Dialkyl Effect on Enantioselectivity: π-Stacking as a Structural Feature in P,N Complexes of Palladium(II)

A phosphino,oxazoline P,N-bidentate ligand, 4, containing 3,5-di- tert -butylphenyl groups has been prepared. In the Heck arylation of dihydrofuran, 4 is shown to afford higher ee's than either 2 or 3, the unsubstituted and m -dimethylphenyl analogues, respectively. Several Pd(0) complexes of 4 are reported. The exchange dynamics of Pd( 4 )(dba) are shown to involve an interconversion of diastereomers via an intramolecular process. The X-ray structure for PdCl 2 ( 4 ), 8, was determined by X-ray diffraction methods. Comparison of data with PdCl 2 ( 2 ), 9, and PdCl 2 ( 3 ), 10, suggests that differing amounts of π−π stacking influence the structures of these relatively simple Pd complexes, with 9 and 10 revealing the strongest π−π interactions. An estimation of the van der Waals energies involved in the interaction supports a ca. 4 kcal/mol stabilization via π−π stacking.