New bimolecular mechanistic pathway for 1,3-hydrogen shift in allenamide and allene system: A theoretical prediction

Thermal isomerization of allenes or allenamides are commonly viewed as a [1,3] hydrogen shift process from a substituent alkyl group to the central sp-hybridized carbon atom in the molecule. Though it is a common believe that the reaction is feasible in antarafacial migration of hydrogen, the mechanistic pathway has to cross a huge activation barrier to carry out the process. Here we have proposed several bimolecular pathways that are found to be energetically more favorable than the commonly believed mechanism of antarafacial migration of hydrogen. The carboxylic acid catalyzed reaction requires minimum activation energy and may be considered as the most favorable process. The mechanism involves the proton transfer through a relay process. Solvent acetonitrile, can catalyze the isomerization through a tandem ene/retro-ene pathway and demands higher activation energy. Other mechanisms through a similar tandem process may be capable to isomerize allenamide and allene system through self catalysis and have to cross similar activation barrier as shown by acetonitrile catalyzed mechanism. All the mechanistic pathways are validated by DFT calculation using B3LYP/6-31G* method.