‘Diopium’, a chiral phosphoniophosphine derived from Kaganʹs diop. Rhodium complexes and reducing catalytic properties

Cationic chiral monophosphines are devised as a novel type of ligands for catalytic transformations of polar substrates with polar reagents. A phosphonio-phosphine derived from (R,R)-diop (‘methyldiopium’) proved to act as a chiral version of Bairdʹs ω-phosphonio-phosphine ligands (‘phophos’) in Rh(I) complexes in both 1:1 and 2:1 P:Rh stoichiometry. Their versatile structure in CDCl3 solution has been studied by NMR spectroscopy in the presence of various anions (Cl−, I−, BF4−, PF6−). The in situ 2.5:1 methyldiopium–Rh catalytic system slowly hydrogenated (Z)-α-acetamidocinnamic acid in 63% conversion and 5% ee. This system was, however, specifically active in a novel hydrogen transfer procedure by a 1:1 HCOOH:NEt3 mixture under mild conditions (40 °C, in the absence of DMSO), for which the corresponding diop–Rh system was not active. Itaconic acid was thus quantitatively reduced to racemic methylsuccinic acid. (Z)-α-acetamidocinnamic acid was reduced in up to 85% conversion to N-acetylphenylalanine in various solvents. Though still moderate, the eeʹs depend on the solvent: 10% in (R) product in THF, 14% in (S) product in acetonitrile.