Oxa-Michael Addition Catalyzed by Amide-Based Acidic Ionic Liquids

A series of acidic ionic liquids, including N-methyl-2-pyrrolidonium dihydrogen phosphate ([NMPH]H2PO4), caprolactam dihydrogen phosphate ([NHCH]H2PO4), N,N'-dimethylformamide dihydrogen phosphate ([DMFH]H2PO4), and N,N'-dimethylacetamide dihydrogen phosphate ([DMEH]H2PO4), were synthesized and characterized. Oxa-Michael addition of beta-phenylethanol to methyl vinyl ketone (MVK) was used as a model reaction. The effects of cation structure, ionic liquid amount, beta-phenylethanol/MVK ratio, reaction temperature, and reaction time were measured. Under the optimized conditions of n(beta-phenylethanol):n(MVK) = 1:2, 25 degrees C, 24 h and using [NMPH]H2PO4 as catalyst, the P-phenylethanol conversion was 95%. The ionic liquid was stable and could be reused at least 5 times with a slight loss of activity. All the amide-based acidic ionic liquids gave higher conversion compared with imidazolium acidic ionic liquid, which showed that the effect of cation structure was important. A possible reaction mechanism was proposed and the evidence for the role of amide-based acidic ionic liquids in Oxa-Michael addition was given.