Theoretical calculations of reactivity

There have been many molecular orbital calculations performed on the azoles, the majority of them directed toward determining factors other than their electrophilic reactivities. In view of the wide range of r+ values for a given position, with activation or deactivation depending upon the reaction studied, it is clear that calculations are unlikely to be able to provide more than the positional order of reactivities since they are unable to take account of the structure of a given transition state. The positional orders can, however, be much more simply obtained by application of a few elementary principles, as follows. 

The positional reactivity order in thiazole has been calculated to be 5 

Finally, it densities have been calculated for pyrazole by the CNDO/2 method [70JCS(B)1692], the values being different again from those given above, but the correct order, namely. 4 5 3, is predicted. Methyl at the 3-position raised the density at the 4-position less than did methyl at position 5, consistent with the expected effect of bond fixation, However, methyl at positions 3 or 5 was predicted to lower the density at the corresponding meta-position, which is anomalous. For the pyrazole cation, the tt densities predicted a lower reactivity, the positional order being 4 5 = 3. 

Polycyclic Heteroaromatics Containing a Five-Membered Ring 

This group constitutes a virtually infinite class of heteroaromatic ring systems. However, relatively few of the parent compounds have yet been made, and fewer still have had their quantitative (or indeed qualitative) reactivities measured. The compounds described in this chapter are subdivided as follows Section 2, compounds with one five- and one six-membered ring Section 3, compounds with one five and two six-mem-bered rings Section 4, compounds with two five- and one six-membered rings Section 5, compounds with two five-membered rings and Section 6, compounds with three or more five-membered rings. 

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