Rate-determining Processes in Acid-catalyzed Decarboxylation Reactions
Bibliographic record
Abstract
The acid-catalyzed decarboxylation reactions of indole- and pyrrole-carboxylic acids require the addition of one equivalent of water to the carboxyl group and a proton to the heterocyclic ring carbon at the position α to the carboxyl. Where α-protonation is thermodynamically favoured over β-protonation, the magnitude of the observed 12C/13C kinetic isotope effect (CKIE) is greater than where the β-position is protonated. This can be understood in terms of a mechanism involving a protonated hydrated precursor to carbon-carbon bond cleavage, where the difference in energy of intermediates and transition states control the proportioning of the intermediates. The intrinsic CKIE on the carbon-carbon bond-breaking step that produces protonated carbonic acid (PCA) is independent of the site of protonation. The interpretation of the observed CKIEs can be generalized based on intermediates from isomeric carboxylic acids whose energetics vary predictably with their sites of protonation. The relative free energy barriers to reversion and formation of PCA control the magnitude of the observed CKIEs and correlate with reactivity. The reported data implicate the formation of PCA as the initial product of carbon-carbon bond cleavage. Application of the principle of microscopic reversibility implies that electrophilic aromatic substitution based on PCA should be an accessible route to carboxylation of aromatic substrates. Over the course of the project, new methods were developed for the simultaneous pressure detection and mass spectral analysis of carbon dioxide released as a final product. Specifically, headspace gas analysis and compound-specific isotope analysis of carbon dioxide have been coupled as a result. The evaluation for new decarboxylation mechanisms in general has led to a clearer understanding of how the intervention of hydrated intermediates leads to formation of PCA and its subsequent rapid conversion to carbon dioxide.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.002 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.002 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".