Divalent Samarium Triflate Mediated Stereoselective Pinacol Coupling of Planar Chiral Phosphanyl and Phosphoryl Ferrocenecarbaldehyde

The pinacol coupling reaction of ( Rp )‐2‐diphenylphosphanyl ferrocenecarbaldehyde ( 1 ) was smoothly mediated by divalent samarium triflate to give ( R , R )‐diol 2a predominantly, whereas the use of samarium(II) iodide resulted in low selectivity as described in the previous literature. In contrast, the coupling reaction of ( Rp )‐2‐diphenylphosphoryl ferrocenecarbaldehyde ( 3 ) with Sm(OTf) 2 gave the ( S , S )‐diol as the major isomer, which was the opposite stereochemistry of that obtained in the reaction with 1 . The rhodium complexes of diphosphanes 2a were good catalysts for the asymmetric hydrogenation of α‐acetamidocinnamic acid, and the product was obtained quantitatively with up to 92 %  ee . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)