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

The silyloxycyclopentadienyl hydride complexes [Re(H)(NO)(PR 3 )(C 5 H 4 OSiMe 2 t Bu)] (R= i Pr ( 3 a ), Cy ( 3 b )) were obtained by the reaction of [Re(H)(Br)(NO)(PR 3 ) 2 ] (R= i Pr, Cy) with Li[C 5 H 4 OSiMe 2 t Bu]. The ligand–metal bifunctional rhenium catalysts [Re(H)(NO)(PR 3 )(C 5 H 4 OH)] (R= i Pr ( 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)(PR 3 )(C 5 H 4 O)][NBu 4 ] (R= i Pr ( 4 a ), Cy ( 4 b )) with NH 4 Br. In nonpolar solvents, compounds 5 a and 5 b formed an equilibrium with the isomerized trans ‐dihydride cyclopentadienone species [Re(H) 2 (NO)(PR 3 )(C 5 H 4 O)] ( 6 a,b ). Deuterium‐labeling studies of compounds 5 a and 5 b with D 2 and D 2 O 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 H 2 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.