The role of N-heterocyclic carbene substituents on ruthenium(II) complexes in the catalytic transfer hydrogenation of acetophenone

Abstract

The novel ruthenium(II) complexes [RuCl 2 (NHC)(p-cymene)], 3a-e, containing 1-alkyl-3-benzylimidazol-2-ylidene ligands were prepared. All synthesized compounds were characterized by NMR spectroscopy and elemental analyses. Ru(II)-NHC complexes were tested as catalysts for the transfer hydrogenation of acetophenone, showing modest to high activity in this reaction. The results revealed that efficiencies depend on the substituents of the benzene ring of the benzyl on the N atom of the NHC ring.

Current advances on ruthenium(II) N-heterocyclic carbenes in hydrogenation reactions

“…It has to be mentioned that these turnovers are low compared with previous publications and the medium TONs hint towards average stability of the catalysts. Nevertheless, the results underline the influence of steric demand and concurrent electron-donating properties on the NHC backbone, which are both stated as a reason for the better catalytic performance of 7 [22].…”

“…The beneficial influence of bulky electron-donating wingtip substituents is underlined by investigations of Günay et al, who examined phenyl (Ph) (4), mesityl (Mes) (5), 2,3,5,6tetramethylphenyl (6) and 2,3,4,5,6-pentamethylphenyl (7) as Naryl substituents ( Fig. 2 and Table 2) [22]. The most active catalyst for the conversion of acetophenone proved to be 7, albeit with a TOF of only 46.5 h À1 and a moderate TON of 186 (entry 4).…”

Synthesis of silver(I) and palladium(II) N-heterocyclic carbene complexes and their use as catalysts for the direct C5 arylation of heteroaromatic compounds

“…Additionally, the NHC ligands can be easily modified by changing the substituents on the nitrogen atoms or the carbene ring, which provides various ligands for organometallic chemistry [7][8][9]. Metal complexes of Nheterocyclic carbenes have received much interest as catalysts and display higher stabilities and reactivities than the corresponding phosphine complexes in various catalytic processes [10][11][12][13][14][15]. In particular, palladium NHC complexes have been successfully developed as highly active precatalysts for C-C and C-N coupling, CO-ethylene copolymerization and direct arylation reactions [16][17][18][19].…”