Letter Cite This: Org. Lett. 2018, 20, 6799−6803
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Ruthenium(II)-Catalyzed Regioselective C‑8 Hydroxylation of 1,2,3,4Tetrahydroquinolines Changjun Chen, Yixiao Pan, Haoqiang Zhao, Xin Xu, Zhenli Luo, Lei Cao, Siqi Xi, Huanrong Li, and Lijin Xu* Department of Chemistry, Renmin University of China, Beijing 100872, China
Org. Lett. 2018.20:6799-6803. Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 11/02/18. For personal use only.
S Supporting Information *
ABSTRACT: Ru(II)-catalyzed chelation-assisted highly regioselective C8-hydroxylation of 1,2,3,4-tretrahydroquinolines has been developed. Various 1,2,3,4-tetrahydroquinolines underwent smooth C8−H hydroxylation with cheap and safe K2S2O8 as the oxidant and oxygen source to furnish the corresponding products in good to excellent yields with high tolerance of the functional groups. The choice of a readily installable and removable Npyrimidyl directing group is the key to catalysis. Mechanistic studies suggest the involvement of a six-membered ruthenacycle intermediate in the catalytic cycle. The method can also be extended to the direct hydroxylation of other (hetero)arene C−H bonds.
1,2,3,4-Tetrahydroquinolines are abundant in a number of biologically and pharmacologically active natural products and synthetic compounds.1 Representative examples include the antibiotic alkaloid helquinoline,2a the bioactive natural product (S)-angustureine,2b the highly selective D2 receptor full agonist sumanirole,2c and the novel antiarrhythmic agent nicainoprol2d (Figure 1). Consequently, the efficient construction of 1,2,3,4-
under palladium, ruthenium, or rhodium catalysis. Despite this progress, exploration of simple and efficient methods is still highly appealing. Over the past decade, transition-metal catalyzed hydroxylation of inert C−H bonds has evolved as a powerful tool for phenol syntheses.10 Early in 1987, Fujiwara and co-workers groundbreakingly reported Pd(OAc)2-catalyzed direct hydroxylation of benzene with oxygen, but this method suffered from low reaction efficiency and very harsh reaction conditions.11 In order to overcome these limitations, Pd-catalyzed arene C−H bond hydroxylation has been widely studied, and a number of catalytic systems have been successfully developed by using various directing groups.12 Meanwhile, Ru-catalyzed chelationassisted C−H hydroxylation of arenes has been independently developed by the groups of Rao,13a,e,f,h,k Ackermann,13b−d,g,i Hong,13j and Singh.13l In addition, the catalytic systems based on copper,14 rhodium,15a,b,d and iridium15c complexes also performed efficiently to catalyze C−H hydroxylation of arenes. Inspired by these elegant studies, we became interested in the selective hydroxylation of the C8-position of tetrahydroquinolines because of the potential utility of such products. Actually, methods for obtaining 8-hydroxy-1,2,3,4-tetrahydroquinolines have been less developed, and the known protocols including Povarov reaction of N-arylimines with alkenes and selective reduction of 8-hydroxyquinolones are generally plagued by limited substrate scope, uneasily available starting materials, and the requirement of prefunctionalization (Scheme 1a,b).16 In this context, the development of an efficient and general synthetic pathway to 8-hydroxy-1,2,3,4-tetrahydroquinolines from simple 1,2,3,4-tetrahydroquinolines via transition-metal catalyzed
Figure 1. Some examples of 1,2,3,4-tetrahydroquinoline-containing pharmaceuticals and bioactive molecules.
tetrahydroquinolines has been a topic of sustained interest during the past several decades. Catalytic reduction of quinolines represents one of the most reliable methods to obtain tetrahydroquinolines, and diverse successful catalytic systems have been developed.3 Extensive efforts have also been made to synthesize 1,2,3,4-tetrahydroquinolines via intramolecular amination reactions,4 Povarov reactions,5a,b and other cyclization reactions.5c Notably, with significant advances in catalytic C−H bond functionalization,6 the application of transition-metal-catalyzed C−H functionalization events to access functionalized 1,2,3,4-tetrahydroquinolines in a stepand atom-economic fashion from simple starting materials is of much current research interest.7−9 In particular, by taking advantage of N-directing groups, simple 1,2,3,4-tetrahydroquinolines underwent smooth C−H bond chlorination,9a arylation,9b−f alkenylation,9e,j acylation,9h,i and amination9m at the C8 position with high efficiency and functional group tolerance © 2018 American Chemical Society
Received: September 12, 2018 Published: October 23, 2018 6799
DOI: 10.1021/acs.orglett.8b02926 Org. Lett. 2018, 20, 6799−6803
Letter
Organic Letters Table 1. Optimization of Reaction Conditionsa
Scheme 1. Synthetic Pathways for 8-Hydroxy-1,2,3,4tetrahydroquinolines
regioselective C−H hydroxylation is highly desirable. On the basis of our previous studies on transition-metal catalyzed (hetero)arene C−H bond functionalization,17 herein, we describe such an approach where hydroxyl group is installed to the C8 position of 1,2,3,4-tetrahydroquinoline substrates by using a readily accessible and removable pyrimidyl group as the directing group and cheap and safe K2S2O8 as the oxidant and oxygen source under Ru(II) catalysis (Scheme 1c). This protocol features high yields, a broad substrate scope, excellent regioselectivity, and wide functional group tolerance. Inspired by Rao’s reports on Ru-catalyzed direct hydroxylation of anilides using the N-acylated amine directing group,13e,k our studies were initiated by examining the hydroxylation of various N-acylated 1,2,3,4-tetrahydroquinolines under Ru catalysis. However, a variety of Ru(II) catalytic systems including those that have proven to be effective in catalyzing direct hydroxylation of anilides, benzamides, acetophenones, benzoates, and benzaldehydes turned out to be totally ineffective (see Supporting Information). Recent studies regarding transition-metal catalyzed C−H bond functionalization have exhibited the power of the readily installable and removable N-(2-pyrimidyl) directing group in controlling the site selectivity and promoting the reactivity.18 Then, we envisioned that the installation of a 2-pyrimidyl group on the tetrahydroquinoline N atom may facilitate the direct hydroxylation of C8−H bond. With this in mind, our investigations focused on the direct hydroxylation of 1(pyrimidin-2-yl)-1,2,3,4-tetrahydroquinoline (1a) under Ru catalysis (Table 1 and Supporting Information). With [Ru(pcymene)Cl2]2 as the catalyst and K2S2O8 as the oxidant, we attempted the hydroxylation of 1a in DCE at 100 °C for 24 h. However, only a trace amount of the desired product 2a was observed (Table 1, entry 1). When CHCl2CHCl2 (TCE) was used as the solvent, the yield of 2a was increased to 14% (Table 1, entry 2). Further improvement was seen by switching the solvent to acetone, whereupon 2a was isolated in 23% yield (Table 1, entry 3). Subsequent studies showed that introducing HBF4 as the additive enabled the reactions in TCE and acetone to deliver 2a in better yields of 28% and 40%, respectively, but is ineffective in DCE (Table 1, entries 4−6). With HBF4 as the additive, the yield of 2a was enhanced to 75% by using a mixed solvent of DCE/acetone (1:1), but the solvent system of TCE/ acetone (1:1) only delivered 2a in 33% yield (Table 1, entries 7 and 8). Gratifyingly, performing the reaction in DCE/acetone (2:1) resulted in the formation of 2a in 90% yield (Table 1, entry 9). Further optimization revealed that HBF4 outperformed other commonly employed acids (Table 1, entries 10−13).
entry
solvent
oxidant
additive
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20c
DCE TCE acetone DCE acetone TCE DCE/acetone (1:1) TCE/acetone (1:1) DCE/acetone (2:1) DCE/acetone (2:1) DCE/acetone (2:1) DCE/acetone (2:1) DCE/acetone (2:1) DCE/acetone (2:1) DCE/acetone (2:1) DCE/acetone (2:1) DCE/acetone (2:1) DCE/acetone (2:1) DCE/acetone (2:1) DCE/acetone (2:1)
K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 PhI(OAc)2 PhI(OTFA)2 O2 (1 atm) BQ Oxone none K2S2O8
none none none HBF4 HBF4 HBF4 HBF4 HBF4 HBF4 HPF6 MeSO3H AcOH PivOH HBF4 HBF4 HBF4 HBF4 HBF4 HBF4 HBF4
trace 14 23 trace 40 28 75 33 90 76 34 46 40 trace trace NR NR 83 NR NR
a Reaction conditions: 1a (0.2 mmol), oxidant (0.4 mmol), [Ru(pcymene)Cl2]2 (2.5 mol %), additive (40.0 mol %), solvent (1.5 mL), 100 °C, 24 h, under air. NR: no reaction. bIsolated yield. cNo [Ru(pcymene)Cl2]2.
Screening of oxidants proved that no other oxidant gave a better outcome than K 2 S 2 O 8 (Table 1, entries 14−18). For comparison, the palladium catalytic systems that worked well to catalyze hydroxylation of arenes with N-directing groups were investigated, but none of them were effective (see Supporting Information). Control experiments confirmed that both the ruthenium catalyst and oxidant were essential for this reaction to occur (Table 1, entries 19 and 20). We finally evaluated the performance of other N-protected tetrahydroquinolines under otherwise identical conditions (see Supporting Information). It was found that the use of the N-(2-pyridyl) moiety only led to the formation of a trace amount of the desired product, and the reaction failed to provide any hydroxylation product when using acyl, Me, or Ph as the N-directing group. No reaction was detected in the case of free 1,2,3,4-tetrahydroquinoline. These results obviously confirm the importance of the N-(2-pyrimidyl) directing group for achieving high reactivity and selectivity in this transformation. After determining the optimized reaction conditions, we next explored the scope of tetrahydroquinoline substrates (Scheme 2). The reaction proceeded smoothly for 2-alkyl substituted tetrahydroquinolines (1b−1f) to exclusively deliver the 8hydroxylated products (2b−2f) in high yields, and the lower yield of 2e is attributed to the steric hindrance of the isobutyl group. When 2-phenyl-1-(pyrimidin-2-yl)-1,2,3,4-tetrahydroquinoline (1g) was employed, the reaction provided the corresponding product 2g in 64% yield. The 3-, 4-, and 5substituted tetrahydroquinolines (1h−1l) performed well to afford the hydroxylated products (2h−2l) in good to excellent yields. Good to excellent yields were achieved in the hydroxylation of a variety of 6-substituted tetrahydroquinolines 6800
DOI: 10.1021/acs.orglett.8b02926 Org. Lett. 2018, 20, 6799−6803
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Organic Letters
of the dihydroxylated products was observed. Various N-aryl acetamides (3g−3m) also underwent facile ortho-hydroxylation to deliver the corresponding products (4g−4m) in good yields, regardless of the nature of substituent on the aryl ring. However, when using oxime ethers 3n and 3o, only traces of the hydroxylated products were observed due to the decomposition of the starting materials under acidic conditions. When replacing K2S2O8 with Oxone and obviating the acid additive, the yields of 4n and 4o could be increased to 60% and 57%, respectively. To show the synthetic utility of the current catalytic method, we conducted some derivatizations of the C8-hydroxylated tetrahydroquinoline products (see Supporting Information). For example, treatment of product 2a with Tf2O easily resulted in the formation of 1-(pyrimidin-2-yl)-1,2,3,4-tetrahydroquinolin-8-yl trifluoromethanesulfonate (5), which could be readily functionalized via Pd-catalyzed cross-coupling reactions (Scheme 4a). Furthermore, the pyrimidyl directing group
Scheme 2. Direct Hydroxylation of Various Tetrahydroquinolinesa
a
Reaction conditions: 1a (0.2 mmol), K2S2O8 (0.4 mmol), [Ru(pcymene)Cl2]2 (2.5 mol %), HBF4 (40.0 mol %, ca. 40% w/w aq), DCE/acetone (2/1) (1.5 mL), 100 °C, 24 h, under air, isolated yield. b Gram-scale reaction.
Scheme 4. Synthetic Applications of the Products and Removal of the N-Directing Group
(1m−1u), and the electronic-rich substrates exhibited better reactivity. Notably, the sensitive functional groups including F, Cl, Br, NO2, alkene, and alkyne were well tolerated, thereby providing the opportunity for further elaboration. The structure of 2p was confirmed by single-crystal X-ray diffraction. Finally, the hydroxylation of 1a could be carried out on a gram scale to give the desired product 2a in 85% yield under the standard reaction conditions, clearly demonstrating the scalability of the current transformation. Notably, this catalytic protocol is not restricted to the tetrahydroquinoline substrates but also enables direct hydroxylation of other (hetero)arene C−H bonds (Scheme 3). It was Scheme 3. Direct Hydroxylation of Other (Hetero)arenesa
could be smoothly removed by treating with NaOtBu in DMSO, and the NH-free 8-hydroxyl tetrahydroquinolines could be produced in high yields (Scheme 4b). Preliminary experiments were carried out to gain mechanistic insight (see Supporting Information). The observed H/D scrambling at the C8-position in the deuterium labeling experiments between 1a and D2O under the standard conditions in the presence or absence of K2S2O8 suggested that the C(8)− H bond undergoes reversible activation in the present catalytic system. A kinetic isotope effect (KIE) of kH/kD = 1.38 was obtained from the parallel experiments, indicating that the C−H bond cleavage may not be involved in the turnover-determining step. Conducting the reaction of 1a under argon or 18O2 atmosphere instead of air did not affect the reaction efficiency, and no formation of 18O labeled 1a was detected. The yield of 2a decreased when introducing H218O into the reaction system, but we still did not observe the generation of 18O labeled 1a. Obviously, these results rule out the possibility that the oxygen atom in the produced hydroxyl group originates from O2 in air or water in the solvent, and K2S2O8 serves as the source of oxygen atom. The reaction of 1a proceeded smoothly in the presence of 2 equiv of TEMPO or BHT, thereby excluding the possibility of radical generation in this transformation. Further studies
a Reaction conditions: 1a (0.2 mmol), K2S2O8 (0.4 mmol), [Ru(pcymene)Cl2]2 (2.5 mol %), HBF4 (40.0 mol %, ca. 40% w/w aq), DCE/acetone (2/1) (1.5 mL), 100 °C, 24 h, under air, isolated yield. b Oxone (0.4 mmol) was used to replace K2S2O8 in the absence of acid.
found that 4-(pyrimidin-2-yl)-3,4-dihydro-2H-benzo[b][1,4]oxazine (3a) and 1-(pyrimidin-2-yl)indoline (3b) were competent participants in this transformation, affording the ortho-hydroxylated products (4a, 4b) in 86% and 77% yields, respectively. Moreover, the structure of 4a was confirmed by single-crystal X-ray diffraction. The substrates (3c−3f) containing different N-directing groups were compatible in generating the desired products (4c−4f) in good yields, and no formation 6801
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preparing various C8-functionalized 1,2,3,4-tetrahydroquinolines, especially those biologically interesting compounds, such as nicainoprol. The identical catalytic system was also applicable to the direct hydroxylation of other (hetero)arene C−H bonds. Further investigation focusing on the applications and mechanism of this reaction is underway in our laboratory.
showed that the thermodynamically stable six-membered ruthenacycle C1 could be readily prepared from the reaction of 1a with [Ru(p-cymene)Cl2]2 by employing NaOAc as the base in DCE at 60 °C (Scheme 5a), and its structure was Scheme 5. Mechanistic Studies
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02926. Detailed experimental procedures, characterization data, and copies of 1H and 13C NMR spectra of products (PDF) Accession Codes
CCDC 1858877−1858879 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
confirmed by single crystal X-ray diffraction analysis. Notably, complex C1 reacted stoichiometrically to give the product 2a in a good yield in shorter time (Scheme 5b). Moreover, complex C1 exhibited almost the same catalytic activity in the reaction of 1a as [Ru(p-cymene)Cl2]2 did (Scheme 5c), indicating that this six-membered complex is indeed an active species in the catalytic cycle. Although the exact mechanism of this coupling reaction is still not clear, a plausible mechanism (Scheme 6) is suggested on the
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] Scheme 6. Plausible Reaction Mechanism
ORCID
Lijin Xu: 0000-0003-4067-8898 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from National Natural Science Foundation of China (21372258) and the Fundamental Research Funds for the Central Universities and the Research Funds of Renmin University of China (Program 16XNLQ04) is gratefully acknowledged.
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REFERENCES
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Organic Letters
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DOI: 10.1021/acs.orglett.8b02926 Org. Lett. 2018, 20, 6799−6803
