Palladium-catalyzed late-stage mono-aroylation of the fully substituted pyrazoles via aromatic C–H bond activation

Article abstract of DOI:10.1016/j.cclet.2018.09.022

The palladium-catalyzed late-stage aroylation of 4-methyl-1,5-diaryl-1H-pyrazole-3-carboxylates has been developed via direct and exclusive mono-Csp²–H bond activation with broad substrate scope and good functional group tolerance. A dual-core

Full text of DOI:10.1016/j.cclet.2018.09.022

Accepted Manuscript
Title: Palladium-catalyzed late-stage mono-aroylation of the
fully substituted pyrazoles via aromatic C–H bond activation
Authors: Miao-Miao Chen, Ling-Yan Shao, Li-Jun Lun,
Yu-Liang Wu, Xiao-Pan Fu, Ya-Fei Ji
PII:
DOI:
Reference:
S1001-8417(18)30377-2
https://doi.org/10.1016/j.cclet.2018.09.022
CCLET 4664
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Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
28-8-2018
22-9-2018
29-9-2018
Please cite this article as: Chen M-Miao, Shao L-Yan, Lun L-Jun, Wu Y-Liang,
Fu X-Pan, Ji Y-Fei, Palladium-catalyzed late-stage mono-aroylation of the fully
substituted pyrazoles via aromatic C–H bond activation, Chinese Chemical Letters
(2018), https://doi.org/10.1016/j.cclet.2018.09.022
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Communication
Palladium-catalyzed late-stage mono-aroylation of the fully substituted pyrazoles via
aromatic C–H bond activation
Miao-Miao Chena, Ling-Yan Shaoa, Li-Jun Lunb, Yu-Liang Wub, Xiao-Pan Fua, Ya-Fei Jia
a School of Pharmacy, East China University of Science & Technology, Shanghai 200237, China
b Shandong Keyuan Pharmaceutical Co. Ltd., Shandong Shanghe Economic Development Zone, Jinan 251601, China
Graphical abstract
A Pd-catalyzed late-stage aroylation of 4-methyl-1,5-diaryl-1H-pyrazole-3-carboxylates has been developed via direct Csp2−H
bond activation with exclusive site-selectivity, broad substrate scope and good functional group tolerance.
ARTICLE INFO
ABSTRACT
Article history:
The palladium-catalyzed late-stage aroylation of 4-methyl-1,5-diaryl-1H-pyrazole-3-
carboxylates has been developed via direct and exclusive mono-Csp2−H bond activation with
broad substrate scope and good functional group tolerance. A dual-core dimeric palladacycle is
confirmed by X-ray single crystal crystallography, and probably serves as an active species in
the catalytic cycle.
Received 28 August 2018
Received in revised form 22 September 2018
Accepted 25 September 2018
Available online
Keywords:
Palladium-catalysis
Late-stage functionalization
Aroylation
Fully substituted pyrazoles
C–H bond activation
Cyclopalladated intermediate
Diaryl ketones are prevalent structural motifs broadly distributed in pharmaceuticals, fragrances, agrochemicals, dyes, organic
functional materials, food additives, and natural products [1]. The electrophilic Friedel−Crafts aroylation of arenes as the most classical
access to the architechtures usually needs large amounts of Lewis acids or strong mineral acids, which is extremely limited by
electronically biased substrate, poor regioselectivity, narrow functional group tolerance and stoichiometric waste [2]. Recently,
transition-metal-catalyzed aroylation assisted by auxiliary has emerged as a powerful tool to circumvent the limitations. Considering
atom- and step-economy, many colleagues have successfully developed a series of direct aroylations by using aldehydes [3], alcohols
[4], α-oxocarboxylic acids [5], α-diketones [6a], carboxylic acids [6b], anhydrides [6c], amines [6d], benzylic ethers [6e], toluene
———
 Corresponding author.
E-mail address: jyf@ecust.edu.cn.

derivatives [7] and others [8] as aroyl sources with the assistance of various directing groups. Therein, the excellent directing ability of
pyrazole moiety for aroylation of arenes have been uncovered by Frost’s and other following works (Scheme 1A) [3m,8a,b].
Currently, late-stage functionalization, which conducts a direct transformation in lead or privileged structures for generating greatly
potential drug-like molecules or highly valuable scaffolds [9], has turned into a leading strategy in chemistry community. As a result,
many elegant efforts have been devoted to a variety of late-stage diversification of bioactive compounds [10], drug leads [11] and
molecular materials [12], demonstrating supernormal value of this strategy.
Given that the highly functionalized pyrazoles and thiazoles serve as important pharmacophores in drug discovery [13], We
recently reported efficient late-stage mono-acetoxylation/iodination of fully substituted pyrazoles (Scheme 1B), as well as late-stage
mono-acyloxylation of the highly substituted thiazoles [14]. Albeit with the advances, late-stage functionalization directed by inherent
pharmacophore still remains to be developed. To continue our interest, herein, we disclosed an efficient protocol for the regioselective
late-stage mono-aroylation of multifunctional pyrazoles using aldehydes as aroyl surrogates, through a Pd-catalyzed aromatic C–H
activation with aroyl radical coupling (Scheme 1C).
Scheme 1. Previous related works and our present work.
Our initial effort focused on briefly evaluating the substituted pattern of pyrazole directing group. To our delight, the unsubstituted
1-phenyl-1H-pyrazole 1a with benzaldehyde (2a, 2.0 equiv.) delivered the expected product 3a in 43% yield in the presence of
Pd(TFA)2 (10 mol%) and TBHP (2.5 equiv.) in DCE (2.0 mL) under N2 atmosphere at 90 oC for 12 h (Fig. 1). When an ethoxycarbonyl
group was installed at 3-position, the yield was evidently increased to 57%, implying possible synergic role of the group (3b).
Sequentially, incorporating phenyl into 5-position led to a slight drop with 51% yield (3c). In striking contrast to the substrates (1a−c),
the fully substituted pyrazole dramatically furnished a good yield of 79% (3d). However, the yield was drastically diminished to 34%
upon changing ethoxycarbonyl into carbamyl at 3-position (3e). Even more, no any reaction occurred when the ethoxycarbonyl was
varied as a carboxyl (3f). Thus, the fully substituted 1d was selected as an optimal directing group to effectively conduct the desired
aroylation.

Fig. 1. Directing effect of the substituted pyrazoles. Reaction conditions: 1 (0.3 mmol), 2a (63.7 mg, 0.6 mmol), Pd(TFA)2 (10.0 mg, 10
mol%), TBHP (67.6 mg, 2.5 equiv.), DCE (2.0 mL), N2 atmosphere, 90 oC, 12 h. Isolated yields of 3. DCE = 1,2-dichloroethane; TBHP = tert-
butyl hydroperoxide.
Upon the directing group ensured, we turned our attention to investigating main reaction parameters with the model substrates 1d
and 2a (Table 1). Firstly, screening of Pd salts, including Pd(TFA)2, Pd(OAc)2, Pd(PPh3)2Cl2, PdCl2 and Pd(CH3CN)2Cl2, exhibited
Pd(TFA)2 holding the highest catalytic activity (entries 1−5). Whereas lowering the loading of Pd(TFA)2 to 5 mol% obviously cut
down the yield (entry 1 versus 6). Subsequently, it was found that DCE was the best solvent compared to other common ones (entry 1
versus 7−10). Further testing peroxides, organic and inorganic oxidants such as DCP, TBPB, PhI(OAc)2, K2S2O8 and BQ, indicated that
TBHP was the ideal one (entry 1 versus 11−15). Furthermore, the yield was impaired when decreasing the stoichiometry of TBHP to
2.0 equiv. (entry 1 versus 16). On the other hand, raising the stoichiometry of TBHP to 3.0 equiv only gave rise to a similar yield (entry
1 versus 17). Considering the impact of the reaction temperature, it was observed that the yield slightly increased to 83% under 100 oC
(entry 18). But further elevating the temperature to 110 oC resulted in a drop of yield (entry 19). Besides, the N2 protection proved to be
vital for the transformation (entry 20). Hence, we chose entry 18 as the standard reaction conditions for the following aroylation survey.
At last, the geometrical configuration of 3d was confirmed by X-ray single crystal crystallography (Fig. 2, details see Supporting
information). The crystal structural analysis exhibits that the central pyrazole ring do not share a common plane with 1,5-diphenyl,
consequently, avoiding potential secondary aroylation [14a].
Table 1
Optimization of the reaction conditions.a
Entry Catalyst
Oxidant
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
DCP
TBPB
PhI(OAc)2 DCE
K2S2O8
BQ
TBHP
TBHP
TBHP
TBHP
TBHP
Solvent
DCE
DCE
DCE
DCE
Yield (%)b
79
54
32
26
21
67
44
37
30
22
57
32
25
21
0
73
1
2
3
4
Pd(TFA)2
Pd(OAc)2
Pd(PPh3)2Cl2
PdCl2
5
Pd(CH3CN)2Cl2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
DCE
DCE
6c
7
8
9
Toluene
CH3CN
THF
1,4-dioxane
DCE
10
11
12
13
14
15
16d
17e
18f
19g
20h
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
80
83
72
58

THF = tetrahydrofuran; DCP = dicumyl peroxide; TBPB = tert-butyl peroxybenzoate; BQ = 1,4-benzoquinone.
a
Reaction conditions: 1d (91.9 mg, 0.3 mmol), 2a (63.7 mg, 0.6 mmol), catalyst (10 mol%), oxidant (2.5 equiv), solvent (2.0 mL), N2
atmosphere, 90 oC, 12 h.
b Isolated yield.
c Pd(TFA)2 (5 mol%).
d 2.0 equiv TBHP.
e 3.0 equiv TBHP.
f 100 oC.
g 110 oC.
h Air atmosphere.
Fig. 2. X-ray single crystal of 3d.
With the optimized reaction conditions in hand, we commenced to evaluate the scope of functionalized pyrazoles (Fig. 3). The
substrates with para-electron-donating and electron-withdrawing groups at 1-phenyl could favourably afford the desired products
(3g−l). Therein, the substrate with para-OMe gracefully supplied an excellent yield of 87% (3g). While the substrate containing
strongly electron-withdrawing and coordinating NO2 [3j,7b,15] was successfully transformed into the aroylated product despite in 39%
yield (3l). Generally, the aroylations of meta-substituted substrates appropriately took place at the sterically less hindered ortho-
positions (3m-p). Interestingly, as for the meta-F substituted substrate, the aroylation occurred at both ortho-positions, respectively, in
an overall yield of 79% (3q and q'). Nicely, the reactive Cl, Br, I, NO2 and innate ethoxycarbonyl groups are readily elaborated for
further synthetic transformation.
We paid a special attention to the substrates containing ortho-substituted 1-aryl. Usually, no any aroylated product was detected
because of the highly twisted molecular configuration (3r and 3s) [14a]. Impressively, with respect to much less bulk of F, the
substrate 1t afforded a synthetically useful yield of 27% (3t). Remarkably, the substrate with a crowded meta, meta’-disubstituted
moiety also provided a moderate yield of 64% (3u). The 1-(2-nathphyl)-substituted pyrazole was amenable to the protocol giving the
major product 3v and minor product 3v’ in an overall yield of 71%, demonstrating the reaction compatible with fused arene. With 5-
aryl further tested, the substrates with an electron-donating group (R2 = para-OMe) and an electron-withdrawing group (R2 = para-Cl)
also underwent the protocol providing good yields of 80% and 78%, respectively (3w and x). Furthermore, a respected yield of 43%
was observed for the substrate containing 5-thiophenyl (3y). Besides, the aroylation via six-membered activation mode also worked
well to furnish 63% yield (3z). A lower yield of 69% was observed when the methoxycarbonyl instead of ethoxycarbonyl at 3-position
(3A versus 3d). Lastly, installing ethyl and isopropyl at 5-position only provided 46% and 36% yields, respectively, demonstrating that
aryl group at 5-position is crucial for the transformation (3B and 3C).

Fig. 3. Substrate scope of functionalized pyrazoles. Reaction conditions: 1 (0.3 mmol), 2a (63.7 mg, 0.6 mmol), Pd(TFA)2 (10.0 mg, 10
mol%), TBHP (67.6 mg, 2.5 equiv.), DCE (2.0 mL), 100 oC, 12 h in N2 atmosphere. Isolated yields.
Next, various arylaldehydes were investigated for the aroylation (Fig. 4). It is worth noting that the ortho-hindered aromatic
aldehydes were feasible coupling partners with 63-88% yields, not to show obvious electronic effect (4a−d). Similarly, the meta- and
para-substituted arylaldehydes suffered from the protocol offering good to excellent yields (4e−m). Nevertheless, the strongly
coordinating NO2 [3j,7b,15] substituted benzaldehyde was less reactive, possibly due to lower catalytic overturn (4n). Notably, the
highly hindered methyl trisubstituted substrate could proceed favorably with 77% yield (4o). Moreover, 2-naphthylaldehyde and 1-
naphthylaldehyde were greatly suitable substrates to afford good to excellent yields (4p and q). Besides, electron-rich piperonal was
converted to the corresponding product in moderate yield (4r). Furthermore, the heterocyclic aldehydes also participated in the
protocol providing the aroylated products in synthetically useful to moderate yields (4s and t). However, our additional investigation
indicated that aliphatic aldehydes such as propionaldehyde and phenylacetaldehyde did not take effect under the work conditions. It
should be noted that the reaction only happened at single ortho-position to exclusively afford the mono-aroylation products in all cases,
due to the twisted configurations.

Fig. 4. Substrate scope of arylaldehydes. Reaction conditions: 1g (100.9 mg, 0.3 mmol), 2 (0.6 mmol), Pd(TFA)2 (10.0 mg, 10 mol%), TBHP
(67.6 mg, 2.5 equiv), DCE (2.0 mL), 100 oC, 12 h in N2 atmosphere. Isolated yields.
To gain insight into the reaction mechanism, the reaction between 1d and 2a was carried out in the presence of 2.0 equiv. TEMPO
[(2,2,6,6-tetramethylpiperidin-1-yl)oxy] as a radical scavenger. The TEMPO ester 5a was isolated in 63% yield with no aroylated
product (3d) observed, suggesting the aroyl radical generated in the protocol (Scheme 2) [16]. In addition, an intermolecular
competition reaction between 1d and 1d' (1d-d5) with 2a was conducted in one vessel exhibited a notable primary kinetic isotopic
effect (kH/kD = 3.0, see Supporting information). The result implies that the Csp2–H bond cleavage might be involved in the rate-
limiting step (Scheme 3).
Scheme 2. Trapping of aroyl radical with TEMPO.
Scheme 3. Kinetic isotope effect study.
Gladly, we successfully prepared a crystallizable palladacycle I according to the literature [17]. Its dual-core dimeric structure was
unambiguously confirmed by X-ray single crystal crystallography (Scheme 4A, details see Supporting information). Omitting
Pd(TFA)2 and 1d, the dimer I smoothly converted into the target product 3d in 69% yield under other identical conditions (Scheme 4B,
details see Supporting information). Therefore, the dimer I was considered to be a key palladacycle in the process.

Scheme 4. Preparation and functionlization of the dual-core dimeric cyclopalladated intermediate I.
Scheme 5. Plausible reaction mechanism.
On the basis of the experimental results and previous reports [3a,b,g,h,j,k,l], a plausible reaction mechanism is proposed in Scheme
5. Firstly, the substrate 1 coordinates with Pd(II) catalyst followed by a Csp2–H bond activation to form a dual-core dimeric
cyclopalladated intermediate II. Then, the dimer II undergoes an oxidative addition with aroyl radical, generated in situ by a hydrogen
abstraction, to produce the highly active Pd(IV) species III. In the process, Pd(II) and the radical conduct a single electron transfer
(SET), followed by an oxidation of Pd(III) into Pd(IV) with TBHP [18]. Herein, TBHP would play the roles of both radical initiator
and oxidant [3c,7a]. Finally, the adduct III is subjected to a following reductive elimination to release the aroylated product 3 or 4, and
regenerate the Pd(II) catalyst.
In summary, we have developed a Pd-catalyzed late-stage aroylation of 4-methyl-1,5-diaryl-1H-pyrazole-3-carboxylates via direct
Csp2–H bond activation with broad substrate scope and good functional group tolerance. The protocol is featured by the highly
functionalized substrates, exclusive mono-site-selectivity and simple operation. Particularly, a dual-core dimeric palladacycle is
confirmed by X-ray crystallography to profitably reveal a plausible reaction mechanism. It is expected that the protocol can serve as a
potential tool for further elaboration from pharmaceutically important and highly functionalized pyrazoles.
Acknowledgment
We gratefully thank the National Natural Science Foundation of China (Nos. 21476074 and 21676088) for financial support.
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