Electrophilic aromatic substitution. Part 26. The effect of the cyclopropyl substituent in aromatic detritiation

Abstract

The effects of the ortho- and para-cyclopropyl and para-1-methylcyclopropyl substituents in acid-catalysed hydrogen exchange have been determined through detritiation of a range of disubstituted aromatics in various acetic acid–trifluoroacetic acid media at 70°. The p-cyclopropyl substituent gives f 13 900 and σ⁺–0.473, the latter being in excellent agreement with values obtained in other reactions. The o-cyclopropyl substituent is much less activating, f 1 630, and this confirms that the very high ortho: para ratios obtained in nitration (especially in acetic anhydride) are anomalous, as they are for certain other substituents. The value for log f_o/log f_p for cyclopropylbenzene in hydrogen exchange (0.77) is significantly lower than for other alkyl- and cycloalkyl-benzenes (each of which gives a value of –0.875 ± 0.01) implying that the bisected conformation of the o-cyclopropyl group is not quite achieved in the (normally) unhindered exchange reaction. The p-1-methylcyclopropyl substituent is more activating than the p-cyclopropyl substituent (f_p 39 200, σ⁺–0.525) just as p-t-butyl is more activating than p-isopropyl in trifluoroacetic acid media, and may be attributed to the same cause viz. the greater inductive and hyperconjugative effect of the methylated species. In these poorly solvating media, the greater importance of C–C hyperconjugation over C–H hyperconjugation is not masked by steric hindrance to solvation. All the cyclopropyl substituents are slightly more activating (relative to methyl) in media containing less trifluoroacetic acid, suggesting that the former are more polarisable. However, since this behaviour parallels, though to a much lesser extent, that of the methoxy-substituent (which is strongly hydrogen-bonded in trifluoroacetic acid), we cannot exclude the possibility that cyclopropyl is similarly, but slightly, hydrogen-bonded. Rate factors were therefore determined in media in which hydrogen bonding is insignificant even for methoxy. The rate versus acidity profile for exchange in a wide range of acetic acid–trifluoroacetic acid media has been determined and shows that at 70°, exchange in acetic acid is 10^8.2 times slower than in trifluoroacetic acid. Between acetic acid and trifluoromethan esulphonic acid, a reactivity range of 10²⁰ is therefore encompassed.