New Insights on Kinetic Versus Thermodynamic Ratios in Catalyzed Alkene Isomerization

Larsen, Casey R.; Grotjahn, Douglas B.

doi:10.1007/s11244-010-9572-y

Cited by 6 publications

(6 citation statements)

References 18 publications

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“…Although the isomerization of terminal alkenes to internal alkenes is thermodynamically favorable, catalytic reactions often give mixtures of internal alkenes. − Further, the products are typically mixtures of the E - and Z -isomers in the ratio expected from the relative free energies (for simple alkenes, this ratio is generally greater than 3:1 favoring the E -isomer). ,− Several catalysts are known for selective formation of the E -isomer with very good selectivity. ,,,− On the other hand, selectivity for the less thermodynamically stable internal isomer is rare. Some alkene isomerization catalysts form the Z -isomers at a very early stage, but the mixture quickly converges to the predominantly E thermodynamic ratio .…”

Section: Introductionmentioning

confidence: 99%

Z-Selective Alkene Isomerization by High-Spin Cobalt(II) Complexes

et al. 2014

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The isomerization of simple terminal alkenes to internal isomers with Z-stereochemistry is rare, because the more stable E-isomers are typically formed. We show here that cobalt(II) catalysts supported by bulky β-diketiminate ligands have the appropriate kinetic selectivity to catalyze the isomerization of some simple 1-alkenes specifically to the 2-alkene as the less stable Z-isomer. The catalysis proceeds via an "alkyl" mechanism, with a three-coordinate cobalt(II) alkyl complex as the resting state. β-Hydride elimination and [1,2]-insertion steps are both rapid, as shown by isotopic labeling experiments. A steric model explains the selectivity through a square-planar geometry at cobalt(II) in the transition state for β-hydride elimination. The catalyst works not only with simple alkenes, but also with homoallyl silanes, ketals, and silyl ethers. Isolation of cobalt(I) or cobalt(II) products from reactions with poor substrates suggests that the key catalyst decomposition pathways are bimolecular, and lowering the catalyst concentration often improves the selectivity. In addition to a potentially useful, selective transformation, these studies provide a mechanistic understanding for catalytic alkene isomerization by high-spin cobalt complexes, and demonstrate the effectiveness of steric bulk in controlling the stereoselectivity of alkene formation.

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Section: Introductionmentioning

confidence: 99%

Z-Selective Alkene Isomerization by High-Spin Cobalt(II) Complexes

et al. 2014

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“…The Grotjahn group has in the past been investigating ruthenium catalysts such as cationic complex 522 − (Scheme ) for their propensity to isomerize alkenes over one, , or many positions, , and even engage in productive tandem “internal-to-terminal” isomerization-CM processes . In addition, these researchers have been intrigued as to why this complex demonstrates such excellent stereoselectivity for the E -isomers of the isomerized allyl groups. , In terms of application, Erdogan and Grotjahn reported that the ruthenium complex 522 was able to perform a biphasic proton–deuterium exchange at the allylic position of estragole 7 , to obtain after isomerization mainly the E isomer 523 (i.e., isomerizations do not occur by way of a metal–hydride species) (Scheme ). This occurred when using (2–5 mol %) of the catalyst 522 and at moderate temperatures (25–70 °C) .…”

Section: Transition Metal-mediated Isomerizations Of Allylbenzenesmentioning

confidence: 99%

“…Cationic Ruthenium Complexes. The Grotjahn group has in the past been investigating ruthenium catalysts such as cationic complex 522 664−666 (Scheme 136) for their propensity to isomerize alkenes over one, 667,668 or many positions, 669,670 and even engage in productive tandem "internal-to-terminal" isomerization-CM processes. 671 In addition, these researchers have been intrigued as to why this complex demonstrates such excellent stereoselectivity for the Eisomers of the isomerized allyl groups.…”

Section: Ruthenium-catalyzed Isomerizationsmentioning

confidence: 99%

Isomerization of Allylbenzenes

et al. 2015

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“…This selectivity for isomerization is different than catalysts of Ru and Rh, which typically give a mixture of the most stable isomers. 9 The catalysis proceeds via an "alkyl" mechanism, with a three-coordinate cobalt(II) alkyl complex as the resting state. β-Hydride elimination and [1,2]-insertion steps are rapid ("chain walking"), as shown by isotopic labeling experiments.…”

Section: Resultsmentioning

confidence: 99%

High-Spin Cobalt Hydrides for Catalysis

Holland¹

2013

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Research Goals Explore the catalytic ability of low-coordinate cobalt complexes in small-molecule catalysis. Determine the mechanisms of organometallic transformations at metals with high-spin electronic configurations. The low coordination number and unusual spin states are anticipated to give novel and useful reactivity patterns, leading to exciting transformations that make catalysts more efficient. Significant Achievements and Results • Synthesis of a "Masked Two-Coordinate" Cobalt Complex: We isolated the highly unsaturated species L tBu Co, which "slips" one of its imine donors to form an η 6 interaction with an arene. The isomerization is reversible upon addition of donor ligands. We established the mechanism of the ligand coordination/decoordination, including intermediate slipping in a CO complex. • Bimetallic C-F Bond Cleavage: L tBu Co reacts with aryl fluorides to give L tBu CoAr and [L tBu CoF] 2. We established the mechanism of this rare example of multimetallic C-F bond activation using kinetic studies. Another Co-F compound exists as an unusual Co-Sn-Co assembly. • Catalytic Alkene Isomerization by Cobalt Complexes: L tBu CoR complexes catalyze the isomerization of simple terminal alkenes specifically to Z-2-alkenes. This is an unprecedented case of selective alkene isomerization to give a single isomer that is not the thermodynamic product. The mechanism of this transformation, and the determinants of selectivity, were established using a combination of computational and mechanistic studies. • Carbon Dioxide is Reduced by the Cobalt Complexes. The cobalt(I) complex reduces CO 2 to carbonate and to a dicobalt(II) oxo complex. The mechanism of this reaction has been established.

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New Insights on Kinetic Versus Thermodynamic Ratios in Catalyzed Alkene Isomerization

Cited by 6 publications

References 18 publications

Z-Selective Alkene Isomerization by High-Spin Cobalt(II) Complexes

Z-Selective Alkene Isomerization by High-Spin Cobalt(II) Complexes

Isomerization of Allylbenzenes

High-Spin Cobalt Hydrides for Catalysis

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