Quinolinophane-derived alkyldiphenylphosphines: two homologous P,N-planar chiral ligands for palladium-catalysed allylic alkylation

Two novel homologous [2]paracyclo[2](5,8)quinolinophane-derived P,N-bidentate planar chiral ligands have been synthesised and their effectiveness in asymmetric catalysis tested in palladium-catalysed allylic malonylation of 1,3-diphenylpropenyl acetate. The extent of asymmetric induction depends linearly on the ligand/metal molar ratio, indicating the involvement of P,P-type and chelate P,N-type π-allylpalladium complexes in equilibrium. The length of the chain that binds the diphenylphosphine group to the quinolinophane moiety, while appreciably affecting the P,N-complex stability, has little effect on the extent of asymmetric induction, which is due to the greater reactivity of the [PN]- exo–syn–syn – ([PN] xss ) with respect to the [PN]- endo – syn – syn – ([PN] nss ) π-allylpalladium complex.