Density functional theory investigation of Ru(ii) and Os(ii) asymmetric transfer hydrogenation catalysts. (14th February 2022)

• Record Type: Journal Article
• Authors: Bolitho, Elizabeth M.; Coverdale, James P. C.; Wolny, Juliusz A.; Schünemann, Volker; Sadler, Peter J.
• Is Part Of: Faraday discussions. Volume 234(2022)
• Page Start: 264
• Page End: 283
• Publication Date: 2022-02-14
• Subjects: Chemistry -- Periodicals; Metallurgy -- Periodicals; Electrochemistry -- Periodicals; 540
• DOI: 10.1039/d1fd00075f
• Languages: English
• ISSNs: 1359-6640

Abstract

Abstract : Density functional theory calculations reveal subtle differences between the mechanism of asymmetric transfer hydrogenation of a ketone by half-sandwich Ru(ii ) and Os(ii ) arene catalysts with 5 chiral centres with formate as the hydride source. Abstract : Transition metal ions have a unique ability to organise and control the steric and electronic effects around a substrate in the active site of a catalyst. We consider half-sandwich Ru(ii ) (Noyori-type) and Os(ii ) sulfonyldiamine 16-electron active catalysts [Ru/Os(η 6 - p -cymene)(TsDPEN-H2 )], where TsDPEN is N -tosyl-1, 2-diphenylethylenediamine containing S, S or R, R chiral centres, which catalyse the highly efficient asymmetric transfer hydrogenation of aromatic ketones to chiral alcohols using formic acid as a hydride source. We discuss the recognition of the prochiral ketone acetophenone by the catalyst, the protonation of a ligand NH and transfer of hydride from formate to the metal, subsequent transfer of hydride to one enantiotopic face of the ketone, followed by proton transfer from metal-bound NH2, and regeneration of the catalyst. Our DFT calculations illustrate the role of the two chiral carbons on the N, N -chelated sulfonyldiamine ligand, the axial chirality of the π-bonded p -cymene arene, and the chirality of the metal centre. We discuss new features of the mechanism, including how a change in metal chirality of the hydride intermediate dramatically switches p -cymene coordination from η 6 to η 2 . Moreover, the calculations suggest a step-wise mechanism involving substrate docking to the bound amine NH2 followed by hydride transfer prior to protonation of the O-atom of acetophenone and release of the enantio-pure alcohol. This implies that formation and stability of the M–H hydride intermediate is highly dependent on the presence of the protonated amine ligand. The Os(ii ) catalyst is more stable than the Ru(ii ) analogue, and these studies illustrate the subtle differences in mechanistic behaviour between these 4d 6 and 5d 6 second-row and third-row transition metal congeners in group 8 of the periodic table.