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

Abstract

The silyloxycyclopentadienyl hydride complexes [Re(H)(NO)(PR(3))(C(5)H(4)OSiMe(2)tBu)] (R=iPr (3 a), Cy (3 b)) were obtained by the reaction of [Re(H)(Br)(NO)(PR(3))(2)] (R=iPr, Cy) with Li[C(5)H(4)OSiMe(2)tBu]. The ligand-metal bifunctional rhenium catalysts [Re(H)(NO)(PR(3))(C(5)H(4)OH)] (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)(PR(3))(C(5)H(4)O)][NBu(4)] (R=iPr (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.

Transfer Hydrogenation of Aldehydes, Allylic Alcohols, Ketones, and Imines Using Molybdenum Cyclopentadienone Complexes

“…Mechanistic studies reveal that these Mo complexes catalyze transfer hydrogenation by a mechanism analogous to that of the Shvo-type complexes of Ru, Re, and Fe. 13,14 These studies reveal that the cooperative behavior of redox-and proton-active tetracyclone ligands with metal hydrides, as pioneered by Shvo, 4,8,9 is a general strategy for the design of bifunctional transfer hydrogenation catalysts. Efforts to improve the stability and reactivity by modification of the cyclopentadienone ligand are currently in progress.…”

“…Ruthenium tetraphenylcyclopentadienone (tetracyclone) complexes, first developed by Shvo, ,, represent a class of metal–ligand bifunctional catalysts, where the transfer of separate hydrogen atoms occurs from the metal center and the hydroxyl group on the cyclopentadienyl ligand. Subsequently, several other transition metal analogues have been reported (Fe, Os, Co, Rh, Re, Ir, Ni); ,− the Fe and Re analogues were shown to be active transfer hydrogenation catalysts. Herein, we report that the molybdenum tetraphenylcyclopentadienone complex 1a , functions as an efficient catalyst precursor for the transfer hydrogenation of aldehydes, ketones, and imines (Scheme ).…”

Rhenium – A Tuneable Player in Tailored Hydrogenation Catalysis

“…Concerning rhenium, following the pioneering work of Ephritikhine et al., [120] wherein the dehydrogenation of alkanes to alkenes was studied, a limited number of examples of catalytic hydrogenations of alkenes were described with rhenium nitrosyl hydride complexes [121,122] . In 2012, Berke et al., who have significantly contributed to the aforementioned studies, also developed a bifunctional rhenium complex to perform transfer hydrogenation catalysis in the reduction of ketones and imines [123] …”

Imino Transfer Hydrogenation Reductions

“…In common with the Shvo catalysts, DFT calculations indicated the operation of an outer-sphere mechanism for the reaction. [118] Cyclometallated complexes of Ru, Rh, Ir have been used as ATH catalysts; Some examples of this class of catalyst have been prepared and reported earlier, however their applications have now been extended to a broader range of target imines, both cyclic and acyclic ( Figure 35). [119] Ru-Pybox complexes have been used to achieve imine ATH in IPA; these complexes gave some excellent results for imine derivatives of acetophenone with products of up to 99% e.e.…”