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Transition state analysis of enantioselective Brønsted base catalysis by chiral cyclopropenimines.

Bandar JS, Sauer GS, Wulff WD, Lambert TH, Vetticatt MJ - J. Am. Chem. Soc. (2014)

Bottom Line: Experimental (13)C kinetic isotope effects have been used to interrogate the rate-limiting step of the Michael addition of glycinate imines to benzyl acrylate catalyzed by a chiral 2,3-bis(dicyclohexylamino) cyclopropenimine catalyst.The reaction is found to proceed via rate-limiting carbon-carbon bond formation.The origins of enantioselectivity and a key noncovalent CH···O interaction responsible for transition state organization are identified on the basis of density functional theory calculations and probed using experimental labeling studies.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Columbia University , 3000 Broadway, New York, New York 10027, United States.

ABSTRACT
Experimental (13)C kinetic isotope effects have been used to interrogate the rate-limiting step of the Michael addition of glycinate imines to benzyl acrylate catalyzed by a chiral 2,3-bis(dicyclohexylamino) cyclopropenimine catalyst. The reaction is found to proceed via rate-limiting carbon-carbon bond formation. The origins of enantioselectivity and a key noncovalent CH···O interaction responsible for transition state organization are identified on the basis of density functional theory calculations and probed using experimental labeling studies. The resulting high-resolution experimental picture of the enantioselectivity-determining transition state is expected to guide new catalyst design and reaction development.

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Comparison of experimental (black) and predicted (red) 13C KIEs.
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fig6: Comparison of experimental (black) and predicted (red) 13C KIEs.

Mentions: To interpret the experimental KIE resultsdescribed above, 13C KIEs were computed from the scaledvibrational frequencies of the two lowest energy transition structuresleading to the major enantiomer (TS5bSE and TS5aSE, as a weighted average based on their energies) using the programISOEFF98.18,19 A one-dimensional tunneling correction20 was applied to the predicted 13CKIEs. The predicted KIEs, along with the experimental values obtainedfor each carbon atom, are shown in Figure 6. The excellent agreement of experiment and theory validates thetransition state model and supports carbon–carbon bond formationto be the rate-limiting step of the reaction.


Transition state analysis of enantioselective Brønsted base catalysis by chiral cyclopropenimines.

Bandar JS, Sauer GS, Wulff WD, Lambert TH, Vetticatt MJ - J. Am. Chem. Soc. (2014)

Comparison of experimental (black) and predicted (red) 13C KIEs.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4230825&req=5

fig6: Comparison of experimental (black) and predicted (red) 13C KIEs.
Mentions: To interpret the experimental KIE resultsdescribed above, 13C KIEs were computed from the scaledvibrational frequencies of the two lowest energy transition structuresleading to the major enantiomer (TS5bSE and TS5aSE, as a weighted average based on their energies) using the programISOEFF98.18,19 A one-dimensional tunneling correction20 was applied to the predicted 13CKIEs. The predicted KIEs, along with the experimental values obtainedfor each carbon atom, are shown in Figure 6. The excellent agreement of experiment and theory validates thetransition state model and supports carbon–carbon bond formationto be the rate-limiting step of the reaction.

Bottom Line: Experimental (13)C kinetic isotope effects have been used to interrogate the rate-limiting step of the Michael addition of glycinate imines to benzyl acrylate catalyzed by a chiral 2,3-bis(dicyclohexylamino) cyclopropenimine catalyst.The reaction is found to proceed via rate-limiting carbon-carbon bond formation.The origins of enantioselectivity and a key noncovalent CH···O interaction responsible for transition state organization are identified on the basis of density functional theory calculations and probed using experimental labeling studies.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Columbia University , 3000 Broadway, New York, New York 10027, United States.

ABSTRACT
Experimental (13)C kinetic isotope effects have been used to interrogate the rate-limiting step of the Michael addition of glycinate imines to benzyl acrylate catalyzed by a chiral 2,3-bis(dicyclohexylamino) cyclopropenimine catalyst. The reaction is found to proceed via rate-limiting carbon-carbon bond formation. The origins of enantioselectivity and a key noncovalent CH···O interaction responsible for transition state organization are identified on the basis of density functional theory calculations and probed using experimental labeling studies. The resulting high-resolution experimental picture of the enantioselectivity-determining transition state is expected to guide new catalyst design and reaction development.

Show MeSH