Research Article: Hydrogenation of Enamides with Cobalt Precatalysts

In another collaboration with Princeton University (USA), Ljiljana Pavlovic has studied the mechanism behind the hydrogenation of prochiral enamides by cobalt precatalysts.

Abstract:
The mechanism of the asymmetric hydrogenation of prochiral enamides by well-defined, neutral bis(phosphine) cobalt(0) and cobalt(II) precatalysts has been explored using(R,R)-^iPrDuPhos ((R,R)-^iPrDuPhos = (+)-1,2-bis[(2R,5R)-2,5-diisopropylphospholano]benzene) as a representative chiral bis(phosphine) ligand. A series of (R,R)-(^iPrDuPhos)Co(enamide) (enamide = methyl-2-acetamidoacrylate (MAA), methyl(Z)-α-acetamidocinnamate (MAC), and methyl(Z)-acetamido(4-fluorophenyl)acrylate (^4FMAC)) complexes (1-MAA, 1-MAC, and 1-^4FMAC), as well as a dinuclear cobalt tetrahydride, [(R,R)-(^iPrDuPhos)Co]₂(μ₂-H)₃(H) (2), were independently synthesized, characterized, and evaluated in both stoichiometric and catalytic hydrogenation reactions. Characterization of (R,R)-(^iPrDuPhos)Co(enamide) complexes by X-ray diffraction established the formation of the pro-(R) diastereomers in contrast to the (S)-alkane products obtained from the catalytic reaction. In situ monitoring of the cobalt-catalyzed hydrogenation reactions by UV–visible and freeze-quench electron paramagnetic resonance spectroscopies revealed (R,R)-(^iPrDuPhos)Co(enamide) complexes as the catalyst resting state for all the three enamides studied. Variable time normalization analysis kinetic studies of the cobalt-catalyzed hydrogenation reactions in methanol established a rate law that is first order in (R,R)-(^iPrDuPhos)Co(enamide) and H₂ but independent of the enamide concentration. Deuterium-labeling studies, including measurement of an H₂/D₂ kinetic isotope effect and catalytic hydrogenations with HD, established an irreversible H₂ addition step to the bound enamide. Density functional theory calculations support that this step is both rate and selectivity determining. Calculations, as well as HD-labeling studies, provide evidence for two-electron redox cycling involving cobalt(0) and cobalt(II) intermediates during the catalytic cycle. Taken together, these experiments support an unsaturated pathway for the [(R,R)-(^iPrDuPhos)Co]-catalyzed hydrogenation of prochiral enamides.

Organometallics cover and Research Article

The newest issue of Organometallics, with the theme “Sustainable Organometallic Chemistry” features a cover designed by our own Kathrin H. Hopmann, alongside Jennifer V. Obligacion and Jeff H. Ward. Hopmann and Obligacion present the issue as editors.

The research papers presented in the issue include Ashot Gevorgyan’s article on Buchwald–Hartwig aminations in lipid solvents, an article on Low-valent molybdenum PNP pincer complexes by our CO₂PERATE collaborators at LIKAT, Germany, and a new research article by Ljiljana Pavlovic on enamides hydrogenation:

The article, titled “Cobalt-Catalyzed Asymmetric Hydrogenation of Enamides: Insights into Mechanisms and Solvent Effects” is a collaboration with the group of Paul J. Chirik at Princeton University, USA. It focuses on “the mechanistic details of the (^PhBPE)Co-catalyzed asymmetric hydrogenation of enamides”.

Abstract:
The mechanistic details of the (^PhBPE)Co-catalyzed asymmetric hydrogenation of enamides are investigated using computational and experimental approaches. Four mechanistic possibilities are compared: a direct Co(0)/Co(II) redox path, a metathesis pathway, a nonredox Co(II) mechanism featuring an aza-metallacycle, and a possible enamide–imine tautomerization pathway. The results indicate that the operative mechanism may depend on the type of enamide. Explicit solvent is found to be crucial for the stabilization of transition states and for a proper estimation of the enantiomeric excess. The combined results highlight the complexity of base-metal-catalyzed hydrogenations but do also provide guiding principles for a mechanistic understanding of these systems, where protic substrates can be expected to open up nonredox hydrogenation pathways.

Research Article: Room-Temperature-Stable Magnesium Electride

The CHOCO group has collaborated with ICIQ (Spain) to investigate the reactivity of bypiridine ligands in Ni-catalysed reductive coupling reactions.

Abstract:
Herein, we report the synthesis of highly reduced bipyridyl magnesium complexes and the first example of a stable organic magnesium electride supported by quantum mechanical computations and X-ray diffraction. These complexes serve as unconventional homogeneous reductants due to their high solubility, modular redox potentials, and formation of insoluble, non-coordinating byproducts. The applicability of these reductants is showcased by accessing low-valent (bipy)₂Ni(0) species that are challenging to access otherwise.

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MSCA Doctoral Networks (DN) 2022 – webinar

Prof. Kathrin H. Hopmann will give a talk Monday 20th June at the MSCA Doctoral Networks (DN) 2022 – webinar, a course arranged by the Research Council of Norway for supporting applications to MSCA DNs.

Researcher Position in Computational Chemistry:
CO₂ activation

Are you our new colleague?

We are pleased to announce the opening of a 2-year researcher position within the CHOCO group on the topic of “New Reactions for Activation of CO₂”. We are looking to fill the position as soon as possible, so prepare to pack your bags and start your arctic adventure in the beautiful city of Tromsø!

Field of Research:
The candidate will use quantum chemical methods to model molecules and reactions and to propose promising new molecules for activation of CO2, especially towards enantioselective carbon-carbon bond formation. The work will be performed in close collaboration with experimental organic chemists.

The candidate can expect to be involved in the supervision of Master and PhD students in the group and may participate in teaching (up to 20%) at the Department of Chemistry.

For more details and application see: https://www.jobbnorge.no/en/available-jobs/job/226003/researcher-in-computational-chemistry-new-reactions-for-activation-of-co2

Research Article: synthesis of butyrolactones from CO2

In this collaboration with the Repo group at Helsinki University, Dat has performed the computational analysis for a Ti(OiPr)₄-mediated multicomponent reaction, which produces 3,4-substituted cis-δ-lactones from alkyl magnesium chloride, benzaldehyde and CO₂. The method is cis-selective.

Abstract:
We report a Ti(OiPr)₄-mediated multicomponent reaction, which produces 3,4-substituted cis-δ-lactones from alkyl magnesium chloride, benzaldehyde and CO₂. The key intermediate, titanacyclopropane, is formed in situ from Ti(OiPr)₄ and a Grignard reagent, which enables 1,2-dinucleophilic reactivity that is used to insert carbon dioxide and an aldehyde. An alternative reaction route is also described where a primary alkene is used to create the titanacyclopropane. A computational analysis of the elementary steps shows that the carbon dioxide and the aldehyde insertion proceeds through an inner-sphere mechanism. A variety of cis-butyrolactones can be synthesized with up to 7 : 1 diastereoselectivity and 77% yield.

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For a list of publications by members of the CHOCO group, see our Publications page 🙂

Research Article: insertion of CO2 into metal element σ-bonds

Ljiljana and Kathrin have collaborated with the Hazari Group from Yale for their study on “the insertion of CO₂ into palladium and nickel methyl complexes supported by ^RPBP […] pincer ligands”. They performed the computational analysis part of the study, where they investigated the plausible pathways for insertion.

Abstract:
The insertion of carbon dioxide into metal element σ-bonds is an important elementary step in many catalytic reactions for carbon dioxide valorization. Here, the insertion of carbon dioxide into a family of group 10 alkyl complexes of the type (^RPBP)M(CH₃) (^RPBP = B(NCH₂PR₂)₂C₆H₄⁻; R = Cy or ^tBu; M = Ni or Pd) to generate κ¹-acetate complexes of the form (^RPBP)M{OC(O)CH₃} is investigated. This involved the preparation and characterization of a number of new complexes supported by the unusual ^RPBP ligand, which features a central boryl donor that exerts a strong trans-influence, and the identification of a new decomposition pathway that results in C–B bond formation. In contrast to other group 10 methyl complexes supported by pincer ligands, carbon dioxide insertion into (^RPBP)M(CH₃) is facile and occurs at room temperature because of the high trans-influence of the boryl donor. Given the mild conditions for carbon dioxide insertion, we perform a rare kinetic study on carbon dioxide insertion into a late-transition metal alkyl species using (^tBuPBP)Pd(CH₃). These studies demonstrate that the Dimroth–Reichardt parameter for a solvent correlates with the rate of carbon dioxide insertion and that Lewis acids do not promote insertion. DFT calculations indicate that insertion into (^tBuPBP)M(CH₃) (M = Ni or Pd) proceeds via an S_E2 mechanism and we compare the reaction pathway for carbon dioxide insertion into group 10 methyl complexes with insertion into group 10 hydrides. Overall, this work provides fundamental insight that will be valuable for the development of improved and new catalysts for carbon dioxide utilization.

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For a list of publications by members of the CHOCO group, see our Publications page 🙂

Research Article: Buchwald–Hartwig in lipid solvents

As a follow-up from the paper “Lipids as versatile solvents for chemical synthesis, Ashot , Kathrin and Annette are now presenting their work on the Buchwald–Hartwig Amination reaction using said solvents. Congratulations to all authors!

Abstract:
The development of green Buchwald–Hartwig aminations has long been considered challenging, due to the high sensitivity of the reaction to the environment. Here we show that food-grade and waste vegetable oils, triglycerides originating from animals, and natural waxes can serve as excellent green solvents for Buchwald–Hartwig amination. We further demonstrate that amphiphiles and trace ingredients present in triglycerides as additives have a decisive effect on the yields of Buchwald–Hartwig aminations.

Read at:

For a list of publications by members of the CHOCO group, see our Publications page 🙂

Recognition: Ashot’s work on vegetable oil solvents

Getting recognised within the scientific community for your work is about the best thing that can happen to your paper after publication, and Ashot’s paper on vegetable oil solvents for catalysis has recently been highlighted in the magazine Chemistry World.

You can read the article at chemistryworld.com

Seminar: Guest researcher Laurent Plasseraud

Dr. Laurent Plasseraud will be visiting our group on the 26th-27th November 2021. at this occasion, we have planned a small seminar together with activities.

Schedule: