New Bidentate Phosphorus Ligands Based on a Norbornane Backbone for Rhodium-Catalyzed Asymmetric Hydrogenation

Abstract

A new class of bidentate diphosphite and diphosphinite ligands has been developed from inexpensive norbornadiene. These ligands exhibited excellent enantioselectivities in the Rh(I)-catalyzed asymmetric hydrogenation of olefin derivatives (up to 99.9% ee).

Key words

asymmetric hydrogenation - bidentate phosphorus ligands - diphosphite ligands - diphosphinite ligand - rhodium

References and Notes

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17

Ligands Preparation (Scheme 1)
The Preparation of Ligand 3
A 10 mL Schlenk flask was charged with diol 2 (100 mg, 0.78 mmol), DMAP (20 mg), Et₃N (160 mg, 1.6 mmol) and THF (5 mL) under nitrogen. The mixture was stirred at 0 °C. A solution of chlorodiphenylphosphine (0.3 mL, 1.56 mmol) in THF (2 mL) was added dropwise with stirring, then stirred at r.t. for 24 h. After filtration, the solvent was removed under vacuum. The crude product was purified through flash chromatography to give pure product as a thick liquid; yield 233 mg (47%). MS (EI): m/z = 496 [M⁺]. ¹H NMR (400 MHz, CDCl₃): δ =7.44-7.13 (m, 20 H, Ph), 4.70-4.69 (b, 2 H, C2 and C5), 2.25 (s, 2 H, bridgehead), 1.92 (d, 2 H, endo of C3 and C5), 1.69 (m, 2 H, exo of C3 and C5), 1.23 (s, 2 H, bridge). ¹³C NMR (100 MHz, CDCl₃): δ = 149.678-122.009 (Ph), 75.628 (O-C), 42.500(bridgehead), 34.849 (bridge), 28.965 (C3 and C5). ³¹P NMR (162 MHz, CDCl₃): δ = 109.130.
Preparation of Ligand 4a
Under nitrogen, the freshly prepared chlorophosphoric acid ester (1.56 mmol) from 2,2-dihydroxy-1,1-biphenyl was dissolved in THF (6 mL) and cooled to -30 °C. Then, Et₃N (3.12 mmol) and DMAP (10 mg) in THF (1 mL) was added dropwise and the reaction was stirred for 30 min. Afterwards, diol 2 (100 mg, 0.78 mmol) in THF (3mL) was added dropwise at -30 °C and the mixture was allowed to warm to r.t. and stirred for 24 h. The mixture was filtrated and the filtrate was then passed through a short alumina column (d = 0.5 cm, L = 3 cm) to give the corresponding product as a white solid; 390 mg (90%). ¹H NMR (400 MHz, CDCl₃): δ = 7.52-7.23 (m,16 H, Ph), 4.42-4.37 (b, 2 H, C2 and C5), 2.31 (s, 2 H, bridgehead), 2.01-1.98 (d, 2 H, endo of C3 and C5), 1.77-1.70 (m, 2 H, exo of C3 and C5), 1.36 (s, 2 H, bridge). ¹³C NMR (100 MHz, CDCl₃): δ = 142.603-128.159 (Ph), 80.685 (C2 and C5), 42.303 (bridgehead), 35.365 (bridge), 29.268 (C3 and C5). ³¹P NMR (162 MHz, CDCl₃): δ = 145.473. ESI-HRMS: m/z calcd for C₃₁H₂₆O₆P₂ [M + K]: 595.0836; found: 595.0846.
Preparation of Ligand 4b Same procedure as for preparation of 4a; yield 78%. ¹H NMR (400 MHz, CDCl₃): δ = 7.96-7.22 (m, 24 H, BINOL), 4.59 (b, 2 H, C2 and C5), 2.25 (s, 2 H, bridgehead), 1.93-1.897 (d, 2 H, endo of C3 and C5), 1.77-1.72 (m, 2H, exo of C3 and C5), 1.24 (s, 2 H, bridge). ¹³C NMR (100 MHz, CDCl₃): δ = 148.556-121.919 (BINOL), 76.02 (C2 and C5), 42.348 (bridgehead), 34.599 (bridge), 28.988 (C3 and C5). ³¹P NMR (162 MHz, CDCl₃): δ = 141.967. ESI-HRMS: m/z calcd for C₄₇H₃₄O₆P₂ [M + Na]: 779.1723; found: 779.1734.
Preparation of Ligand 4c
Same procedure as for preparation of 4a; yield 81%. ¹H NMR (400 MHz, CDCl₃): δ = 7.89-7.16 (m, 24 H, BINOL), 4.48 (b, 2 H, C2 and C5), 2.15 (s, 2 H, bridgehead), 1.87-1.84 (d, 2 H, endo of C3 and C5), 1.58-1.1.53 (m, 2 H, exo of C3 and C5), 1.03 (s, 2 H, bridge). ¹³C NMR (100 MHz, CDCl₃): δ = 148.556-121.919 (BINOL), 75.825 (C2 and C5), 42.454 (bridgehead), 34.955 (bridge), 28.927 (C3 and C5). ³¹P NMR (162 MHz, CDCl₃): δ = 139.921. ESI-HRMS: m/z calcd for C₄₇H₃₄O₆P₂ [M + K]: 795.1462; found: 795.1461.