The photo-Wallach rearrangement. Heavy-atom kinetic isotope effects and mechanism

The photo-rearrangement of mixtures of azoxybenzene 4 and, successively, [15N, 15N′]4, [18O]4, and [2-14C]4 were carried out. Kinetic isotope effects (KIE) were calculated from measurements of isotopic ratios in both recovered 4 and the product, 2-hydroxyazobenzene (6). Analogous rearrangement of mixtures of 2,2′-azoxynaphthalene (8) with [15N, 15N′]8 and [1,1′-13C2]8 were carried out and KIE were calculated from isotope ratios in the product. The results (particularly the lack of nitrogen KIE) collectively indicate that if an oxadiazole-like intermediate is involved in these rearrangements, an activation barrier exists in its formation rather than its decomposition.