N-heterocyclic carbenes bearing a naphthyl substituent and their metal complexes: synthesis, structure, and application in catalytic transfer hydrogenation

Abstract

A series of unsymmetrical imidazolinium bromides (3a-d) bearing naphthyl and benzyl groups (R' = CH$_{2}$C$_{6}$H$_{2}$(CH$_{3})_{3}$-2,4,6 (a); CH$_{2}$C$_{6}$H(CH$_{3})_{4}$-2,3,5,6 (b); CH$_{2}$C$_{6}$(CH$_{3})_{5}$ (c); CH$_{2}$C$_{6}$F$_{5\, }$(d)) at the N$^{1}$ and N$^{3}$ positions were successfully synthesized. [RuCl$_{2}$(NHC)($p$-cymene)] (NHC= N-heterocyclic carbene) complexes (4a-d) were prepared by the reaction of [RuCl$_{2}(p$-cymene)]$_{2}$ with imidazolinium salts (3a-d). The new salts (3a-d) and their ruthenium(II) complexes (4a-d) were characterized by $^{1}$H, $^{13}$C, $^{19}$F NMR, and elemental analysis. The ruthenium(II) complexes (4a-d) were employed as catalysts for the transfer hydrogenation (TH) of ketones in the presence of KOH using 2-propanol as a hydrogen source and the results were compared. The best results in the transfer hydrogenation of ketones were obtained with 4b. [MCl(NHC)L] (M = Ir, L = Cp$^{\ast })$ (5b), cod (6b); M = Rh, L= Cp$^{\ast}$ (5b'), and cod (6b') complexes were prepared and investigated in the TH of ketones. The reactivity of Rh complexes in comparison to those of Ir also appears to be somewhat better. The catalysis appears to be homogeneous.