Photochemical reactions of phenylphosphinic azides having ortho alkyl substituents: diminution of Curtius-like rearrangement; intramolecular nitrene insertion into C–H bonds

Abstract

Two types of reaction are important for the photolysis of the phosphinic azides Ar₂P(O)N₃(Ar = mesityl or 2,4,6-triisopropylphenyl) in MeCN–MeOH: Curtius-like rearrangement, giving ArP(O)(OMe)NHAr, and nitrene insertion into a C–H bond of an ortho substituent (Me or CHMe₂), giving a dihydrobenzazaphosphole oxide 11 or 12. The behaviour of Bu^tArP(O)N₃(Ar = mesityl) is similar except that two rearrangement products are formed (Bu^t or Ar migration; ratio 6:1). Extensive intramolecular insertion is unprecedented for photochemically-generated phosphorus(V) nitrenes. The exceptional behaviour of the present azides probably owes much to steric congestion. It may discourage rearrangement (Ar migration), relative to other phosphinic azides, so more nitrene is made available for insertion reactions; and it may encourage intramolecular reaction, relative to phosphoryl azides, by shielding the nitrene from external molecules (solvent) while ensuring that an internal C–H bond is held in close proximity.