Bifunctional rhenium complexes for the catalytic transfer-hydrogenation reactions of ketones and imines

Article abstract of DOI:10.1002/chem.201103685

The silyloxycyclopentadienyl hydride complexes [Re(H)(NO)(PR ₃)(C₅H₄OSiMe₂tBu)] (R=iPr (3a), Cy (3b)) were obtained by the reaction of [Re(H)(Br)(NO)(PR₃) ₂] (R=iPr, Cy) with Li[C₅H₄OSiMe ₂tBu]. The ligand-metal bifunctional rhenium catalysts [Re(H)(NO)(PR₃)(C₅H₄OH)] (R=iPr (5a), Cy (5b)) were prepared from compounds 3a and 3b by silyl deprotection with TBAF and subsequent acidification of the intermediate salts [Re(H)(NO)(PR ₃)(C₅H₄O)][NBu₄] (R=iPr (4a), Cy (4b)) with NH₄Br. In nonpolar solvents, compounds 5a and 5b formed an equilibrium with the isomerized trans-dihydride cyclopentadienone species [Re(H)₂(NO)(PR₃)(C₅H₄O)] (6a,b). Deuterium-labeling studies of compounds 5a and 5b with D₂ and D ₂O showed H/D exchange at the H_Re and H_O positions. Compounds 5a and 5b 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)(PR ₃)(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.