Investigating endo-exo selectivity in Diels-Alder reactions
Notice bibliographique
Résumé
An important goal for synthetic organic chemists is the synthesis of the desired isomer of a target chemical compound exclusively and in high yield. The value and beauty of the Diels-Alder cycloaddition lies in its capacity for selectivity; that is, the isolation of a single stereoisomer in good yield is possible in many cases. A chemist's knowledge of the typical stereochemical outcomes of a variety of Diels-Alder reactions may be used to predict the product distribution for their system of interest. There are three types of selectivity generally observed in these cycloadditions; the reasons for the selectivity we see are not always clear. Endo-exo selectivity is the main focus of this thesis. -- In Chapter 2, the Diels-Alder reaction of 3,3-disubstituted cyclopropene with butadiene was determined to be exo selective for all of the substituents examined, as was predicted from a previous study of a 3-substituted cyclopropene / butadiene system. The addition of a second substituent at the C3-position resulted in a greater stabilization of the cyclopropene ring when compared to the 3-substituted cyclopropenes; this result can be attributed to the anomeric effect. The activation barriers for the 3,3-disubstituted cyclopropene / butadiene system were below those of the syn 3-substituted system (for both endo and exo addition), mostly due to the substantial relief of ring strain in the 3,3-disubstituted cyclopropene / butadiene transition state structures. -- Long-range and short-range substituent effects on the reactivity of cyclopentene dienophiles were considered in Chapter 3. Disubstitution at C4 (a long-range effect) had essentially no impact on the stabilization of the cyclopentene dienophiles, while disubstitution at C1 and C2 (the reaction center) resulted in significant changes in the lengths of the C1=C2 bonds. The Diels-Alder reactions of the various 4,4- and 1,2-disubstituted dienophiles with both butadiene and cyclopentadiene were examined and the most significant selectivities were observed for the 4,4-disubstituted cyclopentene / butadiene system (exoselective) and the 1,2-disubstituted cyclopentene / cyclopentadiene system (endo selective). Secondary orbital interactions are not possible in the transition state structures of these cyclopentene Diels-Alder systems and thus cannot play a role in determining endo-exoselectivity. The observed selectivities can be rationalized qualitatively by examining the favourable and unfavourable interactions between the diene and dienophile in the transition state structures. -- The investigation of the Diels-Alder reactions involving cyclopentadiene and its substituted analogs in Chapter 4 involved two main objectives: the examination of the endo-exo and facial selectivities for a large number of diene / dienophile combinations, as well as the in-depth analysis of the potential energy curve of the endo dimerization of cyclopentadiene. Endo addition was preferred for all of these cycloadditions, which is the most commonly observed preference. However, a characterization of the reaction coordinate revealed an occurrence considered to be rather atypical for Diels-Alder reactions: the existence of the bispericyclic transition state, first discussed in a 2001 communication by Caramella et al. This transition state leads to a lower energy Cope transition state later on the reaction coordinate. Vibrational frequency data along the curve were examined and product distribution was discussed. In the case of the Diels-Alder reactions of cyclopentadiene and its derivatives, the wealth of p-orbitals available for favourable secondary orbital interactions are likely the major contributing factor in the determination of the endo selectivity observed.
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Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,001 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,001 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,002 | 0,001 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».