Facile Synthesis of Unsymmetrical 9-Phospha- and 9-Arsafluorenes

Unsymmetrical 9-chloro-9-phosphafluorenes (dibenzophospholes) and 9-chloro-9-arsafluorenes (dibenzoarsoles) have been obtained by simple thermolysis of m-terphenyldichlorophosphines and -arsines in close to quantitative yields. The reaction temperatures are about 200 °C for the phosphines and 140 °C for the arsine, and the reactions are complete within 5 min. Alternatively, these compounds can be synthesized through an AlCl₃-catalyzed Friedel−Crafts type ring-closure reaction at low temperatures, but this method suffers from difficult workup procedures. The P(As)−Cl functionality is readily alkylated. Methylation of m-xylyl derivative 4 afforded 1-(3,5-dimethylphenyl)-6,8,9-trimethyl-9-phosphafluorene, 11. The latter compound formed the complexes 11·Fe(CO)₄, 12, and 11·RuCl₂(η⁶-p-cymene), 13, indicating its good donor properties. The new compounds have been characterized by ¹H, ¹³C{¹H}, and ³¹P{¹H} NMR spectroscopy; mass spectrometry; and single-crystal X-ray crystallography in the case of 11, 12, and 13.