In the 1960s and into the 21<sup>st</sup> century, Rolf Husigen established the field of 1,3-dipolar cycloaddition reactions and made many seminal discoveries. In 1980, Gandolfi and Toma reported [8+4], or (8+3) cycloadducts formed from diphenylnitrile imine and 8-substituted-8-azaheptafulvenes. This apparently violates the orbital symmetry rules developed by Woodward and Hoffmann in 1965. In this talk, I will discuss how quantum mechanical studies show how this reaction occurs. Our study reveals that this reaction goes through a special type of ambimodal transition state that connects both [4+2] product and an entropic intermediate. This entropic intermediate then forms the [8+4] product in a highly asynchronous manner so that avoids the high barrier caused by the violation of the orbital symmetry rule. Because of the zwitterionic nature of this intermediate,  the product ratio is strongly influenced by solvent polarity.

A [4+2]/[8+4] ambimodal dipolar cycloaddition: Dynamics and solvent effects

ACS Spring 2022

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