Resumen de Racemization of Isobornyl Chloride via Carbocations:: A Nonclassical Look at a Classic Mechanism
Henry S. S. Rzepa, Charlott S. M. Allan
Our understanding of carbonium ions as intermediates in chemical reaction mechanisms derives from the early work of Julius Stieglitz and the more famous Hans Meerwein, the latter studying the racemization of isobornyl chloride when treated with Lewis acids. This review analyzes how key mechanistic concepts for this reaction evolved and gives the pedagogy a modern slant based on results obtained from accurate quantum mechanical calculations. Thus, originally thought of as involving ionization of the C−Cl bond to form a carbocation that then undergoes a transannular hydride shift, an analysis of modern calculations (using AIM- and ELF-based quantum electronic topology) reveals that a more appropriate description of the bonding at the transition state for this transfer is a nonclassical 3-center-2-electron interaction. We see how other concepts important to organic chemistry such as symmetric intermediates and stereoelectronic control emerge along the way.
