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Catalysts 2016, 6(9), 132; https://doi.org/10.3390/catal6090132

Communication
Synthesis of New Chiral Benzimidazolylidene–Rh Complexes and Their Application in Asymmetric Addition Reactions of Organoboronic Acids to Aldehydes
1,3, 1,3, 1,* and 2,*
1
School of Medicine, Zhejiang University City College, No. 48, Huzhou Road, Hangzhou 310015, China
2
Individualized Medication Key Laboratory of Sichuan Province, Sichuan Academy of Medical Science & Sichuan Provincial People’s Hospital, School of Medicine, Center for Information in Medicine, University of Electronic Science and Technology of China, Chengdu 610072, China
3
College of Pharmaceutical Sciences, Zhejiang University, Hangzhou 310058, China
*
Authors to whom correspondence should be addressed.
Academic Editor: Georgiy B. Shul'pin
Received: 29 July 2016 / Accepted: 25 August 2016 / Published: 3 September 2016

Abstract

:
A series of novel chiral N-heterocyclic carbene rhodium complexes (NHC–Rh) based on benzimidazole have been prepared, and all of the NHC–Rh complexes were fully characterized by NMR and mass spectrometry. These complexes could be used as catalysts for the asymmetric 1,2-addition of organoboronic acids to aldehydes, affording chiral diarylmethanols with high yields and moderate enantioselectivities.
Keywords:
N-heterocycliccarbene; benzimidazolium; Rh-catalyzed; asymmetric 1,2-addition

1. Introduction

Since N-heterocycliccarbenes (NHCs) are excellent σ-donors, and their metal complexes show higher air and thermal stability than phosphane ligands. NHCs are now well established as efficient alternatives to phosphane ligands [1,2,3,4,5,6,7,8,9,10]. Much work has been devoted to the design and development of carbene compounds with new structures to tune their steric and electronic properties, and also to their application in organometallic catalysis. As excellent ligands for transition metals, NHCs have found multiple applications in some of the most important catalytic transformations in the chemical industry. As a logical extension of this development, chiral NHC ligands and their application in asymmetric catalysis are receiving increasing attention [11,12,13,14,15,16]. Despite considerable efforts devoted to this field, the design and synthesis of novel chiral NHCs to enhance their enantioselectivity is still a challenge.
Enantioenriched diarylmethanols are the structural core unit in a considerable number of bioactive compounds and pharmaceuticals [17,18,19,20]. The Rh-catalyzed enantioselective arylation of aromatic aldehydes with organoboronic acids has emerged as a direct and economical route for the synthesis of enantiomerically-enriched diarylmethanols [21]. In 1998, Miyaura and co-workers initially reported the enantioselective Rh-catalyzed addition of phenylboronic acid to naphthaldehyde by using the (S)-MeO–MOP ligand, giving naphthyphenylmethanol in 78% yield and 41% ee [22]. Since then, considerable efforts have been made in this type of reaction [23,24,25,26,27,28,29,30]. However, examples of using chiral N-heterocycliccarbenes in the ligand-catalyzed asymmetric arylation of aldehydes are rare [31,32,33,34,35]. Therefore, developing new chiral N-heterocycliccarbene ligands for the asymmetric 1,2-addition of organoboronic acid to aldehydes is an important synthetic goal. The above-mentioned findings, and our interests in NHCs and C–C forming reactions triggered our efforts to develop new NHC ligands for application in homogeneous catalysis. After our recent report of the synthesis of several chiral benzimidazolium salts for the in situ Rh-catalyzed asymmetric arylation of aldehydes [36], we herein report the synthesis of a series of new NHC–Rh complexes based on benzimidazole and their application in the asymmetric 1,2-addition of arylboronic acids to aldehydes.

2. Results

The synthetic route to the new NHC–Rh complexes based on the benzimidazole skeleton is shown in Scheme 1 and Scheme 2. The NHC complexes were synthesized from enantiomerically-pure benzimidazolium salts (1ag), which in turn can be prepared by following our previous articles [36,37]. Among the NHC precursors prepared, compounds 1c and 1d were new and are reported for the first time in this paper. In the next step, the mild transmetalation developed by Wang and Lin was adopted to prepare rhodium(I) complexes of 1ag. According to this strategy, the benzimidazolium salts 1 were treated with Ag2O in anhydrous CH2Cl2 at room temperature in the darkness. Then direct addition of [Rh(COD)Cl]2 to the freshly prepared solution of silver complexes yielded the corresponding chiral complexes 2ag upon workup, which could be purified by chromatography on silica gel (Scheme 2). The complexes were characterized by 1H NMR, 13C NMR, and high-resolution mass spectrometry (HRMS), and the absence of an N–CNHC–N resonance in the 1H NMR spectra confirmed the formation of the carbene complexes.
With the chiral NHC–Rh complexes in hand, we examined their application in the asymmetric addition of organoboronic acids to aldehydes. Firstly, all of the NHC–Rh complexes were tested in enantioselective phenylation of 2-naphthaldehyde (3a) with PhB(OH)2. The reaction was performed with 3.0 mol % of NHC–Rh complex in DME/H2O (5:1) at 80 °C for 12 h. As shown in Table 1, diarylmethanol 4a was obtained in high yield with each of the NHC–Rh complexes, and compound 2g gave the best result (18% ee).
We then optimized the experimental conditions using 2g as catalyst. By screening bases in DME/H2O (5:1), we found that the addition of excess KF (6.0 equiv.) significantly improved the enantioselectivity as well as yield (Table 2, entry 6). Next, variation of the solvent indicated that the 5:1 mixture of EtOH/DME was the best choice of solvent (Table 2, entry 16). Further screening of reaction temperature showed that lower temperature afforded the product with similar enantioselectivities but inferior yields (Table 3, entries 20–22).
Having optimized reaction conditions, we examined the reactions with various aldehydes, and the results are summarized in Table 3. The arylations with either electron-rich or electron-deficient benzaldehydes proceeded smoothly to afford the corresponding diarylmethanols in excellent yields and moderate enantioselectivities. The best enantioselectivity was obtained starting from o-anisaldehyde (46% ee, entry 3).

3. Materials and Methods

3.1. General

MS spectra were measured on a Finnigan LCQDECA XP instrument (ThermoFinnigan Co., California, CA, USA) and an Agilent Q-TOF 1290LC/6224 MS system (Aglient Technologies Inc., California, CA, USA); 1H and 13C NMR spectra were obtained on Bruker AVANCE III 500 MHz and 600 MHz spectrometers (Bruker Co., Faellanden, Switzerland) with TMS as the internal standard; silica gel GF254 and H (10–40 mm, Qingdao Marine Chemical Factory, Qingdao, China) were used for TLC and CC. Unless otherwise noted, all reactions were carried out under an atmosphere of argon or nitrogen.

3.2. Preparation of Benzimidazolium Salt 1ag

The NHC precursors 1ag were synthesized following our previous paper [36], and the 1H NMR spectra of 1ab and 1ef were identical to those reported in the literature [36,37]. 1c: 1H NMR (500 MHz, CDCl3) δ: 11.18 (s, 1H), 8.24–7.08 (m, 14H), 2.53 (d, J = 6.9 Hz, 3H), 2.39 (s, 3H), 2.07 (s, 3H), 2.00 (s, 3H). 1d: 1H NMR (500 MHz, CDCl3) δ: 10.73 (s, 1H), 8.21–7.42 (m, 16H), 7.11 (q, J = 6.8 Hz, 1H), 2.56 (d, J = 6.8 Hz, 3H).

3.3. Preparation of NHC–Rh Complexes 2ag

To a solution of imidazolinium salt 1a (364.0 mg, 1.00 mmol) in CH2Cl2 (25 mL) was added silver(I) oxide (115.9 mg, 0.50 mmol) in one portion. The suspension was stirred for 3 h in the darkness, during which the black color gradually diminished. The reaction mixture was filtered through a small pad of Celite, [Rh(COD)Cl]2 (246.5 mg, 0.50 mmol) was added in one portion, and the reaction mixture was stirred for an additional 16 h. The solvent was evaporated, and the residue was purified by flash chromatography on silica gel with CH2Cl2 as eluent. After evaporation of volatiles, the residue was purified by column chromatography (CH2Cl2) to give 2a (515.0 mg, 88% yield). 1H NMR (500 MHz, CDCl3) δ: 7.52–7.43 (m, 2H), 7.19–7.08 (m, 2H), 5.86 (m, 1H), 5.73 (m, 1H), 5.18 (m, 1H), 5.04 (m, 1H), 3.66 (t, J = 7.2 Hz, 1H), 3.41 (d, J = 7.5 Hz, 1H), 2.58–1.79 (m, 15H), 1.71 (m, 6H), 1.52–0.86 (m, 15H); 13C NMR (125 MHz, CDCl3) δ: 134.17, 133.92, 121.77, 121.53, 112.10, 112.01, 98.81, 98.76, 98.66, 98.61, 77.28, 76.78, 68.79, 68.67, 67.40, 67.28, 63.73, 62.82, 42.57, 42.35, 33.29, 32.35, 32.07, 31.93, 31.34, 30.68, 30.37, 29.70, 29.36, 28.23, 26.53, 26.42, 26.40, 26.37, 26.05, 25.94, 22.70, 19.06, 18.43, 14.12; HR-ESIMS: m/z 549.2792 [M–Cl]+ (calcd. for C31H46N2Rh, 549.2716).
Analogous compounds 2bg were prepared according to the similar procedure for 2a. HR-ESIMS, 1H and 13C NMR data see Supplementary Materials. 2b: 97% yield; 1H NMR (500 MHz, CDCl3) δ: 9.08 (q, J = 7.3 Hz, 1H), 8.57 (m, 2H), 8.03–7.51 (m, 9H), 7.36 (m, 5H), 7.18 (m, 2H), 7.11–7.00 (m, 1H), 5.16 (m, 1H), 5.03 (m, 1H), 3.38 (s, 1H), 3.02 (t, J = 7.2 Hz, 1H), 2.67 (d, J = 7.2 Hz, 3H), 2.35 (m, 1H), 2.25 (d, J = 7.3 Hz, 3H), 2.15–1.98 (m, 1H), 1.96–1.77 (m, 2H), 1.55 (m, 1H), 1.51–1.41 (m, 1H), 1.20 (d, J = 10.3 Hz, 1H), 0.95–0.88 (m, 1H); 13C NMR (125 MHz, CDCl3) δ: 138.83, 136.20, 135.84, 134.96, 134.13, 134.06, 130.80, 130.03, 129.10, 129.03, 128.77, 128.01, 127.07, 126.59, 126.12, 126.03, 125.21, 125.14, 124.19, 123.90, 123.73, 122.35, 122.32, 122.24, 113.93, 112.26, 99.20, 97.82, 77.28, 76.78, 70.74, 70.62, 67.45, 67.33, 61.11, 56.76, 32.68, 31.94, 31.75, 29.71, 29.35, 27.39, 22.70, 22.33, 20.83, 14.13; HR-ESIMS: m/z 529.1152 [M–Cl–COD]+ (calcd. for C31H26N2Rh, 529.1151). 2c: 91% yield; 1H NMR (500 MHz, CDCl3) δ: 8.76–8.56 (m, 2H), 8.21 (d, J = 5.5 Hz, 2H), 7.96 (d, J = 8.1 Hz, 1H), 7.78 (m, 3H), 7.61 (m, 4H), 7.44 (d, J = 8.0 Hz, 1H), 7.39–7.24 (m, 4H), 5.01 (s, 2H), 2.60–2.52 (m, 1H), 2.48 (m, 1H), 2.31 (d, J = 7.4 Hz, 3H), 1.91–1.80 (m, 1H), 1.70–1.61 (m, 1H), 1.55–1.40 (m, 2H), 1.31 (m, 2H), 1.09–0.99 (m, 1H), 0.65 (m, 1H); 13C NMR (125 MHz, CDCl3) δ: 139.48, 138.06, 136.15, 134.82, 133.99, 130.09, 129.10, 128.97, 128.52, 127.98, 127.29, 126.78, 126.14, 125.25, 123.87, 123.01, 122.74, 122.15, 113.24, 111.01, 99.67, 99.62, 98.81, 98.76, 77.28, 76.77, 69.64, 69.53, 68.89, 68.77, 59.83, 53.42, 31.98, 31.77, 29.70, 28.07, 27.96, 20.95; HR-ESIMS: m/z 559.2367 [M–Cl]+ (calcd. for C33H32N2Rh, 559.1621). 2d: 89% yield; 1H NMR (500 MHz, CDCl3) δ: 8.91 (q, J = 7.3 Hz, 1H), 8.65 (d, J = 8.6 Hz, 1H), 7.96 (d, J = 8.1 Hz, 1H), 7.85–7.69 (m, 3H), 7.61 (t, J = 7.5 Hz, 1H), 7.33–7.27 (m, 2H), 7.24–7.11 (m, 3H), 7.01–6.86 (m, 2H), 4.98 (m, 1H), 4.89–4.78 (m, 1H), 3.07–2.93 (m, 1H), 2.72 (t, J = 7.2 Hz, 1H), 2.51 (s, 3H), 2.43 (s, 3H), 2.37 (d, J = 7.3 Hz, 3H), 2.02–1.88 (m, 1H), 1.65 (m, 4H), 1.54–1.28 (m, 4H), 1.10 (s, 1H), 0.67 (m, 1H); 13C NMR (125 MHz, CDCl3) δ: 140.28, 139.09, 138.04, 136.73, 135.17, 134.61, 134.05, 132.92, 130.17, 130.04, 128.93, 128.37, 127.78, 126.65, 126.11, 125.33, 124.15, 122.82, 122.70, 121.60, 113.30, 110.80, 98.82, 98.77, 98.48, 98.42, 77.29, 76.78, 69.57, 69.45, 67.98, 67.86, 59.99, 32.79, 31.94, 31.32, 29.70, 29.67, 29.37, 28.96, 27.19, 22.70, 21.18, 21.06, 19.68, 17.72, 14.12; HR-ESIMS: m/z 601.4336 [M–Cl]+ (calcd. for C36H38N2Rh, 601.2090). 2e: 83% yield; 1H NMR (500 MHz, CDCl3) δ: 8.45 (m, 1H), 7.60–7.47 (m, 2H), 7.29 (m, 1H), 7.22–7.08 (m, 3H), 6.95 (d, J = 7.9 Hz, 1H), 5.74–5.64 (m, 1H), 5.03–4.88 (m, 2H), 3.78 (d, J = 6.1 Hz, 3H), 3.56 (s, 1H), 2.89 (s, 1H), 2.44–2.16 (m, 4H), 1.98–1.61 (m, 11H), 1.47–1.34 (m, 4H), 1.19–1.05 (m, 2H), 0.89 (t, J = 6.9 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ: 154.29, 136.69, 133.17, 132.68, 130.21, 126.85, 122.06, 121.99, 121.09, 111.75, 111.69, 111.55, 110.98, 110.91, 98.84, 98.79, 98.09, 98.04, 77.28, 76.77, 68.84, 68.72, 67.61, 67.49, 64.12, 63.25, 55.59, 55.42, 42.65, 42.18, 33.21, 32.88, 31.93, 31.66, 31.16, 30.96, 29.70, 29.66, 29.37, 28.60, 28.51, 28.35, 26.58, 26.48, 26.39, 26.25, 25.92, 22.70, 18.94, 18.03, 14.12; HR-ESIMS: m/z 545.2188 [M–Cl]+ (calcd. for C30H38N2ORh, 545.2039). Complex 2f is a known compound, and the NMR and high-resolution mass spectrometry of this compound were identical to those reported in the literature [36]. 2g: 90% yield; 1H NMR (500 MHz, CDCl3) δ: 8.04 (m, 4H), 7.63 (m, 4H), 7.40–7.30 (m, 1H), 7.25–7.07 (m, 3H), 6.16 (m, 1H), 5.05 (m, 2H), 3.75–3.55 (m, 1H), 2.54–2.15 (m, 3H), 1.92 (s, 1H), 1.81 (d, J = 7.2 Hz, 3H), 1.74–1.67 (m, 1H), 1.51–1.41 (m, 2H), 1.31 (s, 9H), 1.21–1.05 (m, 1H); 13C NMR (125 MHz, CDCl3) δ: 135.28, 133.64, 132.76, 128.65, 128.46, 127.81, 126.98, 126.86, 122.47, 122.39, 122.35, 122.24, 113.83, 113.35, 110.93, 110.79, 99.59, 98.86, 97.91, 77.28, 76.77, 70.21, 67.97, 67.36, 67.18, 66.97, 66.86, 36.00, 35.64, 32.68, 32.06, 31.83, 31.27, 29.70, 29.49, 29.39, 29.32, 29.06, 27.72, 27.22, 15.73, 15.14, 14.12; HR-ESIMS: m/z 539.2090 [M–Cl]+ (calcd. for C31H36N2Rh, 539.1934).

3.4. Representative Procedure for the Rh-Catalyzed Asymmetric Arylation of Aldehyde

The NHC–Rh complex 2g (2.2 mg, 0.00375 mmol) was weighted into 1 mL of DME/H2O (5:1) under N2. After stirring at room temperature for 5 min, KF (43.6 mg, 0.75 mmol), phenylboronic acid (30.5 mg, 0.25 mmol), and 2-naphthaldehyde (19.5 mg, 0.125 mmol) were added successively. The resulting mixture was stirred at 80 °C for 12 h. After usual work-up, purification by silica gel column (petroleum/ethyl acetate = 9/1) afforded 4a as a colorless oil (99% yield, 35% ee). The spectral data were comparable to those reported [38]. The ee was determined by HPLC analysis with Daicel Chiralcel OD-H (hexane/PriOH = 90/10, flow rate = 0.8 mL/min, tr (minor) = 19.03 min, tr (major) = 22.46 min).
Analogous compounds 4bl were prepared according to the similar procedure for 4a. 4b: 97% yield, 43% ee. The spectral data were comparable to those reported [24]. The ee was determined by HPLC analysis with Daicel Chiralcel AD-H (hexane/PriOH = 90/10, flow rate = 0.8 mL/min, tr (minor) = 15.0 min, tr (major) = 16.5 min). 4c: 99% yield, 37% ee. The spectral data were comparable to those reported [36]. The ee was determined by HPLC analysis with Daicel Chiralcel AD-H (hexane/PriOH = 90/10, flow rate = 0.8 mL/min, tr (minor) = 11.9 min, tr (major) = 12.8 min). 4d: 85% yield, 46% ee. The spectral data were comparable to those reported [24]. The ee was determined by HPLC analysis with Daicel Chiralcel AD-H (hexane/PriOH = 90/10, flow rate = 0.8 mL/min, tr (major) = 14.0 min, tr (minor) = 15.1 min). 4e: 94% yield, 40% ee. The spectral data were comparable to those reported [24]. The ee was determined by HPLC analysis with Daicel Chiralcel AD-H (hexane/PriOH = 90/10, flow rate = 0.8 mL/min, tr (major) = 7.5 min, tr (minor) = 8.8 min). 4f: 99% yield, 28% ee. The spectral data were comparable to those reported [36]. The ee was determined by HPLC analysis with Daicel Chiralcel OD-H (hexane/PriOH = 90/10, flow rate = 0.8 mL/min, tr (minor) = 10.6 min, tr (major) = 12.4 min). 4g: 99% yield, 36% ee. The spectral data were comparable to those reported [36]. The ee was determined by HPLC analysis with Daicel Chiralcel OD-H (hexane/PriOH = 90/10, flow rate = 0.8 mL/min, tr (minor) = 9.8 min, tr (major) = 10.2 min). 4h: 93% yield, 38% ee. The spectral data were comparable to those reported [36]. The ee was determined by HPLC analysis with Daicel Chiralcel OD-H (hexane/PriOH = 90/10, flow rate = 0.8 mL/min, tr (major) = 8.6 min, tr (minor) = 9.2 min). 4i: 88% yield, 28% ee. The spectral data were comparable to those reported [36]. The ee was determined by HPLC analysis with Daicel Chiralcel OD-H (hexane/PriOH = 90/10, flow rate = 0.8 mL/min, tr (major) = 11.2 min, tr (minor) = 13.2 min). 4j: 94% yield, 28% ee. The spectral data were comparable to those reported [36]. The ee was determined by HPLC analysis with Daicel Chiralcel OD-H (hexane/PriOH = 90/10, flow rate = 0.8 mL/min, tr (major) = 21.5 min, tr (minor) = 23.8 min). 4k: 99% yield, 18% ee. The ee was determined by HPLC analysis with Daicel Chiralcel OD-H (hexane/PriOH = 90/10, flow rate = 0.8 mL/min, tr (minor) = 11.8 min, tr (major) = 12.6 min). 4l: 98% yield, 19% ee. The ee was determined by HPLC analysis with Daicel Chiralcel OD-H (hexane/PriOH = 85/15, flow rate = 0.8 mL/min, tr (minor) = 8.6 min, tr (major) = 9.9 min).

4. Conclusions

In conclusion, seven NHC–Rh complexes (2af) have been prepared. Their applicability in the asymmetric arylation of aromatic aldehydes has been demonstrated, and the corresponding diarylmethanols were obtained with high yields and moderate enantiomeric excesses (up to 46%). Further work is in progress to utilize these complexes in asymmetric 1,2-addition reactions of arylboronic acids with ketones, as well as their applications in fields of nanoscience [39,40].

Supplementary Materials

The following are available online at www.mdpi.com/2073-4344/6/9/132/s1, Figure S1: 1H and 13C NMR Spectra of compounds 2ag, Figure S2: HR-MS Spectra for Compounds 2ag, Figure S3: HPLC data of 4al.

Acknowledgments

We are grateful to the National Natural Science Foundation of China (81302668) and Hangzhou Science and Technology Information Institute of China (20150633B45).

Author Contributions

Jie Li and Jianyou Shi conceived and designed the experiments; Weiping He and Bihui Zhou performed the experiments and analyzed the data; Jie Li and Jianyou Shi wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. Synthesis of benzimidazolium salts 1ag.
Scheme 1. Synthesis of benzimidazolium salts 1ag.
Catalysts 06 00132 sch001
Scheme 2. Synthesis of N-heterocyclic carbene–rhodium NHC–Rh complexes 2ag.
Scheme 2. Synthesis of N-heterocyclic carbene–rhodium NHC–Rh complexes 2ag.
Catalysts 06 00132 sch002
Table 1. Comparison of NHC–Rh complexes.
Table 1. Comparison of NHC–Rh complexes.
Catalysts 06 00132 i001
Entry aLigandYield (%) bee (%) c
12a993
22b991
32c996
42d9817
52e993
62f983
72g9918
8no catalyst
a Reaction condition: ligand (3 mol %), KOtBu (1 equiv.), arylboronic acids (2 equiv.), N2, DME/H2O (5:1), 80 °C, 12 h; b Isolated yields; c Determined by chiral HPLC (CHIRALCEL OD Column) analysis.
Table 2. Optimization of the reaction conditions.
Table 2. Optimization of the reaction conditions.
Catalysts 06 00132 i002
Entry aBaseSolventTemperature (°C)Yield (%) bee (%) c
1NaOtBuDME/H2O (5:1)809918
2LiOtBuDME/H2O (5:1)804229
3LiOHDME/H2O (5:1)809917
4KF (1 equiv.)DME/H2O (5:1)807032
5KF (3 equiv.)DME/H2O (3:1)808032
6KF (6 equiv.)DME/H2O (5:1)809932
7KF (6 equiv.)DME/H2O (10:1)809914
8KF (6 equiv.)DME/H2O (3:1)809918
9KF (6 equiv.)Toluene/H2O (5:1)809922
10KF (6 equiv.)MeOH/DME (5:1)809925
11KF (6 equiv.)t-BuOH/MeOH (5:1)809924
12KF (6 equiv.)MeOH809921
13KF (6 equiv.)i-PrOH809934
14KF (6 equiv.)t-BuOH/EtOH (5:1)809925
15KF (6 equiv.)DME80939
16KF (6 equiv.)EtOH/DME (5:1)809935
17KF (6 equiv.)EtOH809932
18KF (6 equiv.)Dioxane809417
19KF (6 equiv.)i-PrOH/DME (5:1)809934
20KF (6 equiv.)EtOH/DME (5:1)50
21KF (6 equiv.)i-PrOH504333
22KF (6 equiv.)i-PrOH/DME (5:1)504736
a Reaction condition: 2g (3 mol %), base (1 equiv.), arylboronic acids (2 equiv.), N2, 80 °C, 12 h; b Isolated yields; c Determined by chiral HPLC (CHIRALCEL OD Column) analysis.
Table 3. Scope of methodology.
Table 3. Scope of methodology.
Catalysts 06 00132 i003
Entry aAr1Yield (%) bee (%) c
11-Naphthyl 3b97 4b43
22-MeOPh 3c99 4c37
34-MeOPh 3d85 4d46
44-CF3Ph 3e94 4e40
53,4-DiMePh 3f99 4f28
64-EtPh 3g99 4g36
72-FPh 3h93 4h38
83,5-DiFPh 3i88 4i28
94-NO2Ph 3j94 4j28
102-thienyl 3k99 4k18
112-furyl 3l98 4l19
a Reaction condition: 2g (3 mol %), KF (6.0 equiv.), arylboronic acids (2 equiv.), EtOH/DME (5:1), N2, 80 °C, 12 h; b Isolated yields; c Determined by chiral HPLC (CHIRALCEL OD or AD Column) analysis.
Catalysts EISSN 2073-4344 Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert
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