Unravelling the Mysteries of the [3+2] Cycloaddition Reactions

After Huisgen's and Firestone's mechanistic proposals made in the 1960s based on experiments, several theories were proposed during the last century to explain [3+2] cycloaddition (32CA) reactions, most of them still prevailing today. Recent molecular electron density theory (MEDT) studies of 32CA reactions involving representative three-atom components have allowed characterising at least four different electronic structures, which experience a dissimilar chemical reactivity. In this review, the four simplest reactivity models are presented, providing a modern rationalisation of 32CA reactions based on MEDT.