A 21st Century View of Allowed and Forbidden Electrocyclic Reactions

Abstract

In 1965, Woodward and Hoffmann proposed a theory to predict the stereochemistry of electrocyclic reactions, which, after expansion and generalization, became known as the Woodward−Hoffmann Rules. Subsequently, Longuet-Higgins and Abrahamson used correlation diagrams to propose that the stereoselectivity of electrocyclizations could be explained by the correlation of reactant and product orbitals with the same symmetry. Immediately thereafter, Hoffmann and Woodward applied correlation diagrams to explain the mechanism of cycloadditions. We describe these discoveries and their evolution. We now report an investigation of various electrocyclic reactions using DFT and CASSCF. We track the frontier molecular orbitals along the intrinsic reaction coordinate and modeled trajectories and examine the correlation between HOMO and LUMO for thermally forbidden systems. We also investigate the electrocyclizations of several highly polarized systems for which the Houk group had predicted that donor−acceptor substitution can induce zwitterionic character, thereby providing low-energy pathways for formally forbidden reactions. We conclude with perspectives on the field of pericyclic reactions, including a refinement as the meaning of Woodward and Hoffmann's "Violations. There are none!" Lastly, we comment on the burgeoning influence of computations on all fields of chemistry.

Quantum Tunneling Instability in Pericyclic Reactions

“…In these an oss transition state will provide the lower energy pathway, but with a compromised accuracy, as explained above. In contrast, 7 for example is a typical allowed [4 + 2] reaction occurring without crossing and with a closed-shell transition state. While very similar to the cis Ac cases, the trans At reactions do not pass through a system crossing due to the Möbius type of geometry; this is analogous to the conrotatory/disrotatory outcomes or supra/antarafacial reactivities within Woodward–Hoffmann rules . Being forbidden reactions, the oss cases tend to have higher barriers, but sometimes not enough to avoid the QTI, as shown below. Reaction 4 is the N -heterocyclic equivalent to 1B and therefore can be included in the family of aromatic products.…”

“…While very similar to the cis Ac cases, the trans At reactions do not pass through a system crossing due to the Möbius type of geometry; this is analogous to the conrotatory/disrotatory outcomes or supra/antarafacial reactivities within Woodward–Hoffmann rules . Being forbidden reactions, the oss cases tend to have higher barriers, but sometimes not enough to avoid the QTI, as shown below.…”

Application of Sublimation in the Synthesis and Crystal Growth of Organosulfones

“…The Woodward-Hoffmann (WH) rules state that stereochemical outcome of reactions from alkenes like 1 a with two terminal E-alkenes, should give 4 cis, as disrotatory ring closure is thermally allowed. [19] However, our theoretical modelling of the ring closure predicted that the electrocyclization would give 4 trans, which would be a case of where the WH rule does not apply, examples of which have been recently discussed by Houk et al [20] A slightly simplified triene Ph-(CH) 6 -SO 2 H (Figure 5) was used to model the cyclization reaction. The triene cyclization was found to take place via the ring closure reaction pictured in Figure 5; where the conformer, loosely described by C=CÀ C=C dihedrals of close to 54°, cyclizes in an exothermic reaction of À 89.5 kJ mol À 1 via a 63.8 kJ mol À 1 barrier to give 4 trans.…”