Cycloaromatization reactions: the testing ground for theory and experiment

Abstract

Publisher Summary This chapter describes the testing ground for the theory and experiment for cycloaromatization reactions. An interesting breakdown of the tendency for the conservation of chemical bonds occurs in the so-called cycloaromatization reactions. In these reactions, a σ-bond is formed at the expense of two π-bonds, and thus, the process leads to a net loss of one chemical bond that is intrinsically unfavorable thermodynamically. The role of electron repulsion relative to bond breaking and antiaromaticity effects on a quantitative basis using Natural Bond Orbital (NBO) analysis has been recently analyzed. In the case of the Bergman cyclization and the C1–C5 cyclization of enediynes, both the activation barrier for cyclization as well as the thermodynamics of the reaction became more favorable upon one-electron reduction compared to the thermal counterparts. Even though some of the general factors controlling the Bergman cyclization are applicable to newly discovered cycloaromatization reactions, new phenomena such as communication of orthogonal orbitals in radical-anionic, radical-cationic and dianionic cycloaromatizaton, and cyclorearomatization will continue to stimulate future development of this interesting field.