The key role of the latent N-H group in Milstein's catalyst for ester hydrogenation

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 catalys...

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Bibliographic Details
Published in:Chemical science (Cambridge) 2021-06, Vol.12 (24), p.8477-8492
Main Authors: Pham, John, Jarczyk, Cole E, Reynolds, Eamon F, Kelly, Sophie. E, Kim, Thao, He, Tianyi, Keith, Jason M, Chianese, Anthony R
Format: Article
Language:English
Subjects:
Aldehydes
Catalysts
Chemistry
Computation
Crystallography
Dihydrides
Ethane
Format
Hydrogen-based energy
Hydrogenation
Hydrogenolysis
Molecular structure
NMR
Nuclear magnetic resonance
Ruthenium
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Summary: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. We report a detailed mechanistic study of ester hydrogenation catalyzed by the activated form of Milstein's catalyst. Catalyst activation leads to the replacement of a dialkylamino side group with an NHEt group, which has a key role in catalysis.
ISSN:2041-6520
2041-6539
DOI:10.1039/d1sc00703c