The Brönsted Basicities of N-Heterocyclic Olefins in DMSO: An Effective Way to Evaluate the Stability of NHO–CO₂ Adducts

Abstract

A Bronsted basicity scale (∼24 pK units) for 85 commonly seen imidazole-, imidazoline-, triazole-, and thiazole-based N-heterocyclic olefins (NHOs) in DMSO was established using a well-examined computational model. The influence of substituents on the Bronsted basicities of these NHOs was investigated through basicity comparisons and rationalized by geometric analyses. The Gibbs energy (ΔG r ) of the reaction between NHO and CO 2 was also calculated, which linearly correlates with the basicity of the corresponding NHO, suggesting that the stability of NHO−CO 2 adducts can be evaluated by the basicity of NHOs and a stronger basicity leads to a more stable NHO−CO 2 adduct.

Brönsted Basicities and Nucleophilicities of N-Heterocyclic Olefins in Solution: N-Heterocyclic Carbene versus N-Heterocyclic Olefin. Which Is More Basic, and Which Is More Nucleophilic?

“…Table also shows that the basicity of imidazole-based NHOs is obviously greater than those of imidazoline-based ones by as much as 7.1 p K units ( 1c vs 1a ). The experimental results from this work confirmed previous theoretical predictions of PA and p K aH values, in which the large basicity difference originated from aromatization: the protonation of unsaturated imidazole-based NHOs generates stable aromatic imidazolium cations and thus increased aromaticity, while the reverse is true for the saturated imidazoline-based NHOs, as suggested by the NICS(1) calculation from a recent work by Wang et al Interestingly, NHO precursor 1f is only slightly more acidic than saturated 1a but much more acidic than the rest of the imidazole-based NHO precursors ( 1c–1e ), which is most probably due to the fused benzene ring that significantly stabilizes the mesomeric form with partial charges of the corresponding NHO 2f …”

“… a The structure and abbreviation are shown in Figure . b From ref . c The mean value of at least two independent runs with SD ≤ ±0.05 p K units. d From ref . e SD ≤ ±0.10 p K units. …”

“…As shown in Table , the established basicity scale for NHOs with different backbone structures and flanking substituents covers a range from 14.70 to 24.10. The p K a values of 1a and 1b are <20 in DMSO, comparable to that of phenol (p K a = 18.0 in DMSO). , The size of ring brings about 2 p K units difference (five-membered 1a vs six-membered 1b ), and this is consistent with the results from the theoretical p K a prediction in DMSO Table also shows that the basicity of imidazole-based NHOs is obviously greater than those of imidazoline-based ones by as much as 7.1 p K units ( 1c vs 1a ).…”

“…As shown in Figure , two opposing trends exist; i.e., for those NHCs and NHOs that contain unsaturated aromatic rings (Figure , entries 3–5 and 7–9), the basicity of NHOs is slightly stronger than that of NHCs, whereas the saturated NHCs are more basic than their NHO analogues (Figure , entries 1 and 2). Again, aromatization plays as a “regulator” that intrinsically dictates their basicities because the aromaticity of NHCs (both saturated and unsaturated) remains almost unchanged upon protonation, while that for unsaturated NHOs increases significantly and thus renders the unsaturated NHOs as more basic than their NHC analogues. , …”

“…Mayr and co-workers reported the nucleophilicities of five C2-aryl-substituted NHOs in THF and uncovered the effect of aromaticity on the catalytic activities of N-heterocyclic carbenes . Alternatively, theoretical calculations were also employed to predict the proton affinities (PAs) and the nucleophilic indices of NHOs in the gas phase; in addition, very recently, the basicities of a series of NHOs and the stabilities of NHO–CO 2 adducts were also calculated in solution . To the best of our knowledge, so far there has been no systematic experimental investigation of the basicities and nucleophilicities of NHOs in solution; herein, we report the basicities of several commonly used NHOs in DMSO and their nucleophilicities in THF.…”