Symmetric and dissymmetric N‐heterocyclic carbene rhodium(I) complexes: a comparative study of their catalytic activities in transfer hydrogenation reaction

Abstract

Six new [RhBr(NHC)(cod)] (NHC = N-heterocyclic carbene; cod = 1,5-cyclooctadiene) type rhodium complexes (4-6) have been prepared by the reaction of [Rh(m-OMe)(cod)] 2 with a series of corresponding imidazoli(in)ium bromides (1-3) bearing mesityl (Mes) or 2,4,6-trimethylbenzyl (CH 2 Mes) substituents at N 1 and N 3 positions. They have been fully characterized by 1 H, 13 C and heteronuclear multiple quantum correlation NMR analyses, elemental analysis and mass spectroscopy. Complexes of type [(NHC)RhBr(CO) 2 ] (NHC = imidazol-2-ylidene) (7b-9b) were also synthesized to compare s-donor/p-acceptor strength of NHC ligands. Transfer hydrogenation (TH) reaction of acetophenone has been comparatively studied by using complexes 4-6 as catalysts. The symmetrically CH 2 Mes-substituted rhodium complex bearing a saturated NHC ligand (5a) showed the highest catalytic activity for TH reaction.

The Golden Age of Transfer Hydrogenation

“…Complexes 120 and 121 of the type [Rh­(NHC)­X­(COD)] were well-established, and their catalytic properties have been evaluated in TH of ketones (Scheme ). , Catalyst 120 that contains a perimidin-2-ylidene NHC exhibited low catalytic activity, probably due to the weak 6-donocity of the 1,3-disubstituted perimidin-2-ylidene ligand . On the other hand, catalyst 121 bearing symmetric mesityl (Mes) substituents at N1 and N3 positions of imidazoline or imidazole efficiently promoted the TH reaction, giving almost complete conversion within 2 h with 0.5 mol % of catalyst loading . Comparative studies of the efficiencies indicated that the introduction of the CH 2 Mes group to the nitrogen atoms increased TH performances, and the catalyst 121a with imidazoline induced a faster reaction than 121b with imidazole.…”

“…475 On the other hand, catalyst 121 bearing symmetric mesityl (Mes) substituents at N1 and N3 positions of imidazoline or imidazole efficiently promoted the TH reaction, giving almost complete conversion within 2 h with 0.5 mol % of catalyst loading. 476 Comparative studies of the efficiencies indicated that the introduction of the CH 2 Mes group to the nitrogen atoms increased TH performances, and the catalyst 121a with imidazoline induced a faster reaction than 121b with imidazole. The pyridazine-annelated bis-NHC cationic rhodium complexes 122 showed a good activity in TH of ketones at 80 °C.…”

Selective Hydration of Nitriles to Corresponding Amides in Air with Rh(I)-N-Heterocyclic Complex Catalysts

“…In this work, we explored the applicability of Rh(I)-N-heterocyclic complexes 1-4 ( Figure 1) for catalysis of hydration of aromatic nitriles. We developed a synthetic method for obtaining these known compounds [44,46,48,49,[52][53][54][55][56], which does not require the use of the isolated free carbenes or the use of the corresponding Ag(I)-NHC transmetallating agents. In general, the synthesis of 1-4 (Scheme 1) involved stirring of the respective 1,3diarylimidazolium or 1,3-diarylimidazolinium salt in toluene at 70 °C together with [RhCl(cod)]2 and K2CO3 as an efficient and mild base (A) [47] or with [Rh(OH)(cod)]2 (no base added; B).…”

“…To the best of our knowledge, there are no [RhCl(cod)(NHC)] type Rh(I)-catalysts reported until now for the selective hydration reaction of nitriles with water in aqueous or partly aqueous solvents. Therefore, we initiated a study of catalytic nitrile hydration with the use of the known [RhCl(cod)(NHC)] (1-4) complexes with the NHC ligands IMes, SIMes, IPr, and SIPr, respectively (Figure 1) [44,46,48,49,[52][53][54][55][56]. In this article, we report on a simple, one-step synthetic procedure for obtaining these complexes using [RhX(cod)] 2 (X = Cl -, OH -) as a metal precursor, the respective imidazolium/imidazolinium chlorides, and K 2 CO 3 as the deprotonating agent, in toluene at 70 • C. Successful application of complexes 1-4 for the selective hydration of several aromatic and heteroaromatic nitriles to the corresponding amides is also described in detail below.…”

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

“…It is clear that the introduction of the alkylated benzyl group to the nitrogen atom increased transfer hydrogenation performance. Presumably, the flexible character of N-benzyl systems might be electronically more sensitive and tunable to the need of the substrates to enhance the transfer hydrogenation performance, 27…”