Amine‐Linked N‐Heterocyclic Carbenes: The Importance of an Pendant Free‐Amine Auxiliary in Assisting the Catalytic Reaction

Abstract

We have successfully expanded the library of amino-NHCs with varying substituents on the amine group, leading to insight about the instability of NHCs arising from the intermolecular interaction of the dangling amine side-arm. However, the pendant amine plays an important role with respect to the catalytic process, resuscitating the catalytic activity of unsaturated NHC's through a synergistic effect invoked by the secondary amine. This proof of concept allows us to expand the spectrum of catalysis to C-C, as well as C-B bond formation.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

“…This methodology was subsequently extended to include chiral boronation of α,β-unsaturated ketones, esters, aldehydes, amides, and nitriles . An NHC bearing a pendant amine group was also competent as a catalyst for the borylation of α,β-unsaturated ketones . Likewise, an amine/NHC-catalyzed β-borylation reaction has also been applied in tandem with a Wittig reaction, allowing access to homoallylboronates .…”

N‐Phosphine Oxide‐Substituted Imidazolylidenes (PoxIms): Multifunctional Multipurpose Carbenes

“…In the succeeding study on the phosphorylation of isocyanates and aldehydes, [5d] bis-phosphine oxide-substituted 3g was found to catalyzet he cyclotrimerization of phenyli socyanate, whereas phenyl isocyanate was efficiently phosphorylated by PoxIm 3c (Figure 11). [15] These results demonstrate the furtheru tility of PoxIms as an efficient reagent fort he transformation of heterocumulenes and carbonyls.…”

Preferential N–H⋯:C hydrogen bonding involving ditopic NH-containing systems and N-heterocyclic carbenes

“…The hydrogen bond distances are within the sum of the van der Waal radii, and display closer contacts than the hydrogen bond adducts of the Ong group, where carbenes with pendant secondary amines form dimers in the solid-state. 44,45 By contrast, all attempts to crystallize DPB/2IPr resulted in dissociation of the adduct into individual components. It appears that the poor solubility of DPB leads to preferential precipitation/crystallization of DPB, which is in facile equilibrium with IPr and DPB/2IPr (vide infra).…”