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) The pinacol coupling reaction of ferrocenecarbaldehyde 1 was smoothly mediated by Sm(OTf)2 to give (R,R)‐diol 2a predominantly (58 % de), whereas the use of SmI2 resulted in low selectivity. The rhodiumcomplexes of 2a were good catalysts for the asymmetric hydrogenation of α‐acetamidocinnamic acid, and the product was obtained quantitatively with up to 92 % ee.