Fluorescent Recognition of Functional Secondary Amines in the Fluorous Phase

In a fluorous solvent (perfluorohexane, FC-72), a perfluoroalkyl-substituted 2-hydroxy-1-naphthaldehyde (1) was found to show greatly enhanced fluorescence when treated with certain functional secondary amines but little fluorescence response was observed with unfunctional amines as well as functional primary and tertiary amines. The study of the reaction of 1 with 2-methylamino ethanol (8) reveals a facile cyclocondensation of the aldehyde group of 1 with this 1,2-amino alcohol to form a 5-membered ring oxazolidine product which turns on the fluorescence. The reaction of 1 with the amino alcohol was found to be more favorable in the fluorous solvent than in a common organic solvent. The use of the fluorous phase-based fluorescence measurement in this work provides a potentially more sensitive as well as selective method in amine sensing.