Mo(VI)-catalyzed synthesis of 2-aryl-2H-indazoles using pinacol mediated deoxygenation of nitroaromatics

Article abstract of DOI:10.1246/cl.190490

A molybdenum(VI)-catalyzed protocol for the synthesis of 2-aryl-2H-indazoles using pinacol as a reducing agent under neat reaction conditions has been demonstrated. The developed method gives an easy access to a wide range of 2-aryl-2H-indazoles in excell

Full text of DOI:10.1246/cl.190490

Advance Publication Cover Page
Mo(VI)-Catalyzed Synthesis of 2-Aryl-2H-Indazoles Using Pinacol Mediated Deoxygenation of
Nitroaromatics
Dhananjaya Kaldhi, Raghuram Gujjarappa, Nagaraju Vodnala, Arup K. Kabi,
Nayyef Aljaar, and Chandi C. Malakar*
Advance Publication on the web August 2, 2019
doi:10.1246/cl.190490
© 2019 The Chemical Society of Japan
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1
Mo(VI)-Catalyzed Synthesis of 2-Aryl-2H-Indazoles Using Pinacol Mediated Deoxygenation of
Nitroaromatics
Dhananjaya Kaldhi,a Raghuram Gujjarappa,a Nagaraju Vodnala,a Arup K. Kabi,a and Nayyef Aljaar,b Chandi C. Malakar a*
aDepartment of Chemistry, National Institute of Technology Manipur, Langol, Imphal – 795004, Manipur, India.
bChemistry Department, the Hashemite University, P. O. Box 150459 Zarqa 13115, Jordan.
E-mail: chdeepm@gmail.com, cmalakar@nitmanipur.ac.in
1
2
3
4
5
6
7
A
molybdenum(VI)-catalyzed protocol for the
33 the synthesis of 2H-indazoles are still limited and remains
34 desired due to the difficulties in their preparation.16 Among
35 these reported methods, the reductive cyclization of
36 nitroaromatics,17 the transition metal catalysed C-N18 and N-
37 N19 bond formations, and the low-valent titanium mediated
38 cyclization14b, 20 reactions have been described (Scheme 1).
synthesis of 2-aryl-2H-indazoles using pinacol as a reducing
agent under neat reaction conditions has been demonstrated.
The developed method gives an easy access to wide range
of 2-aryl-2H-indazoles in excellent yields. The present
strategy excludes the use of P(III)-reagents as
deoxygenating agents.
8
9
Keywords: MoO2Cl2(dmf)2, Pinacol, 2-Aryl-2H-Indazoles.
10
Indazole containing molecules received considerable
11 attention in medicinal chemistry and drug discovery
12 research.1 Due to their broad spectrum of useful
13 pharmacological properties, these molecules have found
14 wide applications in an extensive range of biological
15 disorders, such as anti-HIV,2 anti-cancer,3 anti-pyretic,4 anti-
16 inflammatory,5 contraceptive activities,6 and anti-microbial
17 activities.7 Scaffolds embedded with 2-substituted indazoles
18 have been identified as potent 5-HT1A receptors,8
19 imidazoline I2 receptor,9 estrogen receptor β10 and viral
20 polymerase inhibitor.11 The biological importance of these
21 structural motifs (Figure 1) has been also witnessed in
22 marketed drugs such as pazopanib12 and lonidamine,13 those
23 act as VEGF inhibitor and anti-cancer agent respectively.
39 Scheme 1. Approaches for the synthesis of indazoles.
40
Albeit, these protocols are promising and endorsed
41 pervasive practices, their major disadvantages affiliated to
42 namely the use of (i) excess amounts of P(III)-reagents,16a,
43 17b-c, 19a (ii) precious metals (Fe, Cu, Pd, Rh)14a, 16c,f,j,l, 18, 19 and
24 Figure 1. Bioactive molecules embedded with indazole moiety.
44 expensive ligands, (iii) carcinogenic reagents (Sn, CO),16b,
25
Hence, owing to their diverse range of activities and
17a
45
(iv) highly reactive low-valent Ti-reagents.14b, 20 Beside
26 analytical properties, during the past years, the indazole
27 derivatives have found enormous industrial and agricultural
28 applications.14 The potent bioactivities of these scaffolds
29 have evidenced a number of synthetic tools in the literature
30 towards their effective preparation. Most of these methods
31 admit the synthesis of 1H-indazoles.15 Whereas, the efforts
32 concerned towards the development of novel protocols for
46 these difficulties, the earlier reported methods agonize from
47 the deficiencies such as narrow substrate scope, harsh and
48 toxic reaction conditions, long-reaction times, and low
49 yields. The selectivity for the formation of desired products
50 remains supplementary challenge, while many of these
51 reported approaches lead to the mixture of 1H- and 2H-

2
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2
3
4
5
6
7
8
9
indazoles.15g, 16d In 2013, we contributed to the synthesis of
2-arylindazoles using the Mo(VI)-catalyzed reductive
cyclization methodology.21 These reactions were mediated
by an excess amounts of P(III)-reagent under microwave
irradiation at 150 °C that derived the subsequent
complications in purification stage due to the presence of
phosphorous reagents. Hence, a mild method which avoids
the use of phosphorous reagents could be an effective
surrogate for the preparation of these scaffolds. Over the
60 pinacol as reducing agent delivered the highest yield (51%)
61 of the product 2a (Table 1, Entry 1). Afterwards, the
62 reaction of N-(2-nitrobenzylidene)aniline (1a) was
63 performed in the presence of 5 mol% MoO2Cl2(DMF)2 as
64 catalyst and 5.0 equiv. of pinacol as deoxygenating agent
65 under solvent free conditions at 100 °C for 8 hours, leading
66 to the formation of the product 2a in 69% yield (Table 1,
67 Entry 8). Interestingly, the yield of the product 2a has been
68 improved, when 10 mol% of MoO2Cl2(DMF)2 was
69 employed as catalyst along with 5.0 equiv. of pinacol at
70 100 °C for 10 hours (Table 1, Entry 9). It was also observed
71 that when the reaction was performed in absence of pinacol,
72 the product 2a has been observed in traces on TLC (Table 1,
73 Entry 10). On the other hand, the formation of the product
74 2a was restricted in absence of catalyst (Table 1, Entry 11).
75 After extensive optimization of the reaction conditions, it
76 was realized that the 10 mol% of MoO2Cl2(DMF)2 and 5.0
77 equiv. of pinacol at 100 °C for 10 hours obtained highest
78 yield (87%) of the desired product 2a (entry 9).
10 past decades, N-heterocycles are perceived as capital assets
11 in the class of organic compounds and are emerging as
12 decisive ingredients in all facets of pure and applied
13 chemistry.22 Starting from their isolation from natural
14 resources to the development of new methodologies and
15 their operations in the fields of pharmaceutical and industry
16 are the subjects of ongoing interest for chemists and
17 biologists.23 Among the various synthetic tools, the
18 reductive cyclization of nitroaromatics has been used
19 extensively for the synthesis of N-heterocycles.24 The
20 heteroannulation
of
nitroaromatics
followed
by
79
Table 1. Screening of the conditions towards MoO2Cl2(DMF)2
catalyzed reaction of 1a.a
21 deoxygenation for the preparation various N-heterocycles
22 has been known since 1898, which may be realized via
23 single or two steps process.25 A variety of reducing agents
24 have been experimented for this purpose.26 Multiple reaction
25 mechanisms and theories have been taken in accounts and
26 several reactive intermediates have been suggested.24h, 27 The
27 major drawback related to these processes are associated to
28 the hazardous deoxygenating agents and reaction conditions,
Entry
Reducing agents
Conditions
2a,
29 and these demerits restrict the further application in modern
Yield %b
17b-c, 19a, 28
30 synthesis.16a,
It is evident that multiple metal-
Pinacol (2.5 equiv.)
HFIP (20 equiv.)
PhMe, 100 °C, 8 h
100 °C, 8 h
31 oxygen containing inorganic complexes are effective
32 towards deoxygenation process.29 Among these,
33 MoO2Cl2(DMF)2 complex has been used extensively due to
1
2
51
17c
29c
PhMe, 100 °C, 10 h
PhMe, 100 °C, 8 h
PhMe, 80 °C, 8 h
PhMe, 80 °C, 8 h
PhMe, 80 °C, 8 h
100 °C, 8 h
3
Et3SiH (10 equiv.)
34 its simple operation and preparation in laboratory including
29a,c-d
35 our recent reports.21,
These deoxygenation reactions
4
Ascorbic acid (5 equiv.)
NaBH4 (1.5 equiv.)
37
33
36 have been executed in the presence broad range of reducing
37 agents to accomplish the successful transformation.
38 Recently, Sanz et al, Sagar et al and our group have
39 successfully employed the combination of Mo(VI) and
40 pinacol as deoxygenative agent for the synthesis of useful
41 organic molecules.29a,c-d Here, we have described the
42 protocol for the synthesis of 2H-indazole derivatives using
43 Mo(VI)-catalyzed reaction, in which pinacol acts as
44 deoxygenating agent (Scheme 1).
5
NaBH3CN (1.5 equiv.)
N2H4‧H2O (5 equiv.)
Pinacol (5 equiv.)
Pinacol (5 equiv.)
-
41
6
46
7
69d
8
100 °C, 10 h
87d,e
9
100 °C, 10 h
Tracec,d,e
0c,d,f
10
11
Pinacol (5 equiv.)
100 °C, 10 h
80 aUnless otherwise mentioned, all reactions were performed using 1.0
81 mmol 1a and 5 mol% MoO2Cl2(DMF)2 in 2 mL solvent in sealed vial.
82 bIsolated yields. cUnreacted starting material 1a was recovered.
83 dReactions were performed in neat condition. eReactions were
84 performed using 10 mol% MoO2Cl2(DMF)2. fReactions were performed
85 in the absence of MoO2Cl2(DMF)2.
45
The starting materials ortho-nitrobenzylidene amines
46 1a-n were synthesized using previously reported method.21
47 To examine the feasibility of this transformation, the
48 substrate
49 investigated under the influence of
N-(2-nitrobenzylidene)aniline
(1a)
was
5
mol%
50 MoO2Cl2(DMF)2 as catalyst and pinacol as deoxygenative
51 agent in toluene as solvent at 100 °C for 8 hours. It was
52 observed that the desired product 2-phenyl-2H-indazole (2a)
53 has been formed in 51% yield (Table 1, Entry 1). By
54 inspiring with this result, the efficacy of a number of
55 reducing agents (HFIP, Et3SiH, Ascorbic acid, NaBH4,
56 NaBH3CN, N2H4‧H2O) have been explored in details (Table
57 1, Entries 2-7). Among the tested deoxygenating agents,
58 NaBH3CN and N2H4‧H2O contributed similarly in the
59 conversion of 1a to 2a (Table 1, Entries 6-7); however, the
86
With the optimized conditions, a series of ortho-
87 nitrobenzylidene amines 1a-n were examined to explore the
88 scope of this methodology. The described protocol revealed
89 a broad substrate scope and reaction conditions enabled the
90 participation of a diversity of substituents in aromatic rings
91 (o-, m- and p-substituted) containing both electron donating
92 and electron withdrawing substituents (Scheme 2).
93 Experimental results suggested that the yield of the products

3
1
2
3
4
5
6
7
8
9
2a-n remains unaffected regardless of electronic nature of
the functional groups. Interestingly, apart from mono-
functionalized aromatic ring, di-substituted and tri-
substituted aromatic residues tolerated rather well under the
optimized reaction conditions to deliver the desired products
2b, d, h-k in high yields ranging from 85-90%. Notably, the
desired products 2l-m embedded with polycyclic aromatic
hydrocarbons can also be synthesized in high yields (90-
92%) using this protocol. The resent protocol also enabled
35 present protocol excludes the use of P(III)-reagents as
36 deoxygenating agents. The developed reaction conditions
37 were realized using catalytic amount of Mo(VI) and pinacol
38 as reducing agent under solvent free conditions. The method
39 was screened on a variety of ortho-nitrobenzylidene amines
40 having electron-donating and -withdrawing groups to obtain
41 the desired product in the range of 84-95%.
10 substitution on nitrobenzylidene ring to give desired product
11 2n in 87% yield.
42 Scheme 3. Plausible mechanism for the Mo(VI)-catalyzed indazole 2
43 synthesis.
12 Scheme 2. Synthesis of indazoles 2a–n using Mo(VI)-catalyzed
13 reaction conditions.
44 Acknowledgements
14
After realizing the scope of the present protocol, we
45
CCM appreciate Science and Engineering Research
15 focused on demonstrating plausible reaction mechanism
16 based on experimental results and literature evidences.29a,c-d
17 The catalyst MoO2Cl2(DMF)2 (A) on reaction with pinacol
46 Board (SERB), New Delhi and NIT Manipur for financial
47 support in the form of research grant (ECR/2016/000337).
48 DK, RG, NV and AKK grateful to Ministry of Human
49 Resource and Development (MHRD), New Delhi for
50 fellowship support.
18 (B)
generates
a
catalytic
species
19 MoO(pinacolate)Cl2(DMF)2 complex (C) by eliminating a
20 molecule of water. The complex C undergoes oxidative
21 cleavage leaving a Mo(IV) complex D along with two
22 molecules of acetone. As one of the acetone molecule is in
23 week coordination with Mo(IV) complex to obtain the
24 geometric stability of molybdenum, the substrate 1 was
25 inserted into the catalytic cycle to form an unstable
26 intermediate E. Next, deoxygenation of intermediate E
27 resulted in intermediate F and Mo(VI), which is restored
28 into catalytic cycle. The derived nitroso intermediate
29 undergoes 6π electrocyclization to produce N-oxide
30 intermediate G. The catalytic cycle was repeated on
31 intermediate G to furnish the desired product 2 (Scheme 3).
51 Supporting Information
52 A detailed supporting information is available which
53 includes the experimental procedures, H NMR and 13C
1
54 NMR of the final products.
55
Supporting
Information
is
available
on
56 http://dx.doi.org/10.1246/cl.******.
57 References and Notes
58
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In summary, we have investigated a new, efficient and
33 mild protocol towards the synthesis of substituted 2H-
34 indazole derivatives using pinacol as a reducing agent. The

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