FULL PAPER 1 Arene-Immobilized Ru(II)/TsDPEN Complexes: Synthesis and Applications to the Asymmetric Transfer Hydrogenation of Ketones Simon Doherty,* [a] Julian G. Knight,* [a] Hind Alshaikh, [b] James Wilson, [a] Paul G. Waddell, [a] Corinne Wills [a] and Casey M. Dixon [a] [a] Dr. S. Doherty, Dr. J. G. Knight, J. Wilson, Dr. P. G. Waddell, Dr. C. Wills and Dr C. M. Dixon Newcastle University Centre for Catalysis (NUCAT) School of Chemistry, Bedson Building Newcastle University Newcastle upon Tyne NE1 7RU, UK E-mail: simon.doherty@ncl.ac.uk; https://www.ncl.ac.uk/nes/staff/profile/simondoherty.html#background [b] Dr. H. Alshaikh Department of Chemistry King Abdulaziz University, Jeddah, Saudi Arabia Supporting information for this article is given via a link at the end of the document. Abstract: The Noyori-Ikariya (arene)Ru(II)/TsDPEN precatalyst has been anchored to amorphous silica and DAVISIL through the  6 - coordinated arene ligand via a straightforward synthesis and the derived systems, (arene)Ru(II)/TsDPEN@silica and (arene)Ru(II)/TsDPEN@DAVISIL, form highly efficient catalysts for the asymmetric transfer hydrogenation of a range of electron-rich and electron-poor aromatic ketones, giving good conversion and excellent ee’s under mild reaction conditions. Moreover, catalyst generated in situ immediately prior to addition of substrate and hydrogen donor, by reaction of silica-supported [(arene)RuCl2]2 with (S,S)-TsDPEN, was as efficient as that generated from its preformed counterpart [(arene)Ru{(S,S)-TsDPEN}Cl]@silica. Gratifyingly, the initial TOFs (up to 1085 h -1 ) and ee’s (96-97%) obtained with these catalysts either rivalled or outperformed those previously reported for catalysts supported by either silica or polymer immobilized through one of the nitrogen atoms of TsDPEN. While the high ee’s were also maintained during recycle studies, the conversion dropped steadily over the first three runs due to gradual leaching of the ruthenium. Introduction The asymmetric hydrogenation of ketones to alcohols is a pivotal transformation in organic synthesis which is widely used in the production of intermediates and pharmaceuticals. [1] The Noyori arene-Ru(II)/TsDPEN system is among the most versatile and efficient catalysts for asymmetric transfer hydrogenation (ATH), [2] using either an azeotropic mixture of formic acid and triethylamine or propan-2-ol as the hydrogen source, as well as asymmetric hydrogenation (AH) [3] and as such numerous modifications have been reported. [4] Although the Noyori-Ikariya catalyst is highly efficient and has been successfully applied in synthetic methodology, [5] the catalyst can be quite costly due to the high catalyst loadings that are often required (0.5-1.0 mol-%) coupled with the expense of the precious metal and a chiral ligand. As such there has been considerable interest in exploring strategies to immobilize this system onto a solid support to facilitate catalyst separation, recovery and reuse as well as improve product purification and enable integration into a continuous flow process for scale up, all of which will ultimately reduce operating costs. One of the most popular approaches to immobilize these systems has been covalent attachment of a nitrogen-modified TsDPEN to an amorphous or mesoporous silica, while retaining the essential key feature of an ‘active N-H’. [6] In addition, while TsDPEN grafted by covalent attachment of nitrogen to polystyrene [7] and PEG [8] have both been used to immobilize these catalysts with varying levels of success, Xiao developed an alternative approach to immobilizing TsDPEN by attachment to poly(ethylene glycol) via both of its phenyl rings; the corresponding Noyori-Ikariya catalyst is among the most efficient to be reported with fast reaction rates, excellent ee’s and outstanding reusability. [9] Other methods used to immobilize (arene)Ru(II)/TsDPEN and facilitate its recovery and reuse include modification of TsDPEN with an imidazolium [10] or phosphonium [11] group for use in ionic liquids and water, respectively, and incorporation of the diamine into a Fréchet-type core-functionalized dendrimer. [12] In more recent developments, (arene)Ru(II)/TsDPEN has been anchored to a support which also incorporates a palladium-based catalyst for cross-coupling and the resulting switchable bifunctional system used as a catalyst for enantioselective cascade reaction sequences. [13] In contrast to the myriad of examples of solid-supported Noyori-Ikariya-type catalysts immobilized through the TsDPEN ligand, there appears to be only a single report of immobilization through the -arene ring. [14] This system was prepared by polymerization of methacrylate side chain-modified [(arene)RuCl2]2 with ethyleneglycol dimethacrylate and the resulting polymers combined with TsDPEN to form an efficient catalyst for the ATH of ketones. However, to the best of our knowledge there are no reports of silica-supported precatalysts tethered via the  6 -coordinated arene, which is somewhat surprising considering there are numerous advantages associated with the use an ordered mesoporous silica as a support for anchoring chiral transition metal catalysts; these include control of surface area and pore volume, tunable pore dimensions, potential for functionalization and good thermal and mechanical integrity. [15] Thus, our interest in developing such an