Novel P,N Ligands Derived from (R)‐ and (S)‐1‐Phenylethylamine with (2R,5R)‐2,5‐Dimethylphospholanyl Groups (DuPHAMIN) for Asymmetric Catalysis

Abstract

AbstractNovel P,N chelating ligands with three stereogenic centres in positions α to the donor atoms have been prepared in good yields and high enantiomeric purity by a series of reactions beginning with either (R)‐ or (S)‐1‐phenylethylamine. These isomers were first lithiated with tBuLi and converted into aminophosphanes 3a by treatment with (Et2N)2PCl. They were transformed into the enantiomeric primary phosphanes 4 by quantitative solvolysis with ethanol to give the phosphonous acid esters 3b and subsequent reduction with LiAlH4. Stepwise treatment of 4 with nBuLi and the cyclic sulfate of (S,S)‐2,5‐hexanediol gave the isomeric tertiary phosphanes (R,RR)‐5 and (S,RR)‐5. Treatment of the aminophosphane (R)‐3a with (R)‐2,2′‐binaphthol yielded the aminophosphonite (R,R)‐6 in high yields. Cationic (COD)Rh complexes of 5 were prepared by treatment with [Rh(COD)Cl]2, the PF6− salts 7a being isolated. Analogous (2,5‐norbornadiene)Rh complexes 7b were prepared similarly. The chelate nature of the P,N ligands in these salts is indicated by their NMR spectra in solution and is confirmed by the crystal structures of (R,RR)‐7a and (S,RR)‐7a. With the exception of (S,RR)‐7a, the complexes 7a and 7b are shown to catalyse the asymmetric hydrogenation of methyl acetamidocinnamate at room temperature. Differences in the catalytic activity of the complexes are related to the different dynamic behaviour detected by the 1H EXSY spectra for the COD ligands of 7a. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)