Electrophilic aromatic reactivities via pyrolysis of esters. Part 21. σ+ Values for thiazole: the high polarisability of thiazole, and the effect of hydrogen bonding on the reactivity of N-containing heterocycles

Abstract

The rates of gas-phase elimination of acetic acid from 1-arylethyl acetates (aryl = thiazol-2-yl, thiazol-4-yl, thiazol-5-yl, or phenyl) have been measured between 633.2 and 698.5 K. The relative rates of pyrolysis at 650 K are: thiazol-2-yl, 0.271; thiazol-4-yl, 0.491; and thiazol-5-yl, 1.104; coupled with the previously determined ρ factor for the reaction (–0.61 at 650 K), this leads to the corresponding σ⁺ values 0.93, 0.505, and –0.07. The positional reactivity order, and the reactivity relative to benzene, are correctly predicted by π-electron density calcuations. The reactivity of each position is substantially less than in solvolysis of the corresponding 1-aryl-1-chloroethanes, in contrast to the reactivity of thiophene which is closely similar in both pyrolysis and solvolysis reactions. Thiazole is thus much more polarisable than thiophene, and hence particularly susceptible to demands for resonance stabilisation of the respective transition states, the demand being less in the pyrolysis. The difference between the reactivities of the 2-position of thiazole and the corresponding α-position of thiophene (1.72 sigma units) is substantially greater than the difference between the reactivities of the 4-position of thiazole and the β-position of thiophene (0.885 sigma units). This reflects the high 2,3-vs. 3,4-bond orders, and consequent variation in the deactivation by the (ortho) nitrogen atom. Nitrogen deactivates the 4-position more than it deactivates the 5-position, as expected. By contrast, in solvolysis the 5-position is deactivated more than the 4-position. This latter anomaly is attributed to hydrogen bonding, which is attenuated when the probe group is adjacent to the ring nitrogen atom. This explanation also accounts for the anomalously low reactivity of the 3- and 4-position of pyridine in determination of the electrophilic reactivity via solvolysis, as compared with gas-phase data, whereas the reactivity of the 2-position is the same in both reactions.