Donor‐Functionalised Expanded Ring N‐Heterocyclic Carbenes: Highly Effective Ligands in Ir‐Catalysed Transfer Hydrogenation

Abstract

The performances of a number of RhI and IrI complexes of type [M(NHC)(COD)Cl] in the transfer hydrogenation of ketones were tested under a variety of reaction conditions, and with a variety of substrates, allowing comparison of Rh‐ and Ir‐NHC complexes, and also comparison of the influence of the NHC ligand on catalytic performance. Notably, of the RhI and IrI complexes with symmetrically substituted NHCs only those bearing cyclohexyl substituents were active, with RhI complexes of saturated 5‐, 6‐ and 7‐NHCs with N‐Mes substituents, [Rh(5,6,7‐Mes)(COD)Cl], showing no activity in transfer hydrogenation under the test conditions. RhI and IrI complexes of unsymmetrical o‐methoxyphenyl donor‐functionalised NHCs (df‐NHC) with differing carbene ring sizes were also tested in transfer hydrogenations, with the RhI complexes displaying no catalytic activity. However, the corresponding df‐NHC IrI complexes were found to be extremely effective catalysts. Catalyst tests also demonstrated the excellent stability of these complexes.

Oxidation and β‐Alkylation of Alcohols Catalysed by Iridium(I) Complexes with Functionalised N‐Heterocyclic Carbene Ligands

“…In particular, the catalyst precursors based on O‐donor‐functionalised NHC ligands were found to be much more active than the corresponding N‐functionalised ligands 14. 17, 18…”

“…In particular, the catalyst precursors based on O-donor-functionalised NHC ligands weref ound to be much more active than the corresponding N-functionalised ligands. [14,17,18] The aim of this work is to assesst he catalytic activity of selected iridium(I) catalystp recursors having ah eteroditopic NHC ligand, but devoid of ac yclopendadienyl ligand, in the balkylation of secondary alcohols with primary alcohols.I nv iew of the lack of mechanistic studies on this hydrogen autotransfer process, [19] the reactionm echanism has been investigated by DFTc alculations. Remarkably,t hese studies have revealed the key role of the transition-metal catalyst not only in the dehydrogenation and hydrogenation steps but also in the cross-condensation step leading to the formation of the new CÀCbond.…”

Ketone Hydrogenation with Iridium Complexes with “non N–H” Ligands: The Key Role of the Strong Base

“…This behavior is shared by many other catalysts that are devoid of active protons, whether they are used in hydrogenation 20 or transfer hydrogenation. [71][72][73][74][75][76][77][78][79][80] The common view is that the base is needed to eliminate the acidity generated during the catalyst activation step. Contrary to other demonstrating that hydrogenation, rather than transfer hydrogenation, takes place at least under these conditions.…”

Novel ruthenium(II)–N-heterocyclic carbene complexes; synthesis, characterization and catalytic application

“…To date, the authors of previous articles on these type complexes inferred that the catalytic reactivity of these type complexes in transfer hydrogenation reaction originated from both the electron donation function and steric hindrance of NHC ligands [38]. Considering the above explanations, the catalytic activity of 3aee strongly depends upon steric bulky and electronic factors of the NHC ligands.…”

“…Catalyst 3b showed high catalytic activity at low catalyst loading (Table 1, entries 36, 37). However, low reaction temperature was not suitable for 3b reduction (Table 1, entries 35,38).…”