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
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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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 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| 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 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".