ChemRxiv
These are preliminary reports that have not been peer-reviewed. They should not be regarded as conclusive, guide clinical practice/health-related behavior, or be reported in news media as established information. For more information, please see our FAQs.
1/1
7 files

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

preprint
submitted on 20.01.2021, 21:21 and posted on 22.01.2021, 05:24 by John Pham, Cole Jarczyk, Eamon Reynolds, Sophie Kelly, Thao Kim, Tianyi He, Jason Keith, Anthony Chianese

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.


Funding

NSF CHE-1665144

NSF CHE-1954924

NSF CHE-1726308

History

Email Address of Submitting Author

achianese@colgate.edu

Institution

Colgate University

Country

United States

ORCID For Submitting Author

0000-0002-9140-6115

Declaration of Conflict of Interest

The authors declare no conflict of interest.

Exports

ChemRxiv

Exports