A Mechanism Displaying Autocatalysis: The Hydrogenation of Acetophenone Catalyzed by RuH(<i>S</i>-binap)(app) Where app Is the Amido Ligand Derived from 2-Amino-2-(2-pyridyl)propane
Notice bibliographique
Résumé
The 2-(aminomethyl)pyridine (ampy) ligand is known to activate ruthenium complexes for the catalytic hydrogenation of ketones. Here we prepare well-defined catalysts using the new ligand 2-amino-2-(2-pyridyl)propane (appH) in order to elucidate the role of the pyridyl group. The ligand has two methyl groups on the α-carbon to block β-hydride elimination reactions. It reacts with RuHCl( S -binap)(PPh 3 ) to produce the orange-yellow complex RuHCl( S -binap)(appH) ( 2 ). In the presence of a strong base (KO t Bu), complex 2 is converted into an active catalyst for the H 2 -hydrogenation of acetophenone in benzene under mild conditions (20 °C, 5 atm H 2 ). Solutions of 2 rapidly react with KO t Bu under an argon atmosphere to produce a deep red amidohydrido complex RuH( S -binap)(app) ( 3 ), which is an active catalyst. A crystal structure determination of 3 represents the first structure of a Ru-binap hydrido-amido complex. It reveals a five-coordinate Ru(II) center with a short Ru−N(amido) distance (1.962(3) Å) and a trigonal planar geometry at the amido nitrogen. The kinetic experiments using 3 as a catalyst and acetophenone as a substrate in benzene show that the rate of 1-phenylethanol production is dependent on both catalyst and H 2 concentrations. These results parallel the behavior of the conventional Noyori-type Ru(II) catalysts with diamine ligands. However, unique features of catalysis with 3 are as follows: (1) the formation of a dihydride is thermodynamically unfavorable at 1 atm H 2, 20 °C; (2) the rate shows a dependence on the product concentration since it increases as the product builds up during the reaction in an autocatalytic fashion. A significant increase in the initial rate is observed when a critical concentration of rac -1-phenylethanol is present at the beginning of the reaction. The addition of 2-propanol in benzene raises the rate as well, and the fastest H 2 -hydrogenation is achieved if 2-propanol is used as a solvent. This “alcohol effect” is favored by the pyridyl ligand app since it was not observed for the similar catalyst RuH(NHCMe 2 CMe 2 NH 2 )(binap). While 3 is an exceptional catalyst for H 2 -hydrogenation in 2-propanol (TOF > 6700 h -1 at 20 °C, 5 atm H 2 ), it has a lower activity in transfer hydrogenation from the same solvent under comparable conditions (TOF 110 h -1 at 20 °C, 1 atm Ar). DFT calculations on the model amido complex Ru(H)(PH 3 ) 2 (HNCH 2 C 5 H 4 N) ( 4 ) confirm that the splitting of H 2 to give the trans dihydride is the turnover-limiting step and lies 9 kcal/mol in free energy above the transition state for the ketone hydrogenation step. The formation of the dihydride is entropically unfavorable. The theoretical activation barrier for H 2 splitting is lowered by 5 kcal/mol by an alcohol-assisted mechanism but still remains higher in energy than the ketone hydrogenation step. This latter step can also be alcohol-assisted and can result in a different ee in the product alcohol than without alcohol assistance, as observed experimentally for reactions using 2-propanol versus benzene as the solvent. With alcohol present, an alkoxohydridoruthenium(II) complex is calculated to be the catalyst resting state.
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Prédiction distillée sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Apprise à partir de 10 348 étiquettes directes de Codex et de 10 348 étiquettes directes de Gemma. Le mode candidate est l'union des têtes enseignantes seuillées; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont ni des étiquettes humaines ni des étiquettes directes de modèles de pointe.
Scores Codex et Gemma par catégorie
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,001 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,001 | 0,000 |
| Méta-épidémiologie (sens large) | 0,001 | 0,000 |
| Bibliométrie | 0,000 | 0,002 |
| Études des sciences et des technologies | 0,001 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,001 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,001 | 0,000 |
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 tête enseignante, 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 ».