Synthesis of Phosphanyl Sulfoximines Through Phospha-Michael Reaction of Alkenyl Sulfoximines and Their Evaluation as Chiral Bidentate 1,5-N,P Ligands for Palladium in Asymmetric Allylic Alkylation

The intermolecular phospha-Michael reaction of cyclic and acyclic alkenyl sulfoximines proceeds readily and yields the corresponding phosphanyl sulfoximines in good yield. The asymmetric induction provided by sulfoximine group in C-P bond formation is apparently only low. The configuration of three phosphanyl sulfoximine-boranes has been determined by X-ray crystal structure analysis. The ability of the phosphanyl sulfoximines to act as ligand in Pd-catalyzed asymmetric allylic alkylation was studied with pairs of diastereomeric cyclic and acyclic derivatives carrying different substituents at the N atom. This study showed that the substituent at the N atom and both the chirality of the backbone and sulfoximine group play a crucial role in determining the enantioselectivity of the allylic alkylation. The Pd-catalyzed reaction of the racemic 1,3-diphenylallyl acetate with the malonate anion in the presence of a cyclic N-benzyl-substituted phosphanyl sulfoximine gives the corresponding malonate with 97 % ee in 98% yield. The similar alkylation with dialkyl-substituted allylic acetates proceeds only with medium enantioselectivity. The bidentate 1,5-N,P-coordination of the Pd atom by the phosphanyl sulfoximine was confirmed by NMR experiments of a pi-1,3-diphenylallyl-Pd-II complex containing a cyclohexyl phosphanyl sulfoximine as ligand. The cyclohexane ring of the free cyclic phosphanyl sulfoximines adopts in solution and in the crystal a conformation in which the sulfoximine and phosphanyl group are both in a pseudo axial position. The coordination of the ligand to the Pd atom causes an inversion of the cyclohexane ring, which places the two groups in equatorial position.