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

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• 作者：Immo Weber ; Frank W. Heinemann ; Walter Bauer ; Stefano Superchi ; Achim Zahl ; Daniela Richter ; Ulrich Zenneck
• 刊名：Organometallics
• 出版年：2008
• 出版时间：August 25, 2008
• 年：2008
• 卷：27
• 期：16
• 页码：4116-4125
• 全文大小：312K
• 年卷期：v.27,no.16(August 25, 2008)
• ISSN：1520-6041

文摘

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 (S_Ru,R_C)-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 (R_Ru,R_C)-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.