Contemporary problems in the chemistry of 1,3-dipolar reagents: Definition and orbital symmetry of Huisgen 1,3-dipoles

Abstract

An analysis was carried out of the electron and spatial structure of 1,3-dipolar reagents. Their reactivity was shown to be strongly dependent on the nature of atoms of dipolar triad, the nature of substituents, and production conditions. For example, nitrile imines vary in their structure from a classical Huisgen 1,3-dipole to arylazocarbene or a distinct allene structure, thus providing an illustration of orbital isomerism. It was concluded that octet-stabilized Huisgen 1,3-dipoles a +=b−c − and b +=a−c − are 4π — electron mesoionic triads with an open electron shell that are orbital isomers of single-atom α-unsaturated sexstenes (heteroenyl or vinylcarbenes, nitrenes, etc., of general type a − b = c). Octet-nonstabilized Huisgen 1,3-dipoles a +-b-c − are zwitterionic compounds that are orbital isomers of 1,3-diradicals a •-b-c • and a •-b=c •. In the literature, the terms “1,3-dipolar reagent” or “1,3-dipolar system” often denote different reagents, including compounds with closed electron shells, whereas the term “1,3-dipolar cycloaddition” often designates different processes, not including pericyclic.