Synthesis of phthalide–skeleton using selective intramolecular Tishchenko reaction over solid base catalysts

An efficient, economical, and environmentally benign method for the synthesis of phthalide–skeleton using heterogeneous catalytic intramolecular Tishchenko reaction with solid bases is described. Among the solid base catalysts examined, MgO, CaO, and SrO exhibited high catalytic performances for the intramolecular Tishchenko reaction of o-phthalaldehyde to yield phthalide exclusively in excellent yields at 313 K in the short time span of 0.25 h. Application of γ-alumina to the intramolecular Tishchenko reaction of o-phthalaldehyde was also successful; phthalide was obtained selectively in excellent yields at 313 K in 4 h. The employment of KF/alumina and KOH/alumina at 313 K for 4 h resulted in the selective formation of phthalide in moderate yields. The heterogeneous catalytic systems realized by the use of CaO and γ-alumina were also successfully applicable to the selective intramolecular Tishchenko reaction of 2,3-naphthalenedicarbaldehyde to give the corresponding five-membered lactone in excellent yields at 333 K for 2 and 20 h, respectively. Based on the reaction results, the infrared spectra of adsorbed o-phthalaldehyde, and the quantum chemical calculations conducted at the PM3-MO level of theory for elucidation of the molecular and electronic structures of o-phthalaldehyde and the potential intermediates, a plausible reaction mechanism for the intramolecular Tishchenko reaction of o-phthalaldehyde to phthalide over MgO and γ-alumina was proposed. The catalytically active species should be o-MOCH2C6H4CHO (M=Mg or Al).