Reactivity of Small and Large Rings

Chapter 3.5 attempts to give an overview of the reactivity of small or large ring systems treated individually in CHEC and CHEC-II. The great diversity of these systems presents a serious problem of organization. In structuring this chapter on reactivity, the nature of the reaction and the distance of the site of attack from the heteroatoms were used. 

Cations of a different kind may be derived from heterocyclics by removing a leaving group with its bonding electrons, e.g. a halide ion from an V-halo moiety. Such cations (nitrenium ions) were also assumed to be reactive intermediates in reactions of other /V-halo heterocycles. 

Anions of small heterocycles are little known. They seem to be involved in some elimination reactions of oxetan-2-ones (80JA3620). Anions of large heterocycles often resemble their acyclic counterparts. However, anion formation can adjust the number of electrons in suitable systems so as to make a system conform to the Hiickel rule, and render it aromatic if flat geometry can be attained. Anion formation in selected large heterocycles can also initiate transannular reactions (see also Section 3.5.6 below). 

2 THERMAL AND PHOTOCHEMICAL REACTIONS, NOT FORMALLY INVOLVING OTHER SPECIES 

Extrusion (or elimination) of sulfur from thiiranes and thiirenes is a facile process. Virtually all thiiranes and thiirenes, as well as their oxides and dioxides, undergo thermal extrusion of the sulfur moiety with increasing facility according to the trend S SO S02. The thermolytic desulfurization mechanism is more complex than a simple cheletropic extrusion (85TL2789). 

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Reactivity of Small and Large Rings Table 3 pA-, Values of tetraazacycloalkanes (14) (in 0.2 M NaC104). 

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