N-Heterocyclic carbene–carbodiimide (NHC–CDI) betaine adducts: synthesis, characterization, properties, and applications

Abstract

N-heterocyclic carbene-carbodiimide betaine adducts are zwitterionic amidinate-like structures with tunable properties that have applications as ligands, junctions in supramolecular polymers, and stabilizers for radical cations.

Switchable organocatalysis from N-heterocyclic carbene-carbodiimide adducts with tunable release temperature

“…We originally hypothesized that a very dynamic adduct with an N-alkyl-Nʹ-aryl CDI would be required to intercept the NHC for catalysis, so we began by synthesizing N-cyclohexyl-Nʹ-phenyl CDI (2a) and mixing it with the common NHC organocatalyst 1,3bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IMes) to form the corresponding betaine adduct (1a). The distinctive color change [44,45] was observed upon formation of 1a and the structure was confirmed using single crystal X-ray crystallography. [55] We verified that 1a is dynamic by measuring the dissociation constant (k d ) for CDI exchange with the N,Nʹdiaryl CDI 2c to form adduct 1c, which was expected to be less dynamic (Scheme 1 and Figures S1-S2).…”

“…Herein, we establish a new platform for regulating NHC organocatalytic activity based on carbodiimide (CDI) masks, which are isoelectronic to CO 2 and isothiocyanates but are highly tunable due to the two nitrogen substituents (Figure 1C). NHC-CDI betaine adducts [44] have attracted attention as amidinate-type ligands for discrete metal complexes [45][46][47][48] or functionalized nanoparticle surfaces, [49][50][51] in polyzwitterionic materials, [35] as mechanophores, [52] and for C-Cl bond activation, [48,53] but have yet to be applied to masked organocatalysis. We were inspired by the work of Johnson and coworkers [35,54] who showed that NHC-CDI adducts containing N,Nʹ-diaryl CDIs were more stable 2 and less dynamic compared to those with N-alkyl-Nʹ-aryl CDIs.…”

NHC-CDI Betaine Adducts and Their Cationic Derivatives as Catalyst Precursors for Dichloromethane Valorization

“…NHC-CDI’s are an emerging class of dipolar compounds that belong to the chemical class of betaines, electroneutral zwitterions that cannot be represented by any resonance form with full charge cancellation. 15 NHC-CDI’s bearing aryl substituents in the CDI part are readily prepared and exhibit enhanced stability even under the open air, despite the strongly basic and nucleophilic character of their amidinate moiety. 16 Due to these properties, stable NHC-CDI adducts have a promising potential for application in many different fields, including metal ligands in coordination and organometallic chemistry, 17 nanocatalyst design, 18 building blocks for polymers, 19 or in the development of new types of persistent free radicals.…”

“…In this contribution, we disclose a practical application of stable adducts of NHC carbenes and carbodiimides (NHC-CDI) as suitable catalysts for nucleophilic chloride substitution on DCM, providing a convenient route for a variety of symmetrical, methylene-bridged derivatives CH 2 Z 2 . NHC-CDI’s are an emerging class of dipolar compounds that belong to the chemical class of betaines, electroneutral zwitterions that cannot be represented by any resonance form with full charge cancellation . NHC-CDI’s bearing aryl substituents in the CDI part are readily prepared and exhibit enhanced stability even under the open air, despite the strongly basic and nucleophilic character of their amidinate moiety .…”

Switchable organocatalysis from N-heterocyclic carbene-carbodiimide adducts with tunable release temperature

“…Herein, we establish a new platform for regulating NHC organocatalytic activity using carbodiimide (CDI) masks, which are isoelectronic to CO2 and isothiocyanates but are highly tunable due to the two nitrogen substituents (Figure 2B). NHC-CDI betaine adducts 53 have attracted attention as amidinate-type ligands for discrete metal complexes [54][55][56][57] or functionalized nanoparticle surfaces, [58][59][60] in polyzwitterionic materials, 40 as mechanophores, 61 and for C-Cl bond activation, 57,62 but have yet to be applied to masked organocatalysis. We envision that the wider range of adduct stabilities accessible using this platform will enable orthogonal and more precise control over NHC activation for organocatalysis compared to previous systems.…”