[1,2]-ACYLOXY SHIFTS IN RADICALS - A COMPUTATIONAL INVESTIGATION OF SUBSTITUENT AND SOLVENT EFFECTS

Three possible pathways for the acyloxy rearrangement in (acyloxy)ethy l radicals have been investigated with ab initio quantum chemical meth ods. For migrations of formyloxy, acetyloxy, and (trifluoroacetyl)oxy groups the concerted [3,2]-acyloxy shift represents the most favorable pathway. The barrier for the concerted [1,2]-acyloxy shift is higher by approximately 4 kcal/mol in all three cases, while a stepwise mecha nism through a ring-closure/ring-opening sequence is even less favorab le. Calculations using the hybrid Becke3LYP density functional and a v ariety of basis sets predict the barrier for the acetyloxy shift to be higher by 3 kcal/mol than the barrier for the (trifluoroacetyl)oxy sh ift. This value is less than half what has been determined experimenta lly before. The effects of aqueous solvation have been studied with th e AM1-SM2 and the SCIPCM continuum models and by explicitly including two water molecules into the system. In all three cases, only small so lvent effects have been found for the acetyloxy rearrangement, in cont rast to previous experimental evidence. Proton catalysis has been stud ied for the formyloxy rearrangement at the Becke3LYP level and has bee n found to lower the gas phase barrier by more than 10 kcal/mol. On th e basis of these results, it is suggested that acid catalysis might be an important aspect of acyloxy rearrangements in solution.