Research Summary, Chi Chen, Yale University
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[Type here] Research Summary Chi Chen Z-Selective alkene isomerization and anti-Markovnikov alkene hydrosilylation with Low Coordinated Cobalt Complexes Efficient catalytic reactions are a key component of sustainable chemistry, and the most desirable catalytic reactions use sustainable catalysts. Base metals such as Fe, Co, and Ni are more sustainable than precious-metal catalysts because of cost, geopolitical, health, and energy issues. The last decade has seen the discovery of selective base-metal catalysts in hydrocarbon transformations and strong bond activations. Alkene Isomerization C=C double bonds are key functional groups in many natural and industrial products, and E (trans) isomer is typically more thermodynamically stable over Z (cis) isomer. This makes it difficult to specifically prepare the Z isomer, though Z alkenes are of great use in synthesis and often found in natural products. Current methods require either special directing groups (the Still-Gennari modification to the Horner-Wadsworth-Emmons olefination), strong base (Wittig and HWE olefination) or expensive palladium catalysts (cross-coupling and reduction of alkyne over Lindlar catalyst). Though alkene isomerization is an atom-economic transformation, it has not reached its potential due to poor selectivity. We found 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.The catalyst shows moderate functional group tolerance. 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. Alkene Hydrosilylation:
Transcript of Research Summary, Chi Chen, Yale University
[Type here] Research Summary Chi Chen
Z-Selective alkene isomerization and anti-Markovnikov alkene hydrosilylation with Low Coordinated Cobalt Complexes
Efficient catalytic reactions are a key component of sustainable chemistry, and the most desirable catalytic reactions use sustainable catalysts. Base metals such as Fe, Co, and Ni are more sustainable than precious-metal catalysts because of cost, geopolitical, health, and energy issues. The last decade has seen the discovery of selective base-metal catalysts in hydrocarbon transformations and strong bond activations.
Alkene Isomerization
C=C double bonds are key functional groups in many natural and industrial products, and E (trans) isomer is typically more thermodynamically stable over Z (cis) isomer. This makes it difficult to specifically prepare the Z isomer, though Z alkenes are of great use in synthesis and often found in natural products. Current methods require either special directing groups (the Still-Gennari modification to the Horner-Wadsworth-Emmons olefination), strong base (Wittig and HWE olefination) or expensive palladium catalysts (cross-coupling and reduction of alkyne over Lindlar catalyst). Though alkene isomerization is an atom-economic transformation, it has not reached its potential due to poor selectivity. We found 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.The catalyst shows moderate functional group tolerance. 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.
Alkene Hydrosilylation:
Alkene Hydrosilylation is one of the largest homogeneous catalysis in industry. It is used to convert alkenes into silanes and silicones that are used in a wide range of applications including lubricants, rubbers, glues, high-temperature plastics, and stationary phases for chromatography. Currently, alkene hydrosilylation is performed predominantly with Pt catalysts, but inexpensive base-metal catalysts would be preferred as in many large-scale applications (e.g. polymer cross-linking), the Pt catalyst remains in the product, and even a low loading adds up to a large amount of lost Pt. Considerations of sustainability and cost motivate the development of cheaper catalysts. We found an inexpensive Co catalyst system that can be used without added solvent at low temperatures with low loadings, and show excellent functional group tolerance. The cobalt catalyst can be generated in situ from an air-stable precursor that is simple to synthesize from low-cost, commercially available materials. In addition, a mixture of Co catalysts performs a tandem catalytic isomerization/hydrosilylation reaction that converts multiple isomers of hexene to the same terminal product, which is promising for the use of alkene mixtures to form a single silane.
