Palladium-Catalyzed Asymmetric Ring-Opening Reactions of Oxabenzonorbornadienes with Potassium Trifluoroborate Salts

a.
Guangxi Key Laboratory of Agricultural Resources Chemistry and Biotechnology, College of Chemistry and Food Science, Yulin Normal University, Yulin, Guangxi 537000
b.
College of Chemistry and Materials, Nanning Normal University, Nanning 530001
^†These authors contributed equally to this work.

Corresponding author: Huang Guobao, lzjx0915@163.com Li Xiuying, lxya8401@163.com

Received Date: 22 December 2019
Revised Date: 11 March 2020
Available Online: 25 June 2020
Fund Project:

Project supported by the National Natural Science Foundation of China (No. 21961042), the Natural Science Foundation of Guangxi Province (No. 2018GXNSFAA294064) and the Open Fund of Guangxi Key Laboratory of Agricultural Resources Chemistry and Biotechnology Research Grant (No. KF04)

Abstract: A new, versatile and highly efficient palladium-catalyzed asymmetric ring-opening reaction of oxabenzonor-bornadienes with a wide range of potassium trifluoroborate salts was developed. The corresponding cis-2-aryl-1, 2-dihydronaphthalen-1-ol products were obtained in good yields with moderate to good enantioselectivities under mild conditions.

Key words:

1. Introduction

The synthesis of substituted 1, 2-dihydronaphthalenes is a theme of ongoing interest because these compounds possess a wide range of biological activities, ^[1] and they are also important intermediates in organic synthesis.^[2] Among the available methods for hydronaphthalenes synthesis, the transition metal-catalyzed asymmetric ring- opening (ARO) reactions of hetero-bicyclic alkenes with various carbanion and heteroatom nucleophiles are valuable processes, which have been intensively investigated. Many metal catalysts such as Fe, ^[3] Ni, ^[4] Cu, ^[5] Ru, ^[6] Rh, ^[7] Pd, ^[8] Pt^[9] and Ir^[10] were explored for the ring-opening of oxa- and aza-bicyclic alkenes.

How to find a group of novel nucleophiles as the aryl source to react with oxabicyclic alkenes is a long-term challenge to the chemists. Various aryl donators such as aryl halides, ^[11] organometallic reagents, ^[12] arylboronic acids^[13] and arylsulfinate salts^[14] had been used in the ARO reactions. To the best of our knowledge, the study referring to the reaction of aryl halides with oxabenzonorbornadiene is relatively rare because of the low activity of these carbanion nucleophiles. On the contrary, high active organometallic reagents may incur some side effects to give the ring-opening product in low yields.^[12a] In recent years, arylsulfinate salts due to the stability and low toxicity, have been employed as effective carbanion nucleophiles in the ring-opening reactions of oxabicyclic alkenes to afford the corresponding products, but with low enantioselectivities. Arylboronic acids are appropriate reagents in this type of ARO reactions due to their low cost, remarkable stability and ease of handling. However, arylboronic acids still have some drawbacks, such as having high nucleophile equivalence and low reactivity in the air atmosphere. Nevertheless, these results gave us good references.

Potassium trifluoroborate salts are a kind of ideal reagents in organic chemistry, for example, the construction of C—C bonds.^[15] Compared with boronic acids, potassium trifluoroborate salts not only own similar chemical properties, but also are more convenient and stable.^[16] To our surprise, so far there is no report on the ring opening reaction of oxybenzodiene with potassium trifluoborate as the substrate. Herein a novel ARO addition of potassium trifluoroborate salts to oxabicyclic alkenes in the presence of palladium complex to afford the corresponding products of cis-2-aryl-1, 2-dihydronapthalene-1-ols in high yields (up to 98%) with moderate to good enantioselectivities (up to 98% ee) is reported. cis-Configuration, not trans- configuration, of the product was obtained through X-ray diffraction analysis.

2. Results and discussion

To explore the ring opening reaction, preliminary optimization of the reaction conditions was carried out using oxabenzonorbornadiene (1a) and potassium phenyl- trifluoroborate (2a) as model substrates in toluene/water (V:V＝5:1) at room temperature. When 1a reacted with 2a in the presence of 5 mol% Pd(OAc)₂ and 10 mol% DPPP under nitrogen atmosphere, desired ring-opened product 3a was obtained in 25% isolated yield after 24 h (Table 1, Entry 1). Encouraged by this result, several chiral ligands were screened for the ring-opening reaction of 1a under similar conditions, including (S, S)-Me-DuPhos, (S)-DM-SegPhos, (R)-(S)-PPF-PtBu₂, (R)-BINAP and (S)-PipPhos. The results indicated that (R)-BINAP was the most effective ligand among the ligands screened for this reaction compared with the others (Table 1, Entries 2~6), and 3a was afforded in 90% yield with 55% ee (Table 1, Entry 6). Meanwhile, the catalyst loading also had a significant impact. The results indicated that 5 mol% Pd(OAc)₂ with 10 mol% (R)-BINAP would be a more suitable catalyst loading in terms of the yield and enantio- selectivity, whereas increasing or decreasing the amount of catalyst loading improved neither the yield nor ee value of 3a (Table 1, Entries 7~9). Similar yield and enantio- selectivity were obtained whether in a nitrogen or air atmosphere (Table 1, Entry 10). In addition, the effect of solvents was investigated. A mixture of water and CH₃CN was totally ineffective in this reaction (Table 1, Entry 11). When the reaction was carried out in a mixed solvent of water with tetrahydrofuran (THF), expected product 3a was afforded in only 20% yield with 30% ee (Table 1, Entry 12). It was observed that the reaction in a mixed solvent of water with protic solvents MeOH gave satisfactory enantioselectivity (up to 76% ee) but poor conversion (Table 1, Entry 13). Moderate to good yields and enantioselectivities were obtained when a mixed solvent of water with CH₂Cl₂, CHCl₃ and dichloroethane (DCE) were respectively employed as the solvents (Table 1, Entries 14~16). Importantly, the study on solvent effect indicated that the reaction worked very well in a mixed solvent of water with the aromatic solvent of toluene showed the best combination of reactivity and enantioselectivity among all solvents tested (91% yield, 55% ee). However, satisfactory results had not been achieved when the impact of reaction temperature was investigated. The reaction run at 0 ℃ gave a diminished yield and slightly improved ee (Table 1, Entry 17). Raising the temperature to 50 ℃ somewhat decreased the yield and ee even after a prolonged reaction time (Table 1, Entry 18).

Table 1

Table 1. Optimization of reaction conditions for the ARO reaction^a

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Entry	Solvent	Pd(OAc)₂/ mol%	Ligand	Time/h	Yield/%	ee^b/%
1	Toluene	5.0	L1	24	25	－
2	Toluene	5.0	L2	7	49	35
3	Toluene	5.0	L3	7	60	21
4	Toluene	5.0	L4	24	NR^c	－
5	Toluene	5.0	L5	0.5	90	55
6	Toluene	5.0	L6	24	NR^c	－
7	Toluene	2.5	L5	1	80	55
8	Toluene	1.0	L5	2	71	55
9	Toluene	10.0	L5	0.5	92	54
10^d	Toluene	5.0	L5	0.5	91	55
11^d	MeCN	5.0	L5	24	NR^c	－
12^d	THF	5.0	L5	24	20	30
13^d	MeOH	5.0	L5	22	30	76
14^d	CH₂Cl₂	5.0	L5	3	85	45
15^d	CHCl₃	5.0	L5	3	80	50
16^d	DCE	5.0	L5	1	81	52
17^d^{, e}	Toluene	5.0	L5	5	72	58
18^d^{, f}	Toluene	5.0	L5	5	85	42
^a Reaction conditions: 1a (0.2 mmol), 2a (1.2 equiv.), Pd(OAc)₂ and ligand (10 mol%) in solvent and H₂O (V:V＝5:1, 3 mL in total) under nitrogen at room temperature. ^b Determined by HPLC with a chiralcel OD-H column. ^c NR: no reaction. ^d Carried out in air. ^e The reaction at 0 ℃. ^f The reaction at 50 ℃.

Next, with the optimized reaction conditions identified, the scope of potassium trifluoroborate salts was explored for this reaction to evaluate their effect on the reactivity and enantioselectivity (Table 2). From Table 2, it can be seen that the structures of potassium trifluoroborate salts have significant impact on the reactivity and enantioselectivities. In general, potassium trifluoroborate salts with electron-donating substituents on the phenyl rings gave higher yields but lower enantioselectivities than those with electron-withdrawing groups (Table 2, Entries 2~9). When it came to the position property of monosubstituted on the phenyl rings of potassium trifluoroborate salts, it had little effect on the reactivity, and the ring-opening of 1a with parasubstituted potassium trifluoroborate salts offerd better enantioselectivities than those with meta- and ortho-substituted potassium trifluoroborate salts (Table 2, Entries 2, 3, 5~7). It is noteworthy that the addition of alkyl-potassium trifluoroborate salt to 1a was found to give the corresponding product 3j successfully just prolonged the reaction time to 3 h (Table 2, Entry 10). Potassium trifluoroborate salts containing heterocycle were employed in the ring-opening of 1a, the results was also satisfactory (Table 2, Entry 11). Thiophene-2-potassium trifluoroborate salt reacted with 1a and 3k was obtained in high yield (95% yield) with modest enantioselectivity (71% ee).

Table 2

Table 2. Ring-opening of oxabenzonorbornadiene 1a with various potassium trifluoroborate salts^a

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Entry	R	Product	Time/h	Yield/%	ee^b/%
1	C₆H₅	3a	0.5	91	55
2	4-CH₃C₆H₄	3b	0.5	94	62
3	2-CH₃C₆H₄	3c	0.5	92	20
4	4-^tBuC₆H₄	3d	0.5	95	36
5	2-CH₃OC₆H₄	3e	0.5	90	2
6	3-CH₃OC₆H₄	3f	0.5	91	56
7	4-CH₃OC₆H₄	3g	0.5	98	51
8	4-BrC₆H₄	3h	0.5	84	64
9	4-FC₆H₄	3i	0.5	84	60
10	CH₃	3j	3	81	56
11	2-Thienyl	3k	0.5	95	71
^a Reaction conditions: 1a (0.2 mmol), 2 (1.2 equiv.), Pd(OAc)₂ (5 mol%), and (R)-BINAP (10 mol%) in toluene and H₂O (V:V＝5:1, 3 mL in total) in air at room temperature. ^b Determined by HPLC with a chiralcel OD-H column.

The molecular configuration of 3d was unambiguously confirmed by X-ray diffraction analysis (Figure 1). The single crystal was obtained by solvent evaporation from a mixture of CHCl₃ and hexane. The configuration of 3d was assigned as (1S, 2R) and confirmed as the 1, 2-cis- configuration.^[13e]

Figure 1

Figure 1. X-ray structure of the product 3d

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The substrate scope of oxabenzonorbornadiene derivatives was further evaluated, and the results were compiled in Table 3. From Table 3, besides 1a, it can be seen that the reaction of various oxabenzonorbornadienes with potassium trifluoroborate salts proceeded smoothly to generate the anticipated ring-opening products in high yields (up to 93%) with good to excellent enantioselectivity (up to 97% ee). Furthermore, the influence of different potassium trifluoroborate salts on the reaction of each substrate is similar to that of substrate 1a, potassium trifluoroborate salts with electron-donating substituents on the phenyl rings gave higher yields but lower enantioselectivities than those with electron-withdrawing groups. The excellent performance of this catalyst system is highlighted by the reaction of 1b with potassium cyclopropyl trifluorobo- ranuide, which furnished product 4a in excellent yield (90%) with remarkable enantioselectivity (up to 97% ee) (Table 3, Entry 2).

Table 3

Table 3. Ring-opening of oxabenzonorbornadiene 1b and 1c with various potassium trifluoroborate salts^a

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Entry	Substrate	R	Product	Yield/%	ee^b/%
1	1b	Cyclopropyl	4a	90	97
2	1b	4-FC₆H₄	4b	84	52
3	1b	4-CH₃C₆H₄	4c	91	51
4	1c	C₆H₅	5a	85	55
5	1c	Cyclopropyl	5b	90	97
6	1c	4-FC₆H₄	5c	84	68
7	1c	4-CH₃C₆H₄	5d	91	65
8	1c	2-Thienyl	5e	93	87
^a Reaction conditions: 1b, 1c (0.2 mmol), 2 (1.2 equiv.), Pd(OAc)₂ (5 mol%), and (R)-BINAP (10 mol%) in toluene and H₂O (V:V＝5:1, 3 mL in total) in air at room temperature. ^b Determined by HPLC with a chiralcel OD-H column.

Based upon the experimental observations and literatures, ^[17] a proposed mechanism for the palladium(II)- catalyzed asymmetric ring-opening of potassium phenyltrifluoroborate to oxaenzonornadiene 3a is summarized in Scheme 1. The catalytic cycle would be initiated by the coordination of Pd(OAc)₂ and (R)-BINAP to form the active chiral palladium complex A, which leads to intermediate B after addition of potassium phenyltrifluoroborate (2a).^[18] B is followed by insertion of azabenzonorbornadiene (1a) into the carbon-palladium bond from the exo side and produce intermediate C. β-Elimination of oxygen results in opening the furyl ring to give the ring-opened intermediate D which follows by the hydrolysis to liberate ring-opened product 3a.^[19] Meanwhile, the palladium species A is regenerated to promote the next catalytic cycle.

Scheme 1

Scheme 1. Proposed mechanism

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3. Conclusions

In summary, a new palladium(II)-catalyzed asymmetric ring-opening reaction of oxabenzonorbornadienes with potassium trifluoroborate salts was successfully developed. Pd(OAc)₂ and (R)-BINAP were used as the catalyst system in this reaction. A practical and efficient approach was provided to synthesize the optically active cis-2-aryl-1, 2- dihydronaphthalen-1-ol derivatives in high yields with moderate to good enantioselectivities under mild conditions. Further investigations on the applications of palladium catalysts in the asymmetric ring-opening are in progress in our laboratory and will be reported.

4. Experimental section

4.1 General information

¹H NMR, ¹³C NMR and ¹⁹F NMR spectra were recorded on a Bruker Avance nuclear magnetic resonance spectrometer at 400/500 MHz and 100/125 MHz at room temperature in CDCl₃, respectively. HRMS (ion trap) were recorded by Q EXACTIVE mass spectrometry using APCI or ESI. The HPLC analysis was performed on a Waters Alliance e2695 high performance liquid chromatography with a Chiralcel OD-H column or Chiralpak AD-H column. Unless otherwise indicated, all reagents were purchased from commercial suppliers and used without further purification. Toluene was used without any pretreatment. All flasks were flame-dried under a stream of nitrogen and cooled to room temperature before use. Oxabenzonorbornadienes 1a~1c were prepared according to the literature procedures.^[20]

4.2 General procedures for the products

All experiments were carried out under an air atmosphere. Palladium diacetate (2.2 mg, 5 mol%) and (R)-BINAP (12.4 mg, 10 mol%) were added to a 10 mL round-bottomed flask, followed by the addition of toluene/H₂O (V:V＝5:1, 3 mL in total). After the mixture was stirred for about 5 min, oxabenzonorbornadienes 1a~1c (0.2 mmol) and potassium trifluoroborate (1.2 equiv., 0.24 mmol) were put into the reaction system. The resulting mixture was stirred at room temperature until completion monitored by thin-layer chromatography (TLC). The solvent was removed in vacuo, and the crude mixture was then purified by column chromatography on silica gel (silica gel, 200~300 mesh) to afford the target product.

(1S*, 2R*)-2-Phenyl-1, 2-dihydronaphthalen-1-ol (3a): Colorless oil (40.0 mg, 90% yield). R_f＝0.25 on silica gel (ethyl acetate/petroleum ether, V:V＝1:20). The ee was determined to be 55% using HPLC analysis on a Chiralcel OD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 7.474 (major) and 11.708 (minor) min. ¹H NMR (500 MHz, CDCl₃) δ: 7.41~7.28 (m, 8H), 7.22 (d, J＝7.2 Hz, 1H), 6.76 (dd, J＝9.6, 1.7 Hz, 1H), 6.18 (dd, J＝9.6, 4.0 Hz, 1H), 4.97 (d, J＝5.6 Hz, 1H), 3.93~3.90 (m, 1H), 1.62 (s, 1H); ¹³C NMR (125 MHz, CDCl₃) δ: 137.7, 136.1, 132.6, 129.6, 129.2, 128.6, 128.3, 128.2, 128.0, 127.4, 126.7, 126.3, 71.3, 47.3; HRMS (APCI-ion trap) calcd for C₁₆H₁₂O [M－2H]^－220.0899, found 220.0889.

(1S*, 2R*)-2-(4-Methylphenyl)-1, 2-dihydronaphthalen-1-ol (3b): Colorless oil (44.4 mg, 94% yield). R_f＝0.20 on silica gel (ethyl acetate/petroleum ether, V:V＝1:20). The ee was determined to be 62% using HPLC analysis on a chiralcel OD-H column (hexane/2-propanol, V:V＝99:1, 1.0 mL/min, λ＝254 nm). Retention times were 15.346 (major) and 31.153 (minor) min. ¹H NMR (500 MHz, CDCl₃) δ: 7.39~7.15 (m, 8H), 6.74 (dd, J＝9.6, 1.8 Hz, 1H), 6.16 (dd, J＝9.6, 4.1 Hz, 1H), 4.96 (t, J＝6.9 Hz, 1H), 4.05~3.68 (m, 1H), 2.37 (s, 3H), 1.59 (s, 1H); ¹³C NMR (125 MHz, CDCl₃) δ: 137.0, 136.2, 134.4, 132.7, 129.9, 129.3, 129.1, 128.2, 128.0, 127.9, 126.6, 126.3, 71.2, 46.9, 21.0; HRMS (APCI-ion trap) calcd for C₁₇H₁₅O [M－H]^－ 235.1123, found 235.1123.

(1S*, 2R*)-2-(2-Methylphenyl)-1, 2-dihydronaphthalen-1-ol (3c): Colorless oil (43.7 mg, 92% yield). R_f＝0.20 on silica gel (ethyl acetate/petroleum ether, V:V＝1:20). The ee was determined to be 20% using HPLC analysis on a vhiralcel OD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 6.952 (minor) and 11.725 (major) min. ¹H NMR (500 MHz, CDCl₃) δ: 7.53~7.16 (m, 8H), 6.76 (dd, J＝9.6, 2.2 Hz, 1H), 6.11 (dd, J＝9.6, 3.1 Hz, 1H), 4.87 (s, 1H), 4.25~4.23 (m, 1H), 2.47 (s, 3H), 1.56 (s, 1H); ¹³C NMR (125 MHz, CDCl₃) δ: 136.7, 136.6, 135.4, 132.5, 130.7, 130.43 129.3, 128.61 127.9, 127.9, 127.6, 127.3, 126.5, 126.3, 69.6, 43.2, 19.8; HRMS (APCI-ion trap) calcd for C₁₇H₁₇O [M+H]⁺ 237.1279, found 327.1274.

(1S*, 2R*)-2-(4-tertbutylphenyl)-1, 2-dihydronaphthalen-1-ol (3d): Colorless oil (52.9 mg, 95% yield). R_f＝0.20 on silica gel (ethyl acetate/petroleum ether, V:V＝1:20). The ee was determined to be 36% using HPLC analysis on a chiralcel OD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 5.040 (major) and 10.656 (minor) min. ¹H NMR (500 MHz, CDCl₃) δ: 7.42~7.12 (m, 8H), 6.70 (dd, J＝9.6, 2.0 Hz, 1H), 6.14 (dd, J＝9.6, 4.1 Hz, 1H), 5.01~4.82 (m, 1H), 3.87~3.85 (m, 1H), 1.57 (s, 1H), 1.31 (s, 9H); ¹³C NMR (125 MHz, CDCl₃) δ: 150.3, 136.1, 134.4, 132.7, 129.9, 128.9, 128.2, 128.0, 127.9, 126.7, 126.3, 125.6, 71.3, 46.8, 34.4, 31.3; HRMS (APCI-ion trap) calcd for C₂₀H₂₂ONa [M+Na]⁺ 301.1569, found 301.1563.

(1S*, 2R*)-2-(2-Methoxylphenyl)-1, 2-dihydronaphthalen-1-ol (3e): Colorless oil (45.1 mg, 90 % yield). R_f＝0.25 on silica gel (ethyl acetate/petroleum ether, V:V＝1:20). The ee was determined to be 2% using HPLC analysis on a chiralcel OD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 7.753 (major) and 12.228 (minor) min. ¹H NMR (400 MHz, CDCl₃) δ: 7.55~7.04 (m, 8H), 6.71 (dd, J＝9.6, 2.5 Hz, 1H), 6.07 (dd, J＝9.6, 3.2 Hz, 1H), 4.82 (t, J＝5.9 Hz, 1H), 4.20 (dt, J＝5.6, 2.9 Hz, 1H), 2.43 (s, 3H), 1.53 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 136.7, 136.6, 135.4, 132.5, 130.6, 130.4, 129.3, 128.6, 127.9, 127.9, 127.6, 127.2, 126.5, 126.3, 69.6, 43.2, 19.7; HRMS (APCI-ion trap) calcd for C₁₇H₁₅O₂ [M－H]^－ 251.1072, found 251.1071.

(1S*, 2R*)-2-(3-Methoxylphenyl)-1, 2-dihydronaphthalen-1-ol (3f): Colorless oil (45.9 mg, 91% yield). R_f＝0.25 on silica gel (ethyl acetate/petroleum ether, V:V＝1:20). The ee was determined to be 56% using HPLC analysis on a chiralcel OD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 9.644 (major) and 14.246 (minor) min. ¹H NMR (400 MHz, CDCl₃) δ: 7.36 (d, J＝6.7 Hz, 1H), 7.33~7.21 (m, 3H), 7.17 (dd, J＝7.2, 1.4 Hz, 1H), 6.94~6.84 (m, 1H), 6.84~6.81 (m, 2H), 6.70 (dd, J＝9.6, 2.0 Hz, 1H), 6.12 (dd, J＝9.6, 4.0 Hz, 1H), 4.94 (d, J＝5.9 Hz, 1H), 3.92~3.80 (m, 1H), 3.74 (s, 3H), 1.61 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 159.8, 139.3, 136.1, 132.6, 129.6, 129.5, 128.3, 128.2, 128.0, 126.7, 126.4, 121.5, 114.8, 112.9, 71.3, 55.1, 47.3; HRMS (APCI-ion trap) calcd for C₁₇H₁₅O₂ [M－H]^－ 251.1072, found 251.1077.

(1S*, 2R*)-2-(4-Methoxylphenyl)-1, 2-dihydronaphthalen-1-ol (3g): Colorless oil (49.4 mg, 98% yield). R_f＝0.25 on silica gel (ethyl acetate/petroleum ether, V:V＝1:20). The ee was determined to be 51% using HPLC analysis on a Chiralcel OD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 9.063 (major) and 14.563 (minor) min. ¹H NMR (400 MHz, CDCl₃) δ: 7.34 (dd, J＝4.3, 3.6 Hz, 1H), 7.30~7.20 (m, 2H), 7.21~7.10 (m, 3H), 6.95~6.74 (m, 2H), 6.67 (dd, J＝9.6, 1.9 Hz, 1H), 6.09 (dd, J＝9.6, 4.3 Hz, 1H), 4.92 (t, J＝6.7 Hz, 1H), 3.82~3.78 (m, 1H), 3.76 (s, 3H), 1.50 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 159.0, 136.3, 132.7, 130.3, 130.0, 129.1, 128.1, 128.0, 127.9, 126.5, 126.2, 114.1, 71.2, 55.2, 46.4; HRMS (APCI-ion trap) calcd for C₁₇H₁₅O₂ [M－H]^－ 251.1072, found 251.1074.

(1S*, 2R*)-2-(4-Bromophenyl)-1, 2-dihydronaphthalen-1-ol (3h): Colorless oil (50.4 mg, 84% yield). R_f＝0.20 on silica gel (ethyl acetate/petroleum ether, V:V＝1:20). The ee was determined to be 64% using HPLC analysis on a chiralcel OD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 7.446 (major) and 11.562 (minor) min. ¹H NMR (500 MHz, CDCl₃) δ: 7.43 (d, J＝8.3 Hz, 2H), 7.32 (dd, J＝13.6, 7.2 Hz, 2H), 7.26 (t, J＝6.9 Hz, 1H), 7.18 (d, J＝7.2 Hz, 1H), 7.13 (d, J＝8.3 Hz, 2H), 6.71 (dd, J＝9.6, 1.5 Hz, 1H), 6.08 (dd, J＝9.6, 4.0 Hz, 1H), 4.91 (s, 1H), 3.82~3.80 (m, 1H), 1.54 (s, 1H); ¹³C NMR (125 MHz, CDCl₃) δ: 136.9, 135.9, 132.4, 131.6, 131.0, 129.1, 128.5, 128.4, 128.2, 126.5, 126.4, 121.3, 71.1, 46.7; HRMS (APCI-ion trap) calcd for C₁₆H₁₂BrO [M－H]^－ 299.0072, found 298.9905, 299.0066.

(1S*, 2R*)-2-(4-Fluorophenyl)-1, 2-dihydronaphthalen-1-ol (3i): White solid (40.4 mg, 84% yield). m.p. 59~60 ℃. R_f＝0.13 on silica gel (ethyl acetate/petroleum ether, V: V＝1:20). The ee was determined to be 60% using HPLC analysis on a Chiralcel OD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 6.792 (major) and 11.332 (minor) min. ¹H NMR (400 MHz, CDCl₃) δ: 7.42~7.14 (m, 6H), 7.06~6.95 (m, 2H), 6.70 (dd, J＝9.6, 2.0 Hz, 1H), 6.09 (dd, J＝9.6, 4.1 Hz, 1H), 4.92 (d, J＝5.8 Hz, 1H), 3.86~3.83 (m, 1H), 1.59 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.4, 161.0, 136.0, 133.32, 133.29, 132.5, 130.8, 130.7, 129.6, 128.4, 128.3, 128.1, 126.5, 126.4, 115.5, 115.3, 71.3, 46.5; ¹⁹F NMR (376 MHz, CDCl₃) δ: －115.4; HRMS (APCI-ion trap) calcd for C₁₆H₁₂FO [M－H]^－ 239.0872, found 239.0873.

(1S*, 2R*)-2-Methyl-1, 2-dihydronaphthalen-1-ol (3j): Colorless oil (25.9 mg, 81% yield). R_f＝0.21 on silica gel (ethyl acetate/petroleum ether, V:V＝1:20). The ee was determined to be 56% using HPLC analysis on a Chiralcel AD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 6.494 (minor) and 6.841 (major) min. ¹H NMR (400 MHz, CDCl₃) δ: 7.36 (dd, J＝7.2, 1.4 Hz, 1H), 7.32~7.20 (m, 2H), 7.12 (dd, J＝7.1, 1.5 Hz, 1H), 6.51 (dd, J＝9.5, 2.4 Hz, 1H), 5.80 (dd, J＝9.5, 3.1 Hz 1H), 4.84~4.22 (m, 1H), 2.79~2.50 (m, 1H), 1.59 (s, 1H), 1.25 (d, J＝7.4 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 136.6, 132.4, 132.3, 128.4, 127.6, 127.3, 126.5, 126.4, 71.6, 35.2, 14.0; HRMS (APCI-ion trap) calcd for C₁₁H₁₃O [M+H]^－ 161.0966, found 161.0961.

(1S*, 2R*)-2-Thienyl-1, 2-dihydronaphthalen-1-ol (3k): Colorless oil (43.3 mg, 95% yield). R_f＝0.25 on silica gel (ethyl acetate/petroleum ether, V:V＝1:20). The ee was determined to be 71% using HPLC analysis on a chiralcel OD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 8.842 (major) and 11.989 (minor) min. ¹H NMR (500 MHz, CDCl₃) δ: 7.40 (d, J＝7.0 Hz, 1H), 7.34~7.23 (m, 2H), 7.17 (dd, J＝5.9, 1.8 Hz, 2H), 6.97 (dd, J＝4.3, 3.1 Hz, 2H), 6.65 (dd, J＝9.6, 0.9 Hz, 1H), 6.16 (dd, J＝9.5, 4.6 Hz, 1H), 5.01 (t, J＝6.5 Hz, 1H), 4.34~3.95 (m, 1H), 1.86 (s, 1H); ¹³C NMR (125 MHz, CDCl₃) δ: 139.9, 136.2, 132.4, 129.5, 128.18, 128.15, 128.0, 126.9, 126.4, 126.4, 126.2, 125.1, 71.0, 42.5; HRMS (APCI-ion trap) calcd for C₁₄H₁₁OS [M－H]^－ 227.0531, found 227.0531.

(1S*, 2R*)-5, 8-Dimethoxy-2-cyclopropyl-1, 2-dihydro-naphthalen-1-ol (4a): Colorless oil (44.3 mg, 90% yield). R_f＝0.2 on silica gel (ethyl acetate/petroleum ether, V: V＝1:10). The ee was determined to be 97% using HPLC analysis on a chiralcel OD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 11.343 (minor) and 18.273 (major) min. ¹H NMR (400 MHz, CDCl₃) δ: 6.91 (dd, J＝9.8, 3.2 Hz, 1H), 6.77 (d, J＝3.4 Hz, 2H), 6.01~5.94 (m, 1H), 5.04 (s, 1H), 3.84 (s, 3H), 3.79 (s, 3H), 1.77 (d, J＝6.3 Hz, 1H), 1.57~1.52 (m, 1H), 1.32~1.14 (m, 2H), 0.47 (dd, J＝10.1, 6.9 Hz, 1H), 0.39~0.17 (m, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 150.6, 149.6, 130.3, 125.4, 122.8, 120.8, 111.3, 110.6, 63.4, 56.2, 56.1, 46.2, 11.1, 3.9, 3.7; HRMS (ESI-ion trap) calcd for C₁₅H₁₇O₂ [M+H－H₂O]⁺ 229.1223, found 229.1216.

(1S*, 2R*)-5, 8-Dimethoxy-2-(4-Fluorophenyl)-1, 2-dihy-dronaphthalen-1-ol (4b): Colorless oil (50.0 mg, 85% yield). R_f＝0.2 on silica gel (ethyl acetate/petroleum ether, V:V＝1:10). The ee was determined to be 52% using HPLC analysis on a chiralcel OD-H column (hexane/ 2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 17.871 (major) and 32.805 (minor) min. ¹H NMR (400 MHz, CDCl₃) δ: 7.39 (dd, J＝8.6, 5.4 Hz, 1H), 7.19~6.98 (m, 1H), 6.83 (q, J＝9.0 Hz, 1H), 6.16~5.95 (m, 1H), 5.06 (dd, J＝4.7, 1.5 Hz, 1H), 3.83 (d, J＝5.1 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.2, 160.8, 150.7, 149.7, 136.3, 136.2, 130.7, 130.6, 128.8, 122.2, 115.4, 115.2, 111.5, 111.1, 64.4, 56.2, 46.5, 22.6; ¹⁹F NMR (375 MHz, CDCl₃) δ: －116.10; HRMS (ESI- ion trap) calcd for C₁₈H₁₆O₂F [M+H－H₂O]⁺ 283.1129, found 283.1120.

(1S*, 2R*)-5, 8-Dimethoxy-2-(4-Methylphenyl)-1, 2-dihy-dronaphthalen-1-ol (4c): Colorless oil (53.9 mg, 91% yield). R_f＝0.2 on silica gel (ethyl acetate/petroleum ether, V:V＝1:10). The ee was determined to be 51% using HPLC analysis on a Chiralcel OD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 15.041 (major) and 25.944 (minor) min. ¹H NMR (400 MHz, CDCl₃) δ: 7.34 (d, J＝8.0 Hz, 2H), 7.22 (d, J＝7.8 Hz, 2H), 7.09 (dd, J＝9.8, 3.2 Hz, 1H), 6.82 (q, J＝9.0 Hz, 2H), 6.16~6.12 (m, 1H), 5.08 (dd, J＝4.6, 1.2 Hz, 1H), 3.84 (s, 1H), 3.83 (s, 1H), 3.80~3.75 (m, 1H), 2.38 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 150.8, 149.7, 137.2, 136.5, 129.2, 129.1, 128.9, 124.4, 122.6, 121.9, 111.4, 110.9, 64.3, 56.2, 56.2, 46.8, 21.1; HRMS (ESI-ion trap) calcd for C₁₉H₁₉O₂ [M+H－H₂O]⁺ 279.1380, found 279.1372.

(1S*, 2R*)-2-Phenyl-1, 2-dihydrotriphenylen-1-ol (5a): A white olid (54.8 mg, 85% yield). m.p. 157~158 ℃. R_f＝0.23 on silica gel (ethyl acetate/petroleum ether, V:V＝1:10). The ee was determined to be 55% using HPLC analysis on a chiralcel AD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 46.597 (major) and 49.684 (minor) min. ¹H NMR (500 MHz, CDCl₃) δ: 8.87~8.62 (m, 2H), 8.50~8.36 (m, 1H), 8.30~8.18 (m, 1H), 7.78~7.69 (m, 2H), 7.66 (d, J＝9.8 Hz, 1H), 7.64~7.58 (m, 2H), 7.23~7.07 (m, 5H), 6.52~6.48 (m, 1H), 5.58 (d, J＝7.4 Hz, 1H), 4.16 (d, J＝5.9 Hz, 1H), 2.17 (d, J＝7.6 Hz, 1H); ¹³C NMR (126 MHz, CDCl₃) δ: 138.6, 130.8, 130.6, 130.1, 129.9, 128.7, 128.6, 127.8, 127.4, 127.3, 127.0, 126.97, 126.9, 126.5, 126.4, 123.8, 123.6, 123.1, 123.0, 122.3, 69.4, 48.8; ¹HRMS (ESI-ion trap) calcd for C₂₄H₂₉O [M+H]⁺ 323.1431, found 323.1430.

(1S*, 2R*)-2-Cyclopropyl-1, 2-dihydrotriphenylen-1-ol (5b): Colorless oil (51.5 mg, 90% yield). R_f＝0.18 on silica gel (ethyl acetate/petroleum ether, V:V＝1:10). The ee was determined to be 97% using HPLC analysis on a Chiralcel AD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 25.025 (major) and 26.224 (minor) min. ¹H NMR (500 MHz, CDCl₃) δ: 8.82~8.69 (m, 2H), 8.40 (d, J＝7.8 Hz, 1H), 8.32~8.23 (m, 1H), 7.78~7.58 (m, 4H), 7.43 (dd, J＝9.8, 3.2 Hz, 1H), 6.43~6.27 (m, 1H), 5.45 (d, J＝4.5 Hz, 1H), 3.69 (t, J＝6.8 Hz, 1H), 1.91~1.76 (m, 1H), 0.87~0.71 (m, 2H), 0.56~0.31 (m, 2H); ¹³C NMR (125 MHz, CDCl₃) δ: 132.1, 130.5, 130.4, 129.8, 129.6, 128.7, 127.2, 126.8, 126.7, 126.7, 126.3, 124.1, 123.7, 123.0, 122.9, 122.7, 66.5, 46.8, 22.6, 11.1, 3.8; ¹HRMS (ESI-ion trap) calcd for C₂₁H₁₇ [M+H]⁺ 269.1325, found 269.1320.

(1S*, 2R*)-2-(4-Fluorophenyl)-1, 2-dihydrotriphenylen-1-ol (5c): Colorless oil (57.2 mg, 84% yield). R_f＝0.15 on silica gel (ethyl acetate/petroleum ether, V:V＝1:10). The ee was determined to be 68% using HPLC analysis on a Chiralcel AD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 37.883 (major) and 67.269 (minor) min. ¹H NMR (500 MHz, CDCl₃) δ: 8.98~8.63 (m, 2H), 8.40~8.29 (m, 1H), 8.29~8.20 (m, 1H), 7.78~7.62 (m, 4H), 7.59 (dd, J＝9.9, 3.3 Hz, 1H), 7.50 (dd, J＝8.5, 5.4 Hz, 2H), 7.16 (t, J＝8.7 Hz, 2H), 6.44~6.41 (m, 1H), 5.41 (d, J＝4.4 Hz, 1H), 4.08~3.95 (m, 1H); ¹³C NMR (125 MHz, CDCl₃) δ: 163.1, 161.1, 135.9, 130.7, 130.7, 129.7, 128.6, 128.58, 127.3, 127.0, 126.5, 126.4, 124.1, 123.7, 123.1, 123.09, 115.6, 115.5, 67.6, 47.3; ¹⁹F NMR (375 MHz, CDCl₃) δ: －115.57; ¹H RMS (ESI-ion trap) calcd for C₂₄H₁₈OF [M+H]⁺ 341.1336, found 341.1331.

(1S*, 2R*)-2-(4-methylphenyl)-1, 2-dihydrotriphenylen-1-ol (5d). Colorless oil (61.2 mg, 91% yield). R_f＝0.20 on silica gel (ethyl acetate/petroleum ether, V:V＝1:10). The ee was determined to be 65% using HPLC analysis on a Chiralcel AD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 38.762 (major) and 45.206 (minor) min. ¹H NMR (500 MHz, CDCl₃) δ: 8.77~8.70 (m, 2H), 8.54~8.36 (m, 1H), 8.33~8.15 (m, 1H), 7.77~7.70 (m, 2H), 7.66 (d, J＝9.8 Hz, 1H), 7.63~7.58 (m, 2H), 7.01 (dd, J＝13.5, 5.7 Hz, 2H), 6.97~6.90 (m, 2H), 6.65~6.33 (m, 1H), 5.58 (s, 1H), 4.13 (d, J＝4.6 Hz, 1H), 2.20 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ: 138.7, 138.3, 130.8, 130.6, 130.1, 129.9, 128.7, 128.5, 127.8, 127.4, 127.3, 127.0, 126.9, 126.4, 126.4, 124.6, 123.9, 123.6, 123.1, 123.0, 122.2, 69.5, 48.8, 21.4; ¹HRMS (ESI-ion trap) calcd for C₂₅H₂₀ONa [M+Na]⁺ 359.1406, found 359.1401.

(1S*, 2R*)-2-Thienyl-1, 2-dihydrotriphenylen-1-ol (5e): Colorless oil (61.0 mg, 93% yield). R_f＝0.20 on silica gel (ethyl acetate/petroleum ether, V:V＝1:10). The ee was determined to be 87% using HPLC analysis on a Chiralcel AD-H column (hexane/2-propanol, V:V＝90:10, 1.0 mL/min, λ＝254 nm). Retention times were 55.292 (major) and 58.044 (minor) min. ¹H NMR (500 MHz, CDCl₃) δ: 9.01~8.60 (m, 2H), 8.48~8.23 (m, 2H), 7.93~7.64 (m, 4H), 7.59 (dd, J＝9.8, 3.3 Hz, 1H), 7.46~7.33 (m, 1H), 7.27 (d, J＝3.4 Hz, 1H), 7.15 (dd, J＝5.2, 3.4 Hz, 1H), 6.52~6.34 (m, 1H), 5.53 (d, J＝4.2 Hz, 1H), 4.47~4.23 (m, 1H); ¹³C NMR (125 MHz, CDCl₃): δ 142.7, 130.7, 130.66, 130.6, 129.7, 128.9, 128.6, 127.4, 127.1, 127.0, 126.0, 126.4, 126.1, 124.8, 124.1, 124.0, 123.9, 123.1, 123.1, 67.7, 43.5; ¹HRMS (ESI-ion trap) calcd for C₂₂H₁₅S [M－H₂O+H]⁺ 311.0889, found 311.0885.

Supporting Information Copies of ¹³C NMR and ¹H NMR and ¹⁹F NMR spectra for all products, HRMS and HPLC data of all products, crystal structure and data of the product 3d. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn.

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Figure 1 X-ray structure of the product 3d

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Scheme 1 Proposed mechanism

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Table 1. Optimization of reaction conditions for the ARO reaction^a


Entry	Solvent	Pd(OAc)₂/ mol%	Ligand	Time/h	Yield/%	ee^b/%
1	Toluene	5.0	L1	24	25	－
2	Toluene	5.0	L2	7	49	35
3	Toluene	5.0	L3	7	60	21
4	Toluene	5.0	L4	24	NR^c	－
5	Toluene	5.0	L5	0.5	90	55
6	Toluene	5.0	L6	24	NR^c	－
7	Toluene	2.5	L5	1	80	55
8	Toluene	1.0	L5	2	71	55
9	Toluene	10.0	L5	0.5	92	54
10^d	Toluene	5.0	L5	0.5	91	55
11^d	MeCN	5.0	L5	24	NR^c	－
12^d	THF	5.0	L5	24	20	30
13^d	MeOH	5.0	L5	22	30	76
14^d	CH₂Cl₂	5.0	L5	3	85	45
15^d	CHCl₃	5.0	L5	3	80	50
16^d	DCE	5.0	L5	1	81	52
17^d^{, e}	Toluene	5.0	L5	5	72	58
18^d^{, f}	Toluene	5.0	L5	5	85	42
^a Reaction conditions: 1a (0.2 mmol), 2a (1.2 equiv.), Pd(OAc)₂ and ligand (10 mol%) in solvent and H₂O (V:V＝5:1, 3 mL in total) under nitrogen at room temperature. ^b Determined by HPLC with a chiralcel OD-H column. ^c NR: no reaction. ^d Carried out in air. ^e The reaction at 0 ℃. ^f The reaction at 50 ℃.

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Table 2. Ring-opening of oxabenzonorbornadiene 1a with various potassium trifluoroborate salts^a


Entry	R	Product	Time/h	Yield/%	ee^b/%
1	C₆H₅	3a	0.5	91	55
2	4-CH₃C₆H₄	3b	0.5	94	62
3	2-CH₃C₆H₄	3c	0.5	92	20
4	4-^tBuC₆H₄	3d	0.5	95	36
5	2-CH₃OC₆H₄	3e	0.5	90	2
6	3-CH₃OC₆H₄	3f	0.5	91	56
7	4-CH₃OC₆H₄	3g	0.5	98	51
8	4-BrC₆H₄	3h	0.5	84	64
9	4-FC₆H₄	3i	0.5	84	60
10	CH₃	3j	3	81	56
11	2-Thienyl	3k	0.5	95	71
^a Reaction conditions: 1a (0.2 mmol), 2 (1.2 equiv.), Pd(OAc)₂ (5 mol%), and (R)-BINAP (10 mol%) in toluene and H₂O (V:V＝5:1, 3 mL in total) in air at room temperature. ^b Determined by HPLC with a chiralcel OD-H column.

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Table 3. Ring-opening of oxabenzonorbornadiene 1b and 1c with various potassium trifluoroborate salts^a


Entry	Substrate	R	Product	Yield/%	ee^b/%
1	1b	Cyclopropyl	4a	90	97
2	1b	4-FC₆H₄	4b	84	52
3	1b	4-CH₃C₆H₄	4c	91	51
4	1c	C₆H₅	5a	85	55
5	1c	Cyclopropyl	5b	90	97
6	1c	4-FC₆H₄	5c	84	68
7	1c	4-CH₃C₆H₄	5d	91	65
8	1c	2-Thienyl	5e	93	87
^a Reaction conditions: 1b, 1c (0.2 mmol), 2 (1.2 equiv.), Pd(OAc)₂ (5 mol%), and (R)-BINAP (10 mol%) in toluene and H₂O (V:V＝5:1, 3 mL in total) in air at room temperature. ^b Determined by HPLC with a chiralcel OD-H column.

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