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Applications of the Hammett Equation to Heterocyclic Compounds

1976; Elsevier BV; Linguagem: Inglês

10.1016/s0065-2725(08)60851-x

1557-8429

Piotr Tomasik, C. D. Johnson,

Organic Chemistry Cycloaddition Reactions

This chapter describes the applicability of Hammett-type parameters that may be tested for heterocyclic compounds in a number of different ways. Work on specific heterocyclic systems may be grouped under two headings: The ring heteroatom as a substituent and reactivity of heteroatoms, as affected by other substituents. Under the ring heteroatom as a substituent, the heteroatom alone or the whole heterocyclic ring may be treated as a substituent. The chapter examines the additivity of effects. This is a more meaningful way of looking at the phenomena of “transmission through heterocyclic rings,” a terminology, which has been much, used previously, but is difficult to define rigorously or express quantitatively. Since heteroatoms have often an extremely large effect on reactivity with concomitant propensity for solvent interactions, this is also a good testing ground for the evaluation of solvent effects on σ values. Under reactivity of heteroatoms, as affected by other substituents, the heteroatom alone may participate in reaction. For example, in nitrogen protonation or quaternization, or the whole ring may be involved, as when heteroatom protonation leads to subsequent nucleophilic attack and ring cleavage.