Cobalt-Catalyzed Asymmetric Hydrogenation of Enamides: Insights into Mechanisms and Solvent Effects

Pavlovic, Ljiljana; Mendelsohn, Lauren N.; Zhong, Hongyu; Chirik, Paul J.; Hopmann, Kathrin H.

doi:10.1021/acs.organomet.2c00180

Cited by 13 publications

(10 citation statements)

References 66 publications

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“…With 1-MAA as the starting point, three mechanistic pathways were considered: a classical redox Co(0/II) mechanism A , a Co(0/II) imine mechanism B , , and a non-redox metallacycle pathway C where cobalt remains the +2 oxidation state during the whole catalytic cycle . The DFT calculations were performed with an explicit MeOH molecule added to the model, as we have shown for related calculations on ( R , R )-( Ph BPE)Co(MAA) that inclusion of a polar solvent molecule stabilizes the charges at the rate-limiting transition states and thus affects the final energies and enantioselectivities. , …”

Section: Resultsmentioning

confidence: 98%

Mechanistic Investigations of the Asymmetric Hydrogenation of Enamides with Neutral Bis(phosphine) Cobalt Precatalysts

et al. 2022

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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- 4F MAC), 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.

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Section: Resultsmentioning

confidence: 98%

Mechanistic Investigations of the Asymmetric Hydrogenation of Enamides with Neutral Bis(phosphine) Cobalt Precatalysts

et al. 2022

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“…On the other hand, a metallacycle pathway is energetically favorable in the case of Co/ Ph BPE catalyzed MAA hydrogenation along with the classical redox pathway [53a] . Formation of a six‐membered Co(II)‐metallacycle ( B , Scheme 56) is energetically accessible via hydride transfer from the Co(0)‐MAA to the α‐carbon of MAA.…”

Section: Mechanistic Aspectsmentioning

confidence: 99%

“…Then the proton transfer (through a four membered cyclic transition state F , as shown in Scheme 56) would result in the formation of the starting Co‐enamide ( A ), releasing the product. MeOH as solvent plays an important role by stabilizing (through H‐bond formation) the possible intermediates in both the Co/ Ph BPE [53a] and the Co/ iPr DuPhos [53b] catalyzed AH of MAA. This also highlighted the importance of the solvent in asymmetric (hydrogenation) catalysis.…”

Section: Mechanistic Aspectsmentioning

confidence: 99%

Cobalt‐Catalyzed Asymmetric Hydrogenation: Substrate Specificity and Mechanistic Variability

Chakrabortty,

de Bruin,

de Vries

2023

Angew Chem Int Ed

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Asymmetric hydrogenation finds widespread application in academia and industry. And indeed, a number of processes have been implemented for the production of pharma and agro intermediates as well as flavors & fragrances. Although these processes are all based on the use of late transition metals as catalysts, there is an increasing interest in the use of base metal catalysis in view of their lower cost and the expected different substrate scope. Catalysts based on cobalt have already shown their potential in enantioselective hydrogenation chemistry. This review outlines the impressive progress made in recent years on cobalt‐catalyzed asymmetric hydrogenation of different unsaturated substrates. We also illustrate the ligand dependent substrate specificity as well as the mechanistic variability in detail. This may well guide further catalyst development in this research area.

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“…This situation has become still more complicated with the recent rapid development of new hydrogenation techniques applying asymmetric catalysis with complexes of cheap and abundant metals [ 24 , 25 , 26 ]. Numerous mechanistic studies of the catalytic cycles of asymmetric hydrogenations have been performed of catalysis by earth-abundant metals, such as Ni [ 27 , 28 , 29 , 30 , 31 ], Co [ 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 ], Fe [ 48 , 49 , 50 , 51 , 52 ], Mn [ 53 , 54 ], etc. It seems that each metal has its own hydrogenation chemistry, which can in turn be subdivided into various catalytic cycles with different metal oxidation states.…”

Section: Introductionmentioning

confidence: 99%

Co-Catalyzed Asymmetric Hydrogenation. The Same Enantioselection Pattern for Different Mechanisms

Gridnev

2023

IJMS

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The mechanism of the recently reported catalyzed asymmetric hydrogenation of enyne 1 catalyzed by the Co-(R,R)-QuinoxP* complex was studied by DFT. Conceivable pathways for the Co(I)-Co(III) mechanism were computed together with a Co(0)-Co(II) catalytic cycle. It is commonly assumed that the exact nature of the chemical transformations taking place along the actually operating catalytic pathway determine the sense and level of enantioselection of the catalytic reaction. In this work, two chemically different mechanisms reproduced the experimentally observed perfect stereoselection of the same handedness. Moreover, the relative stabilities of the transition states of the stereo induction stages were controlled via exactly the same weak disperse interactions between the catalyst and the substrate.

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Cobalt-Catalyzed Asymmetric Hydrogenation of Enamides: Insights into Mechanisms and Solvent Effects

Cited by 13 publications

References 66 publications

Mechanistic Investigations of the Asymmetric Hydrogenation of Enamides with Neutral Bis(phosphine) Cobalt Precatalysts

Mechanistic Investigations of the Asymmetric Hydrogenation of Enamides with Neutral Bis(phosphine) Cobalt Precatalysts

Cobalt‐Catalyzed Asymmetric Hydrogenation: Substrate Specificity and Mechanistic Variability

Co-Catalyzed Asymmetric Hydrogenation. The Same Enantioselection Pattern for Different Mechanisms

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