Iridium-Catalyzed Enantioselective Formal [4+2] Cycloaddition of Biphenylene and Alkynes for the Construction of Axial Chirality

Abstract

Chiral iridium complex catalyzed a formal [4+2] cycloaddition of biphenylene with disubstituted alkynes, which was initiated by carbon-carbon bond cleavage. Axially chiral 9,10-di­substituted phenanthrenes were enantiomerically obtained.

Key words

alkynes - asymmetric catalysis - cycloadditions - iridium - rhodium

References and Notes

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12

In the presence of [IrCl(cod)]₂ without diphosphine ligand, alkyne 1a was consumed within 2 h, but the yield of 2a was low (28%).

13

General Experimental Procedure [IrCl(cod)]₂ (6.7 mg, 0.010 mmol) was placed in a 30 mL two-neck flask under an argon atmosphere, and a xylene solution (0.4 mL) of (S,S)-Me-BPE (5.2 mg, 0.020 mmol) was added. After the mixture was stirred for 5 min at r.t., a xylene solution (0.6 mL) of biphenylene (18.3 mg, 0.12 mmol) and an alkyne (0.10 mmol) was added. The mixture was stirred at 100 °C, then the solvent was evaporated under reduced pressure, and the resulting crude products were purified by TLC to afford a pure cycloadduct.

14

After being stirred for 4 h, cycloadduct 2c was obtained in higher ee of 27% ee. Actually, gradual racemization of 2c was ascertained at 100 °C.

15

9-Methoxymethyl-10-[(2-trifluoromethyl)phen-yl]phenanthrene (2d) White solid; mp 151-152 °C. IR (CH₂Cl₂): 1315, 1130, 1110, 1097 cm^-1. ¹H NMR (500 MHz, CDCl₃): δ = 3.25 (s, 3 H), 4.22 (d, J = 11.0 Hz, 1 H), 4.89 (d, J = 11.0 Hz, 1 H), 7.13 (d, J = 8.0 Hz, 1 H), 7.40-7.44 (m, 2 H), 7.60-7.72 (m, 5 H), 7.88 (d, J = 7.5 Hz, 1 H), 8.28-8.31 (m, 1 H), 8.73-8.78 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 58.2, 70.4, 122.4, 122.8, 125.2, 126.1, 126.2, 126.4, 126.7, 126.8, 127.1, 128.0, 128.1, 129.7, 130.0, 130.2, 130.3, 130.6, 130.7, 131.6, 132.7, 135.6, 138.0. Anal. Calcd for C₂₃H₁₇F₃O: C, 75.40; H, 4.68. Found: C, 75.47; H, 4.58. HRMS-FAB (positive): m/z calcd for C₂₃H₁₇F₃O: 366.1232 [M]⁺; found: 366.1198 [M]⁺; [α]_D ³⁶ 32.0 (c 1.00, CHCl₃, 99% ee after recrystallization). The ee was determined by chiral HPLC analysis using a chiral column (Daicel Chiralpak IA-H: 4 × 250 mm, 254 nm UV detector, r.t., eluent: 0.5% 2-PrOH in hexane, flow rate: 0.5 mL/min, t _R = 20 min for major isomer and 23 min for minor isomer).

16

9-Methyl-10-[(2-trifluoromethyl)phenyl]phenanthrene (2f)
White solid; mp 109-111 °C. IR (CH₂Cl₂): 1315 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 2.37 (s, 3 H), 7.09 (d, J = 8.4 Hz, 1 H), 7.25 (d, J = 2.4 Hz, 1 H), 7.38-7.43 (m, 1 H), 7.55-7.73 (m, 5 H), 7.89 (d, J = 8.4 Hz, 1 H), 8.13-8.16 (m, 1 H), 8.72 (d, J = 8.4 Hz, 1 H), 8.77-8.80 (m, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 17.7, 122.3, 122.9, 125.1, 125.3, 125.7, 126.2, 126.3, 126.3, 126.5, 126.8, 127.2, 127.7, 129.1, 130.2, 130.7, 131.6, 131.9, 132.2, 132.4, 133.8, 139.5. Anal. Calcd for C₂₂H₁₅F₃: C, 78.56; H, 4.50. Found: C, 78.50; H, 4.40. HRMS-FAB (positive): m/z calcd for C₂₂H₁₅F₃: 336.1126 [M]⁺; found: 336.1105 [M]⁺; [α]_D ²⁶ -28.1 (c 1.00, toluene, 98% ee after recrystallization). The ee was determined by chiral HPLC analysis using a chiral column (Daicel Chiralcel OD-H: 4 × 250 mm, 254 nm UV detector, r.t., eluent: 1% 2-PrOH in hexane, flow rate: 1.0 mL/min, t _R = 8 min for minor isomer and 14 min for major isomer).