Synthesis of 2-Phosphaindolizine and [1,3]Azaphospholo[1,5-a]quinoline

The reaction of (chloromethyl)dichlorophosphine (1) with 2-[(trimethylsilyl)methyl] pyridine (6) and 2-[(trimethylsilyl) methyl] quinoline (12) in THF affords unsubstituted parent 2-phosphaindolizine (5) and [1,3]azaphospholo[1,5-a] quinoline (7). Multinuclear low-temperature NMR spectroscopy was used to investigate the reaction mechanism; a cascade of substitution and cyclization steps involving transient picolylphosphines and phosphorus heterocycles was identified. The molecular and crystal structures of two heterocyclic intermediates and of 7 were determined by single-crystal X-ray diffraction. Moreover, all intermediates and products were characterized by multinuclear H-1, C-13 and P-31 NMR spectroscopy.