An N‐Heterocyclic Carbene with a Saturated Backbone and Spatially‐Defined Steric Impact

Abstract

The synthesis and coordination chemistry of a saturated analogue of a "bulky-yet-flexible" N-heterocyclic carbene (NHC) ligand are described. "SIPaul" is a 4,5-dihydroimidazol-2-ylidene ligand with unsymmetrical aryl N-substituents, and is one of the growing class of "bulky-yet-flexible" NHCs that are sufficiently bulky to stabilize catalytic intermediates, but sufficiently flexible that they do not inhibit productive chemistry at the central metal atom. Here, the synthesis of SIPaul.HCl and its complexes with copper, silver, iridium, palladium,

C 2-Symmetric atropisomeric N-heterocyclic carbene–palladium(ii) complexes: synthesis, chiral resolution, and application in the enantioselective α-arylation of amides

“…In order to gain insight into the values of the rotational barriers, theoretical calculations were run (Scheme 1). 15,17 According to these calculations, cis -conformations ( meso ) are more stable than trans -conformations (chiral). As expected, the more energetically favourable rotations take place with the benzhydryl groups on the side of the imidazolium proton (Δ G ≠ (H) ) since the methyl groups prevent rotation on the backbone side (Δ G ≠ (BB) ).…”

Design Concepts for N-Heterocyclic Carbene Ligands

“…The initial version of this ligand was first synthesized by Markó and coworkers from the corresponding aniline, although an improved and scaled-up synthetic route has since been reported (Figure 4f) [49]. Subsequently, several derivatives have been obtained, including: IPr* tol , IPr* OMe , and IPr ** which vary the arene parasubstitution [50][51][52]; IPr* which replaces phenyl with naphthyl [53]; and IPaul and SIPaul which feature only one diphenylmethyl substituent on the arene [54,55]. These ligands were successfully used in catalysis in combination with several transition metals, including copper, gold, nickel, silver, iridium and palladium.…”

Antimony(III) Halide‐Assisted Stereospecific Coordination of Thione

“…The percent buried volume of IPr* carbene ligand (%V bur 52.1% in the [(IPr*)CuCl]) is significantly larger than that of IPaul carbene ligand (%V bur 45.7% in [(IPaul)CuCl]) [14–16] . Thus, IPaul carbene demonstrates unevenly distributed steric impact across four quadrants (two very bulky quadrants and two very open quadrants), while IPr* carbene exhibits the uniformly distributed steric impact across four quadrants.…”

“…This might lead to enhancing the catalytic activity when coordinating with the metal. As of today, only four transition metal IPaul carbene complexes have been isolated and structurally characterized [14,15] . Surprisingly, all four IPaul‐metal complexes [(IPaul)CuCl], (IPaul)AgCl, [(IPaul)(Cp)NiCl] and [(IPaul)(COD)IrCl] were in syn ‐conformation, while no metal complex was isolated and structurally characterized in anti ‐conformation.…”