Better Performance of Monodentate P-Stereogenic Phosphanes Compared to Bidentate Analogues in Pd-Catalyzed Asymmetric Allylic Alkylations

The cationic allylpalladium complexes 3a-3f, 4a, 4e, 5e of type [Pd(eta(3)-2-Me-C3H4)P-2]PF6 were synthesized using a group of monodentate P-stereogenic phosphanes, P=PPhRR' (a-f) and diphosphanes (PhRPCH2)(2) (1a, 1e) or PhRPCH2Si(Me)(2)CH2PPhR (2e). The analogous cationic complexes with the disubstituted allyl group (eta(3)-1,3-Ph-2-C3H3) and monodentate phosphanes were not isolated as stable solids; only [PdCl(eta(3)-1,3-Ph-2-C3H3)P] (6a, 6d) were obtained. Palladium allyl complexes were screened as precatalysts in the allylic substitution of rac-3-acetoxy-1,3-diphenyl-1-propene (I) and (E)-3-acetoxy-1-phenyl-1-propene (III) with dimethyl malonate as the nucleophile. The various catalytic precursors showed a wide range of activity and selectivity. The bismonodentate phosphane complexes 3 are more active than the bidentate analogues. With regard to the regioselectivity, precursors containing monodentate phosphanes favour the formation of the linear product in the allylic substitution of cinnamyl acetate (III) compared with those containing bidentate phosphanes. With substrate I, compounds with the diphosphanes 1a and 1e, containing a five-membered chelate ring, gave low enantioselectivities (less than 10% ee), but those with the diphosphane 2e, forming a six-membered chelate ring or with two monodentate phosphanes, afforded products with moderate enantioselectivity under standard conditions (ee up to 74 %). The results show that the performance of precursors containing monodentate phosphanes was superior to those containing bidentate ligands in both activity and selectivity.