Mechanistic insights into the full hydrogenation of 2,6-substituted pyridine catalyzed by the Lewis acid C<sub>6</sub>F<sub>5</sub>(CH<sub>2</sub>)<sub>2</sub>B(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>
Bibliographic record
Abstract
The reaction mechanism for the full hydrogenation of 2-phenyl-6-methyl-pyridine catalyzed by the Lewis acid C6F5(CH2)2B(C6F5)2 was investigated in detail by density functional theory calculations. Our calculations show that a plausible reaction pathway of the hydrogenation of pyridine contains five stages: (1) the generation of a new borane C6F5(CH2)2B(C6F5)2 from the hydroboration of the alkene, which forms a frustrated Lewis pair (FLP) with a pyridine; (2) the activation of H2 by FLP to yield an ion pair intermediate; (3) intramolecular hydride transfer from the boron atom to the pyridinium cation in the ion pair intermediate to produce the 1,4-dihydropyridine; (4) hydrogenation of the 1,4-dihydropyridine by the FLP to form the 1,4,5,6-tetrahydropyridine; (5) hydrogenation of the 1,4,5,6-tetrahydropyridine by the FLP to yield the final piperidine. The last two hydrogenation processes follow a similar pathway, which includes four steps: (a) proton transfer from the pyridinium moiety to the substrate; (b) dissociation of the newly generated pyridine; (c) hydride migration from the hydridoborate moiety to the protonated substrate to produce the hydrogenated product; (d) release of the hydrogenated product to regenerate the free borane. The full hydrogenation of pyridine is calculated to be exothermic by 16.9 kcal mol(-1), relative to the starting reactants. The rate-limiting step is the proton transfer in the second hydrogenation step, with a free energy barrier of 28.2 kcal mol(-1) in the gas phase (27.9 kcal mol(-1) in toluene) at room temperature and 1.0 atm. Our results can account for the observed experimental facts.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.005 | 0.000 |
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".