Abstract
Heterogenization of homogenous catalysts on solid support has attracted tremendous attention in organic synthesis due to the key benefits of heterogenized catalysts such as easy recovery and reusability. Although a considerable number of heterogenized catalysts are available, to the best of our knowledge, there is no efficient and reusable heterogenized catalyst reported for asymmetric reactions to date. Herein, we prepared a [RuCl2(η6-p-cymene)]/chiralthiourea ligand covalently bonded to graphene nanosheets (G-CLRu(II), where G represents graphene oxide (GO), CL denotes chiral N-((1-phenylethyl)carbamothioyl)acetamide and Ru(II) symbolizes [RuCl2(η6-p-cymene)]), for the asymmetric transfer hydrogenation of ketones. Five simple steps were involved in the preparation of the G-CLRu(II) catalyst. The structure of G-CLRu(II) was investigated by means of various spectroscopic and microscopic techniques. Coordination mode and covalent bonding involved in the G-CLRu(II) structure we reconfirmed. G-CLRu(II) demonstrated good catalytic performance towards the asymmetric transfer hydrogenation of ketones (conversion of up to 95%, enantiomeric excesses (ee) of up to 99%, and turnover number (TON) and turnover frequency (TOF) values of 535.9 and 22.3 h−1, respectively). A possible mechanism is proposed for the G-CLRu(II)-catalyzed asymmetric transfer hydrogenation of ketones. Recovery (~95%), reusability (fifth cycle, yield of 89% and ee of 81%), and stability of G-CLRu(II) were found to be good. We believe that the present stepwise preparation of G-CLRu(II) opens a new door for designing various metal-centered heterogenized chiral catalysts for asymmetric synthesis.
Highlights
Catalytic asymmetric synthesis is a prime way to produce various pharmaceutically important chiral molecules [1,2]
Five simple consecutive steps were involved in the stepwise wet synthesis of G-CLRu(II)
[RuCl2 (η6 -p-cymene)]/chiralthiourea ligand was constructed on graphene nanosheets by a simple stepwise synthesis
Summary
Catalytic asymmetric synthesis is a prime way to produce various pharmaceutically important chiral molecules [1,2]. Chiral alcohols obtained from the corresponding ketone via enantioselective synthesis are often found to play a significant role as intermediates in the synthesis of pharmaceuticals and agrochemicals [3]. Transition metal-catalyzed asymmetric transfer hydrogenation is a most suitable method to achieve such chiral alcohols in high yield with maximum enantiomeric excess [4]. Numerous transition metal complexes have been developed for homogeneous asymmetric catalysis [5,6,7]. Ru(II)–chiral catalysts showed remarkable catalytic activity with high yield and selectivity. Baratta et al [8], found cyclometalated Ru(II)complexes are highly
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