Irradiation of 5-azido-8-methoxypsoralen† in water–acetonitrile gives nitrenium ion and triplet nitrene transients, but only nitrenium ion products

Abstract

Irradiation of 5-azido-8-methoxypsoralen 10 in water–acetonitrile mixtures gives essentially quantitative yields of 5-imino-5,8-dihydropsoralen-8-one 17 regardless of the composition. This quinone imine is derived from the reaction of water with the nitrenium ion that results from protonation of the initially-formed singlet nitrene. Laser flash photolysis (LFP) in water and 90∶10 water–acetonitrile reveals a transient intermediate with λ_max at 500–520 nm with the characteristics of a nitrenium ion, in particular a very effective quenching by azide ion. The immediate product of the reaction of water and the nitrenium ion is a hemiacetal; the breakdown of this species is also observed. In acetonitrile-rich solutions, the transient is a triplet nitrene (λ_max at 410 nm), and at intermediate compositions, signals for both intermediates can be seen. Since the products are derived from the nitrenium ion, a mechanism is required where the triplet nitrene decays by way of this intermediate. Evidence that this actually happens is seen in an LFP experiment in water–2,2,2-trifluoroethanol (TFE). The decay of the nitrenium ion is slowed by the TFE, and the signal at 510 nm can be observed to grow at the same time as the signal for the triplet nitrene decays at 410 nm. Two mechanisms for the triplet nitrene→nitrenium conversion can be proposed, direct protonation or reversion to the singlet nitrene. The latter is favored because 0.02 mol dm^–3 H⁺ has no effect on the rate constant for the decay of the triplet. This implies that the proton transfer step is already very fast with water as the acid, more consistent with the singlet nitrene as the species being protonated.