Kinetic Rationalization of Nonlinear Effects in Asymmetric Catalytic Cascade Reactions under Curtin-Hammett...

Area of Science:

• Chemical Synthesis
• Catalysis
• Organic Chemistry

Background:

• Nonlinear effects in asymmetric catalysis are typically attributed to multiple catalyst species or complex reaction pathways.
• Understanding these effects is crucial for predicting and controlling product stereochemistry.

Purpose of the Study:

• To demonstrate that nonlinear effects in asymmetric catalysis can arise from factors other than multiple catalyst species.
• To propose an alternative mechanistic model for nonlinear effects in cascade reactions.

Main Methods:

• Theoretical analysis of reaction kinetics in asymmetric catalytic cascade reactions.
• Illustrative example using an organocatalytic enantioselective [10 + 2] cyclization reaction.
• Consideration of pre-equilibrium disruption (Curtin-Hammett equilibrium) and its impact on product enantiomeric excess (ee).

Main Results:

• Nonlinear effects on product enantiomeric excess can be observed in cascade reactions even with a single catalyst species.
• Disruption of reversible pre-equilibria before the rate-determining step can lead to deviations from linear relationships between catalyst enantiopurity and product ee.
• The model accounts for both positive and negative nonlinear effects under various kinetic conditions ('major-minor' and 'lock-and-key').

Conclusions:

• The proposed mechanistic scenario offers a viable alternative explanation for observed nonlinear effects in cascade reactions.
• This model is applicable to a wide range of cascade systems exhibiting similar kinetic features.
• It challenges the conventional interpretation solely based on multiple catalyst species.