Acidic ionic liquids based on phosphonium chloride and metal chlorides-recyclable media and catalysts in the Diels-Alder reaction

Acidic ionic liquids based on trihexyltetradecylphosphonium chloride-P_6.6.6.14 Cl and six different metal chlorides (YCl₃, YbCl₃, MgCl₂, ZnCl₂, CuCl₂, InCl₃) were prepared. The molar fraction of metal chloride (X_MClx) used for the formation of acidic ionic liquids with P_6.6.6.14 Cl was changed in the range from 0.3 to 0.5. The high catalytic activity in the Diels-Alder reaction of cyclopentadiene and various αβ-unsaturated carbonyl compounds showed ionic liquids at X_MClx≤0.4. In general, the highest product yields and endo:exo ratios were achieved in the acidic ionic liquids formed from MgCl₂, YCl₃ and YbCl₃. Thermogravimetric analysis suggested that the prepared ionic liquids were thermally stable up to nearly 400°C. Additionally, they could be reused with only little loss of catalytic activity after the 4th recycle.