Cobalt-Mediated η<sup>5</sup>-Pentadienyl/Alkyne [5 + 2] Cycloaddition. Synthesis and Characterization of Unbridged η<sup>2</sup>,η<sup>3</sup>-Coordinated Cycloheptadienyl Complexes
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Bibliographic record
Abstract
A general new metal-mediated [5 + 2] cycloaddition reaction of η 5 -pentadienyl cobalt complexes and alkynes is reported, the first such cycloaddition reaction to provide controlled incorporation of 1 equiv of alkyne. The reaction of terminally substituted pentadienyl complexes proceeds thermally under exceptionally mild conditions, affording cycloheptadienyl complexes cleanly and in high yield. As a consequence of the unusually low kinetic barrier to the cycloaddition, reactions with acetylene at low temperature produce the cycloheptadienyl ring system exclusively as the η 2,η 3 -isomer, an unprecedented coordination mode for unbridged seven-membered ring complexes. Isomerization of the kinetic product to the fully conjugated η 5 -cycloheptadienyl isomer is observed quantitatively upon heating. Disubstituted alkynes, as represented by 2-butyne, proceed through the η 2,η 3 -intermediate to the thermodynamic η 5 -complex, but the rate of initial alkyne incorporation is attenuated to the extent that isomerization is competitive with the cycloaddition, preventing isolation of the η 2,η 3 -intermediates. The reaction with terminal alkynes is relatively insensitive to modest steric or electronic influences, providing mixtures of regioisomeric products. Consistent with these results, a dissociative mechanism is proposed, initiated by the unusually facile η 5 →η 3 isomerization of the pentadienyl ligand. The pentadienyl complexes themselves arise from protonolysis of an organic 1,4-pentadien-3-ol in the presence of the labile Co(I) complex, (η 5 -pentamethylcyclopentadienyl)cobalt(ethylene) 2, suggesting that the reaction can be considered a novel “interrupted Nazarov” cyclization, amenable to further development in an explicitly organic context.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.002 | 0.001 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.001 | 0.002 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 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 it