Water-soluble and reusable Ru-NHC catalyst for aqueous-phase transfer hydrogenation of quinolines with formic acid.

Water-soluble Ru-NHC complexes were synthesized and their catalytic activity was tested in the transfer hydrogenation of quinoline-type N-heteroarenes using a formic acid/sodium formate buffer solution. The unique multifunctional features of the designed ligand within the catalyst backbone endowed it with excellent durability, reusability and compatibility with a simple aqueous-phase operation. Thus, it was possible to reuse as little as 0.25 mol% of the catalyst for three consecutive catalytic runs to provide an overall turnover number of around 900. A mechanistic investigation suggested that hydride generation was the rate-limiting step, whereas hydride transfer was relatively facile. Furthermore, computational studies supported that the reaction pathway was dominated by 1,4-hydride insertion at the N-heteroarene substrates.

Switchable hydrogenation with a betaine-derived bifunctional Ir-NHC catalyst.

A bifunctional iridium catalyst based on the 'uracil-abnormal NHC' hybrid ligand platform was developed for switchable hydrogenation of quinoxalines. Control studies suggested heterolytic H2 activation via a metal-ligand bifunctional operation to generate Ir-H and an adjacent protic O-H group for facile H+/H- transfer to quinoxaline. The presence of a base blocked the most essential H+-transfer step thus switching off the catalysis, while an acid stimulus reversed the action to switch on the reaction again.