Peroxotantalate-Based Ionic Liquid Catalyzed Epoxidation of Allylic Alcohols with Hydrogen Peroxide.

The efficient and environmentally benign epoxidation of allylic alcohols has been attained by using new kinds of monomeric peroxotantalate anion-functionalized ionic liquids (ILs=[P4,4,4,n ]3 [Ta(O)3 (η-O2 )], P4,4,4,n =quaternary phosphonium cation, n=4, 8, and 14), which have been developed and their structures determined accordingly. This work revealed the parent anions of the ILs underwent structural transformation in the presence of H2 O2 . The formed active species exhibited excellent catalytic activity, with a turnover frequency for [P4,4,4,4 ]3 [Ta(O)3 (η-O2 )] of up to 285 h-1 , and satisfactory recyclability in the epoxidation of various allylic alcohols under very mild conditions by using only one equivalent of hydrogen peroxide as an oxidant. NMR studies showed the reaction was facilitated through a hydrogen-bonding mechanism, in which the peroxo group (O-O) of the peroxotantalate anion served as the hydrogen-bond acceptor and hydroxyl group in the allylic alcohols served as the hydrogen-bond donor. This work demonstrates that simple monomeric peroxotantalates can catalyze epoxidation of allylic alcohols efficiently.