SpinPHOX/Ir(I) Catalyzed Asymmetric Hydrogenation of (E)-2-(hydroxymethyl)-3-Arylacrylic Acids

Optically active 3-aryl-2-hydroxymethylpropionoic acids are highly useful chiral building blocks for the preparation of some drugs, but so far their asymmetric syntheses are still plagued by modest enantioselectivity and/or limited substrate scope. In the present study, the SpinPHOX/Ir(I) complexes (S, S)-1c and (R, S)-1e have been demonstrated to be highly effective for the asymmetric hydrogenation of (E)-2-(hydroxymethyl)-3-arylacrylic acids, affording the corresponding optically active 3-aryl-2-hydroxymethylpropionoic acids in good to excellent enantiopurities (ee up to 95%). Catalyst screening and reaction optimization with (E)-2-(hydroxymethyl)-3-phenylacrylic acid 2a as the model substrate, revealed that the asymmetric hydrogenation was best conducted in dichloromethane at r.t. under 10 atm of hydrogen with (S, S)-1c as the catalyst, and the presence of 1 equiv. of a suitable organic base (such as triethylamine) was essential for full substrate conversion and excellent product enantioselectivity (94% ee). Evaluation of the substrate scope of the protocol was performed using various 2-hydroxymethyl cinnamic acid derivatives 2a similar to 2n with various substituents on the phenyl group. Full conversions and good to high ee values were obtained in most cases, irrespective of the electron-withdrawing or -donating nature of the substituent. Using complex (R, S)-1e as the catalyst under a slightly modified reaction conditions (50 atm H-2, 40 degrees C), several substrates were hydrogenated in excellent ee values but with the sense of chiral induction opposite to those obtained by using (S, S)-1c. The reaction can be readily scaled up to gram-scale with retention of enantioselectivity. Among the optically enriched hydrogenation products obtained from this procedure, (S)-3a, (R)-3a and (-)-3n can be used as chiral building blocks for the asymmetric synthesis of pharmaceuticals Alvimopan, Ecadotril, and Fasidotril, respectively. Using this protocol, various (E)-2-(hydroxymethyl)-3-arylacrylic acids could be readily hydrogenated in high optical purities, thus providing a facile access to both enantiomers of the chiral 3-aryl-2-hydroxymethylpropionoic acids as well as the relevant chiral drugs.