Monodentate phosphoramidites; versatile ligands in catalytic asymmetric intramolecular Heck reactions

The Pd-catalysed intramolecular asymmetric Heck reaction (AHR) of cyclohexadienone monoacetals in the presence of chiral phosphoramidite ligands is described. High enantioselectivities (up to 96% ee) are reached with monodentate ligands. The highest selectivity is observed with Taddol-based phosphoramidites containing a small amine substituent. A study of the effect of the leaving group at the aryl moiety and a number of parameters in the AHR show that this AHR most likely proceeds through a neutral pathway and that two equiv. of ligand are necessary for optimal asymmetric induction.