Ferrocene based chiral binuclear η⁶‐benzene‐Ru(II)‐phosphinite complexes: Synthesis, characterization and catalytic activity in asymmetric reduction of ketones

Abstract

In the present study, a series of chiral C 2 -symmetric ferrocenyl based binuclear η 6 -benzene-Ru(II) complexes bearing diphenylphosphinite and diisopropylphosphinite moieties have been synthesised. The new binuclear η 6 -benzene-Ru(II)-phosphinite complexes were characterised based on nuclear magnetic resonance ( 1 H, 13 C, 31 P-NMR), FT-IR spectroscopy and elemental analysis. Then, these complexes have been screened as catalytic precursors in the transfer hydrogenation of acetophenone with 2-propanol as both the hydrogen source and solvent in the presence of KOH. The corresponding optically active secondary alcohols were obtained in excellent conversion rates between 96 and 99% and moderate to good enantioselectivities (up to 78% ee). The complex 5 was the most efficient catalyst among the four new complexes investigated herein. | INTRODUCTIONAsymmetric catalysis plays a vital role in generating single enantiomeric materials. The development of more efficient methodologies in asymmetric catalysis is a very important area of research in organic chemistry. [1] The optically pure secondary alcohols are important class of intermediates for the fine chemicals especially pharmaceutical, [2] agrochemical, [3] cosmetic, food, and flavour industries. [4] The catalytic reduction of pro-chiral ketones to chiral secondary alcohols by using transfer hydrogenation can be considered an attractive method compared with the common reduction processes which involve hazardous reducing reagents or high hydrogen pressure. [5] Simple equipment, atom economy, low catalyst loading, safe manipulation, high selectivity, environmentally friendly solvents and contribution to industrial applications are key advantages of transfer hydrogenation. [6] An increasing number of chiral compounds and optically pure secondary alcohols are prepared through transition metal-catalysed asymmetric reactions. A number of transition metal complexes are known to be used in asymmetric transfer hydrogenation (ATH) reactions as catalysts. [6] Most often, chiral ligand bearing Ru(II) [7] , Rh(I) [8] and Ir(III) [9] complexes have received great attention due to their exclusive applications in asymmetric transfer hydrogenation. The catalysts used in this reaction are often Ru(II) complexes, especially half-sandwich Ru(II) complexes. [10] Despite being stable and easy to prepare, half-sandwich Ru(II) complexes [RuCl 2 (η 6 -arene)(L)], where L is a chiral monodentate phosphinite ligand, have been rarely used in asymmetric transfer hydrogenation.Numerous chiral ligands with aminoalcohols, [11] diamines, [12] sulphur containing ligands, [13] phosphorus containing ligands (phosphine, [14] phosphinite, [15] phosphite [16] etc.), heterocyclic ligands [17] and ferrocenyl backbones [18] have been designed, synthesised and used successfully in a variety of catalytic applications. Among the recently developed numerous chiral ligands for the