Sulfated Mesoporous Alumina:  A Highly Effective Solid Strong Base Catalyst for the Tishchenko Reaction in Supercritical Carbon Dioxide

Heterogeneous strong base catalysis for the intramolecular Tishchenko reaction of aromatic 1,2-dicarbaldehydes to the corresponding phthalides in supercritical CO2CscCO2 has been realized with mesoporous alumina containing SO4 2- ions in the alumina framework (mesoAl2O3/SO4 2-). Infrared spectroscopy of pyrrole adsorbed on the alumina and strong poisoning by a weak Brönsted acid of methanol revealed that the SO4 2- ions in the framework slightly suppressed the average strength of base sites (O2-) on mesoAl2O3/SO4 2-, but there exists a small number of strong base sites that promote the Tishchenko reaction in scCO2. Although the intramolecular Tishchenko reaction of phthalaldehyde to phthalide in scCO2 was somewhat slower than those in organic solvents such as tetrahydrofuran (THF) and benzene, the addition of a small amount of THF as a cosolvent remarkably increased the reaction rate; the reaction in the scCO2−THF system proceeded 1.5-fold faster than those in pure benzene and THF solvents.