Oppenauer's oxidation by paraformaldehyde of piperonyl alcohol to heliotropine

Abstract

3,4-Methylenedioxybenzaldehyde [1-3-benzodioxole-5-carboxaldehyde or heliotropine (HEL)] is widely used in fragrance and pharmaceutical preparations. The synthesis of HEL by Oppenauer's oxidation of the corresponding 3,4-methylenedioxybenzyl alcohol [piperonyl alcohol (PA)] has been investigated using both homogeneous and heterogeneous catalysts. Commercial paraformaldehyde (PFA) was used as hydrogen acceptor and the oxidation proceeded with high activity and selectivity, operating in liquid phase at 383 K, with toluene as solvent and a molar ratio PFA/PA = 2.5. In all cases, a critical step was the removal of the possible water coming from the reactants. Homogeneous Al iso-propoxide catalyst gave rise to a very fast and selective reaction, although presenting relevant economic and environmental drawbacks. Among the heterogeneous catalysts, best initial results were obtained using commercial zirconia-type catalysts, with high yield values regardless of catalyst amount and reagent concentration. However, these catalysts cannot be regenerated. Calcined Mg/Al commercial hydrotalcites not only showed good catalytic results (with only lower reaction rates lower than zirconia-type ones), but also an almost complete recovery of the initial activity after calcinations at 823 K.