Exploring the aldol reaction using catalytic antibodies and "on water" organocatalysts from QM/MM calculations

Kira Armacost, Orlando Acevedo

Research output: Contribution to journalArticlepeer-review

20 Scopus citations

Abstract

The aldol reaction between benzaldehyde and acetone has been investigated using QM/MM Monte Carlo calculations and free-energy perturbation theory to determine the origin of the enhanced rates and enantioselectivities (% ee) derived from an enamine-based catalytic antibody 33F12 and a chiral organocatalyst. Electrostatic stabilization of the general acid/base TyrL36 by TrpH103, SerH100, and AsnL34 enabled the 33F12 active site to exclusively adopt an si-face benzaldehyde orientation for C-C bond formation with the LysH93-enamine. Whereas preorganization was responsible for the exclusive (S)-aldol product in the antibody, the organocatalyst featuring a chiral diphenyl amino alcohol moiety instead derived its preferred (R)-aldol product from an interplay between sterics and electronic stabilization. The si-face benzaldehyde conformation had unfavorable interactions with the organocatalyst in contrast to the re-face. Gas-phase calculations predicted a 73% ee; however, solution boosted the % ee values despite similar reaction geometries. An "on water" environment, defined as a reaction that proceeds in an aqueous organic emulsion, yielded a computed 94% ee (exptl 93% ee) compared to a calculated 87% ee in "neat" acetone (exptl 85% ee). Specific hydrogen bonding between the interfacial waters and an amide oxygen on the catalyst was found to control the % ee. A more compact si-face transition structure reduced solvent accessibility to the amide oxygen with a "closed state" steric barrier compared to an "open state" for the re-face. New insight into the synthetically important aldol reaction and state-of-the-art methodology is presented herein.

Original languageEnglish (US)
Pages (from-to)147-156
Number of pages10
JournalJournal of the American Chemical Society
Volume136
Issue number1
DOIs
StatePublished - Jan 8 2014
Externally publishedYes

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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