Intramolecular cyclization of β-nitroso-o-quinone methides: A theoretical endoscopy of a potentially useful innate 'reclusive' reaction

Oxidatively generated β-nitroso-o-quinone methides undergo an o- and/or peri-intramolecular cyclization to arene-fused 1,2-oxazoles, 1,2-oxazines or indoles. The reaction, found to be an innate process, has been scrutinized by DFT/B3LYP and MP2 calculations. Due to its rapidity, the process has been termed a 'reclusive' one. Competing o-(1,5)- and peri-(1,6)- or (1,5)-cyclizations advance via successive transition states. Activation barriers are drastically lowered in AcOH, probably through H hopping or tunnelling whereas they are barely reduced in other solvents. Aromaticity indices, such as HOMA, I_A and ABO, have been used to assess the stability of the end-heterocycles and the preponderance of any one of them. Thus, the preferred cyclization mode, that is, the prevalence or exclusive formation of one of the heterocycles, appears to be oxidant-directed rather than determined by the quinone methide geometry. The question of the peri-cyclization, being a primary or a secondary process, has been tackled.