Chiral Spiro Dienes Derived Boranes for Asymmetric Hydrosilylation of Ketones

Citation: Wang Qiaotian, Han Caifang, Feng Xiangqing, Du Haifeng. Chiral Spiro Dienes Derived Boranes for Asymmetric Hydrosilylation of Ketones[J]. Chinese Journal of Organic Chemistry, 2019, 39(8): 2257-2263. doi: 10.6023/cjoc201903076 shu

手性螺环骨架硼烷催化酮的不对称硅氢化反应

王桥天 ^a,b, ,
韩彩芳 ^a,b, ,
冯向青 ^{a,b,
,} ,
杜海峰 ^{a,b,
,}

a.
中国科学院化学研究所北京国家分子科学研究中心中国科学院分子识别与功能重点实验室北京 100190
b.
中国科学院大学北京 100049

通讯作者: 冯向青, fxq@iccas.ac.cn
杜海峰, haifengdu@iccas.ac.cn

基金项目:
国家自然科学基金 21825108

国家自然科学基金（No.21825108）资助项目

摘要: 受阻路易斯酸碱对（frustrated Lewis pairs，FLPs）是目前合成化学的前沿挑战性研究领域之一，为非金属催化的氢化和Piers-type硅氢化反应提供了非常有效的途径.近年来，相关研究取得了重要的研究进展，但是相应的不对称反应发展比较缓慢.缺乏高效、高选择性的手性催化剂仍然是制约这一领域快速发展的重要因素.手性螺环是配体设计中的优势骨架.基于前期所发展的联萘骨架手性FLP催化剂及其在不对称催化氢化和硅氢化反应的应用，设计并合成了基于手性螺环骨架的手性双烯，通过与HB（C₆F₅）₂的硼氢化反应原位制备了新型手性硼烷路易斯酸.利用其与三叔丁基膦形成的手性受阻路易斯酸碱对催化剂，成功地实现了简单酮的不对称Piers-type硅氢化反应，反应的对映选择性最高可达90%.

关键词:

English

Chiral Spiro Dienes Derived Boranes for Asymmetric Hydrosilylation of Ketones

a.
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190
b.
University of Chinese Academy of Sciences, Beijing 100049

Corresponding author: Feng Xiangqing, fxq@iccas.ac.cn Du Haifeng, haifengdu@iccas.ac.cn

Received Date: 30 March 2019
Revised Date: 24 April 2019
Available Online: 25 August 2019
Fund Project:

Project supported by the National Natural Science Foundation of China (No. 21825108)

Abstract: The chemistry of frustrated Lewis pairs (FLPs) is among the challenging frontiers of synthetic chemistry, which provides a powerful approach for metal-free catalytic hyrogenations and Piers-type hydrosilylations. In recent years, a significant progress has been made in this field. However, the deveopment of asymmetric reactions is still sluggish. The lacks of highly effective and enantioselective chiral FLP catalysts represent the key issue. C₂-symmetric 1, 1'-spirobiindane is one privileged framework in chiral ligands and catalysts. On the basis of chiral binaphthyl diene-derived frustrated Lewis pairs (FLPs) developed by our group, in this work, we designed and synthesized a novel class of chiral spiro dienes, which could further react with Piers' borane via the hydroboration reaction to generate chiral boranes in situ. With the combination of chiral borane and tri-tert-butylphosphine as an FLP catalyst, an asymmetric Piers-type hydrosilylation of simple ketones was successfully realized to give the desired secondary alcohols with up to 90% ee.

Key words:

1. Introduction

In 2006, Stephan and co-workers^[1] reported their seminal work on the reversible activation of H₂ with sterically hindered Lewis acid and base pairs, which rapidly opened up a famous field as the chemistry of frustrated Lewis pairs (FLPs). In the past decade, the FLP-catalyzed metal-free hydrogenation and Piers-type hydrosilylation have achieved a great success.^[2] However, in contrast, the development of asymmetric reactions with chiral FLPs is still sluggish.^[3] Only a very few FLP catalysts can give satisfactory enantioselectivities.^{[4, 5]} The difficulty to access highly efficient chiral FLPs with suitable backbones and diverse structures seems to be the major obstacle in this field. At present, two protocols have been developed for the synthesis of chiral boron Lewis acids. One is hydroboration of chiral alkenes with Piers' borane HB(C₆F₅)₂, ^[6] and the other is substitution of boron chlorides with chiral organometallic reagents.^[4] As shown in Figure 1, the chiral frameworks of reported FLPs usually include bicycle, ^{[5b, 5c, 5e]} C₂-symmetric binaphthyl, ^[4b~4f] and ferrocene.^[5d] Recently, we have developed a novel strategy for the in situ synthesis of chiral boranes by the hydroboration of binaphthyl-based chiral dienes with HB(C₆F₅)₂.^[7] Very recently, Wang et al.^[8] reported chiral C₂-symmetric bicyclic [3.3.0] and spiro-bicyclic diboranes. Generally, the chiral frameworks are very likely to influence the reactivity and enantioselectivity largely. Further developing chiral FLPs bearing novel backbones is therefore of great importance.

Figure 1

Figure 1. Representative chiral FLP catalysts

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Spiro chirality has attracted considerable attention for the design of chiral ligands and catalysts, and numerous excellent chiral ligands and catalysts have already been successfully developed.^[9] It is noteworthy that C₂-symmetric 1, 1'-spirobiindane 1 has proven to be one of privileged frameworks.^[10] On the basis of our previous work, we wish to further devote our efforts on the development of novel chiral dienes 2 bearing 1, 1'-spirobiindane frameworks (Scheme 1). The corresponding chiral boranes 3 can be easily accessed via the in situ hydroboration with HB(C₆F₅)₂, which can be utilized in the FLP-catalyzed asymmetric reactions.

Scheme 1

Scheme 1. The development of chiral spiro boranes

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Piers-type hydrosilylation has received intensive attention due to its synthetic and mechanistic interests.^[11] Despite some important advances, the asymmetric Piers-type hydrosilylation is far less developed in the past two decades.^[2i] Especially for ketone substrates, very few examples can afford high enantioselectivities.^[12] In 2016, our group reported an asymmetric hydrosilylation of 1, 2-dicarbonyl compounds using chiral diyne-derived alkenylborane and tricyclohexylphosphine to give up to 99% ee.^[7d] Oestreich and co-workers^[4d] employed the binaphthyl-based chiral borane to realize the asymmetric hydrosilylation of simple ketones with up to 99% ee. Herein, we report our preliminary results on the synthesis of chiral spiro boranes and their application in the asymmetric Piers-type hydrosilylation of simple ketones.

2. Results and discussion

The synthesis of chiral spiro dienes 2 with chiral dicarboxylic acid 4^[13] as starting material was shown in Scheme 2. A Pd-catalyzed directed ortho-C—H iodination of compound 4 gave diiodide 5 in 42% yield.^[14] Further treating diiodide 5 with thionyl dichloride in methanol furnished the corresponding ester 6 in 83% yield. Various aryl substituents can be easily incorporated by Suzuki coupling reactions to afford compounds 7 in moderate to good yields. A sequence DIBAL-H reduction and PCC oxidation gave dialdehydes 8 in reasonable yields. Followed by a Wittig-reaction, chiral dienes 2a~2e were obtained in 81%~85% yields. A single crystal of chiral diene 2e was obtained, and its structure was confirmed by X-ray crystallographic analysis (Figure 2).^[15]

Scheme 2

Scheme 2. Synthesis of the chiral spiro dienes 2

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Figure 2

Figure 2. X-ray structure of (S)-2e

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With novel chiral dienes 2 in hand, the asymmetric Piers-type hydrosilylation of acetophenone (9a) with PhMe₂SiH was next investigated. As shown in Table 1, using chiral boranes generated in situ from dienes 2a~2e (5 mol%) and HB(C₆F₅)₂ (10 mol%) in the presence of ^tBu₃P (10 mol%) as a Lewis base, all the reactions proceeded smoothly to give alcohol 10a in high conversions with low to moderate ee's (Entries 1~5). Chiral diene 2b containing 3, 5-di-tert-butylphenyl substituents gave the optimal enantioselectivity (Table 1, Entry 2). Lewis base components were found to have a large impact on the enantioselectivities (Table 1, Entries 2 vs 6~8). Without Lewis base, a much lower ee was obtained (Table 1, Entries 3 vs 9). Various silanes were subsequently studied, and Ph₂SiH₂ gave a promising 80% ee (Table 1, Entries 2, 10~13). Reducing the catalyst loading from 10 mol% to 5 mol% only resulted in a slight loss of ee (Table 1, Entries 11 vs 14). Several solvents were also examined, and toluene proved to be a more suitable solvent (Table 1, Entries 14~17).

Table 1

Table 1. Optimization of reaction conditions for the asymmetric hydrosilylation^a

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Entry	Chiral diene 2	Phosphine	Silane	Solvent	Conv./%	ee/%
1	2a	^tBu₃P	PhMe₂SiH	Toluene	95	40
2	2b	^tBu₃P	PhMe₂SiH	Toluene	> 99	56
3	2c	^tBu₃P	PhMe₂SiH	Toluene	94	34
4	2d	^tBu₃P	PhMe₂SiH	Toluene	94	43
5	2e	^tBu₃P	PhMe₂SiH	Toluene	93	35
6	2b	Cy₃P	PhMe₂SiH	Toluene	71	0
7	2b	Mes₃P	PhMe₂SiH	Toluene	73	17
8	2b	(C₆F₅)Ph₂P	PhMe₂SiH	Toluene	96	0
9	2c	—	PhMe₂SiH	Toluene	91	23
10	2b	^tBu₃P	Et₃SiH	Toluene	59	47
11	2b	^tBu₃P	Ph₂SiH₂	Toluene	> 99	80
12	2b	^tBu₃P	PhSiH₃	Toluene	> 99	44
13	2b	^tBu₃P	Ph₃SiH	Toluene	95	7
14^e	2b	^tBu₃P	Ph₂SiH₂	Toluene	> 99	79
15^e	2b	^tBu₃P	Ph₂SiH₂	Hexane	> 99	42
16^e	2b	^tBu₃P	Ph₂SiH₂	THF	> 99	0
17^e	2b	^tBu₃P	Ph₂SiH₂	DCM	88	46
^a All reactions were carried out with acetophenone (0.1 mmol), HB(C₆F₅)₂ (10 mol%), chiral spiro diene 2 (5 mol%), ^tBu₃P (10 mol%), and silane (1.5 equiv.) in solvent (1.0 mL) at 60 ℃ for 12 h, followed by removal the silyl group with TBAF at room temperature for 2 h. ^b Determined by crude ¹H NMR. ^c Determined by chiral HPLC. ^e HB(C₆F₅)₂ (5 mol%), 2b (2.5 mol%) and ^tBu₃P (5 mol%) was used.

Various ketones 9a were subjected to the asymmetric hydrosilylation using chiral diene 2b (5 mol%), HB(C₆F₅)₂ (10 mol%), and ^tBu₃P (10 mol %), with Ph₂SiH₂ (1.5 equiv.) in toluene for 12 h followed by the deprotection of silyl group with tetrabutylammonium fluoride (TBAF). As shown in Scheme 3, all these reactions went well to furnish the desired secondary alcohols in 67%~95% yields with 75%~90% ee's. 1-(Benzofuran-2-yl)ethan-1-one (9g) and 1-(naphthalen-1-yl)ethan-1-one (9h) were also effective substrates to afford the corresponding alcohols with 84% and 81% ee, respectively.

Scheme 3

Scheme 3. Chiral FLP-catalyzed asymmetric hydrosilylation of ketones

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3. Conclusions

A variety of chiral dienes bearing C₂-symmetric 1, 1'- spirobiindane framework were prepared in reasonable yields for the first time via a five-step synthesis with chiral dicarboxylic acid as starting material. The corresponding chiral spiro boranes were easily accessed by the in situ hydroboration with HB(C₆F₅)₂. Further application of these chiral boranes with ^tBu₃P as chiral FLP catalysts for the asymmetric Piers-type hydrosilylation of simple ketones to furnish the desired secondary alcohols in 67%~95% yields with 75%~90% ee's. The unique spiro skeleton for this type of chiral boranes makes them potentially useful in other asymmetric reactions, and further efforts on this subject are still underway in our laboratory.

4. Experimental section

4.1 General

All air-sensitive compounds were handled under an atmosphere of argon or in a nitrogen-filled glovebox. All reagents and solvents were analytical grade materials purchased from commercial sources and used as received unless otherwise stated. Reactions were monitored by TLC (Qingdao Haiyang Chemical Co. Ltd. Silica gel 60 F₂₅₄) and detected using an UV/Vis lamp (254 nm). Column chromatography was performed on a Qingdao Haiyang Chemical Co. Ltd. Silica gel 60 (200~300 mesh).

¹H NMR and ¹³C NMR spectra were recorded on a Bruker AV 400 at ambient temperature with CDCl₃ and C₆D₆ as solvent and TMS as internal standard. Chemical shifts (δ) were referenced to the residual proton resonance of TMS (0) or to the carbon resonance of the CDCl₃ (77.23), C₆D₆ (126.85). All solvents were purified by conventional methods, distilled before use. Commercially available reagents were used without further purification.

4.2 Typical procedure for the synthesis of chiral diene 2b

4.2.1 Synthesis of compound 5

Compound (S)-4 (0.2000 g, 0.66 mmol), Pd(OAc)₂ (0.0323 g, 0.132 mmol), iodobenzene diacetate (0.5315 g, 1.65 mmol) and I₂ (0.4191 g, 1.65 mmol) were dissolved in N, N-dimethylformamide (DMF, 6.6 mL) under argon. The tube was sealed with a cap and the reaction mixture was stirred at 100 ℃ for 36 h. The reaction mixture was cooled to room temperature. After the solvent was evaporated to vacuum, saturated Na₂SO₃ (30 mL) was added. The mixture was acidified with 2 mol/L HCl (10 mL), extracted with dichloromethane (DCM, 10 mL×3) and dried over Na₂SO₄. The solvent was removed in a rotary evaporator and the residue (S)-6, 6'-diiodo-2, 2', 3, 3'-tetrahydro-1, 1'- spirobi[indene]-7, 7'-dicarboxylic acid (5) (0.2060 g, 42% yield) was used directly without other purification.

4.2.2 Synthesis of compound 6

To a 50 mL three-neck flash, 5 (0.0900 g, 0.16 mmol) was dissolved in SOCl₂ (2.1 mL) under argon. The reaction mixture was refluxed at 90 ℃ for 8 h, and then solvent was evaporated directly. MeOH (20 mL) was added, and it was refluxed at 90 ℃ for 12 h. The reaction mixture was cooled to room temperature. The solvent was filtered, residue was collected to afford target product 6 as a white solid (0.0764 g, 82% yield). m.p. 151~154 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.68 (d, J＝8.0 Hz, 2H), 7.01 (d, J＝8.0 Hz, 2H), 3.25 (s, 6H), 3.04~2.89 (m, 4H), 2.67~2.56 (m, 2H), 2.27~2.16 (m, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 167.8, 147.3, 145.3, 138.1, 136.4, 127.2, 90.4, 62.7, 51.4, 39.8, 30.3; HRMS (APCI) calcd for C₂₁H₁₉O₄I₂[M+H]⁺ 588.9367, found 588.9360.

4.2.3 Synthesis of compound 7

To a 50 mL three-neck flash, 6 (0.5880 g, 1.0 mmol), (3, 5-di-tert-butylphenyl)boronic acid (0.7020 g, 3.0 mmol), Pd(OAc)₂ (0.0112 g, 0.05 mmol), PPh₃ (0.0393 g, 0.15 mmol), K₂CO₃ (0.4140 g, 3.0 mmol) were dissolved in DMF (25 mL) under argon. The reaction mixture was stirred at 90 ℃ for 12 h. The reaction mixture was cooled to room temperature, the mixture was poured into saturated NH₄Cl (aq.) and extracted with Et₂O (20 mL×2). The organic layers were combined, dried over Na₂SO₄, filtered. After removal of solvents, the residue was purified by flash column chromatography on silica gel to afford dimethyl (S)-6, 6'-bis(3, 5-di-tert-butylphenyl)-2, 2', 3, 3'-tetrahydro-1, 1'-spirobi[indene]-7, 7'-dicarboxylate(7b) (0.6070 g, 82% yield) as a white solid. m.p. 227~229 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.35 (s, 1H), 7.33 (s, 1H), 7.32~7.30 (m, 2H), 7.25 (s, 1H), 7.23 (s, 1H), 7.11 (d, J＝2.5 Hz, 4H), 3.19~3.03 (m, 4H), 2.88 (s, 6H), 2.87~2.78 (m, 2H), 2.43~2.35 (m, 2H), 1.29 (s, 36H); ¹³C NMR (125 MHz, CDCl₃) δ: 169.1, 150.3, 146.3, 144.0, 140.5, 140.4, 129.6, 128.6, 125.2, 122.7, 120.6, 62.2, 50.7, 39.9, 34.8, 31.5, 30.4; HRMS (APCI) calcd for C₄₇H₅₃O₂[M+H]⁺ 649.4040, found 649.4030.

4.2.4 Synthesis of compound 8

To a 50 mL three-neck flash, 7b (0.4000 g, 0.56 mmol) was dissolved in toluene (10 mL) under argon. The reaction mixture was cooled down to －78 ℃, DIBAL-H (1.5 mol/L, 1.5 mL, 1.7 mmol) was dropwise added and then stirred at －78 ℃ for 3 h. After that mixture was warmed up to room temperature and further stirred overnight. The mixture was poured into saturated NH₄Cl (aq.) and extracted with Et₂O (20 mL×2). The organic layers were combined, dried over Na₂SO₄, and filtered. Solvent was removed under reduced pressure to afford a mixture of dialcohols (0.3930 g). The mixture was dissolved in DCM (25 mL) in a 50 mL three-neck flash under argon, then PCC (1.3000 g, 6 mmol), Celite (0.3000 g) were added subsequently. The reaction mixture was stirred at room temperature for 4 h. The resulting suspension solution was filtered through a short pad of Celite, and the solid was washed with CH₂Cl₂ (5.0 mL×2). After removal of the solvents, the residue was purified by flash chromatography on silica gel to afford (S)-6, 6'-bis(3, 5-di-tert-butylphenyl)-2, 2', 3, 3'-tetrahydro-1, 1'-spirobi[indene]-7, 7'-dicarbaldehyde (8b) (0.2450 g, 67% yield) as a white solid. m.p. 121~125 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 9.50 (s, 2H), 7.53 (d, J＝8.0 Hz, 2H), 7.39~7.36 (m, 2H), 7.31 (d, J＝7.6 Hz, 2H), 7.08 (d, J＝1.6 Hz, 4H), 3.31~3.09 (m, 4H), 2.79~2.66 (m, 2H), 2.50~2.40 (m, 2H), 1.29 (s, 36H); ¹³C NMR (125 MHz, CDCl₃) δ: 193.7, 150.5, 149.7, 145.3, 144.8, 138.2, 129.7, 129.4, 128.3, 124.7, 121.4, 63.3, 39.5, 34.8, 31.4, 30.7; HRMS (APCI) calcd for C₄₇H₅₇O₂[M+H]⁺ 653.4353, found 653.4338.

4.2.5 Synthesis of compound 2

To a 10 mL Schlenk tube, PPh₃MeI (1.1000 g, 2.7 mmol) and THF (5.0 mL) was cooled to 0 ℃, then ^tBuOK (0.3020 g, 2.7 mmol) was added. After that, the solution of 8b (0.1740 g, 0.27 mmol) in THF (1.0 mL) was added subsequently. The reaction mixture was stirred for 20 min and quenched with water, extracted with Et₂O (5 mL×2). The organic phase was dried over Na₂SO₄, filtered. After removal of the solvent, the residue was purified by flash column chromatography [V(hexanes):V(DCM)＝100:1] to give chiral diene 2b as a white solid (0.1410 g, 81% yield). Chiral dienes 2a, 2c~2e were synthesized by this method.

(S)-6, 6'-Diphenyl-7, 7'-divinyl-2, 2', 3, 3'-tetrahydro-1, 1'-spirobi[indene] (2a): White solid, m.p. 100~103 ℃; [α]_D²⁷ －457.2 (c 0.5, CH₂Cl₂); ¹H NMR (500 MHz, C₆D₆)δ: 7.33 (d, J＝7.5 Hz, 4H), 7.20~7.12 (m, 6H), 7.09~7.01 (m, 4H), 6.52 (dd, J＝17.5, 11.5 Hz, 2H), 4.79 (dd, J＝19.0, 11.5 Hz, 4H), 2.93~2.72 (m, 4H), 2.33 (q, J＝10.5 Hz, 2H), 2.12~2.03 (m, 2H); ¹³C NMR (125 MHz, C₆D₆) δ: 146.9, 143.1, 142.4, 140.2, 133.9, 133.3, 130.3, 130.1, 127.9, 126.1, 123.5, 120.6, 62.4, 37.8, 30.3; IR (film) ν: 3360, 2929, 2848, 1460, 919, 713 cm^－1; HRMS (APCI) calcd for C₃₃H₃₁ [M+H]⁺ 425.2264, found 425.2260.

(S)-6, 6'-Bis(3, 5-di-tert-butylphenyl)-7, 7'-divinyl-2, 2', 3, 3'-tetrahydro-1, 1'-spirobi[indene](2b): White solid, m.p. 121~123 ℃; [α]_D²⁷－278.7 (c 0.53, CH₂Cl₂); ¹H NMR (300 MHz, CDCl₃) δ: 7.29~7.24 (t, J＝1.6 Hz, 2H), 7.23 (d, J＝2.4 Hz, 1H), 7.20 (d, J＝2.4 Hz, 3H), 7.11 (d, J＝1.8 Hz, 4H), 6.20 (dd, J＝17.7, 11.4 Hz, 2H), 4.76 (dd, J＝11.4, 1.8 Hz, 2H), 4.49(dd, J＝17.7, 1.8 Hz, 2H), 3.14~2.88 (m, 4H), 2.37~2.23 (m, 4H), 1.29 (s, 36H); ¹³C NMR (125 MHz, C₆D₆)δ: 150.0, 147.0, 142.3, 142.2, 141.3, 134.1, 133.6, 130.3, 124.9, 123.4, 120.2, 119.6, 62.5, 37.7, 34.6, 31.4, 30.3; IR (film): 3055, 2961, 2865, 1595, 1249, 917, 746 cm^－1; HRMS (APCI) calcd for C₄₉H₆₁[M+H]⁺649.4768, found 649.4757.

(S)-6, 6'-Bis(3, 5-bis(trifluoromethyl)phenyl)-7, 7'-divinyl-2, 2', 3, 3'-tetrahydro-1, 1'-spirobi[indene] (2c): White solid, m.p. 127~129 ℃; [α]_D²⁷－283.8 (c 0.39, CH₂Cl₂); ¹H NMR (500 MHz, C₆D₆) δ: 7.66 (s, 2H), 7.58 (s, 4H), 6.99 (d, J＝8.0 Hz, 2H), 6.78 (d, J＝7.5 Hz, 2H), 6.19 (dd, J＝17.5, 11.0 Hz, 2H), 4.58 (d, J＝10.0 Hz, 2H), 4.36 (d, J＝16.5, Hz, 2H), 2.85~2.66 (m, 4H), 2.19~2.08 (m, 2H), 2.05~1.97 (m, 2H); ¹³C NMR (125 MHz, C₆D₆)δ: 146.8, 144.7, 143.8, 136.7, 133.8, 132.6, 131.2 (q, J＝32.9 Hz), 130.3, 130.15, 124.0, 123.6 (d, J＝271.0 Hz), 121.9, 119.9, 62.0, 37.5, 30.3; ¹⁹F NMR (376 MHz, CDCl₃) δ: －62.62; IR (film) ν: 2920, 2848, 1644, 1379, 1278, 1133, 740 cm^－1; HRMS (APCI) calcd for C₃₇H₂₃F₂ (M－H) 695.1614; Found: 695.1620.

(S)-6, 6'-Bis(4-isopropoxyphenyl)-7, 7'-divinyl-2, 2', 3, 3'-tetrahydro-1, 1'-spirobi[indene](2d): White solid, m.p. 123~125 ℃; [α]_D²⁷－390.1 (c 0.78, CH₂Cl₂); ¹H NMR (500 MHz, C₆D₆) δ: 7.31~7.27 (m, 4H), 7.25 (d, J＝8.0 Hz, 2H), 7.08 (d, J＝7.5 Hz, 2H), 6.87~6.80 (m, 4H), 6.63~6.54 (m, 2H), 4.88 (dq, J＝14.5, 1.5 Hz, 4H), 4.24~4.14 (m, 2H), 2.94~2.75 (m, 4H), 2.43~2.32 (m, 2H), 2.16~2.07 (m, 2H), 1.11 (d, J＝6.0 Hz, 12H); ¹³C NMR (125 MHz, C₆D₆) δ: 156.0, 146.1, 141.2, 139.1, 134.3, 133.1, 132.8, 130.3, 129.5, 122.7, 119.5, 114.4, 68.2, 61.6, 37.0, 29.5, 20.9(2), 20.9(0); IR (film) ν: 3361, 2975, 2934, 2848, 1512, 1240, 1119, 819 cm^－1; HRMS (APCI) calcd for C₃₉H₄₁O₂ [M+H]⁺ 541.3101, found 541.3097.

(S)-6, 6'-Di(naphthalen-2-yl)-7, 7'-divinyl-2, 2', 3, 3'-tetrahydro-1, 1'-spirobi[indene] (2e): White solid, m.p. 221~223 ℃; [α]_D²⁷－547.8 (c 0.54, CH₂Cl₂); ¹H NMR (400 MHz, CDCl₃) δ: 7.77~7.68 (m, 6H), 7.66 (d, J＝8.4 Hz, 2H), 7.41~7.33 (m, 4H), 7.28 (d, J＝8.4 Hz, 2H), 7.21~7.13 (m, 4H), 6.21 (dd, J＝17.6, 11.6 Hz, 2H), 4.74 (d, J＝12.0, 2H), 4.53 (d, J＝17.6 Hz, 2H), 3.08~2.86 (m, 4H), 2.33~2.18 (m, 4H); ¹³C NMR (100 MHz, CDCl₃) δ: 146.9, 142.8, 140.7, 139.6, 134.1, 133.5, 133.2, 132.0, 130.4, 129.2, 128.2, 127.9, 127.6, 126.9, 125.9, 125.6, 123.5, 121.2, 62.3, 37.9, 30.5; IR (film) ν: 3054, 2939, 1265, 742 cm^－1; HRMS (APCI) calcd for C₄₁H₃₃[M+H]⁺ 525.2577, found 525.2573.

4.3 Typical procedure for the metal-free catalytic asymmetric hydrosilylation of ketone

To a 15 mL sealed tube was added HB(C₆F₅)₂ (0.0107 g, 0.03 mmol), chiral diene 2b (0.0097 g, 0.015 mmol), and dry toluene (3.0 mL) in a nitrogen atmosphere glovebox. The resulting mixture was stirred for 5 min at room temperature followed by addition of Ph₂SiH₂ (0.0830 g, 0.45 mmol), P^tBu₃ (10 wt% in n-pentane, 0.0607 g, 0.03 mmol), and ketone 9a (0.0360 g, 0.3 mmol). The reaction mixture was stirred at 60 ℃ for 12 h. The reaction mixture was cooled to room temperature and TBAF (0.3 mmol, 0.3 mL, 1.0 mol/L in THF) was added and stirred at room temperature for 2 h. The resulting solution was added water (15 mL) and extracted with CH₂Cl₂ (10 mL×3). The resulting residue was purified by column chromatography on silica gel (petroleum ether/ethyl acetate, V/V＝50/1) to afford the desired (S)-10a as a colorless oil (0.0246 g, 67% yield, 80% ee). 10b~10h were obtained in a similar manner.

(S)-1-Phenylethan-1-ol (10a): Colorless oil, 0.0246 g, 67% yield, 80% ee. [α]_D²⁷－29.4 (c 0.76, CHCl₃)[lit.^[16a] [α]_D²⁴+42.92 (c 1.04, CHCl₃) (96% ee for R-isomer)]; ¹H NMR (500 MHz, CDCl₃)δ: 7.37~7.30 (m, 4H), 7.28~7.22 (m, 1H), 4.84 (q, J＝6.5 Hz, 1H), 2.22 (s, 1H), 1.46 (d, J＝6.5 Hz, 3H); ¹³C NMR (125 MHz, CDCl₃) δ: 145.8, 128.5, 127.4, 125.4, 70.3, 25.1. Enantiomeric excess was determined by HPLC with a Chiralcel OD-H column [V(hexane)/V(ⁱPrOH)＝95/5, 1.0 mL/min, 210 nm], minor enantiomer t_r＝8.71 min, major enantiomer t_r＝10.01 min.

(S)-1-(p-Tolyl)ethan-1-ol (10b): Colorless oil, 0.0370 g, 91% yield, 75% ee. [α]_D²⁷－39.2 (c 1.04, CH₂Cl₂)[lit.^[16b] [α]_D²⁶+55.2 (c 0.50, CH₂Cl₂) (99.7% ee for R-isomer)]; ¹H NMR (400 MHz, CDCl₃)δ: 7.22 (d, J＝8.0 Hz, 2H), 7.12 (d, J＝8.0 Hz, 2H), 4.79 (q, J＝6.4 Hz, 1H), 2.32 (s, 3H), 2.22 (br s, 1H), 1.43 (d, J＝6.4 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 143.0, 137.1, 129.2, 125.4, 70.2, 25.1, 21.1; Enantiomeric excess was determined by HPLC with a Chiralcel OJ-H column [V(hexane)/V(ⁱPrOH)＝95/5, 1.0 mL/min, 220 nm], major enantiomer t_r＝12.08 min, minor enantiomer t_r＝13.22 min.

(S)-1-(4-Fluorophenyl)ethan-1-ol (10c): Colorless oil, 0.0360 g, 87% yield, 90% ee. [α]_D²⁷－52.8 (c 1.20, CH₂Cl₂)[lit.^[16b] [α]_D²⁰+49.0 (c 0.20, CH₂Cl₂) (99.9% ee for R-isomer)]; ¹H NMR (400 MHz, CDCl₃)δ: 7.50~7.38 (m, 2H), 7.17~7.06 (m, 2H), 4.98 (q, J＝6.4 Hz, 1H), 1.87 (br s, 1H), 1.56 (d, J＝6.4 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 161.1(d, J＝243.7 Hz), 140.5, 126.0 (d, J＝8.1 Hz), 114.3 (d, J＝21.3 Hz), 68.8, 24.3; ¹⁹F NMR (376 MHz, CDCl₃) δ: －115.5; Enantiomeric excess was determined by HPLC with a Chiralcel OJ-H column [V(hexane)/ V(ⁱPrOH)＝70/30, 1.0 mL/min, 210 nm], minor enantiomer t_r＝23.04 min, major enantiomer t_r＝24.27 min.

(S)-1-(4-Chlorophenyl)ethan-1-ol (10d): Colorless oil, 0.0423 g, 90% yield, 90% ee. [α]_D²⁷－35.7 (c 1.17, CHCl₃)[lit.^[16c] [α]_D²⁶+38.7 (c 1.12, CHCl₃) (93% ee for R-isomer)]; ¹H NMR (400 MHz, CDCl₃)δ: 7.24~7.16 (m, 4H), 4.76 (d, J＝6.4 Hz, 1H), 2.18 (brs, 1H), 1.36 (d, J＝6.4 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 144.3, 133.0, 128.6, 126.8, 69.7, 25.2; Enantiomeric excess was determined by HPLC with a Chiralpak IC-H column [V(hexane)/V(ⁱPrOH)＝99/1, 1.0 mL/min, 210 nm], major enantiomer t_r＝20.60 min, minor enantiomer t_r＝22.87 min.

(S)-1-(4-Bromophenyl)ethan-1-ol (10e): Colorless oil, 0.0558 g, 93% yield, 87% ee. [α]_D²⁷－49.6 (c 1.6, CH₂Cl₂)[lit.^[16d] [α]_D²⁴+30.86 (c 0.915, CHCl₃) (95% ee for R-isomer)]; ¹H NMR (400 MHz, CDCl₃)δ: 7.45 (d, J＝8.4 Hz, 2H), 7.21 (d, J＝8.4 Hz, 2H), 4.82 (q, J＝6.4 Hz, 1H), 2.26 (brs, 1H), 1.44 (d, J＝6.4 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 144.8, 131.5, 127.2, 121.1, 69.7, 25.2; Enantiomeric excess was determined by HPLC with a Chiralcel OD-H column [V(hexane)/V(ⁱPrOH)＝98/2, 1.0 mL/min, 210 nm], major enantiomer t_r＝17.23 min, minor enantiomer t_r＝19.42 min.

(S)-1-([1, 1'-Biphenyl]-4-yl)ethan-1-ol (10f): White solid, m.p. 93~95 ℃; 0.0567 g, 95% yield, 86% ee. [α]_D²⁷－39.3 (c 1.01, CHCl₃)[lit.^[16e] [α]_D²⁰－30.0 (c 0.94, CHCl₃) (70% ee for S-isomer)]; ¹H NMR (400 MHz, CDCl₃)δ: 7.58~7.50 (m, 4H), 7.44~7.35 (m, 4H), 7.35~7.28 (m, 1H), 4.92~4.81 (m, 1H), 2.40 (s, 1H), 1.48 (d, J＝6.4 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 145.0, 140.9, 140.4, 128.9, 127.3, 127.3, 127.2, 126.0, 70.1, 25.2; Enantiomeric excess was determined by HPLC with a Chiralpak AD-H column [V(hexane)/V(ⁱPrOH)＝95/5, 0.8 mL/min, 220 nm], major enantiomer t_r＝35.88 min, minor enantiomer t_r＝41.45 min.

(S)-1-(Benzofuran-2-yl)ethan-1-ol (10g): White solid, m.p. 39~42 ℃; 0.0461 g, 94% yield, 84% ee. [α]_D²⁷－12.4 (c 1.38, CHCl₃)[lit.^[16f] [α]_D²⁶+15.9 (c 0.96, CHCl₃) (93% ee for R-isomer)]; ¹H NMR (500 MHz, CDCl₃) δ: 7.49~7.44 (m, 1H), 7.42~7.38 (m, 1H), 7.24~7.11 (m, 2H), 6.50 (s, 1H), 4.98~4.87 (m, 1H), 3.08 (s, 1H), 1.55 (d, J＝6.5 Hz, 3H); ¹³C NMR (125 MHz, CDCl₃) δ: 160.4, 154.8, 128.3, 124.2, 122.8, 121.1, 111.3, 101.8, 64.1, 21.5; Enantiomeric excess was determined by HPLC with a Chiralpak AS-H column [V(hexane)/V(ⁱPrOH)＝98/2, 1.0 mL/min, 254 nm], major enantiomer t_r＝20.60 min, minor enantiomer t_r＝22.87 min.

(S)-1-(Naphthalen-2-yl)ethan-1-ol (10h): White solid, m.p. 71~73 ℃; 0.0490 g, 95% yield, 81% ee. [α]_D²⁷－42.4 (c 1.05, CH₂Cl₂)[lit.^[16b] [α]_D²⁰+37.0 (c 0.50, CH₂Cl₂) (99% ee for R-isomer)]; ¹H NMR (400 MHz, CDCl₃)δ: 7.84~7.75 (m, 4H), 7.73 (s, 1H), 7.49~7.38 (m, 3H), 4.98 (q, J＝6.4 Hz, 1H), 2.33 (br s, 1H), 1.52 (d, J＝6.4 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 143.3, 133.4, 133.0, 128.3, 128.0, 127.7, 126.2, 125.8, 123.9, 123.8, 70.5, 25.1; Enantiomeric excess was determined by HPLC with a Chiralcel OD-H column [V(hexane)/V(ⁱPrOH)＝98/2, 1.0 mL/min, 210 nm], major enantiomer t_r＝34.64 min, minor enantiomer t_r＝45.86 min.

Supporting Information ¹H NMR and ¹³C NMR spectra of compounds 2a~2e, 5, 6, 7b, 8b, 10a~10h and X-ray crystal structure and data of (S)-2e. Determination of enantiomeric excess of compounds 10a~10h. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn/.

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Figure 1 Representative chiral FLP catalysts

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Scheme 1 The development of chiral spiro boranes

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Scheme 2 Synthesis of the chiral spiro dienes 2

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Figure 2 X-ray structure of (S)-2e

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Scheme 3 Chiral FLP-catalyzed asymmetric hydrosilylation of ketones

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Table 1. Optimization of reaction conditions for the asymmetric hydrosilylation^a


Entry	Chiral diene 2	Phosphine	Silane	Solvent	Conv./%	ee/%
1	2a	^tBu₃P	PhMe₂SiH	Toluene	95	40
2	2b	^tBu₃P	PhMe₂SiH	Toluene	> 99	56
3	2c	^tBu₃P	PhMe₂SiH	Toluene	94	34
4	2d	^tBu₃P	PhMe₂SiH	Toluene	94	43
5	2e	^tBu₃P	PhMe₂SiH	Toluene	93	35
6	2b	Cy₃P	PhMe₂SiH	Toluene	71	0
7	2b	Mes₃P	PhMe₂SiH	Toluene	73	17
8	2b	(C₆F₅)Ph₂P	PhMe₂SiH	Toluene	96	0
9	2c	—	PhMe₂SiH	Toluene	91	23
10	2b	^tBu₃P	Et₃SiH	Toluene	59	47
11	2b	^tBu₃P	Ph₂SiH₂	Toluene	> 99	80
12	2b	^tBu₃P	PhSiH₃	Toluene	> 99	44
13	2b	^tBu₃P	Ph₃SiH	Toluene	95	7
14^e	2b	^tBu₃P	Ph₂SiH₂	Toluene	> 99	79
15^e	2b	^tBu₃P	Ph₂SiH₂	Hexane	> 99	42
16^e	2b	^tBu₃P	Ph₂SiH₂	THF	> 99	0
17^e	2b	^tBu₃P	Ph₂SiH₂	DCM	88	46
^a All reactions were carried out with acetophenone (0.1 mmol), HB(C₆F₅)₂ (10 mol%), chiral spiro diene 2 (5 mol%), ^tBu₃P (10 mol%), and silane (1.5 equiv.) in solvent (1.0 mL) at 60 ℃ for 12 h, followed by removal the silyl group with TBAF at room temperature for 2 h. ^b Determined by crude ¹H NMR. ^c Determined by chiral HPLC. ^e HB(C₆F₅)₂ (5 mol%), 2b (2.5 mol%) and ^tBu₃P (5 mol%) was used.

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