Chinese Chemical Letters  2016, Vol. 27 Issue (10): 1617-1621   PDF    
Palladium-catalyzed multi-acetoxylation of 1,3-disubstituted 1H-pyrazole-5-carboxylates via direct C(sp2)-H or C(sp3)-H bond activation
Ding Jun, Guo Ying, Shao Ling-Yan, Zhao Fei-Yi, Liao Dao-Hua, Liu Hong-Wei, Ji Ya-Fei     
School of Pharmacy, East China University of Science & Technology, Shanghai 200237, China
Abstract: A palladium-catalyzed multi-acetoxylation of 1,3-disubstituted 1H-pyrazole-5-carboxylate derivatives containing multiple potential reactive sites is described. Therein, the sequence of this process has been appropriately investigated. The protocol mainly provides the di- and tri-acetoxylated products for 1,3-diarylpyrazoles. Besides, it is found that the acetoxylation of C(sp3)-H bond is prior to that of C(sp2)-H bond under structurally competitive conditions.
Key words: Palladium catalysis     Functionalized pyrazoles     Multi-acetoxylation     C-H activation     Monodentate directing group    
1. Introduction

Transition-metal-catalyzed,directing-grouπ-assisted C-H bond functionalization has emerged as an efficient strategy for the construction of carbon-carbon and carbon-heteroatom bonds [1]. Particularly,oxidative C-H bond acetoxylation is one of the most straightforward transformations for the formation of C-O bond. Over the past decades,various elegant directing groups have been successfully developed to achieve C(sp2)-H and C(sp3)-H acetoxylation [2-4]. However,most researches have dealt with mono- and di-acetoxylation while reports of multi-acetoxylation still remain scarce [5].

Pyrazoles,especially 1,3-diarylpyrazoles,is an important class of structural moieties found in many pharmaceuticals and drug candidates [6]. The introduction of different functional groups,such as halogen,aryl,alkenyl and alkoxy groups to 1-aryl of 1,3- diarylpyrazoles via the ligand-assisted C-H activation has been extensively explored [7]. With respect to acetoxylation of pyrazoles,in 2004,Sanford and co-workers described a pioneering example of ortho-mono-acetoxylation of 1-phenylpyrazole [8a]. Shortly after,they disclosed the ortho,ortho'-diacetoxylation of 1-phenylpyrazole using PS-I(OAc)2 (poly-styrene immobilized PhI(OAc)2) as the terminal oxidant [8b]. Most recently,Kim and coworkers reported a thorough acetoxylation of 1,3,5-triphenylpyr- azoles,in which the corresponding ortho-tetraacetoxylated products were obtained in the presence of extremely excessive oxidant (Scheme 1) [5a]. Based on our previous work on the synthesis of heterozoles [9] and our ongoing interest in C-H functionalization,herein,we report the multi-acetoxylation of1,3- diaryl (or alkyl)-1 H-pyrazole-5-carboxylate compounds. Therein,the step of multi-acetoxylation has been appropriately investigated. We found that the acetoxylation of C(sp3)-H bond is prior to that of C(sp2)-H bond under structurally competitive conditions.

2. Experimental

Unless otherwise indicated,all reagents were obtained from commercial sources and used as received without further purification. All reactions were carried out in oven-dried glassware and monitored by thin layer chromatography (TLC,pre-coated silica gel plates containing HF254). All solvents were only dried over 4 A molecular sieves. Reaction products were purified via column chromatography on silica gel (100-200 mesh). Melting points were determined using an open capillaries and uncorrected. NMR spectra were determined on Bruker AV400 in CDCl3 with TMS as internal standard for 1H NMR (400 MHz) and 13C NMR (100 MHz),respectively. HRMS were measured on a QSTAR Pulsar I LC/TOF MS mass spectrometer or Micromass GCTTM gas chromatograph-mass spectrometer.

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Scheme. 1. Multi-acetoxylation of 1,3-disubstituted pyrazole compounds.

General procedure for the Pd(OAc)2-catalyzed acetoxylation: A mixture of substrate (0.6 mmol),Pd(OAc)2 (0.06 mmol,13.5 mg) and PhI(OAc)2 (2.4 mmol,773 mg) in AcOH (6 mL) was stirred under ambient air at 100 ℃ for 1 h. Upon completion of the reaction,the mixture was first dropped into saturated aqueous NaHCO3 (50 mL),then the solution was extracted with methylene chloride (30 mL × 3),and the combined organic layers were washed with brine,dried over anhydrous MgSO4,and concentrated in vacuo. The resulting residue was purified by silica gel chromatography to give the desired acetoxylated product (s). The characterization data of all products were provided in Supporting information.

3. Results and discussion

Initially,we treated fully substituted ethyl 4-methyl-1,3- diphenyl-1H-pyrazole-5-carboxylate (1) with Pd(OAc)2 and PhI(OAc)2 in AcOH at 100 ℃ under an atmosphere of air (Table 1). Obviously,the degree of acetoxylation of the products could be influenced by the amount of PhI(OAc)2. For instance,when using 1.0 equiv. of oxidant,only mono-acetoxylated product was obtained in 44% yield,with 41% of the starting material recovered and the original 5-ethoxycarbonyl group unaffected (entry 1). It should be pointed out that the first acetoxylation occurred selectively at the ortho-position of 1-phenyl group,rather than at that of 3-phenyl group,possibly due to a bias toward the formation of an electron-richer cyclopalladated intermediate [9c,10]. Furthermore,we observed that the second and third acetoxylation took place at ortho,ortho'-positions of 3-phenyl group in sequence,when 2.0 and 3.0 equiv. of oxidant were employed,respectively (entries 2 and 3). This behavior might be ascribed to the fact that 3-phenyl group holds a lower torsion potential energy relative to 1-phenyl group. Gratifyingly,the di- and tri-acetoxylated products were obtained as the expected resultants in a combined yield of 83% in the presence of 4.0 equiv. of PhI(OAc)2,with trace amount of the tetra-acetoxylated product (entry 4).

Table 1
Optimization of the reaction conditions preferential for 1b and 1c.a

Among various alternative oxidants,K2S2O8 and Oxone were inferior to PhI(OAc)2 (entries 5 and 6),while AgOAc,t-BuOOH,and IOAc (generated in situ from I2 and PhI(OAc)2) [2c] failed to promote any acetoxylation reaction (entries 7-9). On the other hand,no significant improvement was achieved during the screening of other solvents. When using DCE and AcOH/DCE (1:1) as the solvents,low conversions were observed with 34% and 40% recovery of the substrate,respectively (entries 10 and 11). Compared with AcOH,the cosolvent system AcOH/Ac2O (1:1) became slightly less efficient,affording di- and tri-acetoxylated products in a lower combined yield of 71% (entry 12). Thus,entry 4 with the ideal reaction conditions delivered the best result for the desired products 1b and 1c.

With the standard reaction conditions established,a series of 1,3-diaryl-1H-pyrazole derivatives were examined. As shown in Table 2,these substrates were generally compatible with the reaction protocol to afford the desired di- and/or tri-acetoxylated products. Various functional groups,including methyl,methoxyl, chloro,ethoxycarbonyl,amide and benzyl groups,as well as the late-stage introduced acetoxy group were all well tolerated. In general,the substrates with less bulky hydrogen,methyl or ethyl group at 4-position afforded the similar results (entries 1-3),while the di-acetoxylated compound as a major product was supplied in 69% yield when the substrate bearing a phenyl group at 4-position (entry 4). It is probable that the congestion between 3- and 4- phenyl groups impaired the further acetoxylation here. In the reaction process,the acetoxylation steps meet with the sequence of the first one taking place at ortho-position of 1-phenyl group,the second and the third at ortho,ortho7-positions of 3-phenyl group,in turn. The fourth acetoxylation was nearly undetected under current conditions.

Table 2
Acetoxylation of 1,3-diarylpyrazoles.a

Comparing to the pilot substrate 1,when an electron-donating or electron-withdrawing substituent was installed at 3-phenyl ring,the reactants (5-9) suffered from the protocol offering the corresponding di- and/or tri-acetoxylated products in the slightly decreased combined yields. It is noteworthy that,in these cases,the third acetoxylation occurred preferably at ortho'-position of 1- phenyl group. Therein,the substrate with a methyl group at metaposition of 3-phenyl ring furnished single di-acetoxylated product in 76% yield (7b). Besides,the second acetoxylation preferentially took place at the less sterically hindered ortho-position (7b and 9b). Notably,the best yield of tri-acetoxylated product was observed for 9c.

The second acetoxylation via a six-membered metallacycle intermediate was also investigated. Rewardingly,the 3-benzyl substituted substrate was well suitable for the transformation yielding the single di-acetoxylated product in 68% yield,with the reactive benzylic position intact under the strongly oxidative conditions (10b). In addition to ethoxycarbonyl group,we tested the substrates bearing N-phenyl and N-sec-butyl substituted carbamyl groups at 5-position to deliver the single di- acetoxylated products in 58% and 71% yields,respectively,albeit with a weakening effect of amide groups (11b and 12b) [11]. Besides,the 1,3,5-triphenylpyrazole (13) participated in the reaction smoothly to give a satisfactory combined yield of the di- and tri-acetoxylated products.

Furthermore,we turned our attention to activation of inert C(sp3)-H bond with the pyrazole directing group,since it is still challenging [12]. As shown in Scheme 2,all the substrates underwent the reaction conditions to predominantly provide mono- and/or di-acetoxylated products here,while tri-acetoxylated products were undetected (14-21). With regard to the substrates bearing tert-butyl group at 1- or 3-position (14 and 15),to our delight,the di-acetoxylated products were mainly obtained. To our knowledge,the acetoxylation of C(sp3)-H bond directed by pyrazole group has yet been reported [13]. Surprisingly,an acetoxylation preference for C(sp3)-H bond over C(sp2)-H bond was observed for the substrates with both tert-butyl and phenyl groups (16 and 20),which was distinctly different from other reports [3d, 5b, 14]. Probably,the Thorpe-Ingold effect [15] might enhance the bias toward activating C(sp3)-H bond prior to C(sp2)- H bond,to a certain extent. In addition to a di-acetoxylated product arising from only C(sp3)-H bond activation,another di-acetoxylated product deriving from C(sp3)-H and C(sp2)-H bond functionalization was also obtained (20b and 20b' as a mixture of about 1:1).

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Scheme. 2. Acetoxylation of pyrazoles containing C (sp3)–H bonds.a.

It is interesting that,when replacing 1-tert-butyl with 1-isopropyl group,the first acetoxylation still occurred at inert C(sp3)-H bond,sequentially,the second functionalization at the C(sp2)-H bond of 3-phenyl group (17a and 17b). On the other hand,ethyl and methyl groups were installed at 1-position (18 and 19),the reactions undisputedly afforded the ortho,ortho'-di-acetoxylated products of 3-phenyl group in good yields (18b and 19b),illustrating that 1-alkyl group containing a-quaternary and tertiary centers is necessary for C(sp3)-H bond functionalization [16]. In sharp contrast with the substrates 17 and 20,when an isopropyl group was installed at 3-position (21),the acetoxylation only occurred at C(sp2)-H bond,leaving the C(sp3)-H bond unaffected (21a and 21b). As anticipated,the 1-methyl-3-sec- butylpyrazole (22) was unable to give any product,again indicating that a gem-dimethyl effect was essential to the functionalization of alkyl group at 3-position.

4. Conclusion

In summary,we have conducted a specific investigation into the palladium-catalyzed multi-acetoxylation of 1,3-disubstituted 1H- pyrazole-5-carboxylate derivatives containing multiple potential reactive sites. The acetoxylation sequence was properly examined under the mild reaction conditions. The di- and tri-acetoxylated products were mainly provided for 1,3-diarylpyrazole substrates. The acetoxylation of inert C(sp3)-H bond directed by pyrazole was also achieved for the first time. An acetoxylation preference for C(sp3)-H bond over C(sp2)-H bond was observed under structurally competitive conditions. Further explorations of various functionalizations with different coupling partners are underway in our laboratory.

Acknowledgments

The authors are grateful to the National Natural Science Foundation of China (Nos. 21176074 and 21476074) and the Research Fund for the Doctoral Program of Higher Education of China (No. 20130074110009) for financial support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/jxclet.2016.04.007.

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