COMPUTATIONAL INSIGHTS ON THE LONE PAIR INDUCED BARRIER MODULATION IN THE THERMAL REARRANGEMENT OF 6-HALO-2-PYRONES

Abstract

2-pyrones undergo intramolecular thermal rearrangement resulting in the migration of groups at 3-position to 5-position and vice-versa. Rearrangement of 6-halopyrone is a tandem process involving electrocyclic ring opening and closure (ERO & ERC), rotation, sigmatropic shift. It has been modeled at MP2/6-31g (d) level to understand the migratory aptitude of the halogens. Computations show that electrocyclic transition state and the corresponding intermediate which could not be located in 2-pyrone rearrangement have been located for 6-fluoropyrone and 6-chloropyrone. Halogens effectively modulate the barriers of all the steps involved and NBO (Natural Bond Orbital) analyses clearly reveal the involvement of lone pairs in activating rotation and sigmatropic shift, and deactivating ERO and ERC.