A new route to achiral and chiral 1,2-bis(phosphino)ethanes, 1-arsino-2-phosphinoethanes, and 1,3-bis(phosphino)propanes and the molecular structure and catalytic activity of some rhodium(i) complexes derived thereof

Abstract

A series of unsymmetrical 1,2-bis(phosphino)ethanes R₂PCH₂CH₂PR′₂ (2a–d) and 1-arsino-2-phosphinoethanes R₂AsCH₂CH₂PR′₂ (3a–c) mainly with bulky substituents R and R′ were prepared from the cyclic sulfate 1 by stepwise cleavage of the carbon–oxygen bonds by LiPR₂ and LiPR′₂ or LiAsR₂ and LiPR′₂, respectively. Analogously, racemic mixtures of R₂PCH₂CH(Me)PPh₂ (R = iPr 5a, Cy 5b) as well as the enantiomers (R)-5a, (R)-5b and (R)-tBu₂PCH₂CH(Me)PPh₂ (R)-5c were obtained from the corresponding unsymmetrical cyclic sulfates 4 and (S)-4. On a similar route, the racemates of the 1,3-bis(phosphino)propanes R₂PCH₂CH₂CH(Me)PPh₂ (R = iPr 7, tBu 8), optically pure (R)-8 and (S,S)-iPr₂PCH(Me)CH₂CH(Me)PPh₂ (S,S)-10 were prepared. The reaction of [{RhCl(η⁴-C₈H₁₂)}₂] with chelating ligands L–L, where L–L is R₂PCH₂P(men)₂ (R = iPr, Ph; men = (1S,2R,5S)-menthyl), Cy₂AsCH₂P(men)₂, or (R)-5a, (R)-5b, (R)-5c, (R)-8 and (S,S)-10, in the presence of AgPF₆, gave the complexes [Rh(η⁴-C₈H₁₂)(L–L)]PF₆ (12a–h) which were used as pre-catalysts in the hydrogenation of the methyl ester of α-acetamidocinnamic acid (ACM). Depending on L–L, the solvent, the temperature and the pressure of H₂, optical yields of up to 69% ee were achieved. For two of the rhodium complexes, 12c and 12d, the molecular structures were determined by X-ray crystallography.