Pseudo-enantiomeric carbohydrate-based N-heterocyclic carbenes as promising chiral ligands for enantiotopic discrimination

Rational design of galactopyranoside-substituted N-heterocyclic carbene palladium(ii) complexes. Stable and efficient catalyst for C-C coupling in aqueous media

Following a rational design, three novel palladium(ii) complexes bearing galactopyranoside-based N-heterocyclic carbene ligands have been synthesized via transmetalation of the corresponding Ag(i) complexes. Palladium(ii) complexes have been characterized by NMR, FT-IR and elemental analysis. Catalytic studies, using the Stille and Suzuki-Miyaura cross-coupling reactions as model C-C coupling, reveal that the complexes are active and reusable. The best results in terms of TON values were achieved in aqueous medium using either the in situ deacetylation of the catalyst or the previously deacetylated catalyst. The catalytic condition using in situ deacetylation was more efficient because it avoids an additional deprotection step.

Carbohydrate-Functionalized Triazolylidene Iridium Complexes: Hydrogenation Catalysis in Water with Asymmetric Induction

Two sets of carbohydrate-NHC hybrid iridium complexes were synthesised in order to combine properties of carbohydrates and triazolylidene (trz) ligands in organometallic catalysis. One set features a direct trz linkage to the anomeric carbohydrate carbon, while the second set is comprised of an ethyl linker between the two functional units. Deprotection of the carbohydrate afforded hybrid complexes that efficiently catalyse the direct hydrogenation of ketones in water. The catalytic activity of the hybrid complexes was influenced by the pH of the aqueous medium and surpassed the activity of carbohydrate-free or acetyl-protected analogues (>90 % vs 13 % yield). While no enantiomeric induction was observed for the ethyl-linked hybrids, a moderate enantiomeric excess (ee) was induced by the directly linked systems. Moreover, these carbohydrate-trz hybrid complexes displayed mixed inhibitory activity towards a glycosidase from H. orenii that contain a glucose binding site.