The photohydration of N-alkylypyridinium salts: Theory and experiment

Bicyclic aziridines formed by the irradiation of pyridinium salts in basic solution have recently been recognized to have great synthetic potential. W e have undertaken a joint computational and experimental investigation of t he mechanism of this photoreaction, We have computationally determined the structures and relative energies of the relevant stationary points on the l owest potential energy surface (PES of the pyridinium and methylpyridinium ions. Two important intermediates are shown to he hound minima on the groun d-state PES: azoniabenzvalene and a 6-aza[3.1.0]bicyclic ion with an exo-or iented substituent (analogous to prefulvene). We advance a mechanism which involves initial formation of this exo-bicyclic ion, followed by nitrogen m igration around the ring via the azoniabenzvalene intermediate, Thus, the b arrier separating the two intermediates is the factor that determines the d egree of scrambling observed in the photoproducts when the carbon atoms are labeled with deuterium or substituted with additional methyl groups. For N -methylpyridinium, the exo-methyl bicyclic ion was computed to be approxima te to1 kcal mol(-1) lower in energy than N-methyl-azoniabenzvalene. The tra nsition state was computed to lie several kcal mol(-1) above the exo-methyl bicyclic ion (+8.4 kcal mol(-1). 6-31G* RHF: +3.7 kcal mol(-1), 6-31G* B3L YP), but still well below the energy available from the 254 nm excitation o f the N-methylpyridinium ion. The computed relative energies correspond spl endidly with several experimental findings which include the preference for exo products the results of deuterium labeling, and the impact of addition al substituent methyl groups on the product distribution.