A Novel Axially Chiral Phosphine-Oxazoline Ligand with an Axis-Unfixed Biphenyl Backbone: Preparation, Complexation, and Application in an ­Asymmetric Catalytic Reaction

Abstract

A novel chiral phosphine-oxazoline ligand 3 with an axis-unfixed biphenyl backbone was prepared. This ligand existed as a mixture of two diastereomers in equilibrium in solution. However, when it was coordinated with palladium(II), only one of the two kinds of possible diastereomer complexes with different axial chirality was formed. When this axis-unfixed chiral ligand was used, up to 90% ee was attained for the palladium-catalyzed asymmetric allylic alkylation.

Key words

P,N-ligands - chiral biaryl ligands - oxazoline ligands - axial chirality - palladium-catalyzed allylic alkylation

References and Notes

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5

Compound 3a: ¹H NMR (400 MHz, CDCl₃; major/minor = 57:43 in CDCl₃): δ = 7.89 (dd, J = 1.5, 7.7 Hz, 1 H), 7.88 (dd, J = 1.5, 7.7 Hz, 1 H), 7.38-7.07 (m, 32 H), 6.89 (br d, J = 7.7 Hz, 1 H), 4.09 (dd, J = 8.4, 9.5 Hz, 1 H), 4.01 (dd, J = 6.6, 8.1 Hz, 1 H), 3.90-3.80 (m, 3 H), 3.69 (t, J = 8.4 Hz, 1 H), 1.71-1.62 (m, 1 H), 1.61-1.52 (m, 1 H), 0.84 (d, J = 7.0 Hz, 3 H), 0.80 (d, J = 7.0 Hz, 3 H), 0.84 (d, J = 6.9 Hz, 3 H). ³¹P NMR (CDCl₃, PPh₃ = -6.0 ppm): δ = -14.8 (major), -15.0 (minor). HRMS (EI): m/z calcd for C₃₀H₂₈NOP: 449.1910; found: 449.1905. Anal. Calcd for C₃₁H₃₀NOP: C, 80.16; H, 6.28; N, 3.12. Found: C, 79.78; H, 5.89; N, 3.11. [α]_D ²⁵ = -54.2 (c 0.48, CHCl₃).
Compound 3b: ¹H NMR (400 MHz, CDCl₃, major/minor = 53:47 in CDCl₃): δ = 7.89 (dd, J = 1.1, 7.7 Hz, 1 H), 7.88 (dd, J = 1.1, 7.7 Hz, 1 H), 7.39-7.07 (m, 32 H), 6.88 (br d, J = 8.1 Hz, 1 H), 6.83 (br d, J = 8.1 Hz, 1 H), 4.06-3.93 (m, 3 H), 3.89-3.75 (m, 3 H), 0.81 (s, 9 H), 0.73 (s, 9 H).
³¹P NMR (CDCl₃, PPh₃ = -6.0 ppm): δ = -14.9 (major), -15.0 (minor). HRMS (EI): m/z calcd for C₃₁H₃₀NOP: 463.2067; found: 463.2064. Anal. Calcd for C₃₁H₃₀NOP: C, 80.32; H, 6.52; N, 3.02. Found: C, 80.03; H, 6.62; N, 2.87. [α]_D ²⁵ -28.7 (c 0.52, CHCl₃).

6

Complex 9 is formed with only one kind of axial chirality, but exists as a mixture of two diastereomers with W- and M-type.
Compound 9a: ¹H NMR (400 MHz, CD₂Cl₂, major/minor = 54:46 in CD₂Cl₂): δ = 7.86-6.53 (m, 56 H), 5.91 (br d, J = 8.0 Hz, 1 H), 5.88 (br d, J = 7.6 Hz, 1 H), 5.80-5.68 (m, 2 H), 4.94 (d, J = 10.9 Hz, 1 H), 4.85 (d, J = 12.3 Hz, 1 H), 3.97-3.80 (m, 4 H), 3.34 (t, J = 9.4 Hz, 1 H), 2.48-2.38 (m, 1 H), 1.86-1.77 (m, 1 H), 1.50-1.40 (m, 1 H), 0.71 (d, J = 6.9 Hz, 3 H), 0.51 (d, J = 6.9 Hz, 3 H), 0.38 (d, J = 6.9 Hz, 3 H), 0.08 (d, J = 6.9 Hz, 3 H). ³¹P NMR (CD₂Cl₂, PPh₃ = -6.0 ppm): δ = 28.8 (minor), 24.8 (major).
Compound 9b: ¹H NMR (400 MHz, CD₂Cl₂, major/minor = 58:42 in CD₂Cl₂): δ = 7.87-6.58 (m, 55 H), 6.54 (dd, J = 11.2, 13.1 Hz, 1 H), 5.93-5.83 (m, 3 H), 5.53 (t, J = 10.9 Hz, 1 H), 4.94 (d, J = 11.2 Hz, 1 H), 4.80 (d, J = 12.3 Hz, 1 H), 4.03-3.96 (m, 2 H), 3.87 (dd, J = 9.1, 10.5 Hz, 1 H), 3.68 (dd, J = 5.4, 10.5 Hz, 1 H), 3.09 (t, J = 9.4 Hz, 1 H), 2.23 (br s, 1 H), 0.64 (s, 9 H), 0.43 (s, 9 H). ³¹P NMR (CD₂Cl₂, PPh₃ = -6.0 ppm): δ = 29.1 (minor), 25.2 (major).