Effect of modification of silica surface on the extent of enantiomer self-disproportionation

The interplay between the extent of the phenomenon of self-disproportionation of enantiomers (SDE) and the functionalization of the silica surface was investigated. The elution pattern of individual enantiomers in nonracemic mixtures of citalopram, methyl N-acetyl-phenylalaninate, metalaxyl, methyl p-tolyl sulfoxide, and Binol was analyzed for the adsorbents containing unmodified silica (Si), amino-functionalized silica (NH2) and diol-functionalized silica (Diol). The highest yield for the more abundant enantiomer, which was the separation target, was achieved for the Si adsorbent; it decreased in the sequence: Si, NH2, and Diol. Only for Binol, the separation yields achieved for the Si and NH2 adsorbents were comparable, yet with a very low binding capacity of the former. A mechanistic model was used to quantify the adsorption behavior in each chromatographic system and determine the constants of homo and hetero associations on the adsorbed surface. The results obtained indicated preferable formation of hetero-associates over homo-associates, which underlaid the separation selectivity. This preference diminished with increasing hydrophobicity of the adsorbent surface, with the exception of the most hydrophobic Binol, for which the SDE effect was the most pronounced in the NH2 column. This implies the possibility of adjustment of the silica surface functionality to improve the performance of the SDE-driven separation.