X-ray crystallographic and NMR spectroscopical characterization of intermediates in the Pd-catalyzed allylic substitution reaction with 4-substituted phosphinooxazolines. Correlation between intermediate structure and product configuration

Abstract

The novel P,N-ligand 2-[2-(diphenylphosphino)phenyl]-4,5-dihydro-4-methyl-4-phenyloxazole (2a) has been synthesized. The corresponding [PdII(η3-diphenylallyl) (2a)]PF6 (3a) and [PdII(η3-1,3-dimethylallyl) (2a)]PF6 (4a) complexes have been studied by X-ray analysis and NMR spectroscopy. 3a exists as exo–syn–syn isomer in the solid state. In solution, the same isomer predominates. The X-ray structure of 4a reveals that the oxazoline ligand is coordinated in a pseudo-enantiomeric conformation compared with 3a. A syn–anti arrangement of the allyl substituents is favored in the solid state. NMR spectroscopical investigations suggest a formation of six isomers in solution due to endo–exo orientation of the allyl moiety and syn–anti isomerization of the methyl substituents. NMR data of [Pd0(η2-dimethylfumarate)(phosphinooxazoline)] complexes give evidence that two isomers exist in solution. The isomeric ratio is strongly dependent on the steric bulk of the oxazoline substituents. The solid state structures of [Pd0(η2-dmfu) (2c)] and [Pd0(η2-dmfu) (1a)] confirmed the structures of the main isomer found in solution. The asymmetric allylic substitution reaction of 1,3-diphenylallyl acetate with dimethyl malonate catalyzed by 3a proceeds with a selectivity of 97% ee. The ee induced by 2a in catalytic allylic substitution of 1-methylbutenyl acetate is moderate (18%). A comparison of the intermediate 4a and 5a as model of the actual olefinic intermediate suggests that the poor enantioselectivity achieved with ligand 2a is due to the preferred formation of anti-isomers of the allylic intermediate 4a and the conformational instability of the complex.