Pericyclic Reactions of Mesomeric Betaines

The structural requirements of the mesomeric betaines described in Section III endow these molecules with reactive -electron systems whose orbital symmetries are suitable for participation in a variety of pericyclic reactions. In particular, many betaines undergo 1,3-dipolar cycloaddition reactions giving stable adducts. Since these reactions are moderately exothermic, the transition state can be expected to occur early in the reaction and the magnitude of the frontier orbital interactions, as 1,3-dipole and 1,3-dipolarophile approach, can be expected to influence the energy of the transition state—and therefore the reaction rate and the structure of the product. This is the essence of frontier molecular orbital (EMO) theory, several accounts of which have been published. 16.317 application of the FMO method to the pericyclic reactions of mesomeric betaines has met with considerable success. The following section describes how the reactivity, electroselectivity, and regioselectivity of these molecules have been rationalized. 

Fleming, Frontier Orbitals and Organic Chemical Reactions. Wiley, New York, 1976. K. Fukui, Acc. Chem. Re.s. 4, 57 (1971). 

A simplified form of Eq. (12), which has been used to predict rates of cycloaddition, is Eq. (13) (K is a constant here).  

The structural requirements of the mesomeric betaines described in Section III endow these molecules with reactive -electron systems whose orbital symmetries are suitable for participation in a variety of pericyclic reactions. In particular, many betaines undergo 1,3-dipolar cycloaddition reactions giving stable adducts. Since these reactions are moderately exothermic, the transition state can be expected to occur early in the reaction and the magnitude of the frontier orbital interactions, as 1,3-dipole and 

3-dipolarophile approach, can be expected to influence the energy of the transition state—and therefore the reaction rate and the structure of the product. This is the essence of frontier molecular orbital (FMO) theory, several accounts of which have been published. The application of the FMO method to the pericyclic reactions of mesomeric betaines has met with considerable success. The following section describes how the reactivity, electroselectivity, and regioselectivity of these molecules have been rationalized. 

Fleming, Frontier Orbitals and Organic Chemical Reactions. Wiley, New York, 1976. 

Equation (12) is the mathematical basis of the FMO method and describes the energy change due to frontier orbital interactions when two molecules M and N interact (M + N - MN). AE mo is a measure of transition-state stabilization (or destabilization). The first term describes the interaction between the HOMO of molecule M and the LUMO of N whereas the second term describes the alternative interaction.  

---