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

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

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 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.

The Re-al deal: Bifunctional rhenium complexes [Re(H)(NO)(PR₃)(C₅H₄OH)] (R=Cy, iPr) of Shvo-type were prepared and used as catalysts for the transfer hydrogenation of ketones and imines. TOFs up to 1164 h⁻¹ were obtained for ketones and up to 79 h⁻¹ for imines. DFT calculations suggested a secondary-coordination-sphere mechanism for the transfer hydrogenation of ketones.