Palladium N-heterocyclic carbene catalyzed expected and unexpected C-C and C-N functionalization reactions of 1-aryl-3-methyl-1: H -pyrazol-5(4 H)-ones

Article abstract of DOI:10.1039/c6ra22779a

A palladium N-heterocyclic carbene [Pd(NHC)Cl₂] complex of vitamin B₁ developed earlier in our laboratory was successfully employed as an efficient catalyst for the regioselective C-C and C-N functionalization reactions of 1-aryl-3-methyl-1H-pyrazol-5(4H)-ones 1a-b. The catalyst was attempted for the C-H arylation, acylation and alkoxylation of 1a-b using the respective coupling substrates such as aryl iodides 2a-c, benzylic alcohols 3a-d and methanol/ethanol 4. It was surprisingly noted that the acylation and alkoxylation reactions underwent a diverse pathway to yield some unexpected products rather than the targeted ones. In the case of the arylation reactions only the targeted products have been observed. This has made the protocol very interesting compared to the conventional coupling reactions.

Full text of DOI:10.1039/c6ra22779a

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Palladium N-heterocyclic carbene catalyzed expected and
unexpected C–C and C–N functionalization reactions of 1-aryl-3-
methyl-1H-pyrazol-5(4H)-ones
Received 00th January 20xx,
Accepted 00th January 20xx
Vishal B. Purohit*, Sharad C. Karad, Kirit H. Patel, Dipak K. Raval
DOI: 10.1039/x0xx00000x
www.rsc.org/
significant anti-inflammatory, antiviral, antitumor and
antibacterial properties.4 Among the pyrazolone derivatives, 3-
A
palladium N-heterocyclic carbene [Pd(NHC)Cl2] complex of
methyl-1-phenyl-1H-pyrazol-5(4H)-one 1a itself is
a
neuroprotective drug known as edaravone5 which acts as a
radical scavenger to interrupt the preoxidative chain reactions
and membrane disintegrations associated with brain
ischemia.6 A few examples of transition-metal-catalyzed C–H
functionalization of 1a have appeared in the literature.7 In this
regard, our present study to develop a multiple and divergent
C–H functionalizations approach to readily access analogues of
1a would be valuable for diversity-oriented synthesis (DOS) of
drug candidates and their biological screening.
Vitamin B1 developed earlier in our laboratory was successfully
employed as the efficient catalyst for the regioselective C–C and
C–N functionalization reactions of 1-aryl-3-methyl-1H-pyrazol-
5(4H)-ones 1a-b. The catalyst was attempted for the C–H
arylation, acylation and alkoxylation of 1a-b using respective
coupling substrates such as aryl iodides 2a-b, benzylic alcohols 3a-
d and methanol/ethanol 4. It was surprisingly noted that the
acylation and alkoxylation reactions underwent a diverse pathway
to yield some unexpected products rather than the targeted ones.
In case of the arylation reactions only targeted products have
been observed. This has made the protocol much interesting over
the conventional coupling reactions.
Most of the reported complexes of Pd(II) with NHC ligands
were derived from imidazolium ionic liquids.8 They are
complicated to synthesize,9 toxic and poorly biodegradable.10
Our attempt is to look for new biodegradable and nontoxic
NHC ligand from a natural source, Vitamin B1 to incorporate
,
one of the main principles of green chemistry.11
1. Introduction
Recently, we have developed a well-defined palladium N-
heterocyclic carbene [Pd(NHC)Cl2] complex of Vitamin B1 and
have shown its performance as a good catalyst for the
regioselective halogenation12 and thiolation13 of 1-aryl-3-
methyl-1H-pyrazol-5(4H)-ones via C–H bond activation. These
results inspired us further to investigate its application toward
the C–H arylation, acylation and alkoxylation of 1-aryl-3-
Over
the
past
decade,
transition-metal-catalyzed
functionalization of C–H bonds has emerged as one of the
most sustainable and intriguing protocols to construct C−C and
C−heteroatom (O, S, N, Cl, Br, I) bonds with high chemo-,
stereo- and regioselectivity.1 Complexes and salts of iron,
cobalt, nickel, copper, ruthenium, rhodium and palladium have
been identified as efficient catalysts for regioselective C–H
functionalization reactions.1-2 Amongst them, Pd-catalyzed
regioselective C−C and C−N funcꢀonalizaꢀon reacꢀons are of
greater interest due to their better efficacy and high turnover
numbers.1c, 2c, 3
methyl-1H-pyrazol-5(4H)-ones
using
various
coupling
substrates such as aryl iodides, benzylic alcohols and
methanol/ethanol respectively.
In continuation of our previous research,14 herein we have
attempted the catalytic application of Pd(NHC)Cl2 complex
toward the regioselective C–H arylation, acylation and
alkoxylation of 1-aryl-3-methyl-1H-pyrazol-5(4H)-ones. It was
surprisingly noted that the acylation and alkoxylation reactions
underwent a diverse pathway to yield some unexpected
products rather than the targeted products. In case of the
arylation reactions only targeted products have been observed
with excellent regioselectivity. This is the first ever report for
the regioselective cum unexpected C−C and C−N
The pyrazolone derivatives represent a class of important
privileged five-membered-ring lactams embedded in various
natural and pharmaceutical products possessing a broad
spectrum of biological activities. Many of them exhibited
*Corresponding author. Tel.: +91-02692-226856 - Ext. - 211; Fax: +91-02692
236475
† Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar- 388
120, Gujarat, India
E-mail: vishalpurohit113@gmail.com; dipanalka@yahoo.com
The spectral data of synthesized compound are shown in supplementary data.
This journal is © The Royal Society of Chemistry 20xx
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functionalization reactions of 1a and its analogues using obtained via reductive elimination from
C
leading to the
Pd(NHC)Cl2 as a catalyst.
regeneration of the Pd(II) catalyst.
Scheme 1 Pd(NHC)Cl2 catalyzed regioselective C−H arylaꢀon of 1-aryl-3-methyl-
1H-pyrazol-5(4H)-onesa 1a-b
2. Results and discussion
Initially, the screening reactions for C−H arylaꢀon were
performed with respect to the Pd-sources, oxidants and the
solvents by using 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one 1a
(1 mmol) as the model substrate and 4-iodotoluene 2a (1
mmol) as the arylating reagent in accordance with the Wang’s
arylation procedure.15 The reactions proceeded smoothly to
give the arylation product 5aa in moderate to good yields with
exclusive regioselectivity. Among the Pd-sources used,
Pd(NHC)Cl2 was found to be the most efficient (Table 1, entries
4-7) over the other palladium species such as Pd(OAc)2, PdCl2
and Pd(PPh3)4 (Table 1, entries 9-11). Meanwhile, the effect of
various oxidants, and solvents on the model reaction was also
investigated. Best results were obtained by using Ag2O as the
oxidant in AcOH (Table 1, entries 5-11). Thus, the optimal
arylation conditions were set as 5 mol% of Pd(NHC)Cl2 as the
catalyst with Ag2O (0.5 mmol) as an oxidant in AcOH at 100 °C
using 1 mmol of 1a and 1 mmol 2a forming the product 5aa
with 80% yield in 5 h (Table 1, entry 5).
Yieldb
Entry
R
Ar
Compounds
(%)
1
2
3
4
5
6
-H
-H
-H
-Cl
-Cl
-Cl
4-CH3-C6H4-
4-OCH3-C6H4-
4-CN-C6H4-
5aa
5ab
5ac
5ba
5bb
5bc
80
78
77
77
74
74
4-CH3-C6H4-
4-OCH3-C6H4-
4-CN-C6H4-
Table 1 Optimization of the reaction conditions for the synthesisa of 5aa
aReaction conditions: 1-Aryl-3-methyl-1H-pyrazol-5(4H)-one
1
(1 mmol), aryl
iodide 2 (1 mmol), Pd(NHC)Cl2 (5 mol%), Ag2O (0.5 mmol) and AcOH (2 mL) at 100
°C for 5 h. bIsolated yield.
Entry
Catalyst
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(OAc)2
Oxidant
Oxone
PhI(OAc)2
K2S2O8
Ag2SO4
Ag2O
Solvent
AcOH
AcOH
AcOH
AcOH
AcOH
DMSO
DMF
Yieldb (%)
32
1
2
61
3
28
4
72
5
80
6
Ag2O
73
7
Ag2O
71
8
Ag2O
Toluene
AcOH
AcOH
AcOH
37
9
Ag2O
68
10
11
PdCl2
Ag2O
59
Pd(PPh3)4
Ag2O
62
aReaction conditions: 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one 1a (1 mmol), 4-
iodotoluene 2a (1 mmol), Pd-source (5 mol%), oxidant (0.5 mmol) and solvent (2
mL) at 100 °C for 5 h. bIsolated yield
Scheme 2 Plausible catalytic cycle for Pd(NHC)Cl2 catalyzed C−H arylaꢀon of 1
To further explore the utility of Pd(NHC)Cl2 complex in C–H
activation, more catalytic reactions were performed. As shown
in Scheme 3, we first chose benzyl alcohol 3a (1.0 mmol) as a
acylating reagent to react with 1a (1.0 mmol) in presence of
With the established optimized reaction conditions in hand,
scope and limitations for C−H arylaꢀon of various pyrazol-
5(4H)-ones with substituted aryl iodides was investigated. As
summarized in Scheme 1, the reaction of aryl iodides 2a-c
bearing both electron-donating as well as withdrawing
substituents in the para-position of the iodo group proceeded
smoothly to afford the corresponding arylation products 5aa-
ac/5ba-bc in good to excellent yields.
1.5 mmol tert-Butyl hydroperoxide (TBHP) and
5 mol%
Pd(NHC)Cl2 in AcOH at 90 °C for 6 h by following some widely
used conditions17 for C–H acylation in Pd/TBHP catalytic
systems. However, when the reaction was performed as the
standard reaction conditions above, it was surprisingly noted
that the unexpected products 6aa and 7aa were obtained
instead of expected C–H acylation product 10a. This might be
due to the oxidation of 3a to benzaldehyde in Pd/TBHP which
undergo Knoevenagel condensation with 1a to give 6aa and
6aa further undergo Michael addition by 1a to form 7aa. The
possible mechanism is shown in Scheme 4. The structures of
unexpected products 6aa and 7aa were confirmed well by
NMR spectroscopy.
Based on previous investigations and our results, the
mechanistic pathway of Pd(NHC)Cl2 catalyzed regioselective C-
H arylation of
catalytic cycle2c, 15-16 (Scheme 2). First, Ag2O may oxidize the
ortho-C–H bond of to form the intermediate . A may then
undergo transmetallation with Pd(NHC)Cl2 to generate Pd(II)
intermediate , which upon oxidative addition with aryl iodide
forms a Pd(IV) intermediate . Finally, arylation product is
1 has been assumed to involve a Pd(II) – Pd(IV)
1
A
B
2
C
5
2 | J. Name., 2012, 00, 1-3
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The effectiveness of the model reaction was tested against conditions,18 an oily product was obtained in 78% yield (Table
various oxidants in AcOH. Among them, TBHP was found to be 3, entry 4) with spectral data and elemental analysis
the most effective (Table 2, entry 3) over other oxidants such inconsistent with the expected structure 9a, (R = H, R1 = Me).
as PhI(OAc)2, K2S2O8 and oxone (Table 2, entries 5-7). The Indeed, the data were in good agreement with the structure of
efficiency of the model reaction was also considerably affected an unprecedented product 8a (R = H). This might be due to the
by the choice of solvents (Table 2, entries 1-4). In AcOH 6aa
was formed to be the major product (Table 2, entry 3) while in
EtOH 7aa was the major one (Table 2, entry 4).
Table 2 Optimization of the reaction conditions for the synthesisa of 6aa and 7aa
Yieldb (%)
Entry
Catalyst
Oxidant
Solvent
6aa
37
7aa
40
1
2
3
4
5
6
7
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
TBHP
TBHP
DMF
Dioxane
AcOH
EtOH
n.d.
75
n.d.
13
TBHP
TBHP
10
72
PhI(OAc)2
K2S2O8
Oxone
AcOH
AcOH
AcOH
trace
trace
n.d.
trace
trace
n.d.
Scheme 4 Plausible reaction mechanism for the synthesis of 6 and 7
aReaction conditions: 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one 1a (1 mmol),
benzyl alcohol 3a (1.0 mmol), Pd(NHC)Cl2 (5 mol%), oxidant (1.5 mmol) and
solvent (2 mL) at 90 °C for 6 h. bIsolated yield, n.d., not detected.
coupling reaction of solvent DCE with 1a instead of MeOH in
presence of Pd-catalyst and K2S2O8. It was also noted that the
same product 8a was observed by employing EtOH as the
alkoxylating agent (Table 3, entries 5). When 1,2-
dichloroethane (DCE) was replaced with dichloromethane
(DCM)/dioxane under the same reaction condition, none of
the products 8a or 9a were detected (Table 3, entries 6-7).
Other oxidants such as oxone, PhI(OAc)2 and TBHP gave only a
trace amount of product 8a (Table 3, entries 1-3). With these
optimized conditions, the feasibility of the reaction was also
checked by using substituted pyrazol-5(4H)-one 1b and the
results are shown in Scheme 5.
Under these optimized conditions, a scope of substituted
benzyl alcohols was investigated (Scheme 3). The reaction of
substituted benzyl alcohols 3b-d bearing both electron-
donating as well as withdrawing functions in the para-position
proceeded smoothly to afford the corresponding products
6ab-ad/7ab-ad in moderate to good yields. From the NMR
spectra of 7aa-ad it was observed that both the enol and imine
tautomers of 4-substituted pyrazol-5-one co-existed in the
same molecule.
Scheme 3 Scope of benzyl alcohols for the synthesisa of 6/7
Scheme 5 Reaction Scope for the synthesisa,b of 8a-b
Yieldb (%)
Entry
R2
6
7
1
2
3
4
C6H5-
6aa: 75
6ab: 71
6ac: 73
6ad: 74
7aa: 13
7ab: 11
7ac: 12
7ad: 13
4-CH3-C6H4-
4-OCH3-C6H4-
4-NO2-C6H4-
aReaction conditions: 1-Aryl-3-methyl-1H-pyrazol-5(4H)-one 1 (1 mmol), MeOH (2
mL), Pd(NHC)Cl2 (5 mol%), K2S2O8 (0.5 mmol) and DCE (2 mL) at 55 °C for 5 h.
bIsolated yield
aReaction conditions: 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one 1a (1 mmol),
benzylic alcohol 3 (1.0 mmol), Pd(NHC)Cl2 (5 mol%), TBHP (1.5 mmol) and AcOH
(2 mL) at 90 °C for 6 h. bIsolated yield.
Based on these experimental outcomes and by referring to the
16d, 16e, 19
a tentative mechanism have
leading references,2c,
been proposed for the Pd(NHC)Cl2 catalyzed synthesis of 8a
Scheme 6). First, substrate 1a tautomerized to 1a’ which may
undergo the C–H activation to generate palladacycle
Next, in our effort on Pd(NHC)Cl2 catalyzed C–H alkoxylation of
(
1a with methanol (Scheme 5) under the Wang’s alkoxylation
a
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intermediate
may form Pd(IV) species
reaction to yield intermediate
A
. Oxidative addition of DCE to the palladacycle
may then undergo insertion
. Finally, product 8a is obtained
leading to the regeneration of
3. Conclusion
A
B. B
A novel strategy for the catalytic application of Pd(NHC)Cl2
complex for the regioselective C–C and C–N functionalization
reactions of 1-aryl-3-methyl-1H-pyrazol-5(4H)-ones has been
successfully developed. The protocol was found to be simple,
efficient and unique to demonstrate some unexpected C–H
functionalization reactions of 1-aryl-3-methyl-1H-pyrazol-
5(4H)-ones. Moreover, the present study has not only led
some unexpected products with regioselectivity, but also
provided the analogues of valuable drug candidates for the
pharmacological screening.
C
via reductive elimination from
C
the Pd(II) catalyst.
Scheme 6 Plausible catalytic cycle for Pd(NHC)Cl2 catalyzed synthesis of 8a
Acknowledgements
The authors thank Head, Department of Chemistry, Sardar
Patel University for providing necessary research and analysis
facilities. VBP and SCK acknowledge University Grants
Commission-New Delhi, India for Basic Scientific Research
(UGC-BSR) Fellowship awarded to them during 2015-2018. The
authors also acknowledge DST, New Delhi for the assistance in
the form of mass analysis at PURSE central facility at Sardar
Patel University sponsored under PURSE program grant vide
sanction letter DO. No. SR/59/Z-23/2010/43 dated 16th March
2011.
Table 3 Optimization of the reaction conditions for the synthesisa of 8a
Entry
Catalyst
Oxidant
R1OH
Solvent
Yieldb(%)
1
2
3
4
5
6
7
Pd(NHC)Cl2
Oxone
MeOH
MeOH
MeOH
MeOH
EtOH
DCE
DCE
28
36
Pd(NHC)Cl2 PhI(OAc)2
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
Pd(NHC)Cl2
TBHP
K2S2O8
K2S2O8
K2S2O8
K2S2O8
DCE
20
DCE
78
DCE
73
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aReaction conditions: 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one 1a (1 mmol),
MeOH or EtOH (2 mL), Pd(NHC)Cl2 (5 mol%), oxidant (0.5 mmol) and solvent (2
mL) at 55 °C for 5 h. bIsolated yield, n.d., not detected.
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also observed when a mixture of 1a (1 mmol), Pd(NHC)Cl2 (5
mol%) and Ag2O (0.5 mmol) was stirred in AcOH (2 mL) at 100
°C for
9 h (Scheme 7). This might be due to the
tautomerization of 1a to its enol form in polar solvent.13 The
C4-H of the enol form of pyrazole ring might be activated in
presence of Pd-catalyst resulting the homo-coupling product
1
11a. The formation of 11a was confirmed well by H NMR and
mass spectrometry.
Scheme 7 Pd(NHC)Cl2 catalyzed homo-coupling of 1a
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DOI: 10.1039/C6RA22779A