Boramidine: A Versatile Structural Motif for the Design of Fluorescent Heterocycles

Abstract

Sodium cyanoborohydride-derived <i>N</i>-alkylnitriliumboranes were found to be versatile precursors for the synthesis of novel boron-containing heterocycles. The reaction between <i>N</i>-alkylnitriliumboranes and 2-aminopyridines, imidazoles, oxazoles, or isoxazoles leads to the incorporation of the [B-C] motif into a five-membered boramidine, which exists as a mixture of <i>Z</i> and <i>E</i> isomers. The resulting heterocycles are blue fluorescent in both the solid state and in solution with ca. 2700-8400 cm^-1 Stokes shifts and quantum yields in the 65-74% range in water and in the 42-84% range in organic solvents. The combination of photophysical properties, structural tunability, stability, and solubility in various media is expected to find application in a range of disciplines.