N-heterocyclic Carbene Ring Research Articles

Expanded ring N-heterocyclic carbenes: A comparative study of ring size in palladium (0) catalysed Mizoroki–Heck coupling

The synthesis and characterisation of a range of new 6- and 7-membered expanded ring N-heterocyclic carbene (NHC) complexes of zero valent palladium dvtms (divinyltetramethyldisiloxane) 8-14 is reported, with the first comparative catalytic study probing the effects of large ring N-heterocyclic carbene ligands (6-Mes and 7-Mes) with their analogous 5-membered derivative (IMes) on catalytic performance. The catalytic performances of the [Pd(NHC)(dvtms)] complexes are evaluated in the Mizoroki-Heck coupling of 4-bromoacetophenone and n-butyl acrylate, providing a ready and effective comparison into the influence of ring size and steric congestion on catalytic activity in carbon-carbon cross coupling reactions.

Theoretical investigations of the reactivities of saturated five-membered ring N-heterocyclic carbenes with heavier Group 14 elements

The potential energy surfaces for the chemical reactions of group 14 carbenes have been studied using density functional theory (B3LYP/LANL2DZ). Five saturated five-membered-ring N-heterocyclic carbene Dipp[upper bond 1 start]N(CH(2))(2)N(Dipp)E[upper bond 1 end]: (five-ring-E:) species, where E = C, Si, Ge, Sn and Pb, have been chosen as model reactants in this work. Also, four kinds of chemical reactions; addition of water, methane insertion, alkene cycloaddition and dimerization, have been used to study the chemical reactivities of these group 14 carbenes. The present theoretical investigations suggest that the relative carbenic reactivity decreases in the order: C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the more stable is the carbene towards chemical reactions. This may be the reason that there have been many instances reported of the synthesis and characterization of stable group 14 five-membered-ring N-heterocyclic carbene species with various alkyl protecting substituents at room temperature. Furthermore, the singlet-triplet energy splitting of the five-ring-E:, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made.

Dual reactivity of N-heterocyclic carbenes towards copper(ii) salts

Complexes of N-heterocyclic carbenes (NHCs) with copper(II) halogenides are unstable. Upon formation, these complexes decompose to give haloamidinium salts. Contrastingly, O-substituted copper(II) NHC complexes are fairly stable. A series of new five-, six- and seven-membered ring NHC complexes of Cu(OAc)(2) have been synthesised and characterised in the solid state.

Substituted six-membered ring carbenes: the effects of amino and cyclopropyl groups through DFT calculations

DFT calculations are employed to compare and contrast six-membered ring carbenes including 1,3-dimethyltetrahydropyrimidin-2-ylidene ( 1a), 1-methyl-3-cyclopropyltetrahydropyridine-2-ylidene ( 2a), and 1,3-dicyclopropylcyclohexane-2-ylidene ( 3a) as well as their unsaturated analogues 1b, 2b, 3b, and 2c. The amino groups exert singlet–triplet energy separation (∆ E s−t) of 60.9 kcal/mol to 1a while cyclopropyls induce a ∆ E s−t of 14.8 kcal/mol to 3a. The simultaneous presence of amino and cyclopropyl in 2a leads to a ∆ E s−t of 43.3 kcal/mol. Unsaturation slightly increases the ∆ E s−t of 1a and 3a but not that of 2a. Our thermodynamic, kinetic, and reactivity results are compared with those of synthetic five-membered ring N-heterocyclic carbenes.

Expanded ring N-heterocyclic carbene complexes of zero valent platinum dvtms (divinyltetramethyldisiloxane): Highly efficient hydrosilylation catalysts

The synthesis and characterisation of a series of new 6- and 7-membered NHC (N-heterocyclic carbene) complexes 7–12 of zero valent platinum dvtms (divinyltetramethyldisiloxane), [Pt(NHC)(dvtms)] are reported. A number of the complexes were investigated as catalyst precursors in the hydrosilylation of a range of unsaturated substrates, including alkynes, alkenes and a ketone with triethylsilane and bis(trimethylsiloxy)methylsilane (MD’M), at low catalyst loading (0.005 mol %); in general, the activity, and the selectivity for 1-functionalised product was found to be high.

Electronic and Steric Parameters of 76 N-Heterocyclic Carbenes in Ni(CO)3(NHC)

We report electron-donor and steric properties of a diverse group of representative N-heterocyclic carbene (NHC) ligands quantified with the help of DFT calculations. This study afforded the conventional TEP data (Tolman electronic parameter = νCO (A1) of Ni(CO)3(NHC)), which allowed ranking 76 NHC ligands in order of increasing donor power. The TEP data reveal several general trends concerning the influence of NHC ring size, substitution, and annulation. The calculations also provided reaction enthalpies for CO elimination from the Ni(CO)3(NHC) complexes and formation of the 16-electron Ni(CO)2(NHC) species. This reaction is largely under steric control, which allowed defining a new steric descriptor for NHC ligands, r (“repulsiveness”) = 10 × (7.568 − 0.003172TEP − 0.0446ΔH), ranging from 0.0 for the smallest (ImNH2) to 8.0 for the most repulsive carbene (ImNAd2) of this work. Ni(CO)3L and Os(H2)Cl2(CO)L2 complexes eliminate CO and H2, respectively, more readily with L = NHC vs PR3 ligands. Apparently, ...

Rare [(NHC)2Ni-OH]-Type Terminal Nickel Hydroxo and [(NHC)2Ni]-Type Complexes ofN/O-Functionalized N-Heterocyclic Carbenes as Precatalysts for Highly Desirable Base-Free Michael Reactions in Air at Ambient Temperature

Highly convenient base-free Michael reaction of β-dicarbonyl, β-keto ester, and α-cyano ester compounds with activated ethylene compounds in air at ambient temperature catalyzed by a series of new nickel complexes, 1b−4b, of N/O-functionalized N-heterocyclic carbenes (NHC) are reported. Quite interestingly, of these nickel complexes, 1b and 2b were found to be of the [(NHC)2Ni-OH] type bearing a terminal [Ni-OH] moiety and are among the few known structurally characterized examples of monomeric terminal [Ni-OH] complexes. Compounds 3b and 4b, on the other hand, were found to be of the commonly encountered [(NHC)2Ni]-type complexes. More significantly, both the [(NHC)2Ni-OH]-type complexes 1b and 2b and the [(NHC)2Ni]-type complexes 3b and 4b were synthesized using same synthetic methodology by sheer modulation of the N-substituents on the N-heterocyclic carbene ring by the reaction of the imidazolium chloride salts with NiCl2·6H2O in the presence of K2CO3 as a base.

Expanded ring and functionalised expanded ring N-heterocyclic carbenes as ligands in catalysis

The synthesis of new functionalised 6- and 7-membered NHC (N-heterocyclic carbene) precursors bearing anisidyl or pyridine N-substituents, both symmetrically and non-symmetrically substituted is reported. Their corresponding rhodium(i) and iridium(i) complexes, M(COD)(NHC)Cl, were also prepared and characterised. The unusual Rh(iii)/Rh(i) salt, [Rh(eta(2)-NHC-py)(2)Cl(2)][Rh(COD)Cl(2)], was obtained with one of the pyridyl-functionalised NHC ligands. Single-crystal X-ray analyses have been obtained for the majority of the complexes and NHC salts. The activity of these complexes was tested in the hydrogenation of a range of substrates with molecular hydrogen, including 1-cyclooctadiene and 2-methyl styrene, where they show enhanced activity and stability in comparison to non-functionalised NHC analogues, operating under exceptionally mild conditions (ambient temperature and atmospheric pressure).

Active Trifluoromethylating Agents from Well-Defined Copper(I)−CF3 Complexes

The first examples of isolable and structurally characterized Cu(I)-CF3 complexes are reported. N-Heterocyclic carbene (NHC)-supported copper tert-butoxide complexes reacted with Me3Si-CF3 to afford new (NHC)Cu-CF3 complexes whose structures were dependent on whether or not the C4-C5 positions of the five-membered NHC rings were saturated. In situ generated (SIiPr)Cu-CF3 cleanly transferred its trifluoromethyl group to a number of organic halides under mild conditions.

Synthesis, Structure, and Solution Dynamics of Neutral Allylnickel Complexes of N-Heterocyclic Carbenes

Neutral allylnickel complexes of the general formula [Ni(η3-C3H5)(Im)Br] (where Im = 1,3-di-tert-butylimidazol-2-ylidene (tBu2Im), 1,3,4,5-tetramethylimidazol-2-ylidene (Me4Im), 1,3-dimethylimidazol-2-ylidine (Me2Im)) were prepared from the reaction of the dimer [Ni(η3-C3H5)Br]2 with 2 equiv of the corresponding free N-heterocyclic carbene (NHC). The halide exchange of Br- by I- can be easily performed, giving rise to compounds of the type [Ni(η3-C3H5)(Im)I]. The new complexes [Ni(η3-C3H5) (tBu2Im)Br] (1), [Ni(η3-C3H5)(Me4Im)Br] (2), [Ni(η3-C3H5)(Me2Im)Br] (3), [Ni(η3-C3H5)(Me2Im)I] (4), and [Ni(η3-C3H5)(Me4Im)I] (5) were obtained in good yields and were fully characterized by elemental analysis and NMR spectroscopy. The X-ray crystal structures of 1, 2, 4, and 5 reveal a square-planar geometry at the nickel atom and a tilt angle of the NHC ring (in relation to the Ni square plane) dependent on the bulkiness of both the N substituents and the halogen bound to Ni. Variable-temperature NMR experiments in so...