Synthesis and photolysis of phosphiranes and diphosphiranes carrying sterically protecting groups on the phosphorus atoms

Phosphiranes and diphosphiranes carrying the 2,4,6-tri-t-butylphenyl or 2,6-dimesityl-4-methylphenyl group were synthesized to serve as anticipated phosphinidene-precursors, and their photolyses were investigated by means of ³¹P and ¹H NMR, and EPR spectroscopy. Photolysis of the diphosphirane possessing the 2,4,6-tri-t-butylphenyl group on the phosphorus atoms at ambient temperature proceeded selectively to give the phosphaindan and phosphaethene derivatives. A phosphorus-centered radical was observed in an EPR study of the photolysis at cryogenic temperature, but neither the triplet phosphinidene nor triplet carbene was detected. The 2,4,6-tri-t-butylphenylphosphirane derivatives, which exclusively afforded the phosphaindan by irradiation at room temperature, also generated the same phosphorus-centered radical by the photolysis at cryogenic temperature. A novel 2,6-dimesityl-4-methylphenylphosphirane was synthesized to avoid intramolecular reaction with the ortho substituent, but introduction of the 2,6-dimesityl-4-methylphenyl group in place of the 2,4,6-tri-t-butylphenyl group dramatically decreased the photoreactivity. Thus, photolysis of the phosphirane restated in recovery of the starting material. Participation of the ortho t-butyl group can therefore be regarded as a key feature of the photolysis.