Bifunctional Rhenium Complexes for the Catalytic Transfer‐Hydrogenation Reactions of Ketones and Imines |
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Authors: | Anne Landwehr Dr. Balz Dudle Dr. Thomas Fox Dr. Olivier Blacque Prof. Dr. Heinz Berke |
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Affiliation: | Institute of Inorganic Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich (Switzerland), Fax: (+41)?446354806 |
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Abstract: | The silyloxycyclopentadienyl hydride complexes [Re(H)(NO)(PR3)(C5H4OSiMe2tBu)] (R=iPr ( 3 a ), Cy ( 3 b )) were obtained by the reaction of [Re(H)(Br)(NO)(PR3)2] (R=iPr, Cy) with Li[C5H4OSiMe2tBu]. The ligand–metal bifunctional rhenium catalysts [Re(H)(NO)(PR3)(C5H4OH)] (R=iPr ( 5 a ), Cy ( 5 b )) were prepared from compounds 3 a and 3 b by silyl deprotection with TBAF and subsequent acidification of the intermediate salts [Re(H)(NO)(PR3)(C5H4O)][NBu4] (R=iPr ( 4 a ), Cy ( 4 b )) with NH4Br. In nonpolar solvents, compounds 5 a and 5 b formed an equilibrium with the isomerized trans‐dihydride cyclopentadienone species [Re(H)2(NO)(PR3)(C5H4O)] ( 6 a,b ). Deuterium‐labeling studies of compounds 5 a and 5 b with D2 and D2O showed H/D exchange at the HRe and HO positions. Compounds 5 a and 5 b were active catalysts in the transfer hydrogenation reactions of ketones and imines with 2‐propanol as both the solvent and H2 source. The mechanism of the transfer hydrogenation and isomerization reactions was supported by DFT calculations, which suggested a secondary‐coordination‐sphere mechanism for the transfer hydrogenation of ketones. |
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Keywords: | bifunctional catalysts homogeneous catalysis hydrides rhenium transfer hydrogenation |
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