The effect of electronic factors on the reactivity of heteroaromatic N-oxides

Abstract

On the basis of an analysis of the reasons for the multiplicity of constants for substituents it has been concluded that they are variables, the absolute value and sign of which depend on the nature of the compounds being considered (in particular other substituents), the partner with which reaction occurs, and the medium (the aggregate state of participants of the interaction). The use of modifications of the Hammett equation is caused by the impossibility in principle of only varying the values and sign of to describe universally linear functions of the dependence of rate (or equilibrium) constants of chemical processes from the algebraic sum of the electronic, steric, and other factors, which differ in their nature.At the present time empirical equations of the Hammett and Taft type (log(k/k 0 ) = ) are widely used in organic chemistry in spite of the fact that the multiplicity of constants requires greater care in choosing them for making correlations, and the use for interpreting the structure of a transition state interferes with certain other resistant concepts [1-6]. Thus the Hammett equation is used for assessing the structure of a transition state (according to the size of ) among similar compounds by the variation of substituents and consequently by the change in reactivity. It is therefore assumed that is a constant (but is dependent on the nature of the second reactant and the reaction conditions), i.e. the nature ( ) of a substituent does not affect the structure of the transition state. However according to the Hammond postulate [5] in rapid reactions the transition state is more like the reactants, but in slow reactions it is more like the reaction products, i.e. on changing the reaction rate the structure of the transition state must also be changed.In the monograph [4] V. A. Palm notes that the problem of constructing a precise scale even for induction constants of substituents of the type of functional groups or containing functional groups has no solution. The assumption of even O. A. Reutov, A. L. Kurts, and K. P. Butin [5, Vol. 1, p. 288], that " and are not independent parameters, as suggested, but are linked to one another, while a parameter linking and is reduced in their product", does not only not clarify the situation, but also remains as an additional problem as to the nature of this parameter.

Identification of N‐ or O‐Alkylation of Aromatic Nitrogen Heterocycles and N‐Oxides Using 1H–15N HMBC NMR Spectroscopy

“…Moisture‐stable compounds were isolated – in some instances this was achieved by carrying out separate reactions on significantly larger scale than that used for the 1 H‐ 15 N HMBC NMR characterization experiments . Relatively low conversions and yields were observed in some reactions of MeI due to the weak Lewis basicities of some of the diazines or N ‐oxides employed (Scheme ) . Table shows the δ N values recorded for starting materials 1–6 , for derived N‐alkylation adducts 7 – 12 and for O‐alkylation adducts 13 – 17 using 1 H‐ 15 N HMBC NMR spectroscopy.…”

The molecular structure of 4-methylpyridine-N-oxide: Gas-phase electron diffraction and quantum chemical calculations

“…The biochemical activity of N-oxides is usually considered to be the result of complexation with the metalloporphyrins in living organisms [10e12]. The reactivity of Noxides may vary due to different substituents in the ring [13]. The variation of the substituents gives wide possibilities to chemical modifications of N-oxides and allows to influence their complexing properties, and, as the result, their biological activity.…”

Molecular structure and electron distribution of 4-nitropyridine N-oxide: Experimental and theoretical study of substituent effects

“…According to numerous studies, the biochemical activity of N-oxides is due to the complexation with the metalloporphyrins in living organisms [9e11]. Obviously, the reactivity of N-oxides may vary due to different substituents in the ring [12]. The variation of the substituents gives wide possibilities to chemical modifications of Noxides and allows to change their complexing properties, and, as the result, to influence the biological activity.…”