Mechanisms of α-, β-, and γ-H(D) Exchange Processes in the α-Agostic Alkyltitanocene(IV) Complexes [Cp<sub>2</sub>TiCH<sub>2</sub>CH(CH<sub>3</sub>)(CMe<sub>3</sub>)]<sup>+</sup>and [Cp<sub>2</sub>TiCH<sub>2</sub>CH(CD<sub>3</sub>)(CMe<sub>3</sub>)]<sup>+</sup>: Stark Contrasts with Their γ-SiMe<sub>3</sub>Analogues
Bibliographic record
Abstract
The α-agostic alkyltitanocene(IV) complex [Cp 2 TiCH 2 CH(CH 3 )(CMe 3 )] + ( II ) and its D 3 isotopologue [Cp 2 TiCH 2 CH(CD 3 )(CMe 3 )] + ( II -CD 3 ) undergo reversible, intramolecular H–H(D) exchange among the α-, β-, and γ- (β-Me) positions of the alkyl ligands in addition to concomitant intermolecular H–H(D) exchange with the vinylic and 2-methyl sites of the product of β-hydrogen elimination, CH 2 ═CMeCMe 3, results which are in strong contrast to the exchange behavior of the γ-agostic silyl analogues [Cp 2 TiCH 2 CH(CH 3 )(SiMe 3 )] + ( III ) and [Cp 2 TiCH 2 CH(CD 3 )(SiMe 3 )] + ( III -CD 3 ). As has been previously shown, III undergoes tunnelling-expedited exchange of the β-H (but not α-H) with the hydrogen atoms of the β-methyl group (β-H/γ-H exchange) while III -CD 3 isomerizes reversibly but specifically to the isotopomer [Cp 2 TiCD 2 CD(CH 3 )(SiMe 3 )] +, forgoing β-H/γ-H exchange completely. In this paper we show that all of the exchange processes of II / II -CD 3 and III / III -CD 3 initially involve conventional β-H elimination processes and thus, at some stage, intermediacy of the corresponding hydrido alkene complexes [Cp 2 TiH(CH 2 ═CMeCMe 3 )] + ( VI ) or [Cp 2 TiH(CH 2 ═CMeSiMe 3 )] + (or their D 3 isotopologues). The major reason for the vastly different exchange behaviors of the two structurally very similar alkyltitanocene systems II and III is simply that, for steric and electronic reasons, the alkene in VI dissociates reversibly to the hydride, [Cp 2 TiH] +, and free alkene while that in [Cp 2 TiH(CH 2 ═CMeSiMe 3 )] + does not; CH 2 ═CMeSiMe 3 remains coordinated at the temperature and on the time scale of the experiments. DFT calculations generally support our mechanistic conclusions and furthermore point to a subtle ion pairing effect which hinders intramolecular η 2 -alkene rotation in VI and [Cp 2 TiH(CH 2 ═CMeSiMe 3 )] +, thereby exerting a surprisingly important, heretofore unanticipated, influence on the nature of the chemistry involved.
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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) | 0.002 | 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".