{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,18]],"date-time":"2026-08-18T04:58:31Z","timestamp":1787029111636,"version":"build-2736575974"},"reference-count":52,"publisher":"MDPI AG","issue":"24","license":[{"start":{"date-parts":[[2025,12,9]],"date-time":"2025-12-09T00:00:00Z","timestamp":1765238400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Polish high-performance computing infrastructure PLGrid","award":["PLG\/2025\/018731"],"award-info":[{"award-number":["PLG\/2025\/018731"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Molecules"],"abstract":"<jats:p>We investigated the Diels-Alder cycloaddition of methylcyclopentadiene with conjugated nitroalkenes and examined the influence of solvent polarity and substituent effects on the reaction mechanism. In nonpolar media (toluene), pathways A and C proceed via a pre-reactive molecular complex (MC), two transition states, and a heterocyclic intermediate, whereas pathways B and D follow a single-transition-state route directly to the norbornene product. Moderate increases in solvent polarity (acetone) do not qualitatively alter the energy profiles or mechanistic patterns, whereas highly polar solvents (methanol, acetonitrile, water, nitromethane) induce a fundamental transformation in pathway B, which adopts a stepwise, zwitterionic mechanism. NPA, MEP, and NCI analyses confirm the polar, charge-separated nature of the zwitterionic intermediate, while BET analysis elucidates the sequential electronic reorganization, highlighting early polarization toward the nitro fragment and stepwise formation of the C-C bonds. Substituent effect studies using Hammett \u03c3 parameters reveal that electron-withdrawing groups lower activation barriers, whereas electron-donating groups increase them, indicating that electronic effects dominate over steric factors. Overall, the study demonstrates a general, solvent- and substituent-dependent Diels-Alder mechanism, with pathway B proceeding through a polar, highly asynchronous, stepwise route involving a zwitterionic intermediate.<\/jats:p>","DOI":"10.3390\/molecules30244710","type":"journal-article","created":{"date-parts":[[2025,12,9]],"date-time":"2025-12-09T12:42:43Z","timestamp":1765284163000},"page":"4710","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":4,"title":["Zwitterionic Pathway in the Diels\u2013Alder Reaction: Solvent and Substituent Effects from \u03c9B97XD\/6-311G(d) Calculations"],"prefix":"10.3390","volume":"30","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-9419-1316","authenticated-orcid":false,"given":"Agnieszka","family":"\u0141apczuk","sequence":"first","affiliation":[{"name":"Cracow University of Technology, Department of Organic Chemistry and Technology Warszawska 24, 31-155 Cracow, Poland"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2025,12,9]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"510","DOI":"10.1002\/ange.19370502804","article-title":"Untersuchungen \u00fcber den Verlauf der Diensynthese","volume":"50","author":"Alder","year":"1937","journal-title":"Angew. 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