The role of carboxylic acid impurity in the mechanism of the formation of aldimines in aprotic solvents

Bifunctional Primary Amino‐thiourea Asymmetric Catalysis: The Imine‐Iminium Ion Mechanism in the Michael Addition of Nitromethane to Enone

“…This process corresponds to TS1 in Figure 2, and the respective free energy barrier is 42.0 kcal mol −1 . This result was expected because previous studies have shown that the bimolecular mechanism for the formation of carbinolamine is kinetically unfeasible [23,35] …”

“…Direct elimination of hydroxide ion from carbinolamine intermediate should be difficult because there is not enough solvation to stabilize this high energy ion [33,34] . On the other hand, acids present in the medium, even as an impurity, can work as an efficient catalyst, leading to the elimination of water instead of hydroxide ion [35,36] . As an example, in the case of primary amine reaction, acid‐catalyzed carbinolamine decomposition leads directly to an imine and water products [35] .…”

“…[33,34] On the other hand, acids present in the medium, even as an impurity, can work as an efficient catalyst, leading to the elimination of water instead of hydroxide ion. [35,36] As an example, in the case of primary amine reaction, acid-catalyzed carbinolamine decomposition leads directly to an imine and water products. [35] In the next step, the imine could take the proton from nitromethane, forming the ion pair that goes to the addition product.…”

“…[35,36] As an example, in the case of primary amine reaction, acid-catalyzed carbinolamine decomposition leads directly to an imine and water products. [35] In the next step, the imine could take the proton from nitromethane, forming the ion pair that goes to the addition product. Such a possibility was explored in this work.…”

Nucleophilic catalysis ofp-substituted aniline derivatives in acylhydrazone formation and exchange

“…condensation of aldehyde 1 with anilines) no transition structures describing the direct nucleophilic addition to the aldehyde carbonyl groupeither in its protonated or neutral formscould be located. However, assisting the nucleophilic attack with protic molecules acting as proton shuttles, such as water, the orthonitrophenol moiety of aldehyde 1 or a benzoic acid derivative generated from or present as an impurity of aldehyde 1 not detected by 1 H NMR (labelled as 1′), [38][39][40] did allow locating feasible transition structures for both the nucleophilic addition (ts1 a-d) and water elimination (ts2 a-d) elementary steps (see the ESI †). Hence, catalysis by the benzoic acid derivative 1′ yielded the lowest activation barriers (ΔG ‡ ) for these processes and, more importantly, suggested that water elimination is the rate-limiting step to yield imines 5a-d in agreement with previous reports 17,18 and reproduced the experimental reactivity trends: 4).…”