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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 CO 2 . However, the prediction of the reactivity of C −  with CO 2  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 (ΔG 0 ) of the carboxylation reaction with the basicity of the carbanion C − , expressed as the pK a  of the CH/C −  couple. The analysis identifies a threshold pK a  of ca 36 (in CH 3 CN) for the CH/C −  couple, above which the ΔG 0  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 ΔG 0  of carboxylation in β = −12.8 and −20.0 Kcalmol -1  for flavone and chalcone, respectively, associated with pK a  values for the conjugate acids of 50.6 and 51.8, respectively). Conversely, the one-electron reduced radical anions are not reactive toward carboxylation (ΔG 0  &amp;gt; +20 Kcalmol -1  for both substrates, in either α or β position, consistent with pK a  of the conjugate acids &amp;lt; 18.5). For all the possible intermediates, the plot of calculated ΔG 0  of carboxylation vs. pK a  is consistent with the linear correlation model developed. The application of the ΔG 0  vs. pK a  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 CO 2  with organic intermediates.

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