Stereocontrol in Diphenylprolinol Silyl Ether Catalyzed Michael Additions : Steric Shielding or Curtin-Hammett Scenario?
Földes, T., Madarász, Á., Révész, Á., Dobi, Z., Varga, S., Hamza, A., Nagy, P. R., Pihko, P., & Pápai, I. (2017). Stereocontrol in Diphenylprolinol Silyl Ether Catalyzed Michael Additions : Steric Shielding or Curtin-Hammett Scenario?. Journal of the American Chemical Society, 139(47), 17052-17063. https://doi.org/10.1021/jacs.7b07097
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2017Copyright
© 2017 American Chemical Society. This is a final draft version of an article whose final and definitive form has been published by ACS. Published in this repository with the kind permission of the publisher.
The enantioselectivity of amine-catalyzed reactions of aldehydes with electrophiles is often explained by
simple steric arguments emphasizing the role of the bulky group of the catalyst that prevents the approach of the
electrophile from the more hindered side. This standard steric shielding model has recently been challenged by the
discovery of stable downstream intermediates, which appear to be involved in the rate-determining step of the catalytic
cycle. The alternative model, referred to as Curtin-Hammett scenario of stereocontrol, assumes that the enantioselectivity
is related to the stability and reactivity of downstream intermediates. In our present computational study, we examine the
two key processes of the catalytic Michael reaction between propanal and β-nitrostyrene that are relevant to the proposed
stereoselectivity models, namely the C-C bond formation and the protonation steps. The free energy profiles obtained for
the pathways leading to the enantiomeric products suggest that the rate- and stereo-determining steps are not identical
as implied by the previous models. The stereoselectivity can be primarily controlled by C-C bond formation even though
the reaction rate is dictated by the protonation step. This kinetic scheme is consistent with all observations of
experimental mechanistic studies including those of mass spectrometric back reaction screening experiments, which
reveal a mismatch between the stereoselectivity of the back and the forward reactions.
...


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