Orbital Energy Control of Cycloaddition Reactivity

A reactivity model for concerted cycloaddition reactions is presented which allows a systematization of substituent effects. The treatment is based on the frontier electron theory of Fukui. The consideration of the energy separations of HOMOs (Highest Occupied Molecular Orbitals) and LUMOs (Lowest Unoccupied Molecular Orbitals) leads to three reactivity types in these cycloadditions. For the Diels—Alder addition and 1 ,3-dipolar cycloadditions the occurrence of: 1. HOMO (Diene or Dipole)LUMO (olefin) controlled reactions, 2. HOMO (Diene or Dipole)LUMO (olefin) and HOMO (olefin)— LUMO (Diene or Dipole) controlled additions and 3. LUMO (Diene or Dipole)—HOMO (olefin) controlled cycloadditions is demonstrated. Each type exhibits a characteristic behaviour towards substituents in both reaction partners. A semiquantitative treatment of substituent effects together with an experimental verification is given. INTRODUCTION During the last two decades thermal cycloaddition reactions have been of interest to many chemists. The synthetic scope and the mechanistic implications both played an important part in the development1'2'3 The mechanisms by which cycloadducts arc formed are still under investigation. Experimental criteria have been developed to test whether a cycloaddition takes place by a concerted formation of the new a-bonds or whether an intermediate has to be invoked. These criteria are the stereospecificity in the formation of products, i.e. the retention of configuration of the reactants, the interception of intermediates, the correlation of structure and rate, and the dependence of reaction rates on solvent polarity. The gathered experimental information received a theoretical foundation through the rules of conservation of orbital symmetry by Woodward and Hoffmann (W—H rules)4. Most of the experimental results are in accordance with this principle and many new experiments were stimulated. Another theoretical approach relates the mechanism with the occurrence of aromatic or antiaromatic transition states5. It leads essentially to the same conclusions. It is fallacious to think that there is no problem left as far as the mechanism is concerned. Is the retention of stereochemistry, the impossibility of intercepting intermediates or the independence of reaction rates from solvent polarity a sufficient proof for a concerted mechanism? This refers to the difficulty of defining an intermediate on a reaction coordinate.