Efficient enantioselective fluorination of β-keto esters/amides catalysed by diphenylamine-linked bis(thiazoline)–Cu(OTf)₂ complexes

Abstract

An efficient highly enantioselective fluorination of β-keto esters/amides catalysed by diphenylamine-linked bis(thiazoline)–Cu(OTf)₂ complexes has been developed. The corresponding products could be obtained with good to excellent enantioselectivities (up to > 99% ee) in excellent yields by utilizing N-fluorobisbenzenesulphonimide (NFSI) as fluorination reagent.

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Introduction

Replacement of hydrogen atoms or hydroxyl groups in the parent compounds with fluorine atoms often leads to different characters from the parent compounds due to the unique properties of the carbon–fluorine bond.¹ The development of effective methods to incorporate fluorine into organic compounds has been extensively exploited during the past decade.² Catalytic enantioselective fluorination is one of the most fascinating aspects of organofluorine chemistry.³ Because of the possibility of further derivatization, methods for the catalytic enantioselective fluorination of β-keto esters attracted more and more attention.⁴ The first example of catalytic asymmetric fluorination of β-keto esters was reported by Togni et al.⁵ After this pioneering work, chiral metal complexes^4a,b,e,6 and organocatalysts⁷ were developed for these reactions. Oxazoline metal complexes have been successfully utilized in asymmetric fluorination of β-keto esters.⁸ However, to the best of our knowledge, there has been no report on enantioselective fluorination of β-keto esters using bis(thiazoline) ligands. Herein, we wish to detail our research on the enantioselective fluorination of β-keto esters/amides with NFSI as fluorination reagent, catalysed by bis(thiazoline)–Cu(OTf)₂ complex.

Results and discussion

As a series of chiral bis(oxazoline) ligands were cheap and ready available in our laboratory,⁹ we first examined electrophilic fluorination of indanone carboxylate 1a in the presence of the bis(oxazoline) I–Cu(OTf)₂ complex (10 mol%) in toluene at room temperature. N-Fluorobenzenesulfonimide (NFSI) was used as fluorination reagent. To our delight we found that 2a was obtained in 99% ee (Scheme 1). However, fluorination of tetralone derivative 1b gave the corresponding products 2b in lower stereoselectivity under the same conditions (Table 1, entry 1). Then, our exploratory began with the optimization of the model reaction of 1-tetralone-2-carboxylate 1b. Different chiral bis(oxazoline) ligands (II–VI, Fig. 1) were screened in order to improve the enantioselectivity, but the enantioselectivities were even lower because of the steric effect (Table 1, entries 2–5) and electronic effect (Table 1, entry 6). It was found that ligand I still gave the best result among these bis(oxazoline) ligands (Table 1, entries 1–6). Then, chiral bis(thiazoline) VII^9a and bis(imidazoline) ligands VIII and IX¹⁰ were investigated in this reaction. Remarkably, a substantial improvement of the enantioselectivity was realized when ligand VII was used (92% ee) (Table 1, entry 7). Ligand VIII led to no significant increase in yield and enantioselectivity (Table 1, entry 8). It should be noted that the enantioselectivity dramatically decreased when ligand IX was used (Table 1, entry 9). This result elucidated the crucial role of the NH group in ligands I–VIII. Triphenylamine-tethered bis(thiazoline) ligand X has been synthesized to further check our postulation. Low enantioselectivity was observed as expected when ligand X was used (Table 1, entry 10).

Scheme 1 Enantioselective fluorination of indanone carboxylate 1a.

Table 1 Screening of ligands for the enantioselective fluorination of 1-tetralone-2-carboxylate (1b)^a

Entry	Ligand	Yield^b (%)	ee^c (%)
a Reactions were carried out with 1-tetralone-2-carboxylate (0.2 mmol), NFSI (0.24 mmol) and 10 mol% ligand–Cu(OTf)2 in toluene (3 mL). b Isolated yields after column chromatography purification. c Determined by chiral HPLC analysis.
1	I	100	79
2	II	58	12
3	III	76	19
4	IV	51	6
5	V	95	38
6	VI	67	18
7	VII	98	92
8	VIII	93	77
9	IX	79	4
10	X	62	9

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Fig. 1 Chiral tridentate ligands.

To further investigate the reaction conditions, bis(thiazoline) ligand VII was chosen to explore the effect of Lewis acids with 10 mol% catalyst loading. The results are summarized in Table 2. A series of Lewis acid were screened. With CuOTf and Cu(ClO₄)₂, both of the yield and enantioselectivity decreased slightly (Table 2, entries 2 and 3). While Cu(OAc)₂ and La(OTf)₃ gave moderate yields and enantioselectivities (Table 2, entries 4 and 10). The reaction gave no enantioselectivity when Ni(ClO₄)₂·6H₂O was used as Lewis acid (Table 2, entry 5). Besides, Zn(OTf)₂, LiOTf, Mg(OTf)₂ and In(OTf)₃ gave very low enantioselectivities with moderate to high yields (Table 2, entries 6–9). The complex VII–Cu(OTf)₂ was found to be the best choice of catalyst affording excellent yield and high enantioselectivity (Table 2, entry 1).

Table 2 Optimization of reaction conditions for the enantioselective fluorination of 1-tetralone-2-carboxylate (1b)^a

Entry	Solvent	Lewis acid	Yield^b (%)	ee^c (%)
a Reactions were carried out with 1-tetralone-2-carboxylate (0.2 mmol), NFSI (0.24 mmol) and 10 mol% VII–Lewis acid in solvent (3 mL). b Isolated yields after column chromatography purification. c Determined by chiral HPLC analysis. d The reaction temperature was 0 °C. e 5 mol% VII–Cu(OTf)2 was used. f 12 mol% of VII and 10 mol% Cu(OTf)2 was used.
1	Toluene	Cu(OTf)2	98	92
2	Toluene	CuOTf	94	90
3	Toluene	Cu(ClO4)2·6H2O	90	88
4	Toluene	Cu(OAc)2·H2O	67	54
5	Toluene	Ni(ClO4)2·6H2O	78	0
6	Toluene	Zn(OTf)2	100	12
7	Toluene	LiOTf	69	7
8	Toluene	Mg(OTf)2	90	5
9	Toluene	In(OTf)3	89	12
10	Toluene	La(OTf)3	66	60
11	Xylene	Cu(OTf)2	99	90
12	Chlorobenzene	Cu(OTf)2	92	91
13	α,α,α-Trifluorotoluene	Cu(OTf)2	91	91
14	CH2Cl2	Cu(OTf)2	90	89
15	ClCH2CH2Cl	Cu(OTf)2	96	91
16	CHCl3	Cu(OTf)2	99	93
17	THF	Cu(OTf)2	86	72
18	Cycohexane	Cu(OTf)2	94	79
19^d	CHCl3	Cu(OTf)2	99	94
20^e	CHCl3	Cu(OTf)2	93	90
21^f	CHCl3	Cu(OTf)2	94	93

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In order to improve the enantioselectivity of this asymmetric fluorination reaction, we further screened different solvents, temperature and catalyst loading. Utilizing aromatic solvent gave the product 2b in high to excellent yields and high enantioselectivities (Table 2, entries 1 and 11–13). When alkyl halides were used as solvent, 1b reacted smoothly to afford the product with high yields and high enantioselectivities (Table 2, entries 14–16). Chloroform was the best choice of solvent with 99% yield and 93% ee (Table 2, entry 16). Solvent effect was observed when THF and cyclohexane were utilized as solvent (Table 2, entries 17 and 18). The enantioselectivity of the product 2b was improved slightly by lowering the reaction temperature (94% ee) (Table 2, entry 19). When the catalyst loading was decreased to 5 mol%, the enantioselectivity decreased slightly (90% ee) and 93% yield was obtained (Table 2, entry 20). No improvement of the enantioselectivity was observed when 12 mol% VII and 10 mol% Cu(OTf)₂ was used (Table 2, entry 21).

With the optimum reaction conditions in hand, we explored the substrate scope for this reaction. The results are summarized in Table 3. All the substrates reacted smoothly to afford products 2 with good to excellent yields. The alkoxy group in the β-keto ester has no influence on enantioselectivity of the product. Fluorination of tetralone derivatives such as 1c and 1d gave the corresponding products in high enantioselectivities (94% ee and 93% ee, respectively) (Table 3, entries 3 and 4). It was found that excellent enantioselectivities could be obtained for all indanonecarboxylate derivatives (Table 3, entries 1 and 5–11). The enantioselectivity of the reaction was above 99% ee when the ester alkoxy substituent was isopropoxy group (Table 3, entry 5). Electron-rich substituent such as, methoxy group on the aromatic ring of indanonecarboxylate derivative caused slight decrease in enantioselectivity (Table 3, entry 9). Indanone carboxamides were also evaluated in this reaction. Excellent yields and lower enantioselectivities were obtained. The alkylamino groups in the indanonecarboxamides have important influence on enantioselectivities. In the case of 1l (Table 3, entry 12), the enantioselectivity of the product 2l has a moderate decrease (83% ee). N-Phenyl-carboxamide 1m caused slight decrease in enantioselectivity (94% ee) (Table 3, entry 13). When N-benzyl-carboxamide 1n was used as substrate, the enantioselectivity decreased to 73% ee (Table 3, entry 14). Acyclic substrates, such as 1o–1r (Fig. 2) were also evaluated in this reaction. Unfortunately, no corresponding products were obtained when 1p–1r were used as substrates. Substrate 1o afforded racemic product in 42% yield. Besides, aliphatic cyclopentanone-derived ester 1s and cyclohexanone-derived ester 1t were also evaluated in this reaction. Racemic product was obtained with 46% yield in the case of 1s. While 1t did not afford the corresponding product in the same condition. 1,3-Dione 1u afforded the corresponding product 2u with no enantioselectivity and in 79% yield. The above experimental results indicated that the substrate structures influenced the yields and enantioselectivities, tetralone and indanone carboxylate derivatives with fused aromatic ring demonstrate excellent reactivity and enantioselectivity, this may be ascribed to the existence of aromatic π–π interaction between the substrate and phenyl substituent in oxazoline ligand. This interaction has also been demonstrated by the ligand screening results in Table 1 (entries 1, 7 and 8), ligands I, VII and VIII which have phenyl groups in oxazoline ring, afforded good catalytic performance compared with other substituted ligands in same reaction.

Table 3 Scope of the enantioselective fluorination of β-keto esters/amides 1^a

Entry	R^1	R^2	R^3	n	Substrate	Product	Yield^b (%)	ee^c (%)
a Reactions were carried out with 1 (0.2 mmol), NFSI (0.24 mmol) and 10 mol% VII–Cu(OTf)2 in CHCl3 (3 mL). b Isolated yields after column chromatography purification. c Determined by chiral HPLC analysis. d The absolute configuration of 2f was assigned as S based on comparison of the optical rotation with previous report.^8c
1	OEt	H	H	1	1a	2a	100	99
2	OEt	H	H	2	1b	2b	99	93
3	OBn	H	H	2	1c	2c	92	94
4	OMe	H	H	2	1d	2d	100	93
5	OiPr	H	H	1	1e	2e	96	>99
6^d	OtBu	H	H	1	1f	2f	93	99 (S)
7	OBn	H	H	1	1g	2g	100	99
8	OEt	Me	H	1	1h	2h	100	99
9	OEt	OMe	OMe	1	1i	2i	91	98
10	OEt	H	Cl	1	1j	2j	89	99
11	OEt	H	Br	1	1k	2k	96	99
12	NHnBu	H	H	1	1l	2l	100	83
13	NHPh	H	H	1	1m	2m	100	94
14	NHBn	H	H	1	1n	2n	95	73

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Fig. 2 Acyclic, aliphatic β-keto ester and 1,3-dione substrates evaluated in this reaction.

On the basis of the configuration of the product 2f and the activate model of the similar ligand we have published before,⁹ we proposed a transition state model for the fluorination as illustrated in Fig. 3. The bis(thiazoline)–Cu(OTf)₂ catalyst acts in a bifunctional fashion, the Lewis acid activates the β-keto ester through coordination of the copper cation with the two oxygen atoms in β-keto ester. The NH–O hydrogen bond interaction and π–π interaction directs the NFSI attack from the Si-face and result in the (S)-configured product 2f.

Fig. 3 Proposed transition state model.

Conclusions

In conclusion, we have developed an efficient enantioselective fluorination reaction of β-keto esters/amides 1 catalysed by a tridentate bis(thiazoline) VII–Cu(OTf)₂ complex. The corresponding products were obtained in good to excellent yields with good to excellent enantioselectivities (up to > 99% ee). This reaction provides a valuable alternative route to fluoro-bearing β-keto esters or amides. The asymmetric fluorination or related reactions using bis(thiazoline), bi(oxazoline) or bis(imidazoline) ligand–metal complexes will be further investigated in our laboratory.

Experimental

General information

Unless otherwise stated, commercially available compounds were used without further purification. Column chromatography was carried out with silica gel (200–300 mesh). Melting points were measured with a MT-4 melting point apparatus without correction. ¹H NMR spectra were recorded with a Varian Mercury-plus 400 MHz spectrometer. Chemical shifts were reported in ppm with the internal TMS signal at 0.0 ppm as a standard. The data are reported as follows: chemical shift (ppm), and multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet or unresolved, br s = broad singlet), coupling constant(s) in Hz, integration assignment. ¹³C NMR spectra were recorded at 100 MHz. Infrared spectra were obtained with a Perkin Elmer Spectrum One spectrometer. The high resolution MS spectra were obtained with ESI positive ionization using a Bruker APEX IV FT-MS spectrometer. Optical rotations were measured with a Krüss P8000 or WZZ-3 polarimeter at the indicated concentration with unit g per 100 mL. The enantiomeric excesses were determined by chiral HPLC using an Agilent 1200 LC instrument with Daicel Chiralpak IA, IB and AD-H columns, or Daicel Chiralcel OB-H and OD-H columns.

Materials

Chiral bis(oxazoline) I–VI1,⁹ chiral bis(thiazoline) VII,^9b chiral bis (imidazoline) VIII,^10a chiral bis (imidazoline) IX,^10b β-keto esters 1 (ref. 11) and β-keto amides 1 (ref. 12) were prepared according to the reported procedures.

2-((S)-4,5-Dihydro-4-phenylthiazol-2-yl)-N-(2-((S)-4,5-dihydro-4-phenylthiazol-2-yl)phenyl)-N-phenylbenzenamine (X). To a 100 mL flask were added 2,2′-dicarboxyltriphenylamine (666 mg, 2 mmol), EDCI–HCl (900 mg, 4.5 mmol), DMAP (25 mg, 0.2 mmol), and CH₂Cl₂ (35 mL). Then N-methylmorpholine (0.72 mL, 6.5 mmol) was added, and the mixture became clear. The (S)-2-amino-2-phenylethanol (617 mg, 4.5 mmol) was added, and the solution was stirred at room temperature for 48 h. The reaction was quenched by addition of 30 mL of saturated NH₄Cl (aq.). The phase was separated, and the organic phase was washed with saturated NaHCO₃ (aq.) and brine. After drying over anhydrous Na₂SO₄, the solvent was removed in vacuo and the crude product was purified by silica gel column chromatography using CH₂Cl₂/CH₃OH (50 : 1) as eluent. The product bis(hydroxyamide) was obtained (549.7 mg, 48% yield) as yellow oil. To a solution of bis(hydroxyamide) (549.7 mg, 0.96 mmol) in dry pyridine (10 mL) was added P₂S₅ (852.5 mg, 3.84 mmol) and the mixture was refluxed for 22 h. Then the reaction was cooled and 20% KOH solution (10 mL) was added. The aqueous phase was extracted with DCM (10 mL × 3). The organic phase was combined, washed by 2 N HCl, dried over anhydrous Na₂SO₄ and concentrated under reduced pressure to give the crude product. Purified by silica gel column chromatography using petroleum ether/ethyl acetate (15 : 1) as eluent. The product was obtained (53.8 mg, 10% yield) as a yellow solid. M.p. 76–79 °C; [α]²⁸_D −95.4 (c 2.00, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 7.56 (d, J = 7.6 Hz, 2H, ArH), 7.35–7.27 (m, 4H, ArH), 7.20–7.14 (m, 5H, ArH), 7.10 (t, J = 7.4 Hz, 3H, ArH), 7.03 (t, J = 7.2 Hz, 3H, ArH), 6.96 (d, J = 7.6 Hz, 3H, ArH), 6.81 (t, J = 6.8 Hz, 1H, ArH), 6.74 (d, J = 8.4 Hz, 2H, ArH), 5.03 (t, J = 9.8 Hz, 2H, CH), 3.31 (t, J = 9.4 Hz, 2H, CH₂), 2.71 (t, J = 11.0 Hz, 2H, CH₂) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 167.1, 147.9, 146.0, 141.5, 131.4, 131.0, 130.4, 129.4, 128.3, 128.2, 127.3, 126.8, 124.4, 121.9, 121.0, 79.9, 41.5 ppm; IR (KBr): ν 3061, 3028, 2924, 1714, 1590, 1481, 1442, 1361, 1316, 1276, 1221, 1019, 947, 913, 757, 695 cm⁻¹. HRMS (ESI positive): m/z calcd for C₃₆H₃₀N₃S₂ [M + H]⁺ 568.18757, found: 568.18641.

General procedure for the asymmetric fluorination of β-keto esters/amides

To a dried Schlenk tube were added Cu(OTf)₂ (7.1 mg, 0.02 mmol) and ligand VII (9.8 mg, 0.02 mmol) under argon followed by addition of the CHCl₃ (3 mL). The solution was stirred at room temperature for 0.5 h and then β-keto esters/amides 1 (0.2 mmol) was added. The mixture was stirred for 10 min, then NFSI (75.7 mg, 0.24 mmol) was added. After stirring for 15 h at room temperature, the solvent was removed under vacuum. Purification by column chromatography afforded the desired product 2.

Ethyl 2-fluoro-2,3-dihydro-1-oxo-1H-indene-2-carboxylate (2a)^7c. The title compound 2a was obtained according to the general procedure as colorless oil (44.4 mg, 100% yield). HPLC (Daicel Chiralpak IB, n-hexane–2-propanol 98 : 2, flow rate 1.0 mL min⁻¹, detection at 254 nm): t_major = 11.3 min, t_minor = 12.3 min, 99% ee. [α]²⁸_D −7.7 (c 1.63, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 7.84 (d, J = 7.6 Hz, 1H, ArH), 7.71 (t, J = 7.6 Hz, 1H, ArH), 7.51 (d, J = 8.0 Hz, 1H, ArH), 7.47 (t, J = 7.6 Hz, 1H, ArH), 4.29 (q, J = 7.2 Hz, 2H, CH₂), 3.80 (dd, J₁ = 17.6 Hz, J₂ = 11.6 Hz, 1H, CH₂), 3.44 (dd, J₁ = 23.2 Hz, J₂ = 17.6 Hz, 1H, CH₂), 1.26 (t, J = 7.0 Hz, 3H, CH₃) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 195.2 (d, ²J_CF = 18.1 Hz), 167.2 (d, ²J_CF = 27.2 Hz), 150.8 (d, ³J_CF = 3.6 Hz), 136.6, 133.1, 128.5, 126.5, 125.5, 94.4 (d, ¹J_CF = −200.3 Hz), 62.5, 38.2 (d, ²J_CF = 23.7 Hz), 13.9 ppm. lit.^7c [α]²⁰_D −18.4 (c 0.14, CHCl₃, 90% ee).

Ethyl 2-fluoro-1,2,3,4-tetrahydro-1-oxonaphthalene-2-carboxylate (2b)^6d,7a. The title compound 2b was obtained according to the general procedure as colorless oil (46.5 mg, 99% yield). HPLC (Daicel Chiralpak OB-H, n-hexane–2-propanol 90 : 10, flow rate 1.0 mL min⁻¹, detection at 254 nm): t_major = 17.0 min, t_minor = 21.4 min, 93% ee. [α]²⁸_D −2.2 (c 4.63, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 8.08 (d, J = 8.0 Hz, 1H, ArH), 7.56 (t, J = 7.6 Hz, 1H, ArH), 7.37 (t, J = 7.6 Hz, 1H, ArH), 7.29 (d, J = 7.6 Hz, 1H, ArH), 4.30 (q, J = 7.2 Hz, 2H, CH₂), 3.23–3.16 (m, 1H, CH₂), 3.12–3.05 (m, 1H, CH₂), 2.80–2.68 (m, 1H, CH₂), 2.60–2.50 (m, 1H, CH₂), 1.28 (t, J = 7.0 Hz, 3H, CH₃) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 188.6 (d, ²J_CF = 18.6 Hz), 167.3 (d, ²J_CF = 25.5 Hz), 143.1, 134.5, 130.5, 128.7, 128.4, 127.2, 93.1 (d, ¹J_CF = −192.8 Hz), 62.4, 31.8 (d, ²J_CF = 22.0 Hz), 24.8 (d, ³J_CF = 7.3 Hz), 14.0 ppm. lit.^6d [α]²⁰_D +0.2 (c 1.25, MeOH, 20% ee); lit.^7a [α]²⁵_D +7.8 (c 1.0, CHCl₃, 40% ee) for opposite enantiomer.

Benzyl 2-fluoro-1,2,3,4-tetrahydro-1-oxonaphthalene-2-carboxylate (2c)^8a. The title compound 2c was obtained according to the general procedure as colorless oil (54.9 mg, 92% yield). HPLC (Daicel Chiralpak OD-H, n-hexane–2-propanol 70 : 30, flow rate 1.0 mL min⁻¹, detection at 254 nm): t_major = 12.0 min, t_minor = 17.6 min, 94% ee. [α]²⁸_D +1.8 (c 2.74, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 8.07 (d, J = 8.0 Hz, 1H, ArH), 7.54 (t, J = 7.4 Hz, 1H, ArH), 7.33–7.23 (m, 7H, ArH), 5.29 (d, J = 12.4 Hz, 1H, CH₂), 5.21 (d, J = 12.4 Hz, 1H, CH₂), 3.18–3.11 (m, 1H, CH₂), 3.03–2.96 (m, 1H, CH₂), 2.78–2.65 (m, 1H, CH₂), 2.58–2.49 (m, 1H, CH₂) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 188.4 (d, ²J_CF = 18.3 Hz), 167.1 (d, ²J_CF = 25.8 Hz), 143.0, 134.6, 134.5, 130.5, 128.7, 128.5, 128.4, 128.3, 127.9, 127.2, 93.2 (d, ¹J_CF = −192.9 Hz), 67.6, 31.8 (d, ²J_CF = 21.9 Hz), 24.7 (d, ³J_CF = 7.3 Hz) ppm. lit.^8a [α]²⁰_D +24.0 (c 1.2, MeOH, 38% ee).

Methyl 2-fluoro-1,2,3,4-tetrahydro-1-oxonaphthalene-2-carboxylate (2d)^4f,7a. The title compound 2d was obtained according to the general procedure as colorless oil (44.4 mg, 100% yield). HPLC (Daicel Chiralpak OB-H, n-hexane–2-propanol 90 : 10, flow rate 1.0 mL min⁻¹, detection at 254 nm): t_major = 23.2 min, t_minor = 29.4 min, 93% ee. [α]²⁸_D −2.3 (c 3.75, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 8.08 (d, J = 7.6 Hz, 1H, ArH), 7.56 (t, J = 7.4 Hz, 1H, ArH), 7.38 (t, J = 7.6 Hz, 1H, ArH), 7.29 (d, J = 7.6 Hz, 1H, ArH), 3.83 (s, 3H, CH₃), 3.23–3.16 (m, 1H, CH₂), 3.13–3.05 (m, 1H, CH₂), 2.80–2.68 (m, 1H, CH₂), 2.60–2.51 (m, 1H, CH₂) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 188.4 (d, ²J_CF = 18.4 Hz), 167.7, 143.1, 134.6, 130.4, 128.7, 128.4, 127.2, 93.2 (d, ¹J_CF = −192.8 Hz), 53.0, 31.8 (d, ²J_CF = 22.0 Hz), 24.8 (d, ³J_CF = 7.1 Hz) ppm. lit.^4f [α]²⁰_D +6.12 (c 0.4, CHCl₃, 41% ee); lit.^7a [α]²⁵_D +4.72 (c 1.0, CHCl₃, 30% ee) for opposite enantiomer.

Propyl 2-fluoro-2,3-dihydro-1-oxo-1H-indene-2-carboxylate (2e)^7c. The title compound 2e was obtained according to the general procedure as colorless oil (45.1 mg, 96% yield). HPLC (Daicel Chiralpak AD-H, n-hexane–2-propanol 98 : 2, flow rate 1.0 mL min⁻¹, detection at 254 nm): t_minor = 10.2 min, t_major = 10.8 min, 99.6% ee. [α]²⁸_D −23.3 (c 2.26, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 7.84 (d, J = 8.0 Hz, 1H, ArH), 7.71 (t, J = 7.6 Hz, 1H, ArH), 7.51 (d, J = 7.6 Hz, 1H, ArH), 7.47 (t, J = 7.6 Hz, 1H, ArH), 5.18–5.12 (m, 1H, CH), 3.77 (dd, J₁ = 17.6 Hz, J₂ = 11.6 Hz, 1H, CH₂), 3.43 (dd, J₁ = 23.4 Hz, J₂ = 17.8 Hz, 1H, CH₂), 1.26 (d, J = 6.0 Hz, 3H, CH₃), 1.23 (d, J = 6.4 Hz, 3H, CH₃) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 195.4 (d, ²J_CF = 18.0 Hz), 166.8 (d, ²J_CF = 27.4 Hz), 150.9 (d, ³J_CF = 3.3 Hz), 136.6, 133.3, 128.5, 126.5, 125.5, 94.4 (d, ¹J_CF = −199.5 Hz), 70.7, 38.2 (d, ²J_CF = 23.9 Hz), 21.5, 21.4 ppm. lit.^7c [α]²⁰_D −12.51 (c 0.12, CHCl₃, 81% ee).

(S)-tert-Butyl 2-fluoro-2,3-dihydro-1-oxo-1H-indene-2-carboxylate (2f)^7b,8c. The title compound 2f was obtained according to the general procedure as colorless oil (46.6 mg, 93% yield). HPLC (Daicel Chiralpak AD-H, n-hexane–2-propanol 98 : 2, flow rate 1.0 mL min⁻¹, detection at 254 nm): t_minor = 9.4 min, t_major = 10.6 min, 99% ee. [α]²⁸_D −5.9 (c 2.33, CH₂Cl₂); ¹H NMR (400 MHz, CDC_l3): δ 7.83 (d, J = 8.0 Hz, 1H, ArH), 7.69 (t, J = 7.4 Hz, 1H, ArH), 7.50 (d, J = 7.6 Hz, 1H, ArH), 7.46 (t, J = 7.4 Hz, 1H, ArH), 3.73 (dd, J₁ = 17.6 Hz, J₂ = 10.8 Hz, 1H, CH₂), 3.40 (dd, J₁ = 22.8 Hz, J₂ = 17.6 Hz, 1H, CH₂), 1.43 (s, 9H, CH₃) ppm; ¹³C NMR (100 MHz, CDCl₃): 195.7 (d, ²J_CF = 18.2 Hz), 166.2 (d, ²J_CF = 27.3 Hz), 150.9 (d, ³J_CF = 4.0 Hz), 136.4, 133.5, 128.4, 126.4, 125.3, 94.3 (d, ¹J_CF = −200.2Hz), 84.0, 38.3 (d, ²J_CF = 24.0 Hz), 27.7 ppm. lit.^8c [α]²⁴_D −3.93 (c 0.41, CHCl₃, 99% ee); lit.^7b [α]²⁹_D −3.34 (c 0.35, CHCl₃, 96% ee).

Benzyl 2-fluoro-2,3-dihydro-1-oxo-1H-indene-2-carboxylate (2g)^7c,8a. The title compound 2g was obtained according to the general procedure as colorless oil (56.9 mg, 100% yield). HPLC (Daicel Chiralpak IB, n-hexane–2-propanol = 98 : 2, flow rate 1.0 mL min⁻¹, detection at 254 nm): t_major = 18.3 min, t_minor = 20.2 min, 99% ee. [α]²⁸_D −6.3 (c 2.86, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 7.83 (d, J = 7.6 Hz, 1H, ArH), 7.69 (t, J = 7.6 Hz, 1H, ArH), 7.49–7.44 (m, 2H, ArH), 7.35–7.23 (m, 5H, ArH), 5.27 (d, J = 12.4 Hz, 1H, CH₂), 5.21 (d, J = 12.4 Hz, 1H, CH₂), 3.77 (dd, J₁ = 17.8 Hz, J₂ = 11.4 Hz, 1H, CH₂), 3.43 (dd, J₁ = 23.0 Hz, J₂ = 17.8 Hz, 1H, CH₂) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 195.0 (d, ²J_CF = 18.6 Hz), 167.1 (d, ²J_CF = 27.9 Hz), 150.7 (d, ³J_CF = 3.5 Hz), 136.7, 134.6, 133.2, 128.6, 128.6, 128.5, 127.9, 126.5, 125.6, 94.6 (d, ¹J_CF = −200.8 Hz), 67.8, 38.2 (d, ²J_CF = 23.7 Hz) ppm. lit.^8a [α]²⁰_D −6.0 (c 1.99, MeOH, 35% ee); lit.^7c [α]²⁰_D −5.0 (c 0.10, CHCl₃, 95% ee).

Ethyl 2-fluoro-2,3-dihydro-6-methyl-1-oxo-1H-indene-2-carboxylate (2h). The title compound 2h was obtained according to the general procedure as yellow oil (47.0 mg, 100% yield). HPLC (Daicel Chiralpak IB, n-hexane–2-propanol 98 : 2, flow rate 1.0 mL min⁻¹, detection at 254 nm): t_major = 10.8 min, t_minor = 12.7 min, 99% ee. [α]²⁸_D −19.7 (c 2.35, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 7.63 (s, 1H, ArH), 7.53 (dd, J₁ = 8.0 Hz, J₂ = 1.6 Hz, 1H, ArH), 7.40 (d, J = 8.0 Hz, 1H, ArH), 4.28 (q, J = 7.2 Hz, 2H, CH₂), 3.74 (dd, J₁ = 17.6 Hz, J₂ = 11.2 Hz, 1H, CH₂), 3.38 (dd, J₁ = 23.2 Hz, J₂ = 17.6 Hz, 1H, CH₂), 2.43 (s, 3H, CH₃), 1.26 (t, J = 7.2 Hz, 3H, CH₃) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 195.3 (d, ²J_CF = 18.5 Hz), 167.3 (d, ²J_CF = 27.9 Hz), 148.3 (d, ³J_CF = 3.5 Hz), 138.7, 138.0, 133.3, 126.2, 125.3, 94.8 (d, ¹J_CF = −199.9 Hz), 62.5, 37.9 (d, ²J_CF = 23.8 Hz), 21.0, 13.9 ppm; IR (KBr): ν 2984, 2931, 1766, 1726, 1616, 1584, 1495, 1425, 1370, 1284, 1225, 1193, 1104, 1074, 1014, 958, 860, 826, 796, 761, 690 cm⁻¹. HRMS (ESI positive): m/z calcd for C₁₃H₁₄FO₃ [M + H]⁺ 237.09215, found: 237.09207.

Ethyl 2-fluoro-2,3-dihydro-5,6-dimethoxy-1-oxo-1H-indene-2-carboxylate (2i). The title compound 2i was obtained according to the general procedure as yellow solid (51.3 mg, 91% yield). M.p. 95–97 °C. HPLC (Daicel Chiralpak OJ-H, n-hexane–2-propanol 70 : 30, flow rate 1.0 mL min⁻¹, detection at 210 nm): t_major = 31.4 min, t_minor = 50.0 min, 98% ee. [α]²⁸_D −99.3 (c 2.56, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 7.22 (s, 1H, ArH), 6.92 (s, 1H, ArH), 4.29 (q, J = 7.2 Hz, 2H, CH₂), 4.01 (s, 3H, OCH₃), 3.93 (s, 3H, OCH₃), 3.71 (dd, J₁ = 17.4 Hz, J₂ = 10.6 Hz, 1H, CH₂), 3.34 (dd, J₁ = 22.4 Hz, J₂ = 17.2 Hz, 1H, CH₂), 1.28 (t, J = 7.2 Hz, 3H, CH₃) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 193.5 (d, ²J_CF = 18.4 Hz), 167.5 (d, ²J_CF = 27.5 Hz), 157.0, 150.1, 146.9 (d, ³J_CF = 4.0 Hz), 125.9, 107.2, 105.3, 94.9 (d, ¹J_CF = −199.8 Hz), 62.4, 56.4, 56.1, 37.9 (d, ²J_CF = 24.0 Hz), 14.0 ppm; IR (KBr): ν 3081, 2981, 2941, 2840, 1763, 1713, 1605, 1590, 1505, 1466, 1443, 1427, 1370, 1325, 1275, 1225, 1192, 1103, 1074, 996, 920, 862, 776 cm⁻¹. HRMS (ESI positive): m/z calcd for C₁₄H₁₆FO₅ [M + H]⁺ 283.09763, found: 283.09793.

Ethyl 5-chloro-2-fluoro-2,3-dihydro-1-oxo-1H-indene-2-carboxylate (2j). The title compound 2j was obtained according to the general procedure as yellow solid (45.5 mg, 89% yield). M.p. 58–60 °C. HPLC (Daicel Chiralpak IB, n-hexane–2-propanol 98 : 2, flow rate 1.0 mL min⁻¹, detection at 254 nm): t_major = 12.1 min, t_minor = 13.5 min, 99% ee. [α]²⁸_D −62.7 (c 2.11, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 7.78 (d, J = 8.0 Hz, 1H, ArH), 7.52 (s, 1H, ArH), 7.46 (d, J = 8.0 Hz, 1H, ArH), 4.29 (q, J = 7.2 Hz, 2H, CH₂), 3.78 (dd, J₁ = 18.0 Hz, J₂ = 11.2 Hz, 1H, CH₂), 3.42 (dd, J₁ = 22.8 Hz, J₂ = 17.6 Hz, 1H, CH₂), 1.27 (t, J = 7.2 Hz, 3H, CH₃) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 193.8 (d, ²J_CF = 18.4 Hz), 166.8 (d, ²J_CF = 27.2 Hz), 152.2 (d, ³J_CF = 3.8 Hz), 143.4, 131.6, 129.5, 126.8, 126.6, 94.3 (d, ¹J_CF = −200.7 Hz), 62.7, 37.9 (d, ²J_CF = 24.2 Hz), 14.0 ppm. IR (KBr): ν 2984, 1767, 1729, 1599, 1578, 1421, 1324, 1289, 1211, 1189, 1070, 924, 858, 838, 784 cm⁻¹. HRMS (ESI positive): m/z calcd for C₁₂H₁₁ClFO₃ [M + H]⁺ 257.03753, found: 257.03754.

Ethyl 5-bromo-2-fluoro-2,3-dihydro-1-oxo-1H-indene-2-carboxylate (2k). The title compound 2k was obtained according to the general procedure as white solid (57.7 mg, 96% yield). M.p. 80–83 °C. HPLC (Daicel Chiralpak IB, n-hexane–2-propanol 98 : 2, flow rate 1.0 mL min⁻¹, detection at 254 nm): t_major = 12.6 min, t_minor = 14.2 min, 99% ee. [α]²⁸_D −75.6 (c 3.00, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 7.71–7.69 (m, 2H, ArH), 7.62 (d, J = 8.4 Hz, 1H, ArH), 4.29 (q, J = 7.2 Hz, 2H, CH₂), 3.78 (dd, J₁ = 18.0 Hz, J₂ = 11.2 Hz, 1H, CH₂), 3.42 (dd, J₁ = 23.0 Hz, J₂ = 17.8 Hz, 1H, CH₂), 1.27 (t, J = 7.2 Hz, 3H, CH₃) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 194.0 (d, ²J_CF = 18.9 Hz), 166.8 (d, ²J_CF = 28.1 Hz), 152.2 (d, ³J_CF = 3.4 Hz), 132.3, 132.0, 129.9, 126.6, 109.7, 94.2 (d, ¹J_CF = −200.9 Hz), 62.7, 37.8 (d, ²J_CF = 24.3 Hz), 14.0 ppm. IR (KBr): ν 3456, 2925, 2854, 1766, 1731, 1595, 1465, 1417, 1370, 1262, 1210, 1058, 923, 857, 796 cm⁻¹. HRMS (ESI positive): m/z calcd for C₁₂H₁₁BrFO₃ [M + H]⁺ 300.98701, found: 300.98729.

N-Butyl-2-fluoro-2,3-dihydro-1-oxo-1H-indene-2-carboxamide (2l). The title compound 2l was obtained according to the general procedure as white solid (50.0 mg, 100% yield). M.p. 85–88 °C. HPLC (Daicel Chiralpak IB, n-hexane–2-propanol 90 : 10, flow rate 1.0 mL min⁻¹, detection at 254 nm): t_major = 8.4 min, t_minor = 9.2 min, 83% ee. [α]²⁸_D −37.1 (c 2.41, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 7.84–7.78 (m, 1H, ArH), 7.67 (t, J = 7.4 Hz, 1H, ArH), 7.55–7.40 (m, 2H, ArH), 6.65 (s, 1H, CONH), 3.96 (dd, J₁ = 17.6 Hz, J₂ = 11.6 Hz, 1H, CH₂), 3.36–3.26 (m, 3H, CH₂), 1.59–1.52 (m, 2H, CH₂), 1.43–1.34 (m, 2H, CH₂), 0.94 (t, J = 7.2 Hz, 3H, CH₃) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 196.7 (d, ²J_CF = 17.6 Hz), 166.7 (d, ²J_CF = 21.7 Hz), 151.9, 136.6, 133.3, 128.3, 126.4, 125.3, 96.8 (d, ¹J_CF = −202.4 Hz), 39.2, 37.4 (d, ²J_CF = 22.6 Hz), 31.4, 19.9, 13.6 ppm. IR (KBr): ν 3275, 3081, 2950, 2929, 2864, 1735, 1663, 1607, 1589, 1553, 1466, 1372, 1301, 1215, 1202, 1155, 1073, 912, 750, 720 cm⁻¹. HRMS (ESI positive): m/z calcd for C₁₄H₁₇FNO₂ [M + H]⁺ 250.12378, found: 250.12358.

2-Fluoro-2,3-dihydro-1-oxo-N-phenyl-1H-indene-2-carboxamide (2m). The title compound 2m was obtained according to the general procedure as colorless solid (53.9 mg, 100% yield). M.p. 198–202 °C. HPLC (Daicel Chiralpak IB, n-hexane–2-propanol 90 : 10, flow rate 1.0 mL min⁻¹, detection at 254 nm): t_minor = 12.4 min, t_major = 16.1 min, 94% ee. [α]²⁸_D −78.0 (c 2.74, CH₂Cl₂); ¹H NMR (400 MHz, (CD₃₎₂SO): δ 10.45 (s, 1H, CONH), 7.85–7.80 (m, 2H, ArH), 7.72 (dd, J₁ = 8.8 Hz, J₂ = 1.2 Hz, 2H, ArH), 7.68 (d, J = 7.6 Hz, 2H, ArH), 7.55 (t, J = 7.6 Hz, 1H, ArH), 7.34 (t, J = 8.0 Hz, 2H, ArH), 7.13 (t, J = 7.4 Hz, 1H, ArH), 3.90 (dd, J₁ = 18.0 Hz, J₂ = 12.4 Hz, 1H, CH₂), 3.53 (dd, J₁ = 24.8 Hz, J₂ = 18.0 Hz, 1H, CH₂) ppm; ¹³C NMR (100 MHz, (CD₃)₂SO): δ 196.9 (d, ²J_CF = 18.1 Hz), 165.7 (d, ²J_CF = 23.2 Hz), 152.1 (d, ³J_CF = 3.5 Hz), 137.3, 136.8, 132.7, 128.4, 126.8, 124.5, 124.4, 120.6, 95.9 (d, ¹J_CF = −205.3 Hz), 37.8 (d, ²J_CF = 23.1 Hz) ppm. IR (KBr): ν 3353, 3060, 1723, 1667, 1599, 1587, 1537, 1445, 1303, 1212, 1152, 1093, 1070, 921, 752, 717, 687 cm⁻¹. HRMS (ESI positive): m/z calcd for C₁₆H₁₃FNO₂ [M + H]⁺ 270.09248, found: 270.09247.

N-Benzyl-2-fluoro-2,3-dihydro-1-oxo-1H-indene-2-carboxamide (2n). The title compound 2n was obtained according to the general procedure as white solid (53.9 mg, 95% yield). M.p. 107–110 °C. HPLC (Daicel Chiralpak IB, n-hexane–2-propanol 90 : 10, flow rate 1.0 mL min⁻¹, detection at 254 nm): t_major = 13.9 min, t_minor = 16.1 min, 73% ee. [α]²⁸_D +7.7 (c 2.62, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃): δ 7.81 (d, J = 7.6 Hz, 1H, ArH), 7.68 (t, J = 7.4 Hz, 1H, ArH), 7.50 (d, J = 7.6 Hz, 1H, ArH), 7.43 (t, J = 7.4 Hz, 1H, ArH), 7.38–7.28 (m, 5H, ArH), 6.92 (s, 1H, CONH), 4.53 (d, J = 5.6 Hz, 2H, CH₂), 3.99 (dd, J₁ = 17.4 Hz, J₂ = 12.2 Hz, 1H, CH₂), 3.34 (dd, J₁ = 24.0 Hz, J₂ = 17.6 Hz, 1H, CH₂) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 196.5 (d, ²J_CF = 17.8 Hz), 166.8 (d, ²J_CF = 22.8 Hz), 151.8 (d, ³J_CF = 4.0 Hz), 137.1, 136.6, 133.2, 129.7, 129.4, 128.7, 128.3, 127.6, 126.5, 125.4, 96.7 (d, ¹J_CF = −201.9 Hz), 43.4, 37.4 (d, ²J_CF = 22.6 Hz) ppm. IR (KBr): ν 3333, 3065, 3032, 2932, 1730, 1667, 1607, 1588, 1534, 1467, 1454, 1427, 1301, 1216, 1197, 1082, 917, 749, 733, 698 cm⁻¹. HRMS (ESI positive): m/z calcd for C₁₇H₁₅FNO₂ [M + H]⁺ 284.10813, found: 284.10830.

Acknowledgements

We are grateful for financial support from the National Natural Science Foundation of China (Grant no. 20772006, 21072020), the Science and Technology Innovation Program of Beijing Institute of Technology (Grant no. 2011CX01008).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Copies of ¹H and ¹³C NMR spectra of new compounds, and HPLC chromatograms. See DOI: 10.1039/c3ra45438j

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