Enantioselective Mannich-Type Reaction Catalyzed by a Chiral Phosphoric Acid Bearing an (S)-Biphenol Backbone

Abstract

A novel chiral phosphoric acid bearing a biphenol backbone was synthesized and its catalytic activity was investigated in the enantioselective Mannich-type reaction of ketene silyl acetals with aldimines.

Key words

asymmetric synthesis - chiral Brønsted acid - Mannich-type reaction - biphenol - phosphoric acid

References and Notes

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12

( S )-6,6′-Dimethyl-3,3′-bis(4-nitrophenyl)-1,1′-biphenyl-2,2′-yl Phosphate (2a)
[α]_D ^²¹ +365 (c 1.0, CHCl₃). ^¹H NMR (400 MHz, CDCl₃): δ = 8.06-8.04 (m, 4 H), 7.55-7.53 (m, 4 H), 7.39-7.37 (m, 4 H), 2.58 (s, 1 H), 2.35 (s, 6 H). ^³¹P NMR(400 MHz, CDCl₃): δ = 0.74. ^¹³C NMR (75 MHz, CDCl₃): δ = 146.9, 144.7, 143.1, 140.3, 130.2, 130.0, 129.7, 128.4, 127.7, 123.3, 20.0. ^³¹P NMR (162 MHz, CDCl₃): δ = 0.74. Anal. Calcd (%) for C₂₆H₁₉O₈P: C, 60.24; H, 3.69; N, 5.40. Found: C, 60.13; H, 3.66; N, 5.45%.

13

( R )-Methyl 3-( N -2-Hydroxyphenylamino)-2,2-dimethyl-3-phenylpropionate [α]_D ^²¹ +1.4 (c 0.45, CHCl₃; 87% ee); R _f = 0.4 (hexane-EtOAc = 3:1). ^¹H NMR (400 MHz, CDCl₃): δ = 7.29-7.19 (m, 5 H), 6.69 (1 H, dd, J = 7.7, 1.5 Hz), 6.61 (1 H, ddd, J = 7.7, 7.7, 1.5 Hz), 6.53 (1 H, ddd, J = 7.7, 7.7, 1.5 Hz), 6.38 (1 H, dd, J = 7.7, 1.5 Hz), 5.80 (br s, 1 H), 4.55 (br s, 1 H), 4.55 (s, 1 H), 3.69 (s, 3 H), 1.24 (s, 3 H), 1.22 (s, 3 H). ^¹³C NMR (100 MHz, CDCl₃): δ = 177.7, 144.3, 139.0, 135.5, 128.3, 127.9, 127.41, 121.0, 117.9, 114.4, 113.9, 64.6, 52.2, 47.4, 24.4, 20.0.

14

When 1a was employed in the reaction with the aldimine derived from benzaldehyde, the corresponding adduct was obtained in 87% ee.^³

15

The enantioselectivities obtained with 1a: 96% (Ar = Ph), 81% (Ar = 4-MeC₆H₄), 88% (Ar = 4-MeOC₆H₄), 84% (Ar = 4-FC₆H₄), 90% (Ar = PhCH=CHC₆H₄).^³