Base‐Ionizable Anionic NHC Ligands in Pd‐catalyzed Reactions of Aryl Chlorides

Abstract

Aryl chlorides, due to their affordability and accessibility, are preferred reagents in Pd‐catalyzed arylation reactions. However, the reactivity of aryl chlorides is often reduced compared to aryl bromides and iodides due to the significantly higher barriers of the oxidative addition stage. This research introduces a novel design for NHC ligands, which notably enhances the efficiency of Pd/NHC catalytic systems in reactions where oxidative addition of aryl chloride is the rate‐limiting step. This design leverages a synergy between specific steric characteristics and the anionic nature of the newly fashioned 1,2,4‐triazol‐5‐ylidene ligands. These ligands, inspired by Nitron‐type designs, can be ionized under basic conditions due to their NH‐acidic aryl(alkyl)amino groups. Detailed experimental and DFT studies revealed that the deprotonation of these NHCs promotes electron donation to the metal center, promoting the oxidative addition of aryl chloride. The specially optimized ATPr ligand, featuring 2,6‐diisopropylphenyl groups, displayed remarkable catalytic efficacy in the Suzuki‐Miyaura reaction and improved outcomes in ketone α‐arylation and Buchwald‐Hartwig reactions with unactivated aryl chlorides. The insights and strategies established in this study provide rational considerations for further advancements in NHC designs and their applications in metal‐catalyzed reactions.

Synthesis and comparative study of (NHCF)PdCl2Py and (NHCF)Ni(Cp)Cl complexes: investigation of the electronic properties of NHC ligands and complex characteristics

“…To investigate the influence of the fluorine position in the substituent on the π-contribution to the M/NHC bond, a series of selenones 5a–j were synthesized by reacting an imidazolium salt with elemental selenium in the presence of triethylamine in DMF under air conditions. 50 NMR spectra of 77 Se were recorded to determine the chemical shift of 77 Se, which correlates with the π-acceptor properties of the ligand. 51,52 The order of increase in π-acceptance was 5f > 5g > 5d > 5j > 5i > 5c ≈ 5e > 5h > 5b > 5a (Fig.…”

“…40,41 Selenium derivatives 5 were prepared as previously described in the literature. 50 The samples for the ESI-TOF-HRMS experiments were prepared in 1.8 mL glass vials with screw-top caps fitted with Teflon-lined septa (Agilent Technologies). NMR spectra were recorded by using Bruker Avance-NEO 300 or Bruker Fourier 300HD spectrometers operating at 300.1 MHz for 1 H, 75 MHz for 13 C, 57 MHz for 77 Se and 282.4 MHz for 19 F. 1 H and 13 C NMR chemical shifts are reported relative to the solvent signals as internal standards: 2.5 ppm/39.5 ppm for DMSO- d 6 and 7.26 ppm/77.16 for CDCl 3 .…”