Mechanism of Pechmann condensation reaction – A quantum mechanical case study with choline-based ionic liquid

Abstract

At present, many researchers are aiming to develop an efficient and re-usable green catalyst for industrially relevant chemical reactions. Particularly, Pechmann condensation reaction is an important reaction that is used to prepare Coumarin derivatives which find applications in biology and medicine. In this regard, choline based ionic liquids (CILs) are reckoned as effective eco-friendly catalysts for many organic transformations. Specifically, N,N-dimethylaminoethanol [N112OH][HSO4] (NS-CIL) was found to be the effective catalyst for promoting Pechmann condensation in an efficient manner. The present work demonstrates the detailed mechanistic investigation on the three possible pathways (A, B and C) of Pechmann condensation using NS-CIL as catalyst [RSC Adv, 2014, 4, 22946–22950]. Our computed results reveal that the pathway B which involves Electrophilic attack → Water-elimination → Trans-esterification (EWT – Route) is both thermodynamically and kinetically most favourable pathway with a relatively small activation energy of 20.23 kcal/mol at the rate-determining step. Further, NS-CIL has been identified to be a vital entity in decreasing the activation barriers by acting as proton-shuttle and in stabilizing the intermediates and transition states with weak intermolecular hydrogen bonding interactions via most-acidic N–H and O–H protons of cationic unit of NS-CIL. Thus, the mechanistic insights gained through this work will aid and impel the development of more efficient ionic liquid based green catalysts for Pechmann condensation reaction and similar other industrially important reactions.