Treatment of 2-ethynylanilines with P(OPh)3 gives either 2,2-diphenoxy-2-λ(5)-phosphaquinolines or 2-phenoxy-2-λ(5)-phosphaquinolin-2-ones under transition-metal-free conditions. This reaction offers access to an underexplored heterocycle, which opens up the study of the fundamental nature of the N=P(V) double bond and its potential for delocalization within a cyclic π-electron system. This heterocycle can serve as a carbostyril mimic, with application as a bioisostere for pharmaceuticals based on the 2-quinolinone scaffold. It also holds promise as a new fluorophore, since initial screening reveals quantum yields upwards of 40%, Stokes shifts of 50-150 nm, and emission wavelengths of 380-540 nm. The phosphaquinolin-2-ones possess one of the strongest solution-state dimerization constants for a D-A system (130 M(-1)) owing to the close proximity of a strong acceptor (P=O) and a strong donor (phosphonamidate N-H), which suggests that they might hold promise as new hydrogen-bonding hosts for optoelectronic sensing.
Keywords: alkynes; dimerization; fluorescent probes; phosphazenes; phosphorus heterocycles.
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