Structure Determination of a Key Intermediate of the Enantioselective Pd Complex Catalyzed Allylic Substitution Reaction

The structure of a catalytic intermediate with important implications for the interpretation of the stereochemical outcome of the palladium complex catalyzed allylic substitution with phosphino–oxazoline (PHOX) ligands is determined by liquid state NMR. The complex displays a novel structure that is highly distorted compared with other palladium η2-olefin complexes known so far. The structure has been determined from nuclear overhauser data (NOE), scalar coupling constants, and long range projection angle restraints derived from dipole dipole cross-correlated relaxation of multiple quantum coherence. The latter restraints have been implemented into a distance geometry protocol. The projection angle restraints yield a higher precision in the determination of the relative orientation of the two molecular moieties and are essential to provide an exact structural definition of the olefinic part of the catalytic intermediate with respect to the ligand.