Basicity as a Thermodynamic Descriptor of Carbanions Reactivity with Carbon Dioxide: Application to the Carboxylation of α,β-Unsaturated Ketones

Abstract

The utilization of carbon dioxide as a raw material represents nowadays an appealing strategy in the renewable energy, organic synthesis, and green chemistry fields. Besides reduction strategies, carbon dioxide can be exploited as a single-carbon-atom building block through its fixation into organic scaffolds with the formation of new C-C bonds (carboxylation processes). In this case, activation of the organic substrate is commonly required, upon formation of a carbanion C−, being sufficiently reactive toward the addition of CO2. However, the prediction of the reactivity of C− with CO2 is often problematic with the process being possibly associated with unfavorable thermodynamics. In this contribution, we present a thermodynamic analysis combined with density functional theory calculations on 50 organic molecules enabling the achievement of a linear correlation of the standard free energy (ΔG0) of the carboxylation reaction with the basicity of the carbanion C−, expressed as the pKa of the CH/C− couple. The analysis identifies a threshold pKa of ca 36 (in CH3CN) for the CH/C− couple, above which the ΔG0 of the carboxylation reaction is negative and indicative of a favorable process. We then apply the model to a real case involving electrochemical carboxylation of flavone and chalcone as model compounds of α,β-unsaturated ketones. Carboxylation occurs in the β-position from the doubly reduced dianion intermediates of flavone and chalcone (calculated ΔG0 of carboxylation in β = −12.8 and −20.0 Kcalmol-1 for flavone and chalcone, respectively, associated with pKa values for the conjugate acids of 50.6 and 51.8, respectively). Conversely, the one-electron reduced radical anions are not reactive toward carboxylation (ΔG0 > +20 Kcalmol-1 for both substrates, in either α or β position, consistent with pKa of the conjugate acids < 18.5). For all the possible intermediates, the plot of calculated ΔG0 of carboxylation vs. pKa is consistent with the linear correlation model developed. The application of the ΔG0 vs. pKa correlation is finally discussed for alternative reaction mechanisms and for carboxylation of other C=C and C=O double bonds. These results offer a new mechanistic tool for the interpretation of the reactivity of CO2 with organic intermediates.

A Proton-Coupled Electron Transfer Strategy to the Redox-Neutral Photocatalytic CO2 Fixation

“…This mechanism has been recently established as a general and reliable strategy for the generation of reactive nucleophilic intermediates. , In this case the presence of an aryl group is key to the stabilization of the carbanion. The benzyl carbanion 7a is finally capable of reacting with CO 2 (Scheme ). …”

“…To get insights into the impact of the H-bonding network in the one electron oxidation of the 4BnDHP/TBD adduct, we performed DFT calculations. The one-electron oxidation of the 4a within the 4BnDHP/TBD was modeled at the B3LYP/6-311+g­(d,p) level of theory, with a polarizable continuum model of MeCN solvent. , Figure c reports the optimized geometry of the 4BnDHP/TBD adduct (optimized as a neutral, singlet species), revealing the N–H···N hydrogen bond with N–H and H···N distances of 1.04 and 1.87 Å, respectively, and a NĤN angle of 178.8°. , A second geometry with different relative orientations of 4a and TBD was considered, showing a similar energy and similar distances and angles within the H-bonding network, see Figure S14 in the Supporting Information. In agreement with MS-PCET manifold, when optimizing the 4BnDHP/TBD adduct upon one electron oxidation (positively charged species with doublet multiplicity), the proton from the N–H group in 4a migrates to the nitrogen of TBD.…”

“…These data matched with those previously reported in the literature. 65 1,2-Diphenylpropan-1-one (33). Synthesized following a procedure from a literature report, 66 starting from 26.7 mg of 18 (0.18 mmol, 1.0 equiv) Pure 33 was obtained using flash chromatography (hexane:ethyl acetate 95:5) as a colorless oil in 91% yield (24.5 mg, 0.12 mmol, 66% over 2 steps).…”

Computational Insights into Malononitrile-Based Carbanions for CO2 Capture

“…27 Computational studies have shown that the basicity of carbanion and the solvation environments contribute to the reaction ener-getics. 28,29 Tao et al synthesized carbanion-based ionic liquids that were able to chemically bind CO with binding energies from −51 to −78 kJ/mol. 30 These previous works all lead to the question: can the carbanion be designed as a CO 2 sorbent?…”

“…Recently, Kong et al discovered reversible decarboxylation reaction of various arylacetate salts, suggesting that the CO 2 affinity of carbanion may be utilized in reversible CO 2 binding . Computational studies have shown that the basicity of carbanion and the solvation environments contribute to the reaction energetics. , Tao et al synthesized carbanion-based ionic liquids that were able to chemically bind CO with binding energies from −51 to −78 kJ/mol . These previous works all lead to the question: can the carbanion be designed as a CO 2 sorbent?…”