Environmentally-benign palladium(II)-exchanged hydroxyapatite-catalyzed allylic alkylation of allyl methyl carbonate in water Original Research Article

Palladium(II)-exchanged hydroxyapatite (PdHAP; for example, PdHAP-B has a Pd/Ca ratio = ca 1/200) was prepared by ion-exchanging with Ca2+ from calcined hydroxyapatite and Pd(NO3)2 in water at 70 °C for 24 h. The ion-exchange was revealed by inductively coupled plasma analysis; Pd2+ was almost absent in any filtrate that included over 1.6 equimolar amounts of Ca2+ relative to the amount of Pd2+ consumed. The PdHAP (1 mol% Pd) functioned as a catalyst for the allylic alkylation of allyl methyl carbonate with active methylene compounds such as diethyl malonate, 2-ethoxycarbonylcyclopentanone, 2-ethoxycarbonylcyclohexanone, and 2,2-dimethyl-1,3-dioxane-4,6-dione at 50 °C in water under air. Over 97% of the PdHAP was recovered by centrifugation followed by decantation, and the material could be reused. The catalytic activity of PdHAP accompanied by satisfactory yields of monoallylated products was maintained in ten repetitive uses for the allylic alkylation of allyl methyl carbonate with diethyl malonate at 50 °C for 24 h in water under air. From the standpoints of yield and handling, using water as a solvent was superior to using an organic medium such as ethanol, THF or DMF. Thus, this heterogeneous PdHAP-catalyzed allylic alkylation will be one of the environmentally-benign organic syntheses.