Quantum chemical modeling of superbase-catalyzed reactions of acetophenone and methyl mesityl ketone with acetylene

The mechanisms of furan formation and the competitive reaction of 1,5-diketone formation from 1-mesitylbut-2-en-1-one ( 1a ) and 1-phenylbut-2-en-1-one ( 1b ) were studied using the B3LYP/6-311++G**//B3LYP/6-31+G* quantum chemical approach taking into account solvation effects in terms of the IEFPCM model. The addition to 1a of the ethynide ion to form β-acetylene enolate ion ( 2a ) occurs with the activation energy Δ G ≠ = 13.3 kcal mol –1 , while the further intramolecular O -vinylation with the closure of 2a to the dihydrofuran cycle of the dihydrofuran carbanion is the rate-determining step in the reaction pathway of substituted furan formation needing the activation energy G ≠ = 19.5 kcal mol –1 . The alternative addition of the mesitylethenolate ion to 1a to form the 1,5-diketone carbanion is associated with a higher (by 3.4 kcal mol –1 ) free activation energy than that of ethynide ion addition. In contrast, in the case of 1b , the addition at the C=C bond of the phenylethenolate ion rather than of the ethynide ion to form the 1,5-diketone carbanion is more preferable, since its activation free energy is 1.5 kcal mol –1 lower. The quantum chemical calculation results are consistent with the experimental data.