Dihydroimidazolium Salts Research Articles

Macrocyclic N-Heterocyclic Carbenes: Synthesis and Catalytic Applications

A synthesis of two macrocyclic N-heterocyclic carbenes (NHCs) is reported. An advanced intermediate could be differentiated to access either imidazolium or dihydroimidazolium salts. Deprotonation g...

Rhodium‐NHC Hybrid Silica Materials as Recyclable Catalysts for [2+2+2] Cycloaddition Reactions of Alkynes

AbstractBis‐silylated dihydroimidazolium salt 1 and monosilylated imidazolium salt 2 are transformed to (NHC)RhCl(COD) complexes 3 and 4, allowing the preparation of hybrid silica materials either by sol‐gel or grafting processes. Full characterization of the materials by means of solid state NMR, N2‐sorption measurements, thermogravimetric analysis (TGA) and elemental analysis was followed by evaluation of catalytic activity in the [2+2+2] cycloaddition of alkynes. Excellent yields of the cycloadducts are obtained for up to six consecutive cycles with the grafted material, using simple filtration to recover the catalyst. Both conventional and microwave heating prove effective for the process described.

New Phosphine-Functionalized NHC Ligands: Discovery of an Effective Catalyst for the Room-Temperature Amination of Aryl Chlorides with Primary and Secondary Amines

We report convenient and high-yielding syntheses of new phosphine-functionalized dihydroimidazolium salts and demonstrate their utility as ligand precursors for Buchwald–Hartwig amination. Several examples of the general formula [1-Mes-3-{2-(PR2)phenyl}imidazolidin-2-ylium][BF4] have been prepared, where phosphines of varying steric and electronic properties (R = Ph (9), Cy (10), 1-Ad (11)) are tethered by an o-phenylene group. The synthesis was not adaptable to N-aryl groups other than mesityl, giving unexpected phosphonium salt species instead. The synthesis was adapted to flexible benzyl-linked variants of the formula [1-Ar-3-{2-(PCy2)benzyl}imidazolidin-2-ylium][BF4], which allowed more steric variation of the dihydroimidazolium N-aryl group (Ar = Mes (21), Dipp (22)). A preliminary study of these hybrid NHC/P ligands in Buchwald–Hartwig amination catalysis (in situ precatalyst formation) revealed 11 to be the most active of the series. Premixing the isolated free NHC ligand 1-Mes-3-{2-(PAd2)phenyl}imidazolidin-2-ylidene (23) with [Pd(cinnamyl)Cl]2 provided a highly active precatalyst that performed well at room temperature and 1 mol % catalyst loading. The system was shown to have an unprecedented ability to arylate both primary alkylamines (monoarylation) and secondary dialkylamines with aryl chlorides at room temperature. Electron-rich and -poor aryl and heteroaryl halides, as well as those featuring ortho substitution, were well tolerated, while substrates featuring both primary and secondary amine groups were selectively arylated at the NH2 position. Furthermore, a preliminary examination of performance in ammonia arylation and acetone α-arylation showed promising results, giving good conversion and high selectivity for monoarylation in both cases.

Unexpected rearrangements in the synthesis of an unsymmetrical tridentate dianionic N-heterocyclic carbene

Starting from the same ethylenediamine species, three valuable carbene precursors were synthesized under differing conditions: a tridentate dianionic N-heterocyclic carbene bearing an aniline, a phenol and a central dihydroimidazolium salt, its benzimidazolium isomer by intramolecular rearrangement and a dicationic benzimidazolium-benzoxazolium salt by changing the Bronsted acid from HCl to HBF_4. A DFT study was performed to understand the rearrangement pathway. The structure of a bis[(NCO)carbene] zirconium complex was determined.

Synthesis and characterisation of quinoline functionalised imidazolium salts

A series of quinoline based imidazolium salts were prepared and characterised as nucleophilic heterocyclic carbene (NHC) ligand precursors. The symmetrically saturated imidazolium salt prepared by ring closure of achiral tetradentate diquinolyl-diamine with triethyl orthoformate is a tridentate ligand and can function as pincer ligand while the two asymmetric imidazolium salts are bidentate and have different steric bulk which may have interesting implication in catalysis. The solubility of the symmetrical dihydroimidazolium salt was enhanced by replacing the counter ion tetraflouroborate with tetrakis (3,5-bis(triflouromethyl)phenyl)borate (Barf ) and its structure confirmed by X- ray crystallography. Keywords: Nucleophilic heterocyclic carbene (NHC), Imidazolium salt, ligand, ring closure.

Preparation and reaction of uracil substituted cyclen and cyclam: formation of tricyclic guanidinium and dihydroimidazolium salts

Di-uracil substituted cyclen derivatives were prepared by the reaction of cyclen with 6-chloro-1-methyluracil or 6-chloro-1,3-dimethyluracil. The reaction of cyclam with 6-chloro-1,3-dimethyluracil gave a similar di-uracil substituted cyclam. The 1,7-di-uracil substituted cyclen was converted to the tricyclic guanidinium salt and acylurea upon heating in DMSO in the presence of weak acid. The 1,8-di-uracil substituted cyclam gave a tricyclic dihydroimidazolium salt under the same conditions. These reactions can be explained by an intramolecular uracil ring-breaking reaction mechanism.

Improved Chiral Olefin Metathesis Catalysts: Increasing the Thermal and Solution Stability via Modification of a C1‐Symmetrical N‐Heterocyclic Carbene Ligand

AbstractFour new ruthenium‐based olefin metathesis catalysts that possess an N‐heterocyclic carbene (NHC) ligand with benzyl (Bn) or or n‐propyl (n‐Pr) N‐alkyl groups have been prepared. The synthetic routes developed for the synthesis of the required dihydroimidazolium salts are general. Catalysts bearing larger NHC ligands with larger N‐alkyl groups displayed improved thermal and solution state stability up to 80 °C. The reactivity of the new catalysts in ring‐closing metathesis is directly related to the increased steric bulk of the NHC ligand. The new catalysts have been evaluated in desymmetrization reactions and the nature of the N‐alkyl group of the NHC ligands has been shown to have an important effect on the observed enantioselectivities.

Organic–inorganic hybrid silica materials containing imidazolium and dihydroimidazolium salts as recyclable organocatalysts for Knoevenagel condensations

Organic–inorganic hybrid silica materials containing imidazolium and dihydroimidazolium salts prepared from monosilylated and disilylated monomers by sol–gel methodologies are active and reusable organocatalysts for the Knoevenagel condensation of aromatic aldehydes with malononitrile and ethyl cyanoacetate under solvent-free conditions. Our immobilized systems present higher activities than related homogeneous bis-imidazolium salts, showing the cooperative effect of the matrix surface and the additional advantage of easy recycling. The best performances were obtained with the material derived from polycondensation of a disilylated dihydroimidazolium salt in the absence of tetraethoxysilane (TEOS).

Recoverable Palladium Catalysts for Suzuki–Miyaura Cross‐ Coupling Reactions Based on Organic‐Inorganic Hybrid Silica Materials Containing Imidazolium and Dihydroimidazolium Salts

AbstractThe systems formed by palladium acetate [Pd(OAc)2] and hybrid silica materials prepared by sol‐gel from monosilylated imidazolium and disilylated dihydroimidazolium salts show catalytic activity in Suzuki–Miyaura cross‐couplings with challenging aryl bromides and chlorides. They are very efficient as recoverable catalysts with aryl bromides. Recycling is also possible with aryl chlorides, although with lower conversions. In situ formation of palladium nanoparticles has been observed in recycling experiments.

Design and Synthesis of Planar Chiral Heterocyclic Carbene Precursors Derived from [2.2]Paracyclophane

A unique family of planar chiral symmetrical N-heterocyclic carbene precursors with restricted flexibility derived from [2.2]paracyclopane were obtained by a new synthetic route. The resolution of 4-amino-13-bromo[2.2]paracyclophane was achieved with relatively high efficiency. Starting from (4 S p,13 R p)-4-amino-13-bromo[2.2]paracyclophane, the planar chiral pseudogem-disubstituted [2.2]paracyclophanyl dihydroimidazoliums were prepared in a four-step sequence with good yields. The resulting dihydroimidazolium salts were fully characterized with a series of methods including single-crystal X-ray diffraction technique.

Chiral tetradentate amine and tridentate aminocarbene ligands: Synthesis, reactivity and X-ray structural characterizations

(1 R,2 R)-diaminocyclohexane ( 1) and (1 R,2 R)-diaminodiphenylethylenediamine ( 2) were used as building blocks for the synthesis of chiral tetradentate diquinolyl-diamine and related diquinolyl-dihydroimidazolium salts. A neutral chiral palladium(II) complex was synthesized by reaction of palladium acetate with the tetradentate diquinolyl diamine derived from 2 and used as a homogeneous catalyst for the Heck reaction between styrene and haloarenes. A chiral tridentate aminocarbene was generated in situ by deprotonation of the dihydroimidazolium salt derived from 1 and allowed to react with CuI to give a new chiral quinolyl-carbene copper(I) complex.

Efficient Suzuki–Miyaura Coupling Reactions between Lithium Alkynyltrimethylborates and Aryl Chlorides

AbstractThe Pd‐catalysed alkynylation of alkynylboronic esters (generated in situ) with various aryl chlorides in the presence of Pd2(dba)3, the sterically hindered dihydroimidazolium salt 4 as the precatalyst and CsF is reported. Under these conditions, electron‐poor and electron‐rich aryl chlorides undergo efficient cross‐coupling reactions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Monolithic Materials: New High-Performance Supports for Permanently Immobilized Metathesis Catalysts.

Back to the roots: Ring-opening metathesis polymerization allows the synthesis of monolithic materials, which may be subject to an in situ derivatization with dihydroimidazolium salts through metathesis graft polymerization. This method offers an attractive route to highly reactive, permanently immobilized metathesis initiators. The picture shows an electron microscope image of the microstructure of the monolithic support.