A highly efficient acyl-transfer approach to urea-functionalized silanes and their immobilization onto silica gel as stationary phases for liquid chromatography.

An alternative method for efficient synthesis of urea-functionalized silanes was proposed on the basis of an N, N'-carbonyldiimidazole-mediated acyl-transfer reaction between various amino-containing building blocks. The employment of different parent aminosilanes and alkylamines afforded an array of urea-containing silanes, which were subsequently immobilized onto silica gel to form corresponding urea-embedded alkyl stationary phases for high-performance liquid chromatography. The different substituents on the silicon core of the derivatized silane were found to significantly influence the final chromatographic behaviors. The comparative chromatographic characterization of thus-prepared silica packings with conventional octadecyl (C18) stationary phases revealed that the urea group was beneficial to suppress silanol activity towards basic probes, as well as to increase the water-compatibility of the alkyl stationary phases. The combination of a polar urea moiety and a non-polar long alkyl chain was favorable for an enhanced steric selectivity towards shape-constrained isomers. The polarizability-sensitive feature of such stationary phases made them good candidates for efficient separation of nitro-containing polar substances.