page_title: Woodward–Hoffmann rules
section_title: Exceptions
hierarchical_section_title: Woodward–Hoffmann rules / Exceptions
caption_attribution_description: English: The 4 electron electrocyclic ring opening reaction can be influenced by mechanical stress, from Hickenboth (2007).
context_page_description: The Woodward–Hoffmann rules, devised by Robert Burns Woodward and Roald Hoffmann, are a set of rules used to rationalize or predict certain aspects of the stereochemistry and activation energy of pericyclic reactions, an important class of reactions in organic chemistry. The rules are best understood in terms of the concept of the conservation of orbital symmetry using orbital correlation diagrams, for which an elementary description is given in Section 10.4 of a student level text book. The Woodward–Hoffmann rules are a consequence of the changes in electronic structure that occur during a pericyclic reaction and are predicated on the phasing of the interacting molecular orbitals. They are applicable to all classes of pericyclic reactions, including electrocyclizations, cycloadditions, sigmatropic reactions, group transfer reactions, ene reactions, cheletropic reactions, and dyotropic reactions. Due to their elegance, simplicity, and generality, the Woodward–Hoffmann rules are credited with first exemplifying the power of molecular orbital theory to experimental chemists.
context_section_description: In Chapter 12 of The Conservation of Orbital Symmetry, entitled "Violations," Woodward and Hoffmann famously stated:
  There are none!  Nor can violations be expected of so fundamental a principle of maximum bonding.
  This pronouncement notwithstanding, it is important to recognize that the Woodward–Hoffmann rules are used to predict relative barrier heights, and thus likely reaction mechanisms, and that they only take into account barriers due to conservation of orbital symmetry.  Thus it is not guaranteed that a WH symmetry-allowed reaction actually takes place in a facile manner.  Conversely, it is possible, upon enough energetic input, to achieve an anti-Woodward-Hoffmann product.  This is especially prevalent in sterically constrained systems, where the WH-product has an added steric barrier to overcome.  For example, in the electrocyclic ring-opening of the dimethylbicyclo[0.2.3]heptene derivative (1), a conrotatory mechanism is not possible due to resulting angle strain and the reaction proceeds slowly through a disrotatory mechanism at 400ᵒ C to give a cycloheptadiene product.  Violations may also be observed in cases with very strong thermodynamic driving forces.  The decomposition of dioxetane-1,2-dione to two molecules of carbon dioxide, famous for its role in the luminescence of glowsticks, has been scrutinized computationally.  In the absence of fluorescers, the reaction is now believed to proceed in a concerted (though asynchronous) fashion, via a retro-[2+2]-cycloaddition that formally violates the Woodward–Hoffmann rules.
  Similarly, a recent paper describes how mechanical stress can be used to reshape chemical reaction pathways to lead to products that apparently violate Woodward–Hoffman rules.  In this paper, they use ultrasound irradiation to induce a mechanical stress on link-functionalized polymers attached syn or anti on the cyclobutene ring.  Computational studies predict that the mechanical force, resulting from friction of the polymers, induces bond lengthening along the reaction coordinate of the conrotatory mechanism in the anti-bisubstituted-cyclobutene, and along the reaction coordinate of the disrotatory mechanism in the syn-bisubstituted-cyclobutene.  Thus in the syn-bisubstituted-cyclobutene, the anti-WH product is predicted to be formed.
  This computational prediction was backed up by experiment on the system below.  Link-functionalized polymers were conjugated to cis benzocyclobutene in both syn- and anti- conformations.  As predicted, both products gave the same (Z,Z) product as determined by quenching by a stereospecific Diels-Alder reaction with the substituted maleimide.  In particular, the syn-substituted product gave the anti-WH product, presumably as the mechanical stretching along the coordinate of the disrotatory pathway lowered the barrier of the reaction under the disrotatory pathway enough to bias that mechanism.
caption: Computationally predicted products of 4e electrocyclic ring opening under thermal, photo, and mechanical control.
exif: {}