CHIRAL PHOSPHINE-LIGANDS MODIFIED BY CROWN ETHERS - AN APPLICATION TO PALLADIUM-CATALYZED ASYMMETRIC ALLYLATION OF BETA-DIKETONES

Chiral ferrocenylphosphine ligands modified by monoaza or diaza crown ethers of varying ring sizes and linker chain lengths (8a-e, 9) were synthesized. The reaction of the phosphine ligand modified by monoaza-18-crown-6 (8b) and the di-mu-chlorobis(pi-allyl)dipalladium(II) complex in CDC13 produced the pi-allylpalladium(II) complex chelated by the two phosphorus atoms of 8b, leaving the crown ether moiety free. The H-1{H-1} nuclear Overhauser effect study of the pi-allylpalladium(II) complex suggests that the aza crown ether moiety of chiral ligand 8b is located at the proper position to interact with an incoming nucleophile. The palladium catalyst which was prepared in situ by mixing the crown ether-modified chiral ligands and Pd2(dba)3.CHCl3 was examined for stereoselectivity and catalytic activity in the asymmetric allylation of unsymmetrically substituted beta-diketones under solid-liquid, two-phase reaction conditions using potassium fluoride as an insoluble base in mesitylene. The ligands bearing monoaza-18-crown-6 or 1, 10-diaza-18-crown-6 with an appropriate length of linker chain (8b and 8d, respectively) significantly accelerated the allylation and showed fairly high enantioselectivity (up to 75% ee). It is proposed that a ternary complex involving a crown ether, a potassium cation, and an enolate anion attacks a pi-allyipalladium(II) intermediate.