Neutral p-cymene ruthenium complexes with P-stereogenic monophosphines. New catalytic precursors in enantioselective transfer hydrogenation and cyclopropanation |
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Authors: | Arnald Grabulosa Alberto Mannu Antonio Mezzetti Guillermo Muller |
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Institution: | 1. Departament de Química Inorgànica, Universitat de Barcelona, Martí i Franquès 1-11, E-08028 Barcelona, Spain;2. Department of Chemistry and Applied Biosciences, ETH Zürich, CH-8093, Zürich, Switzerland |
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Abstract: | A family of eight neutral, pseudotetrahedral piano-stool ruthenium complexes C, of the type RuCl2(p-cymene)(PArPhR)] (Ar = 1-naphthyl, 9-phenanthryl and 2-biphenylyl; R = Me, i-Pr, OMe, –CH2SiMe3 and –CH2SiPh3) have been prepared and characterised, including the X-ray crystal structure for C6 (Ar = 2-biphenylyl; R = i-Pr). These complexes catalyse the asymmetric hydrogen transfer reaction of acetophenone in refluxing 2-propanol in the presence of potassium tert-butoxyde, reaching full conversions and up to 45% ee after 24 h towards the S enantiomer of 1-phenylethanol. Cationic complexes formed upon treatment of C with one equivalent of AgSbF6 or (Et3O)PF6 are active in the cyclopropanation reaction of styrene and α-methylstyrene by ethyl diazoacetate. Low to moderate conversions (up to 58%), diastereoselectivities (up to 40% de), and moderate enantioselectivities (up to 69% ee) have been found. For both reactions, bulky complexes and C6 in particular lead to the best results. |
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