An Insight into Transfer Hydrogenation Reactions Catalysed by Iridium(III) Bis‐N‐heterocyclic Carbenes

Abstract

A variety of [M(L) 2 (L') 2 {κC,C'-bis(NHC)}]BF 4 complexes (M = Rh or Ir; L = CH 3 CN or wingtip group; L' = I -or CF 3 COO -; NHC = N-heterocyclic carbene) have been tested as precatalysts for the transfer hydrogenation of ketones and imines. The conversions and TOF's obtained are closely related to the nature of the ligand system and metal centre, more strongly coordinating wingtip groups yielding more active and recyclable catalysts. Theoretical calculations at the DFT level support a classic stepwise metal-hydride pathway against the concerted Meerwein-PonndorfVerley (MPV) mechanism. The calculated catalytic cycle involves a series of ligand rearrangements due to the high trans effect of the carbene and hydrido ligands, which are more stable when situated in mutual cis positions. The reaction profiles obtained for the complexes featuring an iodide or a trifluoroacetate in one of the apical positions agree well with the relative activity observed for both catalysts.

Strong Solvent Effects on Catalytic Transfer Hydrogenation of Ketones with [Ir(cod)(NHC)(PR3)] Catalysts in 2-Propanol-Water Mixtures

“…(11) and Ir(2)-C(29) 2.043(10); C(9)=C(10) 1.306 (18) (15); Ir(1)-C(9)-Cl(1) 89.18 (16). (11) and Ir(2)-C(29) 2.043(10); C(9)=C(10) 1.306 (18) and C(30)=C(31) 1.297 (19); Ir(1)-Cl(1) 2.181 (11) and Ir(2)-Cl(2) 2.349(3); Ir(1)-C(9)-Cl(1) 89.2(3) and Ir(2)-C(29)-Cl(2) 88.4(3). Catalysts 2020, 10, 17…”

“…An important advantage of transfer hydrogenations over reductions with H 2 is in the use of an H-donor which is usually a liquid and much less flammable than gaseous H 2 . No wonder, that transfer hydrogenation has a long and fruitful history, and complexes of several transition metals, such as those of Ru [1][2][3][4][5][6][7][8][9][10][11][12][13], Rh [14][15][16] and Ir [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34], and many others have been used to catalyse hydrogen transfer reductions. The various aspects of transfer hydrogenation are covered by several excellent reviews [1][2][3][4][20][21][22]35].…”

“…Iridium complexes, have a long history in catalysis and were applied also as transfer hydrogenation catalysts [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]42]. Complexes with NHC ligands have proved extremely versatile catalysts in this field, too.…”

“…In case of cyclohexanone, the optimum substrate/catalyst/base ratio was found to be 1000/1/5 (with KOH as a base at 80 • C), and the most efficient catalyst was [Ir(cod)(NCCH 3 )(1-methyl-3-(2methoxybenzyl)imidazole-2-ylidene)] + . In their later studies, Ir(III)-bis-NHC complexes were also synthesized and applied as catalysts for transfer hydrogenation of ketones [18,19]. Among them, [Ir(I) 2 (CH 3 CN) 2 {κ 2 C,C -bis(NHC Me )}]BF 4 (bis(NHC Me ) = methylene-bis(N-methyl)imidazole-2-ylidene) was used as catalyst in transfer hydrogenation of acetophenone and afforded 98% conversion in 5 h with a S/C = 100, in 2-PrOH at 80 • C [19].…”

“…In their later studies, Ir(III)-bis-NHC complexes were also synthesized and applied as catalysts for transfer hydrogenation of ketones [18,19]. Among them, [Ir(I)2(CH3CN)2{κ 2 C,C'-bis(NHC Me )}]BF4 (bis(NHC Me ) = methylene-bis(N-methyl)imidazole-2-ylidene) was used as catalyst in transfer hydrogenation of acetophenone and afforded 98% conversion in 5 h with a S/C = 100, in 2-PrOH at 80 °C [19]. With the aim of establishing the electronic and steric effects of the NHC ligands on the catalytic properties of Ir(I)-NHC complexes, transfer hydrogenation of acetophenone from 2-PrOH was investigated in detail by Kühn and co-workers with a series of Ir(I)-NHC catalysts containing various NHC ligands based on imidazole, benzimidazole and imidazolidine [29].…”