The Key Role of the Latent N-H Group in Milstein's Catalyst for Ester Hydrogenation

Abstract

<p>We previously demonstrated that Milstein’s seminal diethylamino-substituted PNN-pincer-ruthenium catalyst for ester hydrogenation is activated by dehydroalkylation of the pincer ligand, releasing ethane and eventually forming an NHEt-substituted derivative that we proposed is the active catalyst. In this paper, we present a computational and experimental mechanistic study supporting this hypothesis. Our DFT analysis shows that the minimum-energy pathways for hydrogen activation, ester hydrogenolysis, and aldehyde hydrogenation rely on the key involvement of the nascent N-H group. We have isolated and crystallographically characterized two catalytic intermediates, a ruthenium dihydride and a ruthenium hydridoalkoxide, the latter of which is the catalyst resting state. A detailed kinetic study shows that catalytic ester hydrogenation is first-order in ruthenium and hydrogen, shows saturation behavior in ester, and is inhibited by the product alcohol. A global fit of the kinetic data to a simplified model incorporating the hydridoalkoxide and dihydride intermediates and three kinetically relevant transition states showed excellent agreement with the results from DFT. <b></b></p><br>

Investigating the antioxidant activity enhancer effect of Cyamopsis tetragonoloba seed extract on phenolic phytochemicals

“…Due to their low cost and positive environmental impact, their natural bioactive compounds are currently a trend in the food processing industry ( Riaz et al., 2022 ). As consumers become more aware of the nutritional and nutraceutical composition of legumes, their global demand continues to rise ( Pham and Luan, 2021 ).…”

“…These compounds are differentially distributed in the cotyledon (mainly non-flavonoid phenolics) and the seed coat (flavonoids) of legumes ( Amarowicz, 2020 ). The distinctive bioactive potential, color and flavor of legumes are due to the most abundant phenolic compound, flavonoids, which are composed of two aromatic rings linked by a 3-C bridge, in the form of heterocyclic C ring ( Pham and Luan, 2021 ).…”