Unusually Facile Thermal Homodienyl-[1,5]-Hydrogen Shift Reactions in Photochemically Generated Vinyl Aziridines

A range of photochemically generated tri- and tetracyclic vinyl aziridines have been found to undergo a general and surprisingly low temperature ring opening through a [1,5]-hydrogen shift reaction. The rate of the process was found to be highly dependent on the structure and substitution around the azirdine ring and the alkene terminus, with some substrates being observed to undergo ring opening at temperatures as low as 25ºC. The rigid nature of these polycyclic systems precludes a conformational explanation of these rate differences, and an Eyring study confirmed a negligible entropic barrier to the reaction. However, the Eyring plots for two different aziridines systems showed a significant difference in their enthalpies of activation. It is therefore believed that the levels of aziridine ring strain, as well as electronic effects, are the dominant factors in this sequence.