Ruthenium-Arene Complexes

Abstract

Stereoselective metal complexation reactions to form ruthenium-arene complexes are frequently used in organic synthesis; however, these reactions can be extremely expensive to perform. In prior research, there has been success in the stereoselective synthesis of [Cp*Ru(n6 -arene)]X and [CpRu(n6-arene)]X complexes (Cp*=pentamethyl-cyclopentadienyl, Cp=cyclopentadienyl). For example, research done by Uemura et al. showed the formation of [CpRu(n6–arene)]PF6 complexes with high diastereoselectivity (95:5, 1H NMR) from chiral ortho-substituted benzylic alcohols. X-ray crystallography was used to verify relative stereochemistry of the Ru complexes allowing these researchers to propose a binding model that correctly predicts the relative stereochemistry. The major limitation in this research was the use of Cp[Ru(NCMe)3]PF6, which comes with a commercially high cost (approximately $400/g) and requires several synthetic steps to prepare. In this study, a onepot binding procedure published by Lindel et al. was used to see if the Cp*Ru complexes could be formed with comparable stereoselectivities and yields to the complexes formed by Uemura using identical substrates, which drastically reduces the expense of preparation. In this vein, 2’-methylacetophenone was reduced to 1-(2-methylphenyl)ethanol via sodium borohydride to use as a model substrate for binding. The [Cp*Ru(n6–arene)]PF6 complex of 1-(2-methylphenyl)ethanol was then formed using the one-pot procedure. The 1H NMR spectrum of the crude reaction mixture suggests that one diastereomer is forming with unknown relative stereochemistry, which was subsequently isolated pure after chromatography. Moving forward, attempts will be made to verify the relative stereochemistry of this complex by X-ray crystallography to determine if it is consistent with Uemura’s model.