Elucidating the Multicomponent Reaction Pathway of 2‑Pyrrolidone Synthesis

TL;DR

This study clarifies the reaction pathway for making 2-pyrrolidone using a combination of advanced analytical techniques and computational methods.

Contribution

The first experimentally supported mechanism for 2-pyrrolidone synthesis via a multicomponent reaction is presented.

Findings

EI-MS and X-ray diffraction confirmed the exclusive formation of 2-pyrrolidone products.

DART-MS identified key intermediates supporting a stepwise reaction mechanism.

Citric acid was found to be the most effective catalyst due to its dual activation role.

Abstract

2-Pyrrolidone derivatives are valuable heterocycles with
significant
biological relevance, yet their synthesis through multicomponent reactions
(MCRs) remains mechanistically ambiguous, particularly due to the
difficulty of distinguishing them from isomeric aryl-substituted furanones.
In this work, we combine electron impact mass spectrometry (EI-MS),
Direct Analysis in Real Time mass spectrometry (DART-MS), and single-crystal
X-ray diffraction to unambiguously confirm the exclusive formation
of 2-pyrrolidone products in the reaction of anilines, benzaldehydes,
and diethyl acetylenedicarboxylate. EI-MS provided a diagnostic fragmentation
profile inconsistent with furanone structures, while X-ray analysis
validated the pyrrolidone core. Time-resolved DART-MS enabled the
detection of key long-lived intermediates–such as imines, hydrated
alkyne adducts, and pyrrolidone-type species–supporting a stepwise
mechanism involving acid-catalyzed imine formation, alkyne hydration,
nucleophilic addition to an iminium ion, and final lactamization.
Complementary experiments employing diethyl oxaloacetate, together
with DFT calculations, further substantiated the pivotal role of alkyne
hydration and the β-nucleophilic attack governing cyclization.
Citric acid emerged as the most effective catalyst due to its dual
activation of the aldehyde and iminium intermediates, while competing
enamine formation rationalizes differences in isolated yields across
the series. Overall, this study provides the first experimentally
supported mechanism for this MCR and establishes a robust analytical
framework for the structural and mechanistic elucidation of pyrrolidone-forming
reactions.

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