Bifunctional Rhenium Complexes for the Catalytic Transfer‐Hydrogenation Reactions of Ketones and Imines

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 H_Re and H_O 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.