Journal of Organic Chemistry 2013-09-20

A chiral bisthiourea as a chiral solvating agent for carboxylic acids in the presence of DMAP.

Guangling Bian, Hongjun Fan, Shiwei Yang, Huifeng Yue, Huayin Huang, Hua Zong, Ling Song

Index: J. Org. Chem. 78(18) , 9137-42, (2013)

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Abstract

A simple chiral bisthiourea has been used as a highly effective and practical chemical solvating agent (CSA) for diverse α-carboxylic acids in the presence of DMAP. Excellent enantiodiscrimination based on well-resolved α-H NMR signals of the enantiomers of carboxylic acids can be obtained without interference from the chiral bisthiourea and DMAP. To check the practicality of the chiral bisthiourea/DMAP for enantiomeric determination, the ee values of mandelic acid (MA) samples over a wide ee range were determined by integration of the α-H signal of MA in (1)H NMR. A discrimination mechanism is proposed, that the formation of two diasteromeric ternary complexes between the chiral bisthiourea and two in situ formed enantiomeric carboxylate-DMAPH(+) ion pairs discriminates the enantiomers of carboxylic acids. Computational modeling studies show that the chemical shift value of α-H of (S)-MA is greater than that of (R)-MA in ternary complexes, which is consistent with experimental observation. 1D and 2D NOESY spectra demonstrate the intermolecular noncovalent interactions between the protons on the aromatic rings of chiral bisthiourea and α-H of the enantiomers of racemic α-methoxy phenylacetic acids in the complexes.