Solvent mediated catalysis and proton-shuttling in the formation of 3-methylphthalide from a ketene intermediate

Abstract

Photoexcitation of o-acetylbenzaldehyde (oABA) in acetonitrile yields, via photoinduced intramolecular hydrogen transfer, the lactone 3-methylphthalide (3MP) with a quantum yield of ca. 0.3. Femtosecond stimulated Raman spectroscopy (FSRS) measurements revealed the formation of a Z-ketene-enol ground state intermediate during this process ca. 2–3 ps after excitation [Phys. Chem. Chem. Phys. 2015, 7, 376]. UV traces furthermore indicate the occurrence of a cyclic enol form shortly before the reaction proceeds towards the lactone. To obtain insight into the reaction mechanism and to clarify the potential role of a cyclic enol, we have identified stationary points along the oABA – 3MP ground-state reaction path with DFT methods applying implicit/explicit solvation models. The computations show that the reaction can efficiently proceed with or without involvement of the cyclic enol by a) catalysis through traces of water in the solvent or b) directly via acetonitrile proton shuttling.