Comprehensive Basicity Scales for N‐Heterocyclic Carbenes in DMSO: Implications on the Stabilities of N‐Heterocyclic Carbene and CO₂ Adducts

Abstract

Av ery broad acidity scale ( % 40 pK units) for about 400 N-heterocyclic carbenep recursors (NHCPs) with various backbones and electronic features, including imidazolylidenes, 1,2,4-triazolylidenes, cyclic diaminocarbenes (CDACs), diamidocarbenes (DACs), thiazolylidenes, cyclic (alkyl)(amino)carbenes (CAACs) and mesoionic carbenes (MICs), was established in DMSO by aw ell examined computational method. Varying the backbone structure or flanking N-substituents can have different extent of acidifying effects, de-pending on both the nature and number of substituent(s). The Gibbs energies (DG r s) for the reactions between the corresponding NHCs and CO 2 were also calculated.T here is a good linear correlation between the pK a so fm ostN HCPs and DG r s, suggestingt hat ag reater basicity of NHC leads to am ore stable NHC-CO 2 adduct. Interestingly,t he nearby asymmetrice nvironment has virtually no differential effect on the acidities of the chiral NHCP enantiomers, but has a pronounced effect on the DG r values.[a] Dr.

Stable Singlet Carbenes as Organic Superbases

“…Altogether, these experimental results allowed us to confidently rank these fifteen carbene motifs by decreasing basicity in the order CBA > a NHC > CAAC‐6 > BiCAAC > NHC‐7 > CAAC‐5 > NHC‐6 > MIC > bz NHC > NHC‐5 > u NHC‐5 > MAC > thio NHC > Enders > DAC . As illustrated in Figure 3, this scale is in good correlation with the p K aH of carbenes for which predicted values are known [18a] . Moreover, matched against the experimental p K aH of popular organic bases, [36] it also highlights the rather broad spectrum of basicity accessible with singlet carbenes.…”

“…Similarly, the reaction of u NHC‐5 H with free CAAC‐5 led to a complete exchange affording free u NHC‐5 and CAAC‐5 H within 20 minutes at room temperature [24] . From these preliminary results, we determined that these carbenes are best ranked according to their decreasing basicity in the order CAAC‐6 > BiCAAC > CAAC‐5 > u NHC‐5 in good correlation with their theoretically predicted basicities [18a] …”

Cationic Group VI Metal Imido Alkylidene N‐Heterocyclic Carbene Nitrile Complexes: Bench‐Stable, Functional‐Group‐Tolerant Olefin Metathesis Catalysts

“…Encouraged by this observation, we successfully coordinated other NHCs to W‐01 , namely IiPr , the sterically very demanding 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene ( IDipp ) and the highly basic 6‐Mes (p K a of the corresponding pyrimidinium salt=24.2 in DMSO) [11] . 1,3‐Dimethyl‐4,5‐dichloroimidazol‐2‐ylidene ( IMeCl 2 ) and 1,3‐dimethylimidazol‐2‐ylidene ( IMe ) were introduced via the corresponding NHC‐AgI complexes.…”

“…Encouraged by this observation, we successfully coordinated other NHCs to W-01,n amely IiPr,t he sterically very demanding 1, 3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IDipp)and the highly basic 6-Mes (pK a of the corresponding pyrimidinium salt = 24.2 in DMSO). [11] 1,3-Dimethyl-4,5-dichloroimidazol-2-ylidene (IMeCl 2 )and 1,3-dimethylimidazol-2-ylidene (IMe)were introduced via the corresponding NHC-AgI complexes.S ince the resulting compounds are all 16electron complexes,wedid not expect them to be catalytically highly active and thus explored the synthesis of corresponding cationic complexes by introduction of the weakly coordinating anion tetrakis [3,5-bis(trifluoromethyl)phenyl]borate,( B-(Ar F ) 4 ). In the case of W-02, W-04, W-05 and W-06,t his was achieved by using NaB(Ar F ) 4 .H owever, W-04 and W-05 required the presence of an additional coordinating nitrile ligand such as pivalonitrile (PivCN) to obtain pure,i solable cationic compounds (Scheme 4).…”

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

“…Although making comparisons between the reactivities of different NHCs can be troublesome, relatively minor differences in the basicity of IPr and IMes can be assumed, for the pK a of their parent imidazoliums' are within error (pK a IPr$HCl ¼ 19.29 AE 0.07, pK a IMes$HCl ¼ 19.40 AE 0.12). 58,59 Though the isolation and authentication of hydrogen bonding adducts of carbenes has thus far been limited to imidazolylidenebased carbenes, hydrogen bonding adducts role in catalysis [60][61][62][63] and X-H insertion chemistry has been postulated for some time. Bertrand recently provided computational evidence for these non-traditional hydrogen bonded species as key intermediates for the reversible insertion of cyclic (alkyl)(amino)(carbene)s (CAACs) into the N-H bond of DPA (Fig.…”