Acylarylnitrosamines. Part VII. [2H] Labelling experiments and the mechanism of formation of benzyne from N-nitrosoacetanilide and benzenediazonium acetate

Abstract

Reaction of N-nitroso[2′,4′,6′-²H₃]acetanilide, formed in situ from [2′,4′,6′-²H₃]acetanilide (99%[²H₃]) and p-chlorobenzoyl nitrite, with tetraphenylcyclopentadienone and acetic acid in benzene gave biphenyl, from which no deuterium had been lost, and 1,2,3,4-tetraphenylnaphthalene which had lost one g atom of deuterium. Similar results were obtained from reaction of [2,4,6-²H₃]benzenediazonium fluoroborate (99%[²H₃]) with potassium acetate in benzene. The corresponding reaction using anthracene as a benzyne trap gave similar result. These results exclude the operation of a ‘pre-equilibrium’ type of E₁cB mechanism in the formation of benzyne from the benzenediazonium acetate ion pair, but do not allow a distinction to be made between E₁cB (irreversible) or concerted E₂ mechanisms, particularly in view of the low (1·5–1·8) isotope effects observed with [2-²H]benzenediazonium salts.