Organopalladium complex promoted asymmetric synthesis of a P-chiral phosphanorbornene in ionic liquids and in organic solvents

Abstract

The organopalladium(II) complex containing the ( S)-form of ortho-palladated (1-(dimethylamino)ethyl)naphthylalene has been successfully utilised as a chiral template to promote the asymmetric endo-cycloaddition reaction between coordinated 3,4-dimethyl-1-phenyl-phosphole and acrolein. The rate of this chiral template promoted reaction is dramatically affected by the solvent and temperature. In dichloromethane, the intermolecular cycloaddition reaction at room temperature gave a 2:1 mixture of the diastereomeric endo-substituted formyl-phosphanorbornene template complexes in 35 days. The major diastereomer could be isolated by fractional crystallization. The absolute configurations and the coordination properties of the endo-formylphosphines in the isolated template complex have been established by X-ray crystallography. The enantiomerically pure endo-cycloadduct (−)-5-(formyl)-2,3-dimethyl-7-phenyl-7-phosphabicyclo[2.2.1]hept-2-ene was obtained by the treatment of the major product with dppe. When the endo-cycloaddition reaction was conducted in the ionic liquid 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim][BF 4]), the same 2:1 diastereomeric mixture was obtained in two days. When the reaction temperature was raised to 85 °C, the reaction generated the two diastereomeric endo-cycloadducts as a 1:1 mixture in 2 h. Similarly, a 1:1 mixture was obtained when the reaction was heated at 85 °C in 1,2-dichloroethane for 2 days.