New Types of o-Carborane-Based Chiral Phosphinooxazoline (Cab-PHOX) Ligand Systems: Synthesis and Characterization of Chiral Cab-PHOX Ligands and Their Application to Asymmetric Hydrogenation

Abstract

o-Carborane-based chiral phosphinooxazoline (Cab-PHOX) ligands were synthesized for the first time and applied to the iridium- and rhodium-catalyzed hydrogenation of unfunctionalized and functionalized olefins with an enantioselectivity of up to 98% and 96%, respectively. The modularity of the Cab-PHOX ligands is highlighted by the facile preparation of a variety of sterically and electronically different ligands.

chiral phosphinooxazoline ligands - o-carborane-based bidentate ligands - asymmetric hydrogenation - unfunctionalized olefins - functionalized olefins

References and Notes

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25

Synthesis of Cab-PHOX 4
To a stirred solution of Cab ^PPh² 1 (0.99 g, 3.0 mmol) in 30 mL of THF, which was cooled to -10 ˚C, was added 2.5 M n-BuLi (1.2 mL, 3.0 mmol) via a syringe. The resulting solution was stirred at -10 ˚C for 1 h and then added 2-bromooxazoline 3 (0.63 g, 3.3 mmol) through a cannula. The reaction temperature was maintained at -10 ˚C for 1 h. Subsequently the reaction mixture was warmed slowly to r.t. After stirring for an additional 12 h, the solvent was removed under vacuum, and the resulting residue was taken up by fresh column chromatography (R _F = 0.6; hexane-benzene, 1:1). Chiral Cab-PHOX 4 was isolated from the reaction solution in 93% yield (1.23 g, 2.8 mmol).
HRMS: m/z calcd for [^¹¹B₁₀ ^¹²C₂₀ ^¹4N^¹H₃₀ ^¹6O^³¹P]⁺: 439.5421; found: 439.5432. Anal. Calcd: C, 54.65; H, 6.88; N, 3.19. Found: C, 54.85; H, 7.02; N, 3.12. IR spectrum (KBr pellet): ν = 2604 (B-H), 1700 (C=N), 2982 (C-H), 2990, 3014
cm^-¹. ^¹H NMR (300 MHz, CDCl₃): δ = 0.92 [d, 3 H, CH(CH ₃)₂, ^³ J _CH-CH3 = 6.6 Hz], 0.99 [d, 3 H, CH(CH ₃)₂, ^³ J _CH-CH3 = 6.9 Hz], 1.92 [m, 1 H, CH(CH₃)₂], 4.16 (m, 1 H, CHN), 4.19 (t, 1 H, CH ₂O, ^² J _C-H = 8.4 Hz), 4.45 (t, 1 H, CH₂O, ^² J _C-H = 8.1 Hz), 7.43-7.82 (m, 10 H, PPh₂ ). ^¹¹B NMR (96.3 MHz, CDCl₃): δ = -3.12 (1 H), -5.74 (1 H), -8.93 (2 H), -12.49 (2 H), -14.02 (4 H). ^¹³C NMR (75.4 MHz, CDCl₃): δ = 14.2, 18.6, 30.5, 67.3, 70.5, 73.8, 81.7, 126.3, 126.5, 127.1, 127.7, 128.3, 128.6, 129.2, 129.7, 130.7, 131.2, 131.6, 132.8, 168.5. ^³¹P NMR (121.5 MHz, CDCl₃): δ = 12.7 (PPh₂).

26

Synthesis of Compound 5
A procedure analogous to the preparation of compound 4 was used but instead starting from Cab ^PCy² 2 (1.02 g, 3.0 mmol). Compound 5 was obtained as pale yellow oil (R _F = 0.5; hexane-benzene, 1:1; 1.19 g, 2.64 mmol, 88%).
HRMS: m/z calcd for [^¹¹B₁₀ ^¹²C₂₀ ^¹4N^¹H₄₂ ^¹6O^³¹P]⁺: 451.6373; found: 451.6387. Anal. Calcd: C, 53.19; H, 9.37; N, 3.10. Found: C, 53.33; H, 9.34; N, 3.11. IR spectrum (KBr pellet): ν = 2600 (B-H), 1698 (C=N), 2985 (C-H), 2996 cm^-¹. ^¹H NMR (300 MHz, CDCl₃): δ = 0.87 [d, 3 H, CH(CH ₃)₂, ^³ J _CH-CH3 = 6.3 Hz], 0.97 [d, 3 H, CH(CH ₃)₂, ^³ J _CH-CH3 = 6.9 Hz], 1.25 (m, 1 H, P-cyclo-CH), 1.35 (m, 2 H, P-cyclo-CH ₂), 1.79 (m, 2 H, P-cyclo-CH ₂ ), 1.86 (m, 1 H, CHN), 1.99 (m, 2 H, P-cyclo-CH ₂ ), 3.97 (m, 1 H, CHN), 4.08 (t, 1 H, CH ₂O, ^³ J _CH-CH2 = 8.7 Hz), 4.34 (t, 1 H, CH₂ O, ^³ J _CH-CH2 = 9.3 Hz). ^¹¹B NMR (96.3 MHz, CDCl₃): δ = -4.24 (1 B), -6.19 (1 B), -9.48 (2 B), -10.51 (2 B), -14.90 (4 B). ^¹³C NMR (75.4 MHz, CDCl₃): δ = 13.7, 16.9, 23.1, 23.4, 25.4, 25.8, 27.4, 27.7, 29.9, 30.3, 30.8, 64.2, 70.4, 73.5, 84.3, 168.2. ^³¹P NMR (121.5 MHz, CDCl₃): δ = 32.3 (PCy₂).