Should the Woodward-Hoffmann Rules be Applied to Mechanochemical Reactions?

• Published in: Chemphyschem Vol. 16; no. 8; pp. 1593 - 1597
• Main Authors: Wollenhaupt, Miriam, Krupička, Martin, Marx, Dominik
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 08.06.2015; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: electrocyclic reactions; electronic structure analysis; mechanochemistry; reaction mechanisms; Woodward-Hoffmann rules; Woodward-Hoffmann rules; reaction mechanisms; electronic structure analysis; mechanochemistry; electrocyclic reactions
• ISSN: 1439-4235; 1439-7641; 1439-7641
• DOI: 10.1002/cphc.201500054

Since decades, pericyclic reactions have been well‐understood by means of the Woodward–Hoffmann rules and their classification as thermally or photochemically “allowed” or “forbidden”. Recently, stunning results on such reactions subject to mechanochemical activation by external forces instead of heat or light have revealed reaction pathways at sufficiently large forces, which are not expected from the Woodward–Hoffmann rules. This led to the much reiterated idea that the “Woodward–Hoffmann rules are broken in mechanochemistry”. Here, by studying ring‐opening of cyclopropane, we show that the electronic structure underlying the dis‐ and conrotatory pathways, which are greatly distorted upon applying forces to an extent that eventually the “thermally forbidden” process becomes “mechanochemically allowed”, does not change along both pathways. It is rather the mechanical work that lowers the activation barrier of the thermally forbidden conrotatory process relative to the disrotatory one at large forces. Which principle rules? The application of a mechanical force is reported herein to overrule the famous Woodward–Hoffmann rules. By analyzing the allowed and forbidden reaction pathways of cyclopropane (see picture) as a function of the applied force, we show that the underlying electronic structure is not affected by the force, but the mechanical work distorts the potential energy to yield products not expected from Woodward–Hoffmann rules.