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Pd(II) supramolecular cage-catalyzed successive oxidative coupling: One-pot and regioselective synthesis of functionalized carbazoles from indoles
Catalysis Communications 124 (2019) 12–18
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Pd(II) supramolecular cage-catalyzed successive oxidative coupling: One-pot and regioselective synthesis of functionalized carbazoles from indoles
T
Xi-Ren Wua,b, He-Long Penga, Lian-Qiang Weia, Li-Ping Lia, Su-Yang Yaoa, Bao-Hui Yea,
⁎
a b
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China School of Pharmacy, Guangdong Medical University, Dongguan 523808, China
ARTICLE INFO
ABSTRACT
Keywords: Carbazole Indole Oxidative Heck reaction Pd(II) supramolecular cage C-H functionalization
Carbazole is an important compound in pharmaceuticals and functional materials. A new Pd(II) supramolecular cage-catalyzed protocol for the synthesis of multifunctional carbazoles from indoles is described through regioselective successive oxidative Heck reactions. This new protocol is highly efficient, with a low Pd (2.4 mol%) catalyst loading and good compatibility for both N-H free and N-protected indole substrates. Moreover, it can be used to synthesize carbazoles with various functional groups by a one-pot two-step procedure. The excellent catalytic activity can be attributed to the distinct properties of the supramolecular cage structure in uniformly distributive and well-defined Pd(II) active centers on the cage surfaces.
1. Introduction Carbazole is an important compound in many bioactive natural products, pharmaceuticals, and functional materials [1,2]. Thus, various synthetic protocols have been developed to construct functionalized carbazoles during the past several decades [3–6]. Among the protocols, cyclization strategies of direct coupling of indoles with alkene molecules are a great attraction because the parent compound indole is readily available. Indeed, [2 + 4] annulation of indole with dialkene or [2 + 2′ + 2″] annulation of indole with ketone and alkene to produce a carbazole scaffold has been reported using Brønsted acid and/or transition-metal as a catalyst [7–13]. However, most of the alkene substrates need to be prefunctionalized. Alternatively, the oxidative coupling of arenes with alkene, discovered by Fujiwara and Moritani [14–16], is an oxidative Heck reaction and has been developed for regioselective alkenylation on indole [17,18]. The successive alkenylation on indole was successfully performed to prepare carbazole with diverse functionalized groups [19–22]. The advantages of this protocol are step and atom economy, and the substrates are commercially available. However, the present approaches contain some limitations such as a high Pd(II) catalyst loading (10–20 mol%) and occasional use of phosphorus ligands. In addition to that, the indole substrate also has high limitations; hence, either N-protected indoles [19,20,22] or N-H free indoles can be used as a substrate (see Scheme 1) [21]. Therefore, a new catalytic reaction with a substrate that has high catalytic efficiency and excellent compatibility (both N-H free and N-protected indoles) and
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an environmentally friendly solvent is highly desirable. Moreover, different reaction mechanisms were proposed for indole-to-carbazole conversion depending on the substrates and catalysts. Reactions of Nprotected indoles with alkenes or its synthetic equivalent to produce 1,3-disubstituted carbazoles using Pd-Cu-Ag trimetallic [19] or Pd-Cu bimetallic [20] catalysts were reported to occur through a Diels−Alder reaction of 3-alkenyated indoles with alkenes. In contrast, N-H free indoles reacted with alkenes to produce 1,3-disubstituted carbazoles through the successive oxidative Heck reactions using Pd-Cu as catalysts in the presence of the PPh3 ligand [21]. Unlike the aforementioned reports, Laha and Dayal demonstrated the formation of 2,3-disubstituted carbazoles from N-protected indoles and alkenes through two sequentially oxidative Heck reactions at the C-3 and C-2 positions of indole in the presence of the Pd-Ag-K2S2O8 catalyst, followed by thermally intramolecular electrocyclization [22]. To develop a highly efficient catalyst for the oxidative Heck reaction, we pay attention to Pd(II) supramolecular cage, which not only arranges the catalytic center Pd(II) in an isolated single site by coordinating with organic ligands with a well-defined structure but also maximally exposes the active Pd(II) ions to the substrates because of the discrete structure. Moreover, the heterometallic catalyst, which exerts a synergistic effect between Pd(II) and other metal ions through the ligand, has been demonstrated to have much increased catalytic activity [23,24]. Following these strategies, a Pd(II)-Al(III) supramolecular cage [Al2Pd3L6Cl6] (Pd-Al) (where HL is 1-(4-(1H-imidazol-1-yl)phenyl)butane-1,3-dione) was synthesized in our laboratory [25]. In each cage,
Corresponding author. E-mail address: [email protected] (B.-H. Ye).
https://doi.org/10.1016/j.catcom.2019.02.023 Received 26 December 2018; Received in revised form 21 February 2019; Accepted 22 February 2019 Available online 23 February 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.
Catalysis Communications 124 (2019) 12–18
X.-R. Wu, et al.
2.2. General procedure for indole-to-carbazole reaction A mixture of indole (0.5 mmol), Pd-Al catalyst (0.8 mol%), Cu (OAc)2 (1.0 mmol), acrylate (1.5 mmol), and pivalic acid (PivOH, 0.25 mmol) in PEG-300/DMSO (5:1, 2 mL) was stirred at 120 °C for 20 h. The reaction mixture was allowed to cool to room temperature and diluted with ethyl acetate (10 mL). The resulting solution was filtered. The filtrate was washed with saturated brine solution three times (3 × 10 mL). The organic layer was dried over Na2SO4, and then, the solvent was removed under reduced pressure. The residue was purified by column chromatography using ethyl acetate/hexane (1:15–1:5) as an eluent to obtain the corresponding product, which was identified by 1 H and 13C NMR analyses (see the Supporting Information). 3. Results and discussion To identify the optimal reaction conditions for indole-to-carbazole conversion, we began our study with N-H free indole 1a and methyl acrylate 2a as model substrates to screen the reaction conditions in the presence of 1.0 mol% Pd-Al as a catalyst and 1.5 equiv. of Cu(OAc)2 as an oxidant in DMF-DMSO (v/v, 5:1, 2 mL) solution at 100 °C. To our delight, the mono-oxidative Heck product 3aa and 1,3-disubstituted carbazole 4aa were obtained in yields of 66% and 23%, respectively (see entry 1 in Table 1). By increasing the amount of the oxidant Cu (OAc)2 to 2.0 equiv., the yield of carbazole improved to 37%, indicating that increasing the amount of oxidant is beneficial for the formation of carbazole. However, continued increase of Cu(OAc)2 to 2.5 equiv. had no significant effect on the yield of carbazole (see entry 3 in Table 1). Thus, we turned to screen other oxidants such as O2, MnO2, K2S2O8, Ag2CO3, BQ, and TBHP (see entries 4–9 in Table 1). Only trace product was observed when O2 was used as an oxidant. Oxidants such as MnO2, K2S2O8, Ag2CO3, BQ, and TBHP were barely successful in the observed reaction conditions. Then, we investigated the effect of solvent and found that the yield of carbazole sharply decreased to 11%, but the mono-oxidative Heck product 3aa increased to 81% when dioxane (Diox) was used as a co-solvent with DMSO (v/v, 5:1). It seemed that Diox solvent is favorable for the formation of the regioselective monooxidative Heck product 3aa rather than the successive oxidative coupling product 4aa (see entries 10 and 11) [17]. In contrast, a significant improvement in the yield of 4aa to 48% was observed when PEG-300/ DMSO (v/v, 5:1) was used as a co-solvent (see entry 12 in Table 1). However, the yield was still moderate. Therefore, the ratio of PEGDMSO was varied to 3:1 and 9:1 under identical conditions. We found the best ratio of PEG-DMSO as a co-solvent was 5:1 for the formation of product 4aa. To screen the optimal conditions, we focused on reaction temperature because high temperature would be beneficial for the successive oxidative Heck reaction. Indeed, the reaction yield of the product 4aa was improved to 71% when reaction temperature was increased to 120 °C (see entry 15 in Table 1). Inspired by the discovery that acid was beneficial for the alkenylation of indole [17,22], PivOH was added to the reaction mixture as an additive. Surprisingly, the yield of the product 4aa was improved to 83% when 0.5 equiv. PivOH was used as an additive at 120 °C. Extending the reaction time to 24 h or increasing the amount of PivOH to 0.7equiv had no significant effect on the reaction. Finally, catalyst loading tests were performed to determine the efficiency of the catalyst. No significant change in the yield of the product 4aa was observed when catalyst Pd-Al loading was decreased from 1.0 mol% to 0.8 mol% (see entry 19 in Table 1). However, the yield was sharply decreased to 56% when the amount of catalyst was lowered to 0.5 mol%. Additionally, the control experiments showed that Pd(II) ion was indispensable for the formation of the product 4aa. When 3 mol% PdCl2 was employed as a catalyst, the yield of the product 4aa was obtained as 36% (see entry 21 in Table 1). On addition of the ligand HL to the above reaction solution, the yield slightly increased to 41% (see entry 22 in Table 1). Therefore, the formation of heterometallic supramolecular cage Pd-Al, which can
Scheme 1. Synthesis of carbazole through oxidative coupling of indole and alkene.
three β-diketone groups with chelating nature from three HL ligands bind to a hard Al(III) ion, and each Pd(II) ion coordinates with two Ndonors of imidazolate from two L ligands and two labile chlorides in a trans fashion as a potential catalytic center (see Fig. S1 in the Supporting Information). It should be pointed out that the long and rigid rod-like ligand ensures the supramolecular cage possesses large windows, in which substrates could freely pass through and the catalytic centers are located in the appropriate sites separately. The introduction of chloride as a labile ligand is beneficial for catalysis because it is easily displaced by substrates. Moreover, the strong N-donors of imidazolate effectively prevent Pd(II) ion leaching out from the cage. In this contribution, we present our recent results on the successive oxidative coupling of indoles with alkenes to produce functional carbazoles using the supramolecular cage Pd-Al as a catalyst in PEG–DMSO solvent (see Scheme 1e). The catalytic experiments showed that the supramolecular cage Pd-Al is an efficient catalyst for the successive oxidative Heck reactions of indole-to-carbazole with a high regioselectivity and a low Pd(II) catalyst loading as well as excellent compatibility in both N-protected and N-H free indoles. 2. Experimental 2.1. General data All chemicals were commercially available and used as purchased unless otherwise noted. 1H and 13C NMR spectra were recorded with a Bruker AV 400 spectrometer using the solvent as an internal standard. Powder X-ray diffraction patterns were recorded on a D8 ADVANCE diffractometer with Cu-Kα radiation (λ = 1.5409 Å) at a scanning rate of 4° min−1, with 2θ ranging from 5 to 35°. The catalyst Pd-Al was prepared according to our previously published procedure [25]. The phase purity of the as-synthesized samples was confirmed by PXRD, comparing the patterns with the one simulated from single crystal data (see Fig. S2). The as-synthesized Pd-Al sample was activated by heating at 110 °C under vacuum for 6 h. 13
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Table 1 Optimal conditions of oxidative coupling reaction.a
Entry
Catalyst (mol%)
Oxidant (equiv)
Solvent
1 2 3 4 5 6 7 8 9 10 11e 12 13 14 15 16 17 18 19 20 21 22f
Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (0.8) Pd-Al (0.5) PdCl2 (3.0) PdCl2 + HL
Cu(OAc)2(1.5) Cu(OAc)2 (2) Cu(OAc)2(2.5) O2c MnO2(1.5) K2S2O8 (1.5) Ag2CO3 (1.5) BQ (1.5) TBHP (1.5) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2)
DMF-DMSO(5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) Diox-DMSO (5:1) Diox-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (9:1) PEG-DMSO (3:1) PEG-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (5:1)
a b c d e f
Additive (equiv)
Temp (°C)/time (h)
Yield (%)b 3a4a
PivOH PivOH PivOH PivOH PivOH PivOH PivOH
100/20 100/20 100/20 100/20 100/20 100/20 100/20 100/20 100/20 100/20 100/8 100/20 100/20 100/20 120/20 120/20 120/24 120/20 120/20 120/20 120/20 120/20
66 23 49 37 48 38 trace N.D.d N.D. N.D. N.D. N.D. 81 11 95 N.D. 35 48 23 37 33 46 16 71 10 83 10 83 10 82 11 82 28 56 35 36 30 41
(0.5) (0.5) (0.7) (0.5) (0.5) (0.5) (0.5)
Reaction conditions: 1a (0.5 mmol), 2a (1.5 mmol), catalyst, and oxidant in 2 mL of solvent. Isolated yield based on indole. O2 balloon. N.D. = not detected. 2a is 0.55 mmol. PdCl2 (3.0 mol%) + HL(6.0 mol%).
effectively prevent Pd(II) ion agglomeration and maintain Pd(II) ion as an isolated active site, helps to improve the stability of Pd species and catalytic activity. With the optimal reaction conditions in hand, the universality of this protocol for indole-to-carbazole conversion was explored under reaction conditions. First, ethyl acrylate (2b) and butyl acrylate (2c), respectively, were used instead of 2a to react with indole 1a under the identical conditions. Surprisingly, the corresponding carbazole products 4ab and 4 ac were obtained in good yields of 80% and 79%, respectively, as shown in Table 2. This also encouraged us to extend the reaction to other indoles with various functional groups. Indeed, when 5-OCH3 indole (1b) was used to react with acrylates 2a, 2b, and 2c, the reaction proceeded favorably to produce the corresponding products4ba, 4bb, and 4bc in good yields of 85%, 82%, and 80%, respectively. Next, 5-Br indole (1c) with an electron-withdrawing group was used to react with acrylates 2a, 2b, and 2c to examine the reaction universality of this protocol. Although the reaction was slightly sluggish, good yields of 72%, 68%, and 65% were also obtained for the products 4ca, 4cb, and 4 cc, respectively. Despite the aryl bromide group being sensitive to metal catalysis reaction, it remains intact in the reaction and provides opportunities for further functionalization of carbazole. When 5-NO2 indole (1d), a substrate with stronger electronwithdrawing group, was used as a substrate to react with acrylates 2a, 2b, and 2c, moderate yields of 53%, 48%, and 46% was obtained for 4da, 4db, and 4 dc, respectively, indicating that the electron-
withdrawing effect on the indole ring is unfavorable for the formation of carbazole. This was consistent with the finding in the previous report by Verma and co-worker [21]. Finally, the substrate was expanded to Nprotected indole, which was demonstrated to be inapplicable to the approach developed by Verma because of the uniquely directing function of free N-H in indole for the successive oxidative Heck reactions [21]. We were pleased to find that N-methyl indole (1e) reacted with acrylates 2a, 2b, and 2c smoothly to produce the corresponding carbazoles 4ea, 4eb, and 4ec in good yields of 71%, 69%, and 68%, respectively. Furthermore, to examine the applicability of this protocol for N-protected indole substrates, N-propyl indole (1f) and N-benzoyl indole (1g) were used to react with acrylates 2a, 2b, and 2c; the corresponding products 4fa, 4fb, 4fc, 4ga, 4gb, and 4gc were obtained in moderate yields of 44–57% (see Table 2), indicating that our indole-tocarbazole protocol processes good compatibility in substrate, i.e., not only N-H free but also N-protected indoles are appropriate as starting materials. This protocol is different from the previous protocols suitable either for N-H free indole [21] or for N-protected indole substrates [19,20,22]. Moreover, indoles bearing various functional groups including electron-neutral, electron-rich, and electron-deficient substituents are suitable as substrates for the preparation of carbazoles in moderate to good yields under the optimal conditions. Then, this protocol was applied to regioselectively synthesize multifunctional carbazole with diverse groups through a one-pot two-step procedure. Indeed, when 1a reacted with 1.1 equiv. of styrene (2d) in
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Table 2 Substrate scope for the synthesis of carbazoles.a,b
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Table 3 One-pot two-step synthesis of asymmetric carbazoles.a
the presence of 1.2 mol% catalyst Pd-Al at 120 °C for 4 h to produce 5methoxy-3-styryl-1H-indole (3ad), 1.5 equiv. of 2a was added to the reaction to produce the corresponding methyl 3-phenyl-9H-carbazole1-carboxylate (4ada) in yield of 42% for 16 h (see Table 3). When 1b, 1c, and 1d were used instead of 1a to react with 2d, and then 2a, the corresponding products 4bda, 4cda, and 4dda were obtained in yields of 45%, 41%, and 36%, respectively. Moreover, the N-CH3 indole (1e) was also used as a substrate to react with 2d, and then 2a; the yield of the asymmetric product 4eda was given as 44%, indicating that the asymmetrically multifunctional carbazoles can be really prepared using two kinds of alkenes in the one-pot two-step strategy. It should be pointed out that the low yields may be related to the low reaction activity of styrene. Gram-scale reaction was also conducted to evaluate the practicability of the indole-to-carbazole reaction. When N-H free indole 1a (1.17 g, 10.0 mmol) was used to react with ethyl acrylate 2b (3.19 mL, 30 mmol) under the optimal conditions, 4ab (2.33 g) was obtained with the yield of 75% (see eq. 1). A similar case was observed in N-protected indole 1e, obtaining 4eb (2.18 g) with a yield of 67% (see eq. 2).
Three different reaction mechanisms have been reported for the synthesis of indole-to-carbazole depending on the substrates and catalysts. Diels−Alder reaction and thermally intramolecular electrocyclization were proposed for the reaction of N-protected indoles and alkenes [19,20,22], while the successive oxidative Heck reactions were demonstrated for the reaction of N-H free indoles with alkenes [21]. To probe the reaction mechanism, control experiments were performed (see eqs. 3 and 4). The mono-Heck reaction product 3-substituted indole 3ab, regarded as an intermediate in the synthesis of carbazole, was isolated and used as the starting material to react with methyl acrylate 2a and butyl acrylate 2c. Asymmetrically 1,3-disubstituted carbazoles 4aba and 4abc were obtained in yields of 83% and 79%, respectively, under the optimal reaction conditions. In contrast, no carbazole product was observed under the similar conditions but in the absence of Pd-Al catalyst. These observations were consistent with Verma's observations [21] and clearly indicated that the formation of carbazole from indole was through the successive oxidative Heck reactions rather than the Diels−Alder reaction. Similarly, when 3eb, a mono-Heck reaction product of N-methyl-indole, was used instead of 3ab under identical conditions, the corresponding products 4eba and 4ebc were produced in yields of 73% and 70%, respectively. This result was different from the results obtained by Verma [21]. Moreover, no 2,3-disubstitued indole product was observed in our reaction. Thus, the thermal intramolecular cyclization procedure would be ruled out [22].
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Although the real mechanism remains unclear, a plausible mechanism of the reaction of indole with alkyene to produce carbazole using Pd-Al cage as a catalyst is outlined in Scheme 2 on the basis of the control experiments and relative literature [21,22]. First, the 3-alkenyl indole 3 was obtained by the regioselective oxidative Heck reaction by means of the insertion reaction of acrylate 2 into C-3 Pd(II) intermediate (I) to produce intermediate (II), followed by β-hydride elimination [26]. During this period, the reduced Pd(0) complex was oxidized by Cu(OAc)2 to regenerate Pd(II), which further catalyzed the next catalytic cycle. Similarly, the intermediate 5 was produced by the second oxidative coupling reaction through the formation of palladium complex intermediates III and IV [21,27]. Finally, a fast intramolecular oxidative Heck reaction occurred through the formation of intermediates V and VI, leading to the carbazole formation. It is worth mentioning that the distinct coordination in the supramolecular cage structure of Pd-Al could prevent Pd(0) agglomeration and prolong the catalyst lifetime.
4. Conclusions In summary, we have developed a new protocol for the synthesis of functionalized carbazoles from indoles through the regioselective successive oxidative Heck reaction using Al2Pd3 supramolecular cage as a catalyst. Unlike the previous reports, the new protocol described herein is highly efficient with a low Pd(II) catalyst loading and good compatibility for both N-H free and N-protected indole substrates. Moreover, an extension of this approach to the synthesis of multifunctional carbazoles in a one-pot two-step procedure warranted a broad application. The excellent catalytic activity can be attributed to the distinct properties of the supramolecular cage structure in uniformly distributive and well-defined Pd(II) active centers on the cage surfaces. This provides a complementary strategy for designing a new catalyst for specific organic reactions.
Scheme 2. Plausible mechanism of the indole-to-carbazole reaction.
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Acknowledgment
[9] J.R. Stepherson, C.E. Ayala, T.H. Tugwell, J.L. Henry, F.R. Fronczek, R. Kartika, Org. Lett. 18 (2016) 3002–3005. [10] B. Guo, X. Huang, C. Fu, S. Ma, Chem. Eur. J. 22 (2016) 18343–18348. [11] S. Chen, Y. Li, P. Ni, H. Huang, G.-J. Deng, Org. Lett. 18 (2016) 5384–5387. [12] S. Chen, L. Wang, J. Zhang, Z. Hao, H. Huang, G.-J. Deng, J. Org. Chem. 82 (2017) 11182–11191. [13] Y. Gu, W. Huang, S. Chen, X. Wang, Org. Lett. 20 (2018) 4285–4289. [14] I. Moritani, Y. Fujiwara, Tetrahedron Lett. 8 (1967) 1119–1122. [15] C. Jia, T. Kitamura, Y. Fujiwara, Acc. Chem. Res. 34 (2001) 633–639. [16] L. Zhou, W. Lu, Chem. Eur. J. 20 (2014) 634–642. [17] N.P. Grimster, C. Gauntlett, C.R.A. Godfrey, M.J. Gaunt, Angew. Chem. Int. Ed. 44 (2005) 3125–3129. [18] M. Petrini, Chem. Eur. J. 23 (2017) 16115–16151. [19] K. Ozaki, H. Zhang, H. Ito, A. Lei, K. Itami, Chem. Sci. 4 (2013) 3416–3420. [20] T. Guo, Q. Jiang, F. Huang, J. Chen, Z. Yu, Org. Chem. Front. 1 (2014) 707–711. [21] A.K. Verma, A.K. Danodia, R.K. Saunthwal, M. Patel, D. Choudhary, Org. Lett. 17 (2015) 3658–3661. [22] J.K. Laha, N. Dayal, Org. Lett. 17 (2015) 4742–4745. [23] J. Huang, W. Wang, H. Li, ACS Catal. 3 (2013) 1526–1536. [24] L. You, W. Zong, G. Xiong, F. Ding, S. Wang, B. Ren, I. Dragutan, V. Dragutan, Y.G. Sun, Appl. Catal. A 511 (511) (2016) 1–10. [25] X.-R. Wu, S.-Y. Yao, L.-Q. Wei, L.-P. Li, B.-H. Ye, Inorg. Chim. Acta 482 (2018) 605–611. [26] C. Jia, W. Lu, T. Kitamura, Y. Fujiwara, Org. Lett. 1 (1999) 2097–2100. [27] R.K. Saunthwal, M. Patel, S. Kumar, A.K. Danodia, A.K. Verma, Chem. Eur. J. 21 (2015) 18601–18605.
This work was supported by the National Natural Science Foundation of China (No. 21571195). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catcom.2019.02.023. References [1] A.W. Schmidt, K.R. Reddy, H.J. Knolker, Chem. Rev. 112 (2012) 3193–3328. [2] C.L. Wang, H.L. Dong, W.P. Hu, Y.Q. Liu, D.B. Zhu, Chem. Rev. 112 (2012) 2208–2267. [3] B. Robinson, Chem. Rev. 69 (1969) 227–250. [4] H.J. Knolker, K.R. Reddy, Chem. Rev. 102 (2002) 4303–4427. [5] Z. Sadiq, E.A. Hussain, S. Naz, Mini-Rev. Org. Chem. 14 (2017) 469–488. [6] X. Zhang, S. Ma, Israel J. Chem. 58 (2018) 608–621. [7] S.G. Dawande, V. Kanchupalli, J. Kalepu, H. Chennamsetti, B.S. Lad, S. Katukojvala, Angew. Chem. Int. Ed. 53 (2014) 4076–4080. [8] J.-Q. Wu, Z. Yang, S.-S. Zhang, C.-Y. Jiang, Q. Li, Z.-S. Huang, H. Wang, ACS Catal. 5 (2015) 6453–6457.
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Pd(II) supramolecular cage-catalyzed successive oxidative coupling: One-pot and regioselective synthesis of functionalized carbazoles from indoles
T
Xi-Ren Wua,b, He-Long Penga, Lian-Qiang Weia, Li-Ping Lia, Su-Yang Yaoa, Bao-Hui Yea,
⁎
a b
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China School of Pharmacy, Guangdong Medical University, Dongguan 523808, China
ARTICLE INFO
ABSTRACT
Keywords: Carbazole Indole Oxidative Heck reaction Pd(II) supramolecular cage C-H functionalization
Carbazole is an important compound in pharmaceuticals and functional materials. A new Pd(II) supramolecular cage-catalyzed protocol for the synthesis of multifunctional carbazoles from indoles is described through regioselective successive oxidative Heck reactions. This new protocol is highly efficient, with a low Pd (2.4 mol%) catalyst loading and good compatibility for both N-H free and N-protected indole substrates. Moreover, it can be used to synthesize carbazoles with various functional groups by a one-pot two-step procedure. The excellent catalytic activity can be attributed to the distinct properties of the supramolecular cage structure in uniformly distributive and well-defined Pd(II) active centers on the cage surfaces.
1. Introduction Carbazole is an important compound in many bioactive natural products, pharmaceuticals, and functional materials [1,2]. Thus, various synthetic protocols have been developed to construct functionalized carbazoles during the past several decades [3–6]. Among the protocols, cyclization strategies of direct coupling of indoles with alkene molecules are a great attraction because the parent compound indole is readily available. Indeed, [2 + 4] annulation of indole with dialkene or [2 + 2′ + 2″] annulation of indole with ketone and alkene to produce a carbazole scaffold has been reported using Brønsted acid and/or transition-metal as a catalyst [7–13]. However, most of the alkene substrates need to be prefunctionalized. Alternatively, the oxidative coupling of arenes with alkene, discovered by Fujiwara and Moritani [14–16], is an oxidative Heck reaction and has been developed for regioselective alkenylation on indole [17,18]. The successive alkenylation on indole was successfully performed to prepare carbazole with diverse functionalized groups [19–22]. The advantages of this protocol are step and atom economy, and the substrates are commercially available. However, the present approaches contain some limitations such as a high Pd(II) catalyst loading (10–20 mol%) and occasional use of phosphorus ligands. In addition to that, the indole substrate also has high limitations; hence, either N-protected indoles [19,20,22] or N-H free indoles can be used as a substrate (see Scheme 1) [21]. Therefore, a new catalytic reaction with a substrate that has high catalytic efficiency and excellent compatibility (both N-H free and N-protected indoles) and
⁎
an environmentally friendly solvent is highly desirable. Moreover, different reaction mechanisms were proposed for indole-to-carbazole conversion depending on the substrates and catalysts. Reactions of Nprotected indoles with alkenes or its synthetic equivalent to produce 1,3-disubstituted carbazoles using Pd-Cu-Ag trimetallic [19] or Pd-Cu bimetallic [20] catalysts were reported to occur through a Diels−Alder reaction of 3-alkenyated indoles with alkenes. In contrast, N-H free indoles reacted with alkenes to produce 1,3-disubstituted carbazoles through the successive oxidative Heck reactions using Pd-Cu as catalysts in the presence of the PPh3 ligand [21]. Unlike the aforementioned reports, Laha and Dayal demonstrated the formation of 2,3-disubstituted carbazoles from N-protected indoles and alkenes through two sequentially oxidative Heck reactions at the C-3 and C-2 positions of indole in the presence of the Pd-Ag-K2S2O8 catalyst, followed by thermally intramolecular electrocyclization [22]. To develop a highly efficient catalyst for the oxidative Heck reaction, we pay attention to Pd(II) supramolecular cage, which not only arranges the catalytic center Pd(II) in an isolated single site by coordinating with organic ligands with a well-defined structure but also maximally exposes the active Pd(II) ions to the substrates because of the discrete structure. Moreover, the heterometallic catalyst, which exerts a synergistic effect between Pd(II) and other metal ions through the ligand, has been demonstrated to have much increased catalytic activity [23,24]. Following these strategies, a Pd(II)-Al(III) supramolecular cage [Al2Pd3L6Cl6] (Pd-Al) (where HL is 1-(4-(1H-imidazol-1-yl)phenyl)butane-1,3-dione) was synthesized in our laboratory [25]. In each cage,
Corresponding author. E-mail address: [email protected] (B.-H. Ye).
https://doi.org/10.1016/j.catcom.2019.02.023 Received 26 December 2018; Received in revised form 21 February 2019; Accepted 22 February 2019 Available online 23 February 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.
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2.2. General procedure for indole-to-carbazole reaction A mixture of indole (0.5 mmol), Pd-Al catalyst (0.8 mol%), Cu (OAc)2 (1.0 mmol), acrylate (1.5 mmol), and pivalic acid (PivOH, 0.25 mmol) in PEG-300/DMSO (5:1, 2 mL) was stirred at 120 °C for 20 h. The reaction mixture was allowed to cool to room temperature and diluted with ethyl acetate (10 mL). The resulting solution was filtered. The filtrate was washed with saturated brine solution three times (3 × 10 mL). The organic layer was dried over Na2SO4, and then, the solvent was removed under reduced pressure. The residue was purified by column chromatography using ethyl acetate/hexane (1:15–1:5) as an eluent to obtain the corresponding product, which was identified by 1 H and 13C NMR analyses (see the Supporting Information). 3. Results and discussion To identify the optimal reaction conditions for indole-to-carbazole conversion, we began our study with N-H free indole 1a and methyl acrylate 2a as model substrates to screen the reaction conditions in the presence of 1.0 mol% Pd-Al as a catalyst and 1.5 equiv. of Cu(OAc)2 as an oxidant in DMF-DMSO (v/v, 5:1, 2 mL) solution at 100 °C. To our delight, the mono-oxidative Heck product 3aa and 1,3-disubstituted carbazole 4aa were obtained in yields of 66% and 23%, respectively (see entry 1 in Table 1). By increasing the amount of the oxidant Cu (OAc)2 to 2.0 equiv., the yield of carbazole improved to 37%, indicating that increasing the amount of oxidant is beneficial for the formation of carbazole. However, continued increase of Cu(OAc)2 to 2.5 equiv. had no significant effect on the yield of carbazole (see entry 3 in Table 1). Thus, we turned to screen other oxidants such as O2, MnO2, K2S2O8, Ag2CO3, BQ, and TBHP (see entries 4–9 in Table 1). Only trace product was observed when O2 was used as an oxidant. Oxidants such as MnO2, K2S2O8, Ag2CO3, BQ, and TBHP were barely successful in the observed reaction conditions. Then, we investigated the effect of solvent and found that the yield of carbazole sharply decreased to 11%, but the mono-oxidative Heck product 3aa increased to 81% when dioxane (Diox) was used as a co-solvent with DMSO (v/v, 5:1). It seemed that Diox solvent is favorable for the formation of the regioselective monooxidative Heck product 3aa rather than the successive oxidative coupling product 4aa (see entries 10 and 11) [17]. In contrast, a significant improvement in the yield of 4aa to 48% was observed when PEG-300/ DMSO (v/v, 5:1) was used as a co-solvent (see entry 12 in Table 1). However, the yield was still moderate. Therefore, the ratio of PEGDMSO was varied to 3:1 and 9:1 under identical conditions. We found the best ratio of PEG-DMSO as a co-solvent was 5:1 for the formation of product 4aa. To screen the optimal conditions, we focused on reaction temperature because high temperature would be beneficial for the successive oxidative Heck reaction. Indeed, the reaction yield of the product 4aa was improved to 71% when reaction temperature was increased to 120 °C (see entry 15 in Table 1). Inspired by the discovery that acid was beneficial for the alkenylation of indole [17,22], PivOH was added to the reaction mixture as an additive. Surprisingly, the yield of the product 4aa was improved to 83% when 0.5 equiv. PivOH was used as an additive at 120 °C. Extending the reaction time to 24 h or increasing the amount of PivOH to 0.7equiv had no significant effect on the reaction. Finally, catalyst loading tests were performed to determine the efficiency of the catalyst. No significant change in the yield of the product 4aa was observed when catalyst Pd-Al loading was decreased from 1.0 mol% to 0.8 mol% (see entry 19 in Table 1). However, the yield was sharply decreased to 56% when the amount of catalyst was lowered to 0.5 mol%. Additionally, the control experiments showed that Pd(II) ion was indispensable for the formation of the product 4aa. When 3 mol% PdCl2 was employed as a catalyst, the yield of the product 4aa was obtained as 36% (see entry 21 in Table 1). On addition of the ligand HL to the above reaction solution, the yield slightly increased to 41% (see entry 22 in Table 1). Therefore, the formation of heterometallic supramolecular cage Pd-Al, which can
Scheme 1. Synthesis of carbazole through oxidative coupling of indole and alkene.
three β-diketone groups with chelating nature from three HL ligands bind to a hard Al(III) ion, and each Pd(II) ion coordinates with two Ndonors of imidazolate from two L ligands and two labile chlorides in a trans fashion as a potential catalytic center (see Fig. S1 in the Supporting Information). It should be pointed out that the long and rigid rod-like ligand ensures the supramolecular cage possesses large windows, in which substrates could freely pass through and the catalytic centers are located in the appropriate sites separately. The introduction of chloride as a labile ligand is beneficial for catalysis because it is easily displaced by substrates. Moreover, the strong N-donors of imidazolate effectively prevent Pd(II) ion leaching out from the cage. In this contribution, we present our recent results on the successive oxidative coupling of indoles with alkenes to produce functional carbazoles using the supramolecular cage Pd-Al as a catalyst in PEG–DMSO solvent (see Scheme 1e). The catalytic experiments showed that the supramolecular cage Pd-Al is an efficient catalyst for the successive oxidative Heck reactions of indole-to-carbazole with a high regioselectivity and a low Pd(II) catalyst loading as well as excellent compatibility in both N-protected and N-H free indoles. 2. Experimental 2.1. General data All chemicals were commercially available and used as purchased unless otherwise noted. 1H and 13C NMR spectra were recorded with a Bruker AV 400 spectrometer using the solvent as an internal standard. Powder X-ray diffraction patterns were recorded on a D8 ADVANCE diffractometer with Cu-Kα radiation (λ = 1.5409 Å) at a scanning rate of 4° min−1, with 2θ ranging from 5 to 35°. The catalyst Pd-Al was prepared according to our previously published procedure [25]. The phase purity of the as-synthesized samples was confirmed by PXRD, comparing the patterns with the one simulated from single crystal data (see Fig. S2). The as-synthesized Pd-Al sample was activated by heating at 110 °C under vacuum for 6 h. 13
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Table 1 Optimal conditions of oxidative coupling reaction.a
Entry
Catalyst (mol%)
Oxidant (equiv)
Solvent
1 2 3 4 5 6 7 8 9 10 11e 12 13 14 15 16 17 18 19 20 21 22f
Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (1.0) Pd-Al (0.8) Pd-Al (0.5) PdCl2 (3.0) PdCl2 + HL
Cu(OAc)2(1.5) Cu(OAc)2 (2) Cu(OAc)2(2.5) O2c MnO2(1.5) K2S2O8 (1.5) Ag2CO3 (1.5) BQ (1.5) TBHP (1.5) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2) Cu(OAc)2 (2)
DMF-DMSO(5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) DMF-DMSO (5:1) Diox-DMSO (5:1) Diox-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (9:1) PEG-DMSO (3:1) PEG-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (5:1) PEG-DMSO (5:1)
a b c d e f
Additive (equiv)
Temp (°C)/time (h)
Yield (%)b 3a4a
PivOH PivOH PivOH PivOH PivOH PivOH PivOH
100/20 100/20 100/20 100/20 100/20 100/20 100/20 100/20 100/20 100/20 100/8 100/20 100/20 100/20 120/20 120/20 120/24 120/20 120/20 120/20 120/20 120/20
66 23 49 37 48 38 trace N.D.d N.D. N.D. N.D. N.D. 81 11 95 N.D. 35 48 23 37 33 46 16 71 10 83 10 83 10 82 11 82 28 56 35 36 30 41
(0.5) (0.5) (0.7) (0.5) (0.5) (0.5) (0.5)
Reaction conditions: 1a (0.5 mmol), 2a (1.5 mmol), catalyst, and oxidant in 2 mL of solvent. Isolated yield based on indole. O2 balloon. N.D. = not detected. 2a is 0.55 mmol. PdCl2 (3.0 mol%) + HL(6.0 mol%).
effectively prevent Pd(II) ion agglomeration and maintain Pd(II) ion as an isolated active site, helps to improve the stability of Pd species and catalytic activity. With the optimal reaction conditions in hand, the universality of this protocol for indole-to-carbazole conversion was explored under reaction conditions. First, ethyl acrylate (2b) and butyl acrylate (2c), respectively, were used instead of 2a to react with indole 1a under the identical conditions. Surprisingly, the corresponding carbazole products 4ab and 4 ac were obtained in good yields of 80% and 79%, respectively, as shown in Table 2. This also encouraged us to extend the reaction to other indoles with various functional groups. Indeed, when 5-OCH3 indole (1b) was used to react with acrylates 2a, 2b, and 2c, the reaction proceeded favorably to produce the corresponding products4ba, 4bb, and 4bc in good yields of 85%, 82%, and 80%, respectively. Next, 5-Br indole (1c) with an electron-withdrawing group was used to react with acrylates 2a, 2b, and 2c to examine the reaction universality of this protocol. Although the reaction was slightly sluggish, good yields of 72%, 68%, and 65% were also obtained for the products 4ca, 4cb, and 4 cc, respectively. Despite the aryl bromide group being sensitive to metal catalysis reaction, it remains intact in the reaction and provides opportunities for further functionalization of carbazole. When 5-NO2 indole (1d), a substrate with stronger electronwithdrawing group, was used as a substrate to react with acrylates 2a, 2b, and 2c, moderate yields of 53%, 48%, and 46% was obtained for 4da, 4db, and 4 dc, respectively, indicating that the electron-
withdrawing effect on the indole ring is unfavorable for the formation of carbazole. This was consistent with the finding in the previous report by Verma and co-worker [21]. Finally, the substrate was expanded to Nprotected indole, which was demonstrated to be inapplicable to the approach developed by Verma because of the uniquely directing function of free N-H in indole for the successive oxidative Heck reactions [21]. We were pleased to find that N-methyl indole (1e) reacted with acrylates 2a, 2b, and 2c smoothly to produce the corresponding carbazoles 4ea, 4eb, and 4ec in good yields of 71%, 69%, and 68%, respectively. Furthermore, to examine the applicability of this protocol for N-protected indole substrates, N-propyl indole (1f) and N-benzoyl indole (1g) were used to react with acrylates 2a, 2b, and 2c; the corresponding products 4fa, 4fb, 4fc, 4ga, 4gb, and 4gc were obtained in moderate yields of 44–57% (see Table 2), indicating that our indole-tocarbazole protocol processes good compatibility in substrate, i.e., not only N-H free but also N-protected indoles are appropriate as starting materials. This protocol is different from the previous protocols suitable either for N-H free indole [21] or for N-protected indole substrates [19,20,22]. Moreover, indoles bearing various functional groups including electron-neutral, electron-rich, and electron-deficient substituents are suitable as substrates for the preparation of carbazoles in moderate to good yields under the optimal conditions. Then, this protocol was applied to regioselectively synthesize multifunctional carbazole with diverse groups through a one-pot two-step procedure. Indeed, when 1a reacted with 1.1 equiv. of styrene (2d) in
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Table 2 Substrate scope for the synthesis of carbazoles.a,b
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Table 3 One-pot two-step synthesis of asymmetric carbazoles.a
the presence of 1.2 mol% catalyst Pd-Al at 120 °C for 4 h to produce 5methoxy-3-styryl-1H-indole (3ad), 1.5 equiv. of 2a was added to the reaction to produce the corresponding methyl 3-phenyl-9H-carbazole1-carboxylate (4ada) in yield of 42% for 16 h (see Table 3). When 1b, 1c, and 1d were used instead of 1a to react with 2d, and then 2a, the corresponding products 4bda, 4cda, and 4dda were obtained in yields of 45%, 41%, and 36%, respectively. Moreover, the N-CH3 indole (1e) was also used as a substrate to react with 2d, and then 2a; the yield of the asymmetric product 4eda was given as 44%, indicating that the asymmetrically multifunctional carbazoles can be really prepared using two kinds of alkenes in the one-pot two-step strategy. It should be pointed out that the low yields may be related to the low reaction activity of styrene. Gram-scale reaction was also conducted to evaluate the practicability of the indole-to-carbazole reaction. When N-H free indole 1a (1.17 g, 10.0 mmol) was used to react with ethyl acrylate 2b (3.19 mL, 30 mmol) under the optimal conditions, 4ab (2.33 g) was obtained with the yield of 75% (see eq. 1). A similar case was observed in N-protected indole 1e, obtaining 4eb (2.18 g) with a yield of 67% (see eq. 2).
Three different reaction mechanisms have been reported for the synthesis of indole-to-carbazole depending on the substrates and catalysts. Diels−Alder reaction and thermally intramolecular electrocyclization were proposed for the reaction of N-protected indoles and alkenes [19,20,22], while the successive oxidative Heck reactions were demonstrated for the reaction of N-H free indoles with alkenes [21]. To probe the reaction mechanism, control experiments were performed (see eqs. 3 and 4). The mono-Heck reaction product 3-substituted indole 3ab, regarded as an intermediate in the synthesis of carbazole, was isolated and used as the starting material to react with methyl acrylate 2a and butyl acrylate 2c. Asymmetrically 1,3-disubstituted carbazoles 4aba and 4abc were obtained in yields of 83% and 79%, respectively, under the optimal reaction conditions. In contrast, no carbazole product was observed under the similar conditions but in the absence of Pd-Al catalyst. These observations were consistent with Verma's observations [21] and clearly indicated that the formation of carbazole from indole was through the successive oxidative Heck reactions rather than the Diels−Alder reaction. Similarly, when 3eb, a mono-Heck reaction product of N-methyl-indole, was used instead of 3ab under identical conditions, the corresponding products 4eba and 4ebc were produced in yields of 73% and 70%, respectively. This result was different from the results obtained by Verma [21]. Moreover, no 2,3-disubstitued indole product was observed in our reaction. Thus, the thermal intramolecular cyclization procedure would be ruled out [22].
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Although the real mechanism remains unclear, a plausible mechanism of the reaction of indole with alkyene to produce carbazole using Pd-Al cage as a catalyst is outlined in Scheme 2 on the basis of the control experiments and relative literature [21,22]. First, the 3-alkenyl indole 3 was obtained by the regioselective oxidative Heck reaction by means of the insertion reaction of acrylate 2 into C-3 Pd(II) intermediate (I) to produce intermediate (II), followed by β-hydride elimination [26]. During this period, the reduced Pd(0) complex was oxidized by Cu(OAc)2 to regenerate Pd(II), which further catalyzed the next catalytic cycle. Similarly, the intermediate 5 was produced by the second oxidative coupling reaction through the formation of palladium complex intermediates III and IV [21,27]. Finally, a fast intramolecular oxidative Heck reaction occurred through the formation of intermediates V and VI, leading to the carbazole formation. It is worth mentioning that the distinct coordination in the supramolecular cage structure of Pd-Al could prevent Pd(0) agglomeration and prolong the catalyst lifetime.
4. Conclusions In summary, we have developed a new protocol for the synthesis of functionalized carbazoles from indoles through the regioselective successive oxidative Heck reaction using Al2Pd3 supramolecular cage as a catalyst. Unlike the previous reports, the new protocol described herein is highly efficient with a low Pd(II) catalyst loading and good compatibility for both N-H free and N-protected indole substrates. Moreover, an extension of this approach to the synthesis of multifunctional carbazoles in a one-pot two-step procedure warranted a broad application. The excellent catalytic activity can be attributed to the distinct properties of the supramolecular cage structure in uniformly distributive and well-defined Pd(II) active centers on the cage surfaces. This provides a complementary strategy for designing a new catalyst for specific organic reactions.
Scheme 2. Plausible mechanism of the indole-to-carbazole reaction.
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Acknowledgment
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