Diastereoselective Routes to [Amino{σ(P):η6-(ansa-phosphinite)benzene}chlororuthenium(II)] PF6 Salts: Kinetic versus Thermodynamic Preferences

Abstract

A novel chiral phosphinite derivative was designed as a ligand for Ru2+ ions to give access to new enantiomerically pure or diastereomerically enriched [{σ(P):η6-(ansa-phosphinite)benzene}Ru(II) complexes. We are interested in this class of compounds as potential catalysts for enantioselective transfer hydrogenation of organic carbonyl compounds. The preparation of enantiopure (R)-[{σ(P):η6-(ansa-phosphinite)benzene}Ru(II)Cl2] (6) was achieved in good yeld on the basis of (S)-mandelic acid. Four diastereomeric [{σ(P):η6-(ansa-phosphinite)benzene}Ru(II)Cl(NH2R)]+ complex PF6− salts were obtained from neutral 6 by nucleophilic substitution of a chloride ligand with primary amines. For aniline and p-fluoroaniline salts 7 and 8 a significant excess of the (SRu,RC)-diastereomers 7S and 8S was observed in the reaction mixtures as the kinetically preferred species, and 7S could be isolated as a pure diastereomer. No diastereoselectivity was observed in the reaction mixtures of (R)- and (S)-1-phenylethylamine salts 9 and 10 right after preparation. 7, 8, and 9 epimerize at the metal center. This leads to a loss of diastereoselectivity of 7S and 8S but enrichment of (RRu,RC)-diastereomer 9R. Pure diastereomers 7S, 9R, and 10S as well as a 1:1 diastereomeric mixture 8S/8R have been characterized structurally. The experimental setup allowed detailed insight into the specific X-ray crystallographic, CD, and NMR spectroscopic properties of defined stereoisomers of chiral Ru(II) complexes. The kinetic diastereoselectivities of 7S and 8S are explained by a H-bond delivery effect, while pure diastereomer salt 9R is configuratively stable.