Synthesis, X-ray crystal-structure analysis, and NMR studies of (eta(3)-allyl )palladium(II) complexes containing a novel dihydro(phosphinoaryl)oxazine ligand: Application in palladium-catalyzed asymmetric synthesis

The novel chiral P,N-ligand 2-[2-(diphenylphosphino)phenyl]-5,6-dihydro-4-p henyl-4H-1,3-oxazine (4) was synthesized. The corresponding {dihydro[(phosp hino-xP)aryl]oxazine-xN} (eta(3)-diphenylallyl)palladium(II) hexafluorophos phate 5 and the analogous [Pd(eta(3)-1,3-dimethylallyl)] complex 6 were inv estigated by X-ray analysis and 1D- and 2D-NMR spectroscopy. The complex 5 exists as 'exo'-syn-syn isomer in the solid state (Fig. 1). In solution, th e same isomer exceeds with 90%. The X-ray crystal structure of 6 reveals th at the dihydro(phosphinoaryl)oxazine ligand coordinates in a pseudo-enantio meric conformation compared with that of 5 (Fig. 3). A syn-anti arrangement of the allyl substituents of 6 is favored in the solid state. H-1-NMR Spec troscopic investigations suggest that the auxiliary 6 adopts two conformati ons. This conformational instability together with 'exo'/'endo' and syn/ant i isomerization leads to the formation of 6 isomers (Fig. 4). The asymmetri c allylic substitution reaction of 1,3-diphenylallyl acetate with dimethyl malonate in the presence of 4 proceeds with a selectivity of 99% ee. The ee induced by 4 in the catalytic allylic substitution BE 1-methylbut-2-enyl a cetate is moderate (54%).