Ruthenium Complexes of Phosphino-Substituted Ferrocenyloxazolines in the Asymmetric Hydrogenation and Transfer Hydrogenation of Ketones: A Comparison

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• 作者：Afrooz Zirakzadeh ; Raffael Schuecker ; Nikolaus Gorgas ; Kurt Mereiter ; Felix Spindler ; Walter Weissensteiner
• 刊名：Organometallics
• 出版年：2012
• 出版时间：June 11, 2012
• 年：2012
• 卷：31
• 期：11
• 页码：4241-4250
• 全文大小：515K
• 年卷期：v.31,no.11(June 11, 2012)
• ISSN：1520-6041

文摘

Three novel routes have been developed for the synthesis of ferrocenyl-based phosphino-oxazolines in which the phosphino unit is attached to a ferrocenylmethyl or a ferrocenylethyl side chain. In two of the routes the phosphino-substituted ethyl side chain was built up diastereoselectively. Ruthenium complexes of the type [RuCl₂PPh₃(L)] of 12 bidentate phosphine-oxazoline ligands were synthesized, characterized, and tested in the transfer hydrogenation of acetophenone. For the best performing complexes a total of 12 additional ketones were screened in transfer hydrogenations and hydrogenations under transfer hydrogenation conditions. Two catalyst precursors in particular delivered products with an enantiomeric excess of up to 98% in transfer hydrogenations and 99% ee in hydrogenations. The transfer hydrogenation results obtained with all novel ligands were compared to those of two well-established FOXAP ligands. Furthermore, a qualitative comparison with the hydrogenation data was carried out. In both cases surprising similarities in product enantiomeric excess and product absolute configuration were found. Attempts were made to rationalize some of the observed features by considering a transition-state model. The molecular structures of one synthesis intermediate, two catalyst precursors, and two corresponding acetonitrile complexes were studied by X-ray diffraction.