Acyloxy-shifts in open and closed shell systems–intramolecular nucleophilic substitution reactions in disguise
The 1,3-acyloxy shift in allyl formate is compared to the 1,2-acyloxy-shift in the 2-formylethyl radical at various theoretical levels. The Becke3LYP hybrid density functional gives the best agreement with experiment. The barrier for the closed shell rearrangement is 22 to 27 kcal mol–1 higher than for the open shell transformation. Both reactions can be described as intramolecular nucleophilic substitution reactions, in which the formyloxy group functions as both the nucleophile and the leaving group. In the framework of the valence bond curve crossing model, the difference in the barriers is mainly due to the energy required to excite the CC double bond to the triplet state.