Iridium(I) Complexes with Hemilabile N-Heterocyclic Carbenes: Efficient and Versatile Transfer Hydrogenation Catalysts

Abstract

A series of neutral and cationic rhodium and iridium(I) complexes based on hemilabile O-donor-and N-donor-functionalized NHC ligands having methoxy, dimethylamino, and pyridine as donor functions have been synthesized. The hemilabile fragment is coordinated to the iridium center in the cationic complexes [Ir(cod] has been determined by X-ray diffraction. The iridium complexes are efficient precatalysts for the transfer hydrogenation of cyclohexanone in 2-propanol/KOH. A comparative study has shown that cationic complexes are more efficient than the neutral and also that complexes having O-functionalized NHC ligands provide much more active systems than the corresponding N-functionalized ligands with TOFs up to 4600 h À1 . The complexes [Ir(NCCH 3 )(cod)(MeImR)] + (R = 2-methoxyethyl and 2-methoxybenzyl) have been successfully applied to the reduction of several unsaturated substrates as ketones, aldehydes, α,β-unsaturated ketones, and imines. The investigation of the reaction mechanism by NMR and MS has allowed the identification of relevant alkoxo intermediates [Ir(OR)(cod)(MeImR)] and the unsaturated hydride species [IrH(cod)(MeImR)]. The β-H elimination in the alkoxo complex [Ir(OiPr)(cod)(MeIm(2-methoxybenzyl))] leading to hydrido species has been studied by DFT calculations. An interaction between the β-H on the alkoxo ligand and the oxygen atom of the methoxy fragment of the NHC ligand, which results in a net destabilization of the alkoxo intermediate by a free energy of +1.0 kcal/mol, has been identified. This destabilization facilitates the β-H elimination step in the catalytic process and could explain the positive effect of the methoxy group of the functionalized NHC ligands on the catalytic activity.

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

“…The activation energies are 22.9 and 23.2 kcal mol −1 , respectively, and they are almost identical to that found for the simple model used in the study of the full cycle. This suggests that the role of the substituent on the NHC ligand is almost negligible in this step and it must be involved with the alcohol oxidation process, as previously reported by us,16a which is in full agreement with the experimental results shown in Table 3 (entries 2 and 6 vs. entry 9).…”

“…In this regard, it is worth mentioning that this transformation is also a key step in the redox isomerisation of allylic alcohols and the racemisation of secondary alcohols catalysed by ruthenium cyclopentadienyl complexes 27. In fact, we have identified by NMR spectroscopy and mass spectrometry the relevant alkoxo intermediates [Ir(OR)(cod)(MeIm∩Z)] and the unsaturated hydride species [IrH(cod)(MeIm∩Z)] (∩Z=2‐methoxybenzyl) involved in the transfer hydrogenation of unsaturated substrates in 2‐propanol/KOH catalysed by iridium(I) complexes having hemilabile O‐ and N‐ donor‐functionalised NHC ligands 16a. Interestingly, DFT calculations have allowed to identify an interaction between the β‐H atom on the alkoxo ligand and the methoxy fragment of the NHC ligand, which results in a net destabilisation of the alkoxo intermediate, thereby facilitating the β‐H elimination step, leading to the hydride intermediate 16a.…”

“…In fact, we have identified by NMR spectroscopy and mass spectrometry the relevant alkoxo intermediates [Ir(OR)(cod)(MeIm∩Z)] and the unsaturated hydride species [IrH(cod)(MeIm∩Z)] (∩Z=2‐methoxybenzyl) involved in the transfer hydrogenation of unsaturated substrates in 2‐propanol/KOH catalysed by iridium(I) complexes having hemilabile O‐ and N‐ donor‐functionalised NHC ligands 16a. Interestingly, DFT calculations have allowed to identify an interaction between the β‐H atom on the alkoxo ligand and the methoxy fragment of the NHC ligand, which results in a net destabilisation of the alkoxo intermediate, thereby facilitating the β‐H elimination step, leading to the hydride intermediate 16a. Thus, considering the reversibility of both hydrogen‐transfer processes it is likely that the Oppenauer‐type oxidation of alcohols proceeds through the closely related mechanism shown in Scheme .…”

The Golden Age of Transfer Hydrogenation

“…Interestingly, due to the strong reducing-agent nature of glycerol, iridium nanoparticles in the size of 2−3 nm with uniform and narrow size distribution were formed in the TH process, which was confirmed by both transmission electron microscopy (TEM) and UV−vis analysis. 512 Other iridium catalysts with various mono-NHC units were also successfully synthesized and used such as complex 145 containing an abnormal NHC, 513 146 functionalized by a hemilabile NHC, 514 and 147 containing a NHC with an extended ring and a donor substituent. 515 Both 146 and 147 were extremely effective catalysts.…”

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.…”