Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part XII. Total reactivity of isoquinoline

Abstract

All seven 1-(isoquinolyl)ethyl acetates have been prepared and their rates of elimination of acetic acid determined together with that of 1-phenylethyl acetate at temperatures between 351.8 and 426.3°. The results give the first measure of the quantitative electrophilic reactivity of the neutral isoquinoline molecule and the positional reactivity order is: 4 > (phenyl) > 5 = 7 > 8 > 6 > 3 > 1. Each position is less reactive than the corresponding position in naphthalene and more reactive than the corresponding position in pyridine. The electrophilic substituent constants are: –0.025(4); +0.07(5,7); +0.255(8); +0.31(6); +0.41(3); +0.51(1). The reactivity pattern shows that the formation of mainly 4-derivatives in electrophilic substitutions under neutral conditions is not anomalous as hitherto supposed, and the special mechanism invoked to account for this substitution pattern is no longer necessary. Isoquinoline is more reactive overall than quinoline; the benzenoid ring is much less reactive than in quinoline, but the pyridinoid ring is correspondingly much more reactive than in quinoline. The deactivating effect of the nitrogen in quinoline and isoquinoline is closely paralleled by the effects of substituents in hydrogen exchange of naphthalenes. The weak deactivation of the 5-position in quinoline, and the greater deactivation at the 1-relative to the 3-position in isoquinoline are shown to be due to bond fixation effects. As with pyridine and quinoline, Hückel π-electron densities are found to be the best theoretical indices of reactivity, and with an auxiliary inductive parameter of [graphic omitted] 0.018 they predict not only the observed reactivity order, but also the activation at the 4-position. The reactivity order is also predicted almost quantitatively by the difference between the localization energy at position j of i-methylenenaphthalene and that at position j of naphthalene.