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Kinetic evidence of an apparent negative activation enthalpy in an organocatalytic process.

Han X, Lee R, Chen T, Luo J, Lu Y, Huang KW - Sci Rep (2013)

Bottom Line: A combined kinetic and computational study on our tryptophan-based bifunctional thiourea catalyzed asymmetric Mannich reactions reveals an apparent negative activation enthalpy.The formation of the pre-transition state complex has been unambiguously confirmed and these observations provide an experimental support for the formation of multiple hydrogen bonding network between the substrates and the catalyst.Such interactions allow the creation of a binding cavity, a key factor to install high enantioselectivity.

View Article: PubMed Central - PubMed

Affiliation: KAUST Catalysis Center and Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal, Kingdom of Saudi Arabia.

ABSTRACT
A combined kinetic and computational study on our tryptophan-based bifunctional thiourea catalyzed asymmetric Mannich reactions reveals an apparent negative activation enthalpy. The formation of the pre-transition state complex has been unambiguously confirmed and these observations provide an experimental support for the formation of multiple hydrogen bonding network between the substrates and the catalyst. Such interactions allow the creation of a binding cavity, a key factor to install high enantioselectivity.

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Reaction profile diagram for the asymmetric Mannich reaction (mPW1PW91/6-31 + G(d,p)).
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f5: Reaction profile diagram for the asymmetric Mannich reaction (mPW1PW91/6-31 + G(d,p)).

Mentions: When measuring the reaction rate, one is looking at the overall free energy barrier, i.e. TS-I, and it occurs that the overall enthalpy change of transition state TS-I is −1.1 kcal mol−1 relative to starting substrates. Comparing the enthalpy change for intermediate pTS-I to starting materials (−7.2 kcal mol−1) suggests a distinct feature of an exothermic elementary step due to formation of hydrogen bonds with C and substrates. As our experiments revealed inverse dependence of reaction rate with temperature, it is clear from the reaction profile how negative activation enthalpy originated (Figure 5).


Kinetic evidence of an apparent negative activation enthalpy in an organocatalytic process.

Han X, Lee R, Chen T, Luo J, Lu Y, Huang KW - Sci Rep (2013)

Reaction profile diagram for the asymmetric Mannich reaction (mPW1PW91/6-31 + G(d,p)).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3757351&req=5

f5: Reaction profile diagram for the asymmetric Mannich reaction (mPW1PW91/6-31 + G(d,p)).
Mentions: When measuring the reaction rate, one is looking at the overall free energy barrier, i.e. TS-I, and it occurs that the overall enthalpy change of transition state TS-I is −1.1 kcal mol−1 relative to starting substrates. Comparing the enthalpy change for intermediate pTS-I to starting materials (−7.2 kcal mol−1) suggests a distinct feature of an exothermic elementary step due to formation of hydrogen bonds with C and substrates. As our experiments revealed inverse dependence of reaction rate with temperature, it is clear from the reaction profile how negative activation enthalpy originated (Figure 5).

Bottom Line: A combined kinetic and computational study on our tryptophan-based bifunctional thiourea catalyzed asymmetric Mannich reactions reveals an apparent negative activation enthalpy.The formation of the pre-transition state complex has been unambiguously confirmed and these observations provide an experimental support for the formation of multiple hydrogen bonding network between the substrates and the catalyst.Such interactions allow the creation of a binding cavity, a key factor to install high enantioselectivity.

View Article: PubMed Central - PubMed

Affiliation: KAUST Catalysis Center and Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal, Kingdom of Saudi Arabia.

ABSTRACT
A combined kinetic and computational study on our tryptophan-based bifunctional thiourea catalyzed asymmetric Mannich reactions reveals an apparent negative activation enthalpy. The formation of the pre-transition state complex has been unambiguously confirmed and these observations provide an experimental support for the formation of multiple hydrogen bonding network between the substrates and the catalyst. Such interactions allow the creation of a binding cavity, a key factor to install high enantioselectivity.

Show MeSH