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

β-(Z)-Selective alkyne hydrosilylation by a N,O-functionalized NHC-based rhodium(I) catalyst

Neutral and cationic cyclooctadiene rhodium(I) complexes with a lutidine-derived polydentate ligand having NHC and methoxy side-donor functions, [RhBr(cod)(κC-tBuImCH2PyCH2OMe)] and [Rh(cod)(κ2C,N-tBuImCH2PyCH2OMe)]PF6, have been prepared. Carbonylation of the cationic compound yields the dicarbonyl complex [Rh(CO)2(κ2C,N-tBuImCH2PyCH2OMe)]PF6 whereas carbonylation of the neutral compound affords a mixture of di- and monocarbonyl neutral complexes [RhBr(CO)2(κC-tBuImCH2PyCH2OMe)] and [RhBr(CO)(κ2C,N-tBuImCH2PyCH2OMe)]. These complexes efficiently catalyze the hydrosilylation of 1-hexyne with HSiMe2Ph with a marked selectivity towards the β-(Z)-vinylsilane product. Catalyst [RhBr(CO)(κ2C,N-tBuImCH2PyCH2OMe)] showed a superior catalytic performance, in terms of both activity and selectivity, and has been applied to the hydrosilylation of a range of 1-alkynes and phenylacetylene derivatives with diverse hydrosilanes, including HSiMe2Ph, HSiMePh2, HSiPh3 and HSiEt3, showing excellent β-(Z) selectivity for the hydrosilylation of linear aliphatic 1-alkynes. Hydrosilylation of internal alkynes, such as diphenylacetylene and 1-phenyl-1-propyne, selectively affords the syn-addition vinylsilane products. The β-(Z) selectivity of these catalysts contrasts with that of related rhodium(I) catalysts based on 2-picolyl-functionalised NHC ligands, which were reported to be β-(E) selective. An energy barrier ΔG‡ of 19.8 ± 2.0 kcal mol-1 (298 K) has been determined from kinetic studies on the hydrosilylation of 1-hexyne with HSiMe2Ph. DFT studies suggest that the methoxy-methyl group is unlikely to be involved in the activation of hydrosilane, and then hydrosilane activation is likely to proceed via a classical Si-H oxidative addition.

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

The synthesis and characterization of the new Ir(I)-complexes [IrCl(cod)(Bnmim)], [Ir(cod)(emim)(PPh3)]Cl and [Ir(cod)(Bnmim)(mtppms)] are reported. The zwitterionic complexes [Ir(cod)(NHC)(mtppms)] and Na2[Ir(cod)(NHC)(mtppts)] (NHC = emim, bmim or Bnmim; mtppms-Na and mtppts-Na3 = sodium salts of mono- and trisulfonated triphenylphosphine, respectively) were found to be effective precatalysts for transfer hydrogenation of aromatic and aliphatic ketones in basic 2-propanol-water mixtures with initial turnover frequencies up to 510 h−1 at 80 °C, and their catalytic performances were compared to those of [IrCl(cod)(NHC)] complexes (NHC = emim, bmim, Bnmim, IMes) and [Ir(cod)(emim)(PPh3)]Cl. Three of the catalysts were characterized by single-crystal X-ray diffraction. The reaction rates of the transfer hydrogenation of acetophenone and benzophenone showed strong dependence on the water concentration of the solvent, indicating preferential solvation of the catalytically active metal complexes.

Stereoelectronic Profiling of Expanded-Ring N-Heterocyclic Carbenes

Heterobis(carbene) complexes of palladium(II) and gold(I) containing expanded-ring N-heterocyclic carbenes (erNHCs) have been prepared in order to study their electronic properties. erNHCs with mesityl substituents were found to exhibit anisotropic interferences, which hampered ranking of their donicities by ¹³C NMR spectroscopy. The anisotropy effects were found to be stronger in the linear gold complexes, where a smaller coordination number allows the wingtips to spread out more. erNHCs with flexible N-benzyl groups are more suitable, and their donor strengths were found to gradually increase from five- to seven-membered heterocycles. The same trend can also be obtained by comparing the ¹ J(C-H) coupling constants of the respective salts, although significant differences between seven- and eight-membered erNHCs could not be detected. The % V_bur values of erNHCs obtained from structures of their palladium and gold complexes revealed that the anisotropic interferences increase with overall steric bulk.

Simple NMR predictors of catalytic hydrogenation activity for [Rh(cod)Cl(NHC)] complexes featuring fluorinated NHC ligands

A series of imidazolium salts precursors for N-heterocyclic carbenes (NHCs) featuring fluoroaryl substituents have been prepared along with their selenides and rhodium complexes. Tests of the catalytic activity of the [Rh(cod)Cl(NHC)] complexes in the transfer hydrogenation of acetophenone with iPrOH shows that the rhodium complexes bearing fluorinated NHCs are better than their non-fluorinated counterparts. The order of activity being 4-F-C6H4 < 2,4-F2-C6H3 < 2,4,5-F3-C6H2 < 2,6-F2-C6H3 < 2,4,6-F3-C6H2. This order of reactivity is consistent with a number of simple NMR measures of the electronic properties of these systems, including 1JCH of the NHC·HBF4 salts, δ(77Se) of the NHC selenides and 1JRh-C and δ(13Ccarbene) of the [Rh(cod)Cl(NHC)] complexes.

Ring-expanded N-heterocyclic carbenes as ligands in iron-catalysed cross-coupling reactions of arylmagnesium reagents and aryl chlorides

The structure-activity relationship of expanded-ring N-heterocyclic carbenes (NHCs) in the iron-catalysed Kumada aryl-aryl coupling reaction was explored. This was achieved by comparing the catalytic performance of Fe-NHC catalysts generated in situ containing NHCs that differ in steric bulk. In particular, the influences of ring sizes (5-8) and N-aryl substituents were explored in terms of spectroscopic and structural features, which affect their %Vbur values. The three best performing ligands were found on a diagonal of a 5 × 4 structural matrix revealing an optimal steric bulk and significant influences of subtle steric variations on the catalytic activities.

A leap forward in iridium-NHC catalysis: new horizons and mechanistic insights

This review summarises the most recent advances in Ir-NHC catalysis while revisiting all the classical reactions in which this type of catalyst has proved to be active. The influence of the ligand system and, in particular, the impact of the NHC ligand on the activity and selectivity of the reaction have been analysed, accompanied by an examination of the great variety of catalytic cycles hitherto reported. The reaction mechanisms so far proposed are described and commented on for each individual process. Moreover, some general considerations that attempt to explain the influence of the NHC from a mechanistic viewpoint are presented at the end of the review. The first sections are dedicated to the most widely explored reactions that use Ir-NHCs, i.e., hydrogenation and transfer hydrogenation, for which a general overview that tries to compile all the Ir-NHC catalysts hitherto reported for these processes is provided. The next sections deal with hydrogen borrowing, hydrosilylation, water splitting, dehydrogenation (of alcohols, alkanes, aminoboranes and formic acid), hydrogen isotope exchange (HIE), signal amplification by reversible exchange and C-H bond functionalisation (silylation and borylation). The last section compiles a series of reactions somewhat less explored for Ir-NHC catalysts that include the hydroalkynylation of imines, hydroamination, diboration of olefins, hydrolysis and methanolysis of silanes, arylation of aldehydes with boronic acids, addition of aroyl chlorides to alkynes, visible light driven reactions, isomerisation of alkenes, asymmetric intramolecular allylic amination and reactions that employ heterometallic catalysts containing at least one Ir-NHC unit.

Mechanistic studies on the N-alkylation of amines with alcohols catalysed by iridium(i) complexes with functionalised N-heterocyclic carbene ligands

Experimental and theoretical studies give support for an iridium-catalyzed C–N bond formation.

Base free transfer hydrogenation using a covalent triazine framework based catalyst

Isomerisation of allylic alcohols to saturated ketones can be efficiently catalysed by a heterogeneous molecular system resulting from Ir^IIICp* anchoring to a covalent triazine framework.

N-Heterocyclic olefins as ancillary ligands in catalysis: a study of their behaviour in transfer hydrogenation reactions

Ir(i) complexes featuring an N-heterocyclic olefin ligand (NHO) have been tested in the transfer hydrogenation reaction; this representing the first example of the use of NHOs as ancillary ligands in catalysis.

Transfer hydrogenation with abnormal dicarbene rhodium(iii) complexes containing ancillary and modular poly-pyridine ligands

Rhodium(iii) complexes with abnormal dicarbene and diimine ligands are transfer hydrogenation catalysts with visually distinct active and dormant states.

Synthesis and catalytic alcohol oxidation and ketone transfer hydrogenation activity of donor-functionalized mesoionic triazolylidene ruthenium(II) complexes

We report on the synthesis of a variety of C,E-bidentate triazolylidene ruthenium complexes that comprise different donor substituents E (E = C: phenyl anion; E = O: carboxylate, alkoxide; E = N: pyridine at heterocyclic carbon or nitrogen). Introduction of these donor functionalities is greatly facilitated by the synthetic versatility of triazoles, and their facile preparation routes. Five different complexes featuring a C,E-coordinated ruthenium center with chloride/cymene spectator ligands and three analogous solvento complexes with MeCN spectator ligands were prepared and evaluated as catalyst precursors for direct base- and oxidant-free alcohol dehydrogenation, and for transfer hydrogenation using basic iPrOH as a source of dihydrogen. In both catalytic reactions, the neutral/mono-cationic complexes with chloride/cymene spectator ligands performed better than the solvento ruthenium complexes. The donor functionality had a further profound impact on catalytic activity. For alcohol dehydrogenation, the C,C-bidentate phenyl-triazolylidene ligand induced highest conversions, while carboxylate or pyridine donor sites gave only moderate activity or none at all. In contrast, transfer hydrogenation is most efficient when a pyridyl donor group is linked to the triazolylidene via the heterocyclic carbon atom, providing turnover frequencies as high as 1400 h(-1) for cyclohexanone transfer hydrogenation. The role of the donor group is discussed in mechanistic terms.

N-alkyl functionalised expanded ring N-heterocyclic carbene complexes of rhodium(I) and iridium(I): structural investigations and preliminary catalytic evaluation

A series of new N-alkyl functionalised 6- and 7-membered expanded ring N-heterocyclic carbene (NHC) pro-ligands 3-6 and their corresponding complexes of rhodium(I) and iridium(I), [M(NHC)(COD)Cl] 7-14 and [M(NHC)(CO)2Cl] 15-22 are described. The complexes have been characterised by (1)H and (13)C{(1)H} NMR, mass spectrometry, IR and X-ray diffraction. It is noted from X-ray diffraction studies that the N-alkyl substituents are found to orientate themselves away from the metal centre due to unfavourable steric interactions resulting in low percent buried volume (%V(bur)) values in the solid state. The heterocycle ring size is also found to dictate the spatial orientation of the N-alkyl substituents in the neopentyl functionalised derivatives 10 and 14. The 7-membered derivative 14 allows for a conformational 'twist' of the heterocycle ring with the N-alkyl substituents adopting a mutually trans configuration with respect to each other, while the more rigid 6-membered system 10 does not allow for this conformational 'twist' and consequently the N-alkyl substituents adopt a mutually cis configuration. The σ-donor function of this new class of expanded ring NHC ligand has also been probed by measured IR stretching frequencies of the [M(NHC)(CO)2Cl] complexes 15-22. A preliminary catalytic survey of the hydrogenation of functionalised alkenes with molecular hydrogen under mild conditions has also been undertaken with complex , affording an insight into the application of large ring NHC ancillary ligands bearing N-alkyl substituents in hydrogenation transformations.

Hydroxyl and ester functionalized N-heterocyclic carbene complexes of iridium(i): efficient catalysts for transfer hydrogenation reactions

Iridium(i)–NHC catalyzed transfer hydrogenation reaction of aldehydes and ketones with up to 10 000 TON has been achieved.

Expanded ring diaminocarbene palladium complexes: synthesis, structure, and Suzuki-Miyaura cross-coupling of heteroaryl chlorides in water

A series of new 6- and 7-membered N-heterocyclic carbene (NHC) complexes of palladium (NHC)Pd(cinn)Cl (cinn = cinnamyl = 3-phenylallyl) were synthesized and characterized structurally in the solid state. The influence of ring size (5, 6 or 7) and bulkiness of N-aryl substituents (Mes = 2,4,6-trimethylphenyl, or Dipp = 2,6-diisopropylphenyl) in carbenes on palladium catalysed Suzuki-Miyaura cross-coupling was revealed. Due to the unique stereoelectronic properties of expanded ring NHCs, a versatile, highly efficient green protocol of coupling of heteroaromatic chlorides and bromides with boronic acids has been developed. High quantitative yields of biaryls were achieved with water as solvent, under air, using low catalyst and phase transfer agent loadings, and with mild and environmentally benign base NaHCO3.