D. A. Evans. B. Breit. B. Dunn Electrocyclic Reactions: Dewar-Zimmerman Chem 206 The Dewar- Zimmerman analysis is based on identifying transition states Connect as aromatic or antiaromatic. We will not go into the theory behind why Orbitals this treatment works, but it will give the same predictions as FMO or Orbital Symmetry treatments, and is fundamentally equivalent to them Using the Dewar-Zimmerman model Disrotatory y Closure Closure a Choose a basis set of 2p atomic orbitals for all atoms involved(1s for I Assign phases to the orbitals. Any phases will suffice. It is not important to identify this basis set with any molecular orbital I Connect the orbitals that interact in the starting material, before the eaction begins zero phase inversions One Phase Inversion Huckel Topology Mobius Topology L Allow the reaction to proceed according to the geometry 4 electrons in system 4 electrons in system postulated. Connect those lobes that begin to interact that were not Antiaromatic and Aromatic and interacting in the starting materials Allowed a Count the number of phase inversions that occur as the electrons Note that I can change the phase of an abitrary orbital and the analysis flow around the circuit. Note that a phase inversion within an orbital is is still valid! not counted Connect Orbitals a Based on the phase inversions, identify the topology of the system Odd number of phase inversi Mobius topology Even number of phase invers Disrotatory Closure Assign the transition state as aromatic or antiaromatic, based on the number of electrons present Aromat Antiaromatic 4q Two Phase Inversions Three phase Inversions If the transition state is aromatic then the reaction will be allowed thermally. If the transition state is antiaromatic, then the reaction will Huckel Topology Mobius Topology be allowed photochemically electrons in system 4 electrons in syster Antiaromatic and Aromatic and ForbiddenD. A. Evans, B. Breit, T. B. Dunn Electrocyclic Reactions: Dewar-Zimmerman Chem 206 Connect Orbitals Disrotatory Closure Conrotatory Closure Zero Phase Inversions \Hückel Topology 4 electrons in system \ Antiaromatic and Forbidden One Phase Inversion \Möbius Topology 4 electrons in system \ Aromatic and Allowed Note that I can change the phase of an abitrary orbital and the analysis is still valid! Connect Orbitals Disrotatory Closure Conrotatory Closure Two Phase Inversions \Hückel Topology 4 electrons in system \ Antiaromatic and Forbidden Three Phase Inversions \Möbius Topology 4 electrons in system \ Aromatic and Allowed The Dewar-Zimmerman analysis is based on identifying transition states as aromatic or antiaromatic. We will not go into the theory behind why this treatment works, but it will give the same predictions as FMO or Orbital Symmetry treatments, and is fundamentally equivalent to them. Using the Dewar-Zimmerman model: ■ Choose a basis set of 2p atomic orbitals for all atoms involved (1s for hydrogen atoms). ■ Assign phases to the orbitals. Any phases will suffice. It is not important to identify this basis set with any molecular orbital. ■ Connect the orbitals that interact in the starting material, before the reaction begins. ■ Allow the reaction to proceed according to the geometry postulated. Connect those lobes that begin to interact that were not interacting in the starting materials. ■ Count the number of phase inversions that occur as the electrons flow around the circuit. Note that a phase inversion within an orbital is not counted. ■ Based on the phase inversions, identify the topology of the system. Odd number of phase inversions: Möbius topology Even number of phase inversions: Hückel topology ■ Assign the transition state as aromatic or antiaromatic, based on the number of electrons present. System Aromatic Antiaromatic Hückel 4q + 2 4q Möbius 4q 4q + 2 ■ If the transition state is aromatic, then the reaction will be allowed thermally. If the transition state is antiaromatic, then the reaction will be allowed photochemically