Pd-NHC Catalysts Research Articles

ChemInform Abstract: Synthesis of Acyl Pyrroles via Palladium‐Catalyzed Carbonylative Amination of Aryl and Alkenyl Iodides.

A palladium-catalyzed synthesis of acyl pyrroles from aryl and alkenyl iodides is reported. This carbonylative amination requires only atmospheric (balloon) pressure of carbon monoxide and proceeds with Pd(PPh3)4 and Pd-NHC catalysts. Aryl and heteroaryl iodides give the corresponding acyl pyrroles in good to excellent yields, while alkenyl iodides provide the corresponding acyl pyrroles in low to moderate yields.

Synthesis of Acyl Pyrroles via Palladium-Catalyzed Carbonylative Amination of Aryl and Alkenyl Iodides

A palladium-catalyzed synthesis of acyl pyrroles from aryl and alkenyl iodides is reported. This carbonylative amination requires only atmospheric (balloon) pressure of carbon monoxide and proceeds with Pd(PPh(3))(4) and Pd-NHC catalysts. Aryl and heteroaryl iodides give the corresponding acyl pyrroles in good to excellent yields, while alkenyl iodides provide the corresponding acyl pyrroles in low to moderate yields.

Asymmetric C(sp3)-H/C(Ar) coupling reactions. Highly enantio-enriched indolines via regiodivergent reaction of a racemic mixture

N-Aryl, N-branched alkyl carbamates react with in situ generated chiral Pd-NHC catalysts by coupling a Pd-Ar moiety with an aliphatic C–H bond at high temperature to give enantioenriched 2-substituted and 2,3-disubstituted indolines. Prochiral precursors give single products with very high asymmetric induction. Chiral racemic precursors react in a regiodivergent reaction of a racemic mixture to yield enantioenriched indolines resulting from either methyl C–H activation or asymmetric methylene C–H activation. In favorable cases this can result in a complete separation of an enatiomeric mixture into two different highly enantioenriched indolines.

ChemInform Abstract: N‐Heterocyclic Carbene (NHC) Ligands and Palladium in Homogeneous Cross‐Coupling Catalysis: A Perfect Union

AbstractReview: 95 refs.

Characterization of [Pd(pap)(NHC)I2]n (n = +1, 0, −1; pap = phenylazopyridine; NHC = N-heterocyclic carbene): Unusual coordination mode of pap and non-innocent behavior of NHC

The compounds [Pd(pap)(NHC)I2], 1 and 2 (pap=Phenylazopyridine, NHC=1,3-di-n-butyl-benzimidazolin-2-ylidene, 1 and NHC=1-n-butyl-3-isopropyl-benzimidazolin-2-ylidene, 2) were synthesized from the respective 1,3-disubstituted benzimidazolium salts and [Pd(pap)] precursor in the presence of a base. Crystallographic characterization of 1 shows an unusual coordination mode of pap, with the ligand coordinating to the Pd(II) center only through the pyridine nitrogen atom in an η1-N fashion. Both the nitrogen atoms of the azo group remain uncoordinated to the metal center. Cyclic voltammetry of 1 and 2 shows two oxidation and two reduction steps. A combination of UV–Vis and EPR spectroelectrochemistry is used to show that the reduction steps are pap based, proving its known non-innocent character. Investigation of the first oxidation step points to an electron removal from a predominantly NHC based orbital, thus showing the potential non-innocent nature of these ligands. These results can provide new mechanistic insights into the use of Pd–NHC catalysts for cross-coupling reactions, where oxidative addition is one of the deciding steps for catalysis.

N-Heterocyclic carbene (NHC) ligands and palladium in homogeneous cross-coupling catalysis: a perfect union

Cross-coupling reactions using Pd-NHC (NHC = N-heterocyclic carbene) catalysts are discussed in this critical review and examined in terms of catalytic activity and how these have permitted advances in the area as they developed (95 references).

Suzuki Coupling in Water with Pd-NHC Immobilized on Fe3O4/Ionic Liquid

An immobilized Pd-NHC catalyst 1 was prepared according to equation 1. Catalyst 1 was found to promote the Suzuki coupling of aryl bromides 2 and aryl boronic acids 3 to give the corresponding biaryl products 4 in up to 100% yield (28 examples).

Highly Active and Magnetically Recoverable Pd-NHC Catalyst Immobilized on Fe3O4 Nanoparticle-Ionic Liquid Matrix for Suzuki Reaction in Water

Pd-NHC-ionic liquid matrix was immobilized into ionic liquid layers coated on the surface of Fe3O4. The immobilized Pd-NHC was used as a catalyst for Suzuki coupling. The catalyst exhibited both high catalytic activity and stability for the coupling between aryl bromide and arylboronic acid in water. This catalyst was simply recovered by an external permanent magnet and recycled without a significant loss in the catalytic activity.

N-Heterocyclic carbenes as ligands in palladium-catalyzed Tsuji–Trost allylic substitution

A Pd(0)-catalyzed allylic substitution (i.e., Tsuji–Trost reaction) using N-heterocyclic carbene as a ligand was investigated. It has been proven that an imidazolium salt 2d having bulky aromatic rings attached to the nitrogens in its imidazol-2-ylidene skeleton is suitable as a ligand precursor and that a Pd 2dba 3–imidazolium salt 2d–Cs 2CO 3 system is highly efficient for producing a Pd–NHC catalyst in this reaction. Allylic substitution using a Pd–NHC complex differed from that using a Pd–phosphine complex as follows: (1) the reaction using a Pd–NHC complex required elevated temperature (50 °C or reflux in THF), (2) allylic carbonates were inert to a Pd–NHC complex, and (3) nitrogen nucleophiles such as sulfonamide and amine did not react with allylic acetate. It was also found that allylic substitution with a soft nucleophile using a Pd–NHC catalyst proceeds via overall retention of configuration to give the product in a stereospecific manner, the stereochemical reaction course obviously being the same as that of the reaction using a Pd–phosphine complex.

Towards the rational design of palladium- N-heterocyclic carbene catalysts by a combined experimental and computational approach

A combined experimental and computational approach towards the development of Pd–NHC catalysts is described. A range of benzimidazolylidinium ligands incorporating electron-rich and electron-poor substituents were prepared and evaluated in the Suzuki reaction. The most electron-rich ligand showed the highest catalytic activity. Based on this information, the first alkyl–alkyl Negishi cross-coupling reaction protocol was developed. Evaluation of N, N′-diaryl-(4,5-dihydro)imidazolylilidinium ligands showed a strong dependence on the steric topography around the metal centre. A computational study of the most active ligand in the Negishi reaction, its Pd(0) and PdCl 2-complexes and related structures were modelled at the B3LYP/DZVP and HF/3-21G levels of theory. The potential energy hypersurfaces flattened with increase in ligand size. Binding energies were computed for carbene/Pd(0) adducts (in the range ∼31–40 kcal mol −1), roughly double that for PH 3 (∼16 kcal mol −1). Weak intramolecular interactions were found using AIM analyses.