Soft Heteroleptic N-Heterocyclic Carbene Palladium(II) Species for Efficient Catalytic Routes to Alkynones via Carbonylative Sonogashira Coupling

Abstract

N , N ′-Substituted di-isopropyl ( NHC-1 ), benzyl-isopropyl ( NHC-2 ), and adamantyl-isopropyl ( NHC-3 ) benzimidazolium salts react with palladium(II) bromide in pyridine to afford the corresponding trans -dibromidopyridinepalladium(II) complexes Pd-C1 , Pd-C2 , and Pd-C3 in high yields. A distorted square planar geometry for Pd-C2 and Pd-C3 was confirmed by single-crystal X-ray diffraction. The palladium(II) complexes show a remarkably higher catalytic activity and selectivity, compared to the literature data, in carbonylative Sonogashira coupling reactions of aryl iodides and aryl diiodides with aryl alkynes, alkyl alkynes, and dialkynes. Excellent yields with as low as 0.03 mol % loading of the catalyst were obtained. In the series of benzimidazolium (NHC) precursors, the 1 H NMR signals of the α hydrogen show a consistent probing of the N-substituent donor strength. The density functional theory (DFT) quantum mechanical descriptors of the frontier orbitals were calculated. A linear correlation of the calculated absolute softness of the complexes versus the calculated percent buried volume (% V bur ) of their corresponding ligands was obtained. The catalytic activity experimental data are consistent with the hard soft acid base (HSAB)-predicted high affinity of the softest Pd-C3 complex for soft substrates, such as aryl iodides.

Regioselective Synthesis of Chromones via Cyclocarbonylative Sonogashira Coupling Catalyzed by Highly Active Bridged-Bis(N-Heterocyclic Carbene)Palladium(II) Complexes

“… e Heteroleptic N-heterocyclic carbene palladium(II) complexes (Pd–NHC–Py). 59 f THF was used as a solvent. …”

Recent Advances in Benzimidazole Based NHC‐Metal Complex Catalysed Cross‐Coupling Reactions**

“…Recently, Ali et al investigated the catalytic efficiency of heteroleptic NHC-pyridine-Pd(II) complexes derived from benzimidazole having adamantyl, benzyl and isopropyl substituents in carbonylative Sonogashira coupling of aryl iodides and aryl diiodides with aryl alkynes, alkyl alkynes, and dialkynes. [59] Structural studies indicated a distorted square planar geometry for the trans-dibromidopyridine palladium-(II) complexes. Based on the DFT quantum mechanical descriptors of the frontier orbitals complex 32 c owing to its inherent steric and electronic properties provided by the adamantly group was found to be the softest species and its high catalytic activity towards soft substrates like aryl iodides compliments the results.…”

“…Recently, Ali et al . investigated the catalytic efficiency of heteroleptic NHC‐pyridine‐Pd(II) complexes derived from benzimidazole having adamantyl, benzyl and isopropyl substituents in carbonylative Sonogashira coupling of aryl iodides and aryl diiodides with aryl alkynes, alkyl alkynes, and dialkynes [59] . Structural studies indicated a distorted square planar geometry for the trans ‐dibromidopyridine palladium‐(II) complexes.…”

Robust alkyl‐bridged bis(N‐heterocyclic carbene)palladium(II) complexes anchored on Merrifield's resin as active catalysts for the selective synthesis of flavones and alkynones

“…(NHC)palladium(II) complexes were used efficiently in different applications including carbonylative coupling and alkoxycarbonylation and aminocarbonylation reactions. [ 36–40 ] The activity of (NHC)palladium(II) complexes is connected to the bonding nature of the NHC ligands including a strong σ‐electron donation, as well as a significant π acidity and π basicity, contributing to the stability of the different intermediates of the catalytic cycle and preventing deactivation pathways such as the formation of inactive palladium black. [ 41,42 ] The stability of these complexes can be further enhanced by the thermodynamic chelate effect such as the case of the chelating bis(NHCs).…”