8] In general, NBS is used for
bromination,[9a-d] and oxidative dearomatization
reactions,[4h,9e] not only because it is very cheap, but
also because its side product can be easily removed.
As outlined in the Scheme 1, we envisioned that this
privileged structure could be accessed through
sequential NBS-mediated bromination followed by
oxidation in the presence of piperidine as a base.
synthetic elaborations. In addition, 9-phenanthrol was
applied to this reaction condition, which provided the
expected product 3t in 90% yield.
Table 1. Optimization of reaction conditionsa
Results and Discussion
aUnless otherwise stated, the reaction condition: Naphthol
To pursue our goal, we selected 4-alkyl naphthol
1a[6], as a model substrate and used 3.0 equiv. of
NBS in DMSO at room temperature under open air
conditions to discover the suitable condition. 1,2-NQ
was obtained in 10% yield (Table 1, entry 1). The
influences of the other bases were also investigated
(entries 3-7). However, bases such as pyrrolidine,
Et3N, DABCO, and DMAP were ineffective and led
to side products. Interestingly, when using 7.0 equiv.
of piperidine, an improved yield (68%, entry 9) was
observed. Subsequently, the screenings of NBS
analogues and other halogen sources were also
carried out (entries 10-13). Next, the screening of a
range of solvents was investigated (entries 14-16). To
our delight, a 90% isolated yield was achieved in a
DMSO/DCM (1:2) (entry 16). Varying the amount of
NBS and/or piperidine was proved to be ineffective
(entries 17-20). Based on all the above entries, we
concluded that the optimal condition was: 1.0 equiv.
of 1a, 3.0 equiv. of NBS, and 7.0 equiv. of piperidine
in 0.06M DMSO/DCM (1:2).
Brominating
agent
Yield
(%)b
Entry
Base
Solvent
1
2
3
4
5
6
7
8
NBS
NBS
NBS
NBS
NBS
NBS
NBS
NBS
NBS
-
DMSO
DMSO
10
c
K2CO3
16
Pyrrolidinec DMSO
SRd
SRd
13
Et3Nc
DMSO
DMSO
DMSO
DMSO
Pyridinec
DABCOc
DMAPc
SRd
SRd
25
Piperidinec DMSO
9
Piperidine
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO/THF
DCM
68
45
10
11
12
13
14
15
16
17
18
19
20
Br2-dioxane Piperidine
DBDMH
NCS
NIS
NBS
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
10
SRd
17
51
72
NBS
NBS
DMSO/DCMe 90
DMSO/DCMe 28
DMSO/DCMe 73
NBSf
NBSg
NBS
Piperidinec DMSO/DCMe 58
Piperidineh DMSO/DCMe 83
To examine the scope of this protocol, the
optimized condition was applied to various 2-
naphthol derivatives. As shown in Table 2, this
oxidation reaction exhibited a wide scope of
NBS
1a (0.21 mmol), bromination agent (3.0 equiv.) and base
(7.0 equiv.) in solvent (0.18 M) at room temperature for 30
b
c
min under open air conditions. Isolated yield. 5.0 equiv.
substituted
2-naphthols
and
produced
the
d
e
of base was used. SR: Side Reaction. Solvent ratio: 0.06
corresponding products in good to excellent yields.
When the mono-alkyl group was substituted in the 4th
position of the 2-naphthol, the reaction produced
good to excellent yields (3a-g). Substituted phenyl
ring was also tolerated, leading to the corresponding
products in high yields (3h-k). In particular, the
halogen-substituted phenyl ring had relatively
excellent yields (3l-n). Similarly, 3,4-di-substituted
naphthol substrates did not lead to any negative
impacts on the reaction (3o-r). However, the position
of the substituent significantly affected the reaction
yield: only 33% yield was obtained with the 6-chloro
substituted naphthol (3s). On the other hand, 2-
naphthols with electron donating methyl group (as a
mixture of isomers) successfully undergoes this
oxidation reaction and delivered 1,2-NQs 3u-v in
very high yields (90% for 3u and 97% for 3v). It
should be noted that the reaction conditions could
accommodate various functional groups such as ester
and halide; these groups can be utilized for further
f
g
M (DMSO:DCM= 1:2). 2.5 equiv. of NBS was used. 4.0
equiv. of NBS was used. h10.0 equiv of base was used.
We next focused on utilizing unsubstituted 2-
naphthol and 4-phenyl-1-naphthols as shown in
Scheme 2. The corresponding products were obtained
in low yield, highly likely due to the Michael addition
at the 4th position of unsubstituted 2-naphthol 1w and
non-selective bromination of 1-naphthol, respectively.
For example, 4-phenyl-1-naphthol, derived from the
4-bromo-1-naphthol using Sonogashira coupling,
provided the desired 1,2-NQ 3h in 38% yield.
Tactically, we hypothesized a masking (at the 4th
position) to prevent the Michael addition, allowing us
to deliver the 1,2-NQ.[4h,6]
The benzylic position is highly prone to oxidation
and delivers the corresponding oxidation compounds
2
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10.1002/adsc.201701312
Advanced Synthesis & Catalysis
in the presence of NBS.[10a-c] Intriguingly, when we
applied our optimized condition to 4-benzyl-2-
naphthol 4a, the desired 1,2-NQ 5a was obtained in a
chemoselective-manner as shown in Scheme 3. The
substrate scope was extended to heterocyclic systems
to examine the generality of this reaction. A furan-
containing heterocycle 4b[10d] was successfully
oxidized and produced 5b in good yield. With 6-
quinolinol,[10e] chemoselective
oxidation also
occurred on the phenyl ring without N-oxide
formation to afford 2-methyl quinoline-5,6-dione
(5c).
Scheme 2: Reaction with unsubstituted 2-naphthol and 4-
phenyl-1-naphthol
Table 2. Synthesis of 1,2-naphthoquinone derivativesa
Scheme 3: Reaction with benzylic and heterocyclic
systems
Vicinal tricarbonyl compounds also serve as
important building blocks in synthetic chemistry due
to their highly electrophilic nature of the carbonyl
groups. Depending on nitrogen nucleophiles, various
heterocyclic compounds can be constructed.[11]
Consequently, various biologically important
molecules can be synthesized from vicinal
tricarbonyls.[12] Thus, we attempted to expand our
protocol to synthesize vicinal tricarbonyl compounds
from 1,3-dicarbonyl derivatives. This result was
summarized in Scheme 4.
aReaction condition: Naphthols 1 (0.2 mmol) in DMSO
(1.2 mL) and DCM (2.4 mL), piperidine (1.4 mmol, 7.0
equiv.), then NBS (0.6 mmol, 3.0 equiv.) at room
temperature for 30 min in open air condition, yields are
isolated yields.
Scheme 4: Reaction on 1,3-dicarbonyl systems
A gram scale synthesis of 6c was successfully
achieved under the optimized condition, resulting in
about 71% yield of the vicinal tricarbonyl derivative
7c. Similarly, 9-phenanthrol (1t) also reacted
efficiently and afforded the corresponding product 3t
in 84% yield (Scheme 5).
3
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10.1002/adsc.201701312
Advanced Synthesis & Catalysis
utilizing 3.0 equiv. of NBS with 6c. The dibromo
intermediate 10c was converted into the final product
by utilizing 7.0 equiv. of piperidine. However, the
mono-bromo derivative 9c, only produced the
piperidine substituted product 11c, which was
transformed into the final product 7c with 2.0 equiv.
of NBS (Scheme 8).
Scheme 5: Gram scale synthesis
Replacement of organic solvents with water is
highly appreciated in concern of being
environmentally benign. To this end, we also
conducted the reaction in water as a solvent and the
1,2-NQ 3a was obtained in good yield (60%) without
any problems. When the reaction was performed
under an inert atmosphere (N2), it produced nearly the
same yield (90%) with that of open air conditions.
This indicates that this reaction does not depend on
molecular oxygen.[6]
Scheme 8: Control experiment B
Based on the above results, a plausible mechanism
was proposed for the NBS mediated oxidation
reaction as depicted in Scheme 9. Piperidine could
attack the carbonyl group of the dibromo intermediate
2 to afford a hemiaminal, which was spontaneously
transformed into an oxirane intermediate I. Then, the
rearrangement of I, followed by the subsequent
hydrolysis of the resulting iminium intermediate II,
can lead to the final product 3 (Pathway A of Scheme
9).
Alternatively, nucleophilic attack of piperidine
on the dibromo carbon could lead to the formation of
III, which may undergo subsequent elimination of
the bromide, resulting in the intermediate IV.
Subsequent hydrolysis of IV could generate the final
product 3 (Pathway B of Scheme 9).
Scheme 6: Reaction in water and inert conditions
To figure out the reaction pathway, several control
experiments were performed (Schemes 7 and 8). We
only obtained the mono-bromo derivative 8a when
using 1.1 equiv. of NBS. A further reaction with
piperidine failed to produce the naphthoquinone. This
implies that this reaction cannot proceed via the
Kornblum type oxidation. On the other hand, we
obtained dibromo derivative 2a with 97% yield when
using 3.0 equiv. of NBS. Treatment of piperidine or
pyrrolidine to the dibromo compound 2a gave the
final product 3a in 89% yield, whereas reactions
without a base (e.g. DMSO or NBS/DCM condition)
failed to deliver the desired 1,2-NQ.
Scheme 9: Plausible reaction mechanism
Conclusion
We have developed an efficient synthetic
method for 1,2-naphthoquinones from 2-naphthols by
a facile one-pot procedure, utilizing NBS in the
presence of piperidine under metal-free conditions.
This one-pot method would allow the rapid
construction of 1,2-NQs without the use of toxic
metals or expensive complex reagents. Furthermore,
it is also applicable to heterocyclic systems, and can
be utilized for the synthesis of vicinal tricarbonyl
systems.
Scheme 7: Control experiment A
Similarly, in the case of tricarbonyl synthesis, both
mono- and dibromo derivatives were obtained by
4
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10.1002/adsc.201701312
Advanced Synthesis & Catalysis
Conflicts of interest
Mukhopadhyay, J. N. Moorthy, Tetrahedron,
2017, 73, 2210-2216.
There are no conflicts to declare.
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Experimental Section
General procedure: To a solution of 2-naphthol derivatives
1 (0.21 mmol) in DMSO (1.2 mL) and dichlromethane (2.4
mL) was added drop wise piperidine (0.15 mL) and the
reaction mixture was stirred at room temperature. Then,
NBS (114 mg, 0.64 mmol) was added to the reaction
mixture; stirring was continued until reaction was
completed as monitored by TLC (5 min – 30 min). Then
10.0 mL of water was added, and extracted with
dichloromethane, dried over MgSO4. The solvent was
removed under reduced pressure, and the residue was
purified by column chromatography (silica gel, 20-50%
ethyl acetate in hexanes) to afford the 1,2-naphthoquinone
derivatives 3.
Acknowledgements
This work was supported by the National Research Foundation of
Korea grants funded by the Korea Government (MSIP) (MRC
2017R1A5A2015541, and Basic Science Research Program
2017R1D1A1B03) and the International Science and Business
Belt Program through the Ministry of Science, ICT and Future
Planning (2017K000490).
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6
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10.1002/adsc.201701312
Advanced Synthesis & Catalysis
COMMUNICATION
Rapid, operationally simple, and metal-free NBS
mediated one-pot synthesis of 1,2-naphthoquinone
from 2-naphthol
Adv. Synth. Catal. Year, Volume, Page – Page
Jaeuk Sim, Hyeju Jo, Mayavan Viji, Minho Choi,
Jin-Ah Jung, Heesoon Lee, and Jae-Kyung Jung*
7
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