Synthesis and characterization of new chiral P,O ferrocenyl ligands and catalytic application to asymmetric Suzuki–Miyaura coupling

Abstract

The synthesis and characterization of a series of novel chiral P,O ferrocenyl ligands obtained from the reaction of racemic (9) or enantiomerically pure ((R)-9 or (S)-9) 2-thiodiphenylphosphino(hydroxymethyl)ferrocene with alcohols is described. The use of para-methylbenzylalcohol, ethanol and (1R,2S,5R)-menthol gives rise to three ligands (10, 11 and 12 respectively) possessing planar chirality. All compounds have been characterized by multinuclear NMR and mass spectrometry. Compounds 12 and 12a (the protected form of phosphine 12 with a PS bond) present both planar and central chirality giving rise to two diastereoisomers differing in configuration (R or S) of the ferrocenyl fragment. Compound (S)-12a has been characterized by single-crystal X-ray diffraction. Kinetic studies realized at three different temperatures (40, 50 and 60 °C) show good yields in the asymmetric Suzuki–Miyaura synthesis of substituted binaphthalenes using 1-naphthalenboronic acid and 1-bromo-2-methylnaphthalene. The best enantioselectivity (37% ee) was obtained with ligand (R)-12.