Hajos-Parrish-Eder-Sauer-Wiechert reaction: The definitive reaction mechanism deciphered by DFT calculations.

• DFT calculations decipher the complete mechanism of the H-P-E-S-W reaction in DMSO. • The dehydration step of the ketol intermediate proceeds via an E 1 cB mechanism. • In the rate-determining step of ketol intermediate formation a water molecule catalyses the transformation, by acting as a proton shuttle. The use of (S)-proline as a catalyst for intramolecular aldol reactions pioneered by Hajos, Parrish, Eder, Sauer and Wiechert paved the way for modern organocatalysis, which earned MacMillan and List the Nobel Prize in Chemistry in 2021. The work reported herein shed light on mechanistic aspects regarding the "prototypal" H-P-E-S-W reaction that, up to date, are unclear. The DFT calculations determined i) the mechanism of the dehydration step of the ketol intermediate that proceeds via an E 1 cB (elimination, unimolecular, conjugate base) mechanism; ii) the rate-determining steps (RDS) of the H-P-E-S-W reaction that correspond to enamine formation and dehydration; iii) the catalytic role of the water molecule formed during the reaction, that behaves as a proton shuttle during the enamine and C C bond formation and determines a significant lowering of the activation barriers. [Display omitted] [ABSTRACT FROM AUTHOR]