Synthesis, characterization and diastereoselective coordination of a planarly chiral, hybrid ferroceneligand, (Sp)-2-(diphenylphosphino)ferrocenecarboxylic acid

Abstract

Hydrolytic cleavage of the dihydrooxazole ring in (S_p)-2-{2-(diphenylphosphino)ferrocenyl}-4-(1-methylethyl)-4,5-dihydrooxazole affords the planarly chiral functionalized phosphine, (S_p)-2-(diphenylphosphino)ferrocenecarboxylic acid, (S_p)-Hpfc, in two steps and 65% yield. (S_p)-Hpfc, all intermediates and the corresponding phosphine oxides were characterized by elemental analysis, multinuclear NMR and IR spectroscopy, and by optical rotation measurements. Solid-state structures of the phosphine oxides were further studied by X-ray crystallography to reveal extensive hydrogen bonding of various types. Neutralization of (S_p)-Hpfc with tert-BuOK followed by metathesis of the in situ obtained salt with [{RuCl(μ-Cl)(η⁶-p-cymene)}₂] gives a chelate complex [RuCl(pfc-κ²O,P)(η⁶-p-cymene)], 5, as a kinetic 1 : 1 mixture of diastereoisomers. Upon standing in solution, the diastereomeric mixture undergoes a spontaneous resolution to yield the thermodynamically preferred diastereoisomer (R_Ru,S_p)-5 whose configuration was corroborated by X-ray crystallography; the (R_Ru,S_p)-5 diastereoisomer was obtained in 82% yield by crystallization. The epimerization is likely initiated by a reversible Ru–O bond cleavage and suggests a hemilabile coordination of the (S_p)-pfc⁻ anion. However, according to NMR spectra pure (R_Ru,S_p)-5 does not undergo diastereoisomer interconversion in a solution, which is in agreement with an unfavourable geometric arrangement of the (S_Ru)-epimer.