Facile Synthesis and Properties of 2-λ(5)-Phosphaquinolines and 2-λ(5)-Phosphaquinolin-2-ones.

Treatment of 2-ethynylanilines with P(OPh)3 gives either 2,2-diphenoxy-2-λ5-phosphaquinolines or 2-phenoxy-2-λ5-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 NPV 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 (PO) and a strong donor (phosphonamidate NH), which suggests that they might hold promise as new hydrogen-bonding hosts for optoelectronic sensing.