Abstract
Three Ru complexes containing carbohydrate/N-heterocyclic carbene hybrid ligands were synthesized that were comprised of a triazolylidene coordination site and a directly linked per-acetylated glucosyl (5Glc) or galactosyl unit (5Gal), or a glycosyl unit linked through an ethylene spacer (6). Electrochemical and UV-vis analysis indicate only minor perturbation of the electronic configuration of the metal center upon carbohydrate installation. Deprotection of the carbohydrate was accomplished under basic conditions to afford complexes that were stable in solution over several hours, but decomposed in the solid state. Complexes 5 and 6 were used as pre-catalysts for transfer hydrogenation of ketones under basic conditions, i.e. conditions that lead to in situ deprotection of the carbohydrate entity. The carbohydrate directly influences the catalytic activity of the metal center. Remotely linked carbohydrates (complex 6) induce significantly lower catalytic activity than directly linked carbohydrates (complexes 5Glc, 5Gal), while unfunctionalized triazolylidenes are an order of magnitude more active. These observations and substrate variations strongly suggest that substrate bonding is rate-limiting for transfer hydrogenation in these hybrid carbohydrate/triazolylidene systems.
Highlights
N-Heterocyclic carbenes (NHCs) act as versatile ligands for various catalytic systems,[1] as well as for biological and materials applications.[2]
We have demonstrated that deprotected carbohydrate substituents in NHC–Ir(III) complexes are beneficial for base-free alcohol and amine oxidation.[22]
Triazole precursors 1 were synthesized in moderate yields by the CuAAC reaction from 1-hexyne and acetyl-protected anomeric azide derivatives of glucose and galactose (Scheme 1).[63]
Summary
N-Heterocyclic carbenes (NHCs) act as versatile ligands for various catalytic systems,[1] as well as for biological and materials applications.[2]. The introduction of carbohydrate substituents on the triazolylidene scaffold is attractive as this approach introduces functional groups in close proximity to the metal active site Such cooperation of ligand sites and the metal center has been demonstrated in so-called bifunctional catalysts, as introduced elegantly with Noyori-type catalysts containing an amide functionality,[46,47,48] and Shvo’s catalyst featuring a proximal oxygen functionality.[49,50] Bifunctional NHC ligands have shown promise in a range of catalytic transformations including hydrogenations, giving rise to increased catalytic activities when compared to more classical analogs.[13,17,23,51,52,53,54,55]. We report three novel Ru–triazolylidene complexes that incorporate carbohydrate functionality, including their photophysical and electrochemical properties as well as their catalytic activity in transfer hydrogenation of ketones, which revealed that complexes are efficiently deprotected in situ under the basic catalytic conditions
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