Phase transfer catalysis (PTC): search for alternative organic solvents, even environmentally benign

Abstract

The use of aliphatic esters, ethers and ketones such as dimethyl carbonate (DMC), methyl-tert-amyl ether (MTAE), 5-methyl-2-hexanone (MIAK), 4-methyl-2-pentanone (MIBK) as an alternative to traditional solvents (toluene, chlorobenzene, etc.) was explored as a strategy for performing phase transfer catalyzed reactions in organic media, even environmentally benign. Experimental data show that in these media the partition of the catalyst in the organic phase is comparable or higher than that in chlorobenzene, especially in the case of scarcely lipophilic methyltributylammonium (3) or tetrabutylammonium (4) salts (Eq. (1)). Anion reactivity under PTC conditions increases with decreasing the solvent polarity, in the order: ketone<ester≈chlorobenzene<ether. Such a trend is even more pronounced in the anhydrous solvent, where in the absence of any solvation it is possible to evaluate the intrinsic reactivity of the ion pair. Results clearly indicate the oxygenated solvents examined (MTAE, DMC, MIAK, MIBK) as a valid alternative to traditional media for PTC.