Synthesis of Cyclopropyl Pinacol Boronic Esters from Dibromo­cyclopropanes

Abstract

The synthesis of cyclopropyl pinacol boronic esters from dibromocyclopropanes via Matteson–Pasto rearrangement is reported. The method is readily scalable and shows limited levels of stereoinduction, with a selectivity that is in part complementary to that observed in existing stereoselective borylcyclopropanation strategies. The method can be used to rapidly access borylcyclopropanes as interesting building blocks for diversely functionalized cyclopropanes.

Key words

olefins - dibromocyclopropanes - borylcyclopropanes - pinacolborane - Matteson–Pasto rearrangement

Publication History

Received: 15 January 2022

Accepted after revision: 20 March 2022

Article published online:
19 April 2022

• References and Notes


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• 10 General Dibromocyclopropanation ProcedureSubstrate 1 (1 equiv) and benzyltriethylammonium chloride (0.1 equiv) were dissolved in a mixture of bromoform (7 equiv) and dichloromethane (c₁ = 0.75 M) and a solution of sodium hydroxide (25 M in H₂O, 37 equiv) was added dropwise at room temperature and under argon atmosphere. The mixture was stirred at room temperature for 3–4 h. The product was extracted with Et₂O (3×) while discarding the polymeric tar. The combined organic phase was washed with brine, dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The crude was purified over silica gel column chromatography to afford the desired dibromocyclopropane. All dibromocyclopropanes were stored under argon in the freezer.(3a′S,6′R,6a′R)-2,2-Dibromo-6′-methoxy-2′,2′-dimethyldihydro-6′H-spiro{cyclopropane-1,4′-furo[3,4-d][1,3]dioxole} (2)Compound 2a (13.65 g, 38.13 mmol, 71%, orange oil) and compound 2b (1.10 g, 3.07 mmol, 6%, dark orange oil) were synthesized from methyl 2′,3′-isopropylidene-4′-methylidene-d-erythro furanoside (1a, 10 g, 53.70 mmol, 1 equiv) according to the general procedure (3 h stirring at room temperature). The analysis of the ¹H NMR spectrum of the crude mixture indicates a ratio 2a/2b = 93:7. The crude was purified over silica gel column chromatography (elution with petroleum ether (PE) 100% then PE/EtOAc = 95:5 and 90:10) to afford the major isomer 2a followed by the fraction containing the minor isomer 2b. Compound 2a: IR (neat): ν _max = 2988, 2935, 1372, 1200, 1104, 1083, 975, 867, 703, 666 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 5.13 (s, 1 H), 5.05 (d, J = 5.7 Hz, 1 H), 4.71 (d, J = 5.7 Hz, 1 H), 3.56 (s, 3 H), 2.00 (d, J = 9.5 Hz, 1 H), 1.96 (d, J = 9.5 Hz, 1 H), 1.46 (s, 3 H), 1.37 (s, 3 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 113.4, 108.9, 85.5, 82.0, 73.6, 57.2, 33.4, 29.6, 26.7, 26.1 ppm. GC–MS, LC–MS, and HRMS were not conclusive probably due to poor or no ionization.Compound 2b: ¹H NMR (400 MHz, CDCl₃): δ = 5.08 (s, 1 H), 4.73 (d, J = 5.7 Hz, 1 H), 4.65 (d, J = 5.7 Hz, 1 H), 3.33 (s, 3 H), 1.98 (d, J = 8.7 Hz, 1 H), 1.83 (d, J = 8.7 Hz, 1 H), 1.58 (s, 3 H), 1.38 (s, 3 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 113.7, 109.0, 85.9, 84.9, 71.1, 55.4, 35.7, 26.6, 26.0, 22.9 ppm. GC–MS: [M]^+• = 355, 19.85 min. HRMS was not conclusive probably due to poor or no ionization.(±)-{[(2,2-Dibromo-1-methylcyclopropyl)methoxy]methyl}benzene (5)Compound 5 (288.5 mg, 0.86 mmol, 78%, orange oil) was synthesized from allyl ether 1b (178.5 mg, 1.10 mmol, 1 equiv) according to the general procedure (3 h stirring at room temperature). The crude was purified over silica gel column chromatography (elution with pentane/EtOAc = 99:1). IR (neat): ν _max = 2859, 1453, 1361, 1088, 1074, 1028, 966, 850, 735, 694 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.39–7.27 (m, 5 H), 4.61–4.54 (m, 2 H), 3.63 (d, J = 10.1 Hz, 1 H), 3.59 (d, J = 10.1 Hz, 1 H), 1.57 (d, J = 7.6 Hz, 1 H), 1.49 (s, 3 H), 1.46 (d, J = 7.5 Hz, 1 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 138.2, 128.5 (2 C), 127.9 (3 C), 76.1, 73.1, 36.0, 32.9, 29.7, 21.3 ppm. LC–MS: [M + NH₄]^+• = 352, 7.42 min. HRMS was not conclusive probably due to poor or no ionization.(±)-{(1R,3R)-3-[(Benzyloxy)methyl]-2,2-dibromocyclopropyl}benzene (6)Compound 6 (321 mg, 0.81 mmol, 74%, orange oil) was synthesized from allyl ether 1c (247 mg, 1.10 mmol, 1 equiv) according to the general procedure (4 h stirring at room temperature). The crude was purified over silica gel column chromatography (elution with pentane/EtOAc = 99:1). IR (neat): ν _max = 3029, 2858, 1496, 1452, 1360, 1085, 1028, 812, 732, 693 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.42–7.25 (m, 10 H), 4.67 (d, J = 12.0 Hz, 1 H), 4.63 (d, J = 12.0 Hz, 1 H), 3.82–3.80 (m, 2 H), 2.66 (d, J = 8.4 Hz, 1 H), 2.27 (ddd, J = 8.4, 7.0, 6.0 Hz, 1 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 138.0, 135.7, 128.9 (2 C), 128.6 (2 C), 128.5 (2 C), 128.0 (3 C), 127.9, 73.2, 71.5, 39.7, 34.8, 34.7 ppm. LC–MS: [M + NH₄]^+• = 414, 7.60 min. HRMS was not conclusive probably due to poor or no ionization.(±)-(2-{(1S,3R)-3-[(Benzyloxy)methyl]-2,2-dibromocyclopropyl}ethyl)benzene (7)Compound 7 (133 mg, 0.31 mmol, 28%, yellow oil) was synthesized from allyl ether 1d (277.6 mg, 1.10 mmol, 1 equiv) according to the general procedure (3.5 h stirring at room temperature). The crude was purified over silica gel column chromatography (elution with pentane/EtOAc = 99:1). IR (neat): ν _max = 3027, 2921, 2854, 1495, 1453, 1270, 1086, 1027, 734, 695 cm^–1. ¹H NMR (300 MHz, CDCl₃): δ = 7.36–7.20 (m, 10 H), 4.58 (d, J = 11.9 Hz, 1 H), 4.53 (d, J = 12.0 Hz, 1 H), 3.53 (d, J = 6.6 Hz, 2 H), 2.91–2.72 (m, 2 H), 1.96–1.80 (m, 2 H), 1.54–1.49 (m, 1 H), 1.30–1.23 (m, 1 H) ppm. ¹³C NMR (75 MHz, CDCl₃): δ = 141.0, 138.0, 128.6 (2 C), 128.5 (2 C), 128.4 (2 C), 127.8 (2 C), 127.7, 126.1, 72.9, 71.5, 35.9, 32.3, 34.33, 34.31, 34.2 ppm. LC–MS: [M + NH₄]^+• = 442, 7.77 min. HRMS was not conclusive probably due to poor or no ionization.(±)-(R)-{[(2,2-Dibromo-3,3-dimethylcyclopropyl)methoxy]methyl}benzene (8)Compound 8 (307 mg, 0.88 mmol, 80%, yellow oil) was synthesized from allyl ether 1e (194 mg, 1.10 mmol, 1 equiv) according to the general procedure (3 h stirring at room temperature). The crude was purified over silica gel column chromatography (elution with pentane/EtOAc = 99:1). The reaction was also performed on 1 g scale of 1e (5.68 mmol) to afford the desired product 8 (1.32 g, 3.79 mmol, 67%, yellow oil) and on 3 g scale of 1e (17 mmol) to afford the desired product 8 (4.26 g, 12.23 mmol, 72%, yellow oil). IR (neat): ν _max = 2864, 1453, 1374, 1204, 1093, 1074, 994, 815, 734, 696 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.37–7.28 (m, 5 H), 4.60 (d, J = 11.9 Hz, 1 H), 4.53 (d, J = 11.9 Hz, 1 H), 3.67 (dd, J = 10.8, 6.7 Hz, 1 H), 3.51 (dd, J = 10.8, 6.8 Hz, 1 H), 1.62 (t, J = 6.7 Hz, 1 H), 1.42 (s, 3 H), 1.23 (s, 3 H) ppm. ¹³C NMR (75 MHz, CDCl₃): δ = 138.2, 128.6 (2 C), 127.9 (3 C), 73.1, 68.9, 44.8, 38.6, 28.6, 27.3, 19.7 ppm. LC–MS: [M + NH₄]^+• = 366, 7.37 min. HRMS was not conclusive probably due to poor or no ionization.(±)-({[(1R,3R)-2,2-Dibromo-3-propylcyclopropyl]methoxy}methyl)benzene (9)Compound 9 (320 mg, 0.79 mmol, 90% purity, 72%, orange oil) was synthesized from allyl ether 1f (209.3 mg, 1.10 mmol, 1 equiv) according to the general procedure (3 h stirring at room temperature). The crude was purified over silica gel column chromatography (elution with pentane/EtOAc = 99:1). IR (neat): n _max = 2958, 2929, 2856, 1454, 1365, 1092, 1074, 1028, 734, 690 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.37–7.29 (m, 5 H), 4.59 (d, J = 11.8 Hz, 1 H), 4.53 (d, J = 11.9 Hz, 1 H), 3.60 (dd, J = 10.6, 7.0 Hz, 1 H), 3.48 (dd, J = 10.6, 6.7 Hz, 1 H), 1.97 (dt, J = 11.2, 6.9 Hz, 1 H), 1.69 (dt, J = 11.2, 7.0 Hz, 1 H), 1.55–1.36 (m, 4 H), 0.97 (t, J = 7.2 Hz, 3 H) ppm. ¹³C NMR (75 MHz, CDCl₃): δ = 138.1, 128.6 (2 C), 127.9 (3 C), 73.2, 68.1, 34.6, 33.8, 32.6, 29.3, 21.8, 14.0 ppm. LC–MS: [M + NH₄]^+• = 380, 7.64 min. (±)-7,7-Dibromo-2-oxabicyclo[4.1.0]heptane (10)Compound 10 (194 mg, 0.76 mmol, 69%, orange oil) was synthesized from 3,4-dihydro-2H-pyran (1g, 0.1 mL, 1.10 mmol, 1 equiv) according to the general procedure (3 h stirring at room temperature). The crude was purified over silica gel column chromatography (elution with hexane/EtOAc = 95:5). IR (neat): ν _max = 2928, 2859, 1446, 1234, 1150, 1060, 1018, 850, 754, 668 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 3.89 (d, J = 8.1 Hz, 1 H), 3.78 (dt, J = 10.9, 3.7 Hz, 1 H), 3.35 (ddd, J = 11.8, 11.0, 2.4 Hz, 1 H), 2.09–1.98 (m, 1 H), 1.94–1.72 (m, 3 H), 1.38 (dddd, J = 13.7, 8.0, 5.4, 2.6 Hz, 1 H) ppm. ¹³C NMR (75 MHz, CDCl₃): δ = 64.6, 59.9, 35.9, 27.6, 20.6, 17.4 ppm. LC–MS: [M]^+• = 256, 1.21 min. HRMS was not conclusive probably due to poor or no ionization.(±)-7,7-Dibromobicyclo[4.1.0]heptane (11)In a 250 mL three-neck round-bottomed flask, bromoform (0.22 mL, 12.5 mmol, 1.25 equiv) was added dropwise to stirred solution of cyclohexene 1h (1.01 mL, 10 mmol, 1 equiv), benzyltriethylammonium chloride (68.3 mg, 0.30 mmol, 0.03 equiv), and a 50% aqueous solution of sodium hydroxide (0.79 mL, 15 mmol, 1.5 equiv) in dichloromethane (10 mL) at room temperature. The mixture was stirred at 40 °C for 20 h. The reaction mixture was then treated with water (20 mL), and the product was extracted with Et₂O (3 × 20 mL). The combined organic phase was dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The crude was purified over silica gel column chromatography (elution with hexane/EtOAc = 95:5) to afford the desired product 11 (401.5 mg, 1.58 mmol, 16%) as a yellowish oil. Data are in accordance with the literature.^{12 1}H NMR (400 MHz, CDCl₃): δ = 2.05–1.95 (m, 2 H), 1.86–1.81 (m, 2 H), 1.60–1.55 (m, 2 H), 1.39–1.33 (m, 2 H), 1.25–1.14 (m, 2 H) ppm. ¹³C NMR (75 MHz, CDCl₃): δ = 45.0, 27.2 (2 C), 20.8 (2 C), 20.3 (2 C) ppm
• 11 General Procedure AIn an oven dried two-neck round-bottomed flask under argon atmosphere, dibromocyclopropane (1 equiv) was dissolved in anhydrous THF (c = 0.1 M). The solution was cooled to –100 °C (ethanol/liquid nitrogen bath), a solution of BuLi (2.5 M in hexane, 1.2 equiv) was added dropwise, and the reaction was stirred for 45 min. Then, a solution of pinacolborane (1 M in THF, 2 equiv) was added dropwise, and the resulting mixture was warmed to room temperature. Then, the reaction mixture was stirred for 18 h at 50 °C. The reaction was quenched by adding a saturated aqueous solution of NaHCO₃. The product was extracted with EtOAc (3×). The combined organic phases were washed with water and brine. The combined organic phase was dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The crude was purified over silica gel column chromatography to afford the desired cyclopropyl pinacol boronic ester.General Procedure BIn an oven-dried two-neck round-bottomed flask under argon atmosphere, dibromocyclopropane (1 equiv) was dissolved in anhydrous THF (c = 0.1 M). The solution was cooled to –100 °C (ethanol/liquid nitrogen bath), and a solution of BuLi (2.5 M in hexane, 1.2 equiv) was slowly added over 30 min using a syringe pump followed by a slow addition of a solution of pinacolborane (1 M in THF, 2 equiv) over 30 min using a syringe pump. The resulting mixture was stirred for 45 min at –100 °C then warmed to room temperature. Then, the reaction mixture was stirred for 18 h at 50 °C. The reaction was quenched by adding a saturated aqueous solution of NaHCO₃. The product was extracted with EtOAc (3×). The combined organic phases were washed with water and brine. The combined organic phase was dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The crude was purified over silica gel column chromatography to afford the desired cyclopropyl pinacol boronic ester. 2-((1R,3a′S,6′R,6a′R)-6′-Methoxy-2′,2′-dimethyldihydro-6′H-spiro{cyclopropane-1,4′-furo[3,4-d][1,3]dioxol}-2-yl)--4,4,5, 5-tetramethyl-1,3,2-dioxaborolane (3)Compound 3 (153.7 mg, 0.47 mmol, 52%, colorless oil, 3a/3b = 63:37) was synthesized from dibromocyclopropane 2a (322 mg, 0.90 mmol, 1 equiv) according to the general procedure A. The analysis of the ¹H NMR spectrum of the crude mixture indicates a ratio 3a/3b = 66:34. The crude was purified over silica gel column chromatography (elution with PE/EtOAc = 95:5 to 90:10). The reaction was also performed on larger scale of 2a (1 g, 2.79 mmol) to afford 3 (659.8 mg, 2.02 mmol, 72%, 3a/3b = 66:34). For characterization purposes, both isomers were partially separated from each other over silica gel column chromatography (elution with PE/EtOAc = 95:5 to 90:10) to afford isomer 3b as a pure fraction (colorless oil) followed by a mixture of both isomers and then isomer 3a as a mixture containing 25% of minor isomer 3b (colorless oil). Compound 3a: IR (neat): ν _max = 2980, 2930, 1456, 1411, 1370, 1324, 1244, 1210, 1144, 1102, 1041, 1010, 960, 865 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 4.97 (s, 1 H), 4.65 (d, J = 5.9 Hz, 1 H), 4.45 (d, J = 5.9 Hz, 1 H), 3.40 (s, 3 H), 1.49 (s, 3 H), 1.33 (s, 3 H), 1.25 (s, 6 H), 1.24 (s, 6 H), 1.22–1.21 (m, 1 H), 1.17 (dd, J = 11.1, 5.5 Hz, 1 H), 0.31 (dd, J = 11.1, 8.5 Hz, 1 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 112.7, 109.4, 85.7, 85.4, 83.4 (2 C), 73.1, 56.1, 29.8, 26.7, 26.3, 25.5 (2 C), 24.5 (2 C), 10.8 ppm. GC–MS: [M]^+• = 326, 15.24 min.Compound 3b: IR (neat): ν _max = 2978, 2924, 1454, 1408, 1370, 1319, 1245, 1211, 1143, 1104, 1043, 1005, 962, 859 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 4.91 (s, 1 H), 4.79 (d, J = 5.9 Hz, 1 H), 4.61 (d, J = 5.9 Hz, 1 H), 3.28 (s, 3 H), 1.51 (s, 3 H), 1.35 (s, 3 H), 1.28 (d, J = 11.3, 5.6 Hz, 1 H), 1.22 (s, 12 H), 1.04 (dd, J = 8.7, 5.6 Hz, 1 H), 0.35 (dd, J = 11.3, 8.7 Hz, 1 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 112.3, 108.2, 85.7, 83.4 (2 C), 82.0, 72.1, 55.0, 29.8, 26.6, 25.7, 25.0 (2 C), 24.7 (2 C), 10.1 ppm. GC–MS: [M]^+• = 326, 14.39 min. HRMS was not recorded to confirm structural identity, but NMR and GC–MS data are fully in line with the proposed structure.2-((1S,3a′S,6′R,6a′R)-6′-Methoxy-2′,2′-dimethyldihydro-6′H-spiro{cyclopropane-1,4′-furo[3,4-d][1,3]dioxol}-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (4)Compound 4 (66 mg, 0.20 mmol, 22% calculated, yellowish oil) was synthesized from dibromocyclopropane 2b (321 mg, 0.90 mmol, 1 equiv) according to the general procedure A. The analysis of the ¹H NMR spectrum of the crude mixture indicates the presence of a single isomer of the product and several byproducts. The crude was purified over silica gel column chromatography (elution with PE/EtOAc = 98:2, 95:5, and then 90:10) to afford a fraction of a byproduct, then a fraction containing product 4 contaminated with traces of inseparable impurities, followed by a ca. 1:1 mixture of the product 4 and another byproduct, and finally a fraction containing product 4 contaminated with traces of inseparable impurities. ¹H NMR (400 MHz, CDCl₃): δ = 4.88 (s, 1 H), 4.60 (d, J = 5.9 Hz, 1 H), 4.52 (d, J = 5.9 Hz, 1 H), 3.28 (s, 3 H), 1.46 (s, 3 H), 1.27 (s, 3 H), 1.23 (s, 6 H), 1.21 (s, 6 H), 1.02–0.95 (m, 2 H), 0.54 (dd, J = 10.9, 8.2 Hz, 1 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 112.0, 108.1, 85.5, 83.5 (2 C), 83.1, 72.2, 54.8, 26.6, 25.8, 24.92 (2 C), 24.86 (2 C), 17.9 (2 C) ppm. GC–MS: [M]^+• = 326, 14.61 min. HRMS was not recorded to confirm structural identity, but NMR and GC–MS data are fully in line with the proposed structure.(±)-2-{(1R,2S)-2-[(Benzyloxy)methyl]-2-methylcyclopropyl}-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (12)Compound 12 (49.4 mg, 0.16 mmol, 54% combined yield) was synthesized from dibromocyclopropane 5 (100 mg, 0.30 mmol, 1 equiv) according to the general procedure A. The analysis of the ¹H NMR spectrum of the crude mixture indicates a ratio 12a/12b = 55:45. The crude was purified over silica gel column chromatography (elution with pentane/EtOAc = 99:1 to 95:5) to afford isomer 12a (colorless oil), then a mixture of 12a/12b and finally isomer 12b (colorless oil). The individual isomers are described from pure analytical fractions. The NMR data of both isomers are in accordance with the literature.^6dCompound 12a: ¹H NMR (400 MHz, CDCl₃): δ = 7.37–7.31 (m, 4 H), 7.28–7.24 (m, 1 H), 4.55 (d, J = 12.1 Hz, 1 H), 4.47 (d, J = 12.1 Hz, 1 H), 3.49 (d, J = 9.6 Hz, 1 H), 3.42 (d, J = 9.6 Hz, 1 H), 1.23 (s, 3 H), 1.20 (s, 6 H), 1.18 (s, 6 H), 0.72 (dd, J = 6.9, 3.6 Hz, 1 H), 0.68 (dd, J = 9.1, 3.5 Hz, 1 H), –0.12 (dd, J = 9.1, 7.0 Hz, 1 H) ppm. ¹³C NMR (175 MHz, CDCl₃): δ = 139.1, 128.4 (2 C), 127.6 (2 C), 127.4, 83.1 (2 C), 75.6, 72.7, 25.1 (2 C), 24.7 (2 C), 23.7, 23.4, 18.0, 6.7 ppm. LC–MS: [M + NH₄]^+• = 320, 7.65 min.Compound 12b: ¹H NMR (400 MHz, CDCl₃): δ = 7.37–7.24 (m, 5 H), 4.54 (d, J = 12.1 Hz, 1 H), 4.50 (d, J = 12.1 Hz, 1 H), 3.42 (dd, J = 9.9, 0.9 Hz, 1 H), 3.14 (d, J = 9.9 Hz, 1 H), 1.25 (s, 3 H), 1.24 (s, 6 H), 1.22 (s, 6 H), 0.73 (dd, J = 9.6, 3.6 Hz, 1 H), 0.66 (dd, J = 6.5, 3.2 Hz, 1 H), –0.11 (dd, J = 9.6, 6.8 Hz, 1 H) ppm. ¹³C NMR (175 MHz, CDCl₃): δ = 138.9, 128.4 (2 C), 127.7 (2 C), 127.5, 83.2 (2 C), 79.4, 72.6, 25.3 (2 C), 24.7 (2 C), 22.5, 18.0, 17.2, 4.8 ppm. LC–MS: [M + NH₄]^+• = 320, 7.63 min.(±)-{(1R,3R)-3-[(Benzyloxy)methyl]-2,2-dibromocyclopropyl}benzene (13)Compound 13 (73 mg, 0.20 mmol, 67% combined yield) was synthesized from dibromocyclopropane 6 (118.8 mg, 0.30 mmol, 1 equiv) according to the general procedure A. The analysis of the ¹H NMR spectrum of the crude mixture indicates a ratio 13a/13b = 58:42. The crude was purified over silica gel column chromatography (elution with pentane/EtOAc = 98:2 to 90:10) to afford isomer 13b (colorless oil), then a mixture of 13a/13b and finally isomer 13a (yellowish oil). The individual isomers are described from pure analytical fractions. The NMR data of the minor isomer 13b are in accordance with the literature.^6d The stereochemistry of the major isomer 13a was concluded from the stereochemistry of the minor isomer 13b.Compound 13a: IR (neat): ν _max = 2972, 2926, 2861, 1446, 1386, 1324, 1214, 1143, 1062, 1020, 851, 735, 696 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.36–7.10 (m, 10 H), 4.61 (d, J = 12.1 Hz, 1 H), 4.57 (d, J = 12.1 Hz, 1 H), 3.65 (dd, J = 10.5, 5.8 Hz, 1 H), 3.43 (dd, J = 10.5, 6.9 Hz, 1 H), 2.25 (dd, J = 10.5, 5.4 Hz, 1 H), 2.07–2.01 (m, 1 H), 1.02 (s, 6 H), 0.89 (s, 6 H), 0.41 (dd, J = 10.5, 6.6 Hz, 1 H) ppm. ¹³C NMR (175 MHz, CDCl₃): δ = 140.0, 138.8, 128.9 (2 C), 128.5 (2 C), 127.9 (2 C), 127.7 (2 C), 127.6, 126.0, 83.2 (2 C), 74.1, 72.4, 27.9, 24.9 (2 C), 24.5 (2 C), 22.2, 9.2 ppm. LC–MS: [M + NH₄]^+• = 382, 7.77 min. HRMS (ESI-TOF): m/z [M + NH₄]⁺ calcd for C₂₃H₃₃BNO₃: 382.2548; found: 382.2555.Compound 13b: ¹H NMR (300 MHz, CDCl₃): δ = 7.37–7.07 (m, 10 H), 4.57 (d, J = 11.9 Hz, 1 H), 4.52 (d, J = 11.9 Hz, 1 H), 3.79 (dd, J = 10.2, 6.5 Hz, 1 H), 3.59 (dd, J = 10.2, 8.0 Hz, 1 H), 2.12 (dd, J = 6.0, 5.3 Hz, 1 H), 1.78 (dddd, J = 9.8, 7.9, 6.6, 5.0 Hz, 1 H), 1.24 (s, 6 H), 1.22 (s, 6 H), 0.58 (dd, J = 9.8, 6.4 Hz, 1 H) ppm. ¹³C NMR (175 MHz, CDCl₃): δ = 142.6, 138.8, 128.4 (4 C), 127.8 (2 C), 127.6, 125.9 (2 C), 125.8, 83.4 (2 C), 72.6, 71.1, 28.8, 27.8, 25.1 (2 C), 24.8 (2 C), 10.5 ppm. LC–MS: [M + NH₄]^+• = 382, 7.78 min.(±)-2-{(2R,3R)-2-[(Benzyloxy)methyl]-3-phenethylcyclopropyl}-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (14)Compound 14 (46.7 mg, 0.12 mmol, 41%, colorless oil, 14a/14b = 55:45) was synthesized from dibromocyclopropane 7 (123 mg, 0.29 mmol, 1 equiv) according to the general procedure A. The analysis of the ¹H NMR spectrum of the crude mixture indicates a ratio 14a/14b = 57:43. The crude was purified over silica gel column chromatography (elution with pentane/EtOAc = 98:2 to 95:5). The stereochemistry of isomers 14a and 14b could not be determined. Both isomers were described from the mixture, and some NMR signals could be attributed to the corresponding isomer. ¹H NMR (400 MHz, CDCl₃): δ = 7.35–7.14 (m, 20 H), 4.53–4.50 (m, 3 H), 4.47 (d, J = 11.9 Hz, 1 H _14a ), 3.59 (dd, J = 10.1, 6.7 Hz, 1 H _14a ), 3.47–3.39 (m, 2 H), 3.22 (dd, J = 10.5, 7.1 Hz, 1 H _14b ), 2.75–2.63 (m, 4 H), 1.90–1.81 (m, 1 H _14b ), 1.78–1.69 (m, 1 H _14b ), 1.63 (d, J = 7.4 Hz, 1 H _14a ), 1.59 (d, J = 7.5 Hz, 1 H _14a ), 1.24–1.20 (m, 26 H), 1.01–0.84 (m, 2 H), 0.08–0.14 (m, 1 H _14a + 1 H _14b ) ppm. ¹³C NMR (175 MHz, CDCl₃): δ = 142.6, 142.4, 139.0, 138.9, 128.7 (2 C), 128.6 (2 C), 128.5 (2 C), 128.40 (2 C), 128.36 (2 C), 128.3 (2 C), 127.8 (2 C), 127.7 (2 C), 127.6, 127.5, 125.8, 125.7, 83.2 (2 C _14b ), 83.1 (2 C _14a ), 74.7 (C _14b ), 72.5 (C _14a ), 72.3 (C _14b ), 71.5 (C _14a ), 36.8 (C _14a ), 36.3(C _14b ), 36.0(C _14a ), 31.9 (C _14b ), 29.8, 25.5, 25.2 (2 C _14b ), 25.0 (2 C _14a ), 24.84 (2 C _14b ), 24.78 (2 C _14a ), 24.6, 24.3, 23.9 ppm. The boron-bound carbons were not detected due to quadrupolar relaxation. LC–MS: [M _14a + NH₄]^+• = 410, 8.06 min. LC–MS: [M _14b + NH₄]^+• = 410, 8.12 min.(±)-2-{(3R)-3-[(Benzyloxy)methyl]-2,2-dimethylcyclopropyl}-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (15)Compound 15 (70 mg, 0.22 mmol, 73% combined, 15a/15b = 70:30) was synthesized from dibromocyclopropane 8 (104.4 mg, 0.30 mmol, 1 equiv) according to the general procedure B. The analysis of the ¹H NMR spectrum of the crude mixture indicates a ratio 15a/15b = 68:32. The crude was purified over silica gel column chromatography (elution with pentane/EtOAc = 98:2 to 95:5) to afford isomer 15b (colorless oil), then a mixture of 15a/15b and finally isomer 15a (colorless oil). The individual isomers are described from pure analytical fractions. Compound 15a: IR (neat): ν _max = 2925, 2854, 1453, 1370, 1316, 1274, 1214, 1144, 1073, 1028, 959, 852, 734, 696 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.34–7.26 (m, 5 H), 4.53 (d, J = 12.1 Hz, 1 H), 4.50 (d, J = 12.1 Hz, 1 H), 3.66 (dd, J = 10.6, 5.5 Hz, 1 H), 3.29 (dd, J = 10.6, 8.8 Hz, 1 H), 1.31–1.29 (m, 1 H), 1.24 (s, 6 H), 1.21 (s, 6 H), 1.18 (s, 3 H), 1.14 (s, 3 H), –0.38 (d, J = 6.6 Hz, 1 H) ppm. ¹³C NMR (175 MHz, CDCl₃): δ = 139.0, 128.4 (2 C), 127.8 (2 C), 127.6, 83.1 (2 C), 72.4, 71.4, 29.9, 25.3 (2 C), 24.6 (2 C), 24.3, 23.5, 22.2, 13.5 ppm. LCMS: [M + NH₄]^+• = 334, 7.46 min. HRMS (ESI-TOF): m/z [M + NH₄]⁺ calcd for C₁₉H₃₃BNO₃: 334.2548; found: 334.2547.Compound 15b: IR (neat): ν _max = 2922, 2855, 1433, 1389, 1377, 1315, 1238, 1138, 1098, 1073, 976, 861, 734, 697 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.37–7.27 (m, 5 H), 4.56 (d, J = 11.9 Hz, 1 H), 4.49 (d, J = 11.9 Hz, 1 H), 3.66 (d, J = 7.5 Hz, 2 H), 1.23 (d, J = 7.4 Hz, 1 H), 1.20 (s, 6 H), 1.19 (s, 6 H), 1.16 (s, 3 H), 1.15 (s, 3 H), –0.04 (d, J = 9.6 Hz, 1 H) ppm. ¹³C NMR (175 MHz, CDCl₃): δ = 139.1, 128.4 (2 C), 127.9 (2 C), 127.5, 82.9 (2 C), 72.7, 68.9, 30.5, 29.9, 25.1 (2 C), 24.8 (2 C), 22.9, 17.5, 12.2 ppm. LC–MS: [M + NH₄]^+• = 334, 7.47 min. HRMS was not conclusive probably due to poor or no ionization.(±)-2-{(1S,2R,3S)-2-[(Benzyloxy)methyl]-3-propylcyclopropyl}-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (16)Compound 16 (47.5 mg, 0.145 mmol, 48% combined yield) was synthesized from dibromocyclopropane 9 (119.5 mg, 0.30 mmol, 1 equiv) according to the general procedure A. The analysis of the ¹H NMR spectrum of the crude mixture indicates a ratio 16a/16b = 62:38. The crude was purified over silica gel column chromatography (elution with pentane/EtOAc = 99:1 to 95:5) to afford isomer 16a (yellowish oil), then a mixture of 16a/16b and finally isomer 16b (colorless oil). The individual isomers are described from pure analytical fractions. The NMR data of the minor isomer 16b are in accordance with the literature.^6d The stereochemistry of the major isomer 16a was concluded from the stereochemistry of the minor isomer 16b.Compound 16a: IR (neat): ν _max = 2956, 2922, 2856, 1401, 1378, 1316, 1214, 1144, 1094, 1073, 854, 733, 695 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.38–7.27 (m, 5 H), 4.57 (d, J = 11.8 Hz, 1 H), 4.48 (d, J = 11.8 Hz, 1 H), 3.70 (dd, J = 10.1, 7.7 Hz, 1 H), 3.64 (dd, J = 10.1, 7.2 Hz, 1 H), 1.52–1.32 (m, 6 H), 1.21 (s, 6 H), 1.20 (s, 6 H), 0.91 (t, J = 7.2 Hz, 3 H), 0.15 (t, J = 9.4 Hz, 1 H) ppm. ¹³C NMR (175 MHz, CDCl₃): δ = 139.1, 128.4 (2 C), 127.9 (2 C), 127.5, 82.9 (2 C), 72.9, 68.3, 27.9, 25.1 (2 C), 24.9 (2 C), 23.6, 22.7, 21.9, 14.2 ppm. The boron-bound carbon was not detected due to quadrupolar relaxation. LC–MS: [M + NH₄]^+• = 348, 7.97 min. HRMS (ESI-TOF): m/z [M + NH₄]⁺ calcd for C₂₀H₃₅BNO₃: 348.2704; found: 348.2700.Compound 16b: ¹H NMR (400 MHz, CDCl₃): δ = 7.35–7.27 (m, 5 H), 4.55 (d, J = 12.0 Hz, 1 H), 4.49 (d, J = 12.0 Hz, 1 H), 3.59 (dd, J = 10.4, 5.9 Hz, 1 H), 3.37 (dd, J = 10.4, 8.3 Hz, 1 H), 1.48–1.36 (m, 6 H), 1.21 (s, 12 H), 0.90 (t, J = 7.2 Hz, 3 H), –0.50 (t, J = 6.0 Hz, 1 H) ppm. ¹³C NMR (175 MHz, CDCl₃): δ = 138.9, 128.5 (2 C), 127.8 (2 C), 127.6, 83.0 (2 C), 72.6, 70.5, 31.2, 24.8 (4 C), 23.2, 22.8, 22.0, 14.1 ppm. The boron-bound carbon was not detected due to quadrupolar relaxation. LC–MS: [M + NH₄]^+• = 348, 7.96 min.(±)-2-{(1R,6S,7S)-2-Oxabicyclo[4.1.0]heptan-7-yl}-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (17)Compound 17 (34.4 mg, 0.153 mmol, 38%, colorless oil, 17a/17b = 93:7) was synthesized from dibromocyclopropane 10 (102.4 mg, 0.40 mmol, 1 equiv) according to the general procedure A. The analysis of the ¹H NMR spectrum of the crude mixture indicates a ratio 17a/17b = 91:9. The crude was purified over silica gel column chromatography (elution with hexane/EtOAc = 95:5 to 90:10). Only the major isomer 17a from the mixture was described. The NMR data of the major isomer 17a are in accordance with the literature.^6e The stereochemistry of the minor isomer 17b was concluded from the stereochemistry of the major isomer 17a. ¹H NMR (400 MHz, CDCl₃): δ = 3.65 (dd, J = 3.0, 6.7 Hz, 1 H), 3.58–3.54 (m, 1 H), 3.34 (td, J = 10.7, 2.8 Hz, 1 H), 1.97–1.93 (m, 2 H), 1.53–1.39 (m, 2 H), 1.25–1.22 (m, 1 H), 1.19 (m, 12 H), 0.07 (dd, J = 7.6, 3.0 Hz, 1 H) ppm. ¹³C NMR (175 MHz, CDCl₃): δ = 83.1 (2 C), 64.7, 57.2, 24.9 (2 C), 24.8 (2 C), 21.9, 20.5, 17.2, 8.3 ppm. LC–MS: [M + H]^+• = 225, 6.34 min.(±)-2-{Bicyclo[4.1.0]heptan-7-yl}-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (18)Compound 18 (54 mg, 0.24 mmol, 40%, colorless oil, 18a/18b = 93:7) was synthesized from dibromocyclopropane 11 (152.4 mg, 0.60 mmol, 1 equiv) according to the general procedure B. The analysis of the ¹H NMR spectrum of the crude mixture indicates a ratio 18a/18b = 93:7. The crude was purified over silica gel column chromatography (elution with pentane/EtOAc = 98:2). Only the major isomer 18a from the mixture was described. The NMR data of the major isomer 18a are in accordance with the literature.^6e The stereochemistry of the minor isomer 18b was concluded from the stereochemistry of the major isomer 18a. ¹H NMR (400 MHz, CDCl₃): δ = 1.90–1.85 (m, 2 H), 1.68–1.62 (m, 2 H), 1.28–1.25 (m, 2 H), 1.21 (s, 12 H), 1.19–1.16 (m, 2 H), 1.14–1.11 (m, 2 H), –0.38 (t, J = 5.8 Hz, 1 H) ppm. ¹³C NMR (175 MHz, CDCl₃): δ = 82.8 (2 C), 24.9 (4 C), 24.2 (2 C), 21.2 (2 C), 17.3 (2 C), 6.4 ppm
• 12 Grupe S, Von Wangelin AJ. ChemCatChem 2013; 5: 706