Macromol. Theory Simul. 3/2007

Abstract

Cover: The picture shows that diprotonation of carbonyl compounds does not occur in triflic acid. The triflic acid mediated reaction of polyhydroxyalkylation of aromatic hydrocarbons involves only monoprotonated species. The reactivity of carbonyl molecules is governed by electronic effects of the groups linked directly to the carbonyl. Strong donor and strong electron withdrawing groups reduce the reactivity of carbonyl group as electrophile; in the first case the protonation leads to the formation of excessively stable cation, in the second one monoprotonation is impossible thermodynamically.

Superelectrophilic Activation of N‐Substituted Isatins: Implications for Polymer Synthesis, a Theoretical Study

“…[17] The model selection was based on its ability to reproduce the experimentally determined pK a values of different acids, since exact pK a determination implies accurate calculation of the Gibbs free energies (DG) of solvated ionic species. This model is described in detail in Reference [9]. The average error in pK a determination was about 1 pK a unit, corresponding to about 1.4 kcal Á mol…”

“…Moreover, the protonation energies are barely affected by the nature of the N-substituent. The relatively weak basicity of the ketone carbonyl of isatin derivatives compared to acetophenone, [9] for example, is due to the -I effect of the adjacent amide carbonyl. In fact, the basicity of ketone carbonyl in isatin is close to that of 2,2,2-trifluoracetophenone, [9] where the carbonyl group is also affected by an electron-withdrawing trifluormethyl group.…”

“…The relatively weak basicity of the ketone carbonyl of isatin derivatives compared to acetophenone, [9] for example, is due to the -I effect of the adjacent amide carbonyl. In fact, the basicity of ketone carbonyl in isatin is close to that of 2,2,2-trifluoracetophenone, [9] where the carbonyl group is also affected by an electron-withdrawing trifluormethyl group. It has been shown [6,7] that 2,2,2-trifluoracetophenone reacts smoothly with aromatics to give high molecular weight polymers and that monoprotonated species are the principal reactive intermediate.…”

“…On the other hand, isatin can be considered to be a carbonyl-molecule-bearing electron-withdrawing group. It has been shown [9] that, in this case, monoprotonated intermediates are the reactive species in hydroxyalkylation reactions in TFSA. Therefore, the aim of this paper is to study the protonation process of isatin in superacid media and to propose reactive species participating in the hydroxyalkylation of isatin to get deeper insight into the mechanism of its polymerization.…”

“…[8] In the case of triflic acid (TFSA)-catalyzed polyhydroxyalkylation of aldehydes and ketones, the increase of reactivity observed for diprotonated species is not sufficient to compensate for the high Gibbs energy needed to form these species and monocationic species are the principal reaction intermediates. [9] On the other hand, the existence of diprotonated carbonyl molecules in superacids has been proven experimentally [10] when an alternative site for second protonation was available (heteroatom or electron-rich double bond). Thus, the TFSAcatalyzed condensation of 3-pyridinecarboxaldehyde with deactivated aromatic compounds, in which the dication was observed by low-temperature NMR, have been The stability and reactivity of mono-and multi-protonatred N-substituted isatin derivatives were studied at PBE0/aug-cc-pvtz//PBE0/6-31þG ÃÃ level of theory in triflic acid (TFSA) solution.…”