Delineating the Mechanism of Ionic Liquids in the Synthesis of Quinazoline-2,4(1H,3H)-dione from 2-Aminobenzonitrile and CO2
Abstract
Ionic liquids (ILs) are versatile solvents and catalysts for the synthesis of quinazoline-2,4-dione from 2-aminobenzonitrile and CO2. However, the role of the IL in this reaction is poorly understood.
Consequently, we investigated this reaction and showed that the IL cation does not play a significant role in the activation of the substrates, and instead plays a secondary role in controlling the physical properties of the IL. A linear relationship between the pK(a) of the IL anion (conjugate acid) and the reaction rate was identified Tvith maximum catalyst efficiency observed at a pK(a) of > 14.7 in DMSO.
The base-catalyzed reaction is limited by the acidity of the quinazoline-2,4-dione product, which is deprotonated by more basic catalysts:, leading to the formation of the quinazotide anion (conjugate acid pK(a) 14.7). Neutralization of the original catalyst and formation of the quinazolide anion catalyst leads to the observed reaction limit.