Supramolecular Control of Reactivity toward Hydrolysis of 7-Diethylaminocoumarin Schiff Bases by Cucurbit[7]uril Encapsulation

Abstract

A series of aromatic Schiff bases, featuring 7-diethylamino-coumarin and with five different substituents at an adjacent phenyl ring, were synthesized and characterized. With the aim of assessing the stability of these dyes in acidic medium, their hydrolysis reactions were kinetically studied in the absence and presence of the macrocycle cucurbit[7]uril (CB[7]). Our results are consistent with a model containing three different forms of substrates (un-, mono-, and diprotonated) and three parallel reaction pathways. The p K a values and the rate constants were estimated and discussed in terms of the presence of a hydroxyl group at the ortho position and electron-releasing groups on the phenyl ring of the dyes. The kinetic study in the presence of CB[7] led to two different behaviors. Promotion of the reaction by CB[7] was observed for the hydrolysis of the Schiff bases containing only one coordination site toward the macrocycle. Conversely, an inhibitor effect was observed for the hydrolysis of a Schiff base with two coordination sites toward CB[7]. The latter effect could be explained with a model as a function of a prototropic tautomeric equilibrium and the formation of a 2:1 host/guest complex, which prevents the attack of water. Therefore, the kinetic results demonstrated a supramolecular control of the macrocycle toward the reactivity and stability of 7-diethylaminocoumarin Schiff bases in acidic medium.

Selective Photooxidation of Benzyl Alcohol to Benzaldehyde via H-Abstraction by Bi2.15WO6 under Alkaline Conditions

“…Actually, the reaction between aldehydes and amines produces Schiff bases (or imine) in organic solvents, and the Schiff base of N , N ′-dibenzylideneethylenediamine can be readily formed by condensation of benzaldehyde and EDA in ethanol. , The shift of CO of aldehyde at 1698 cm –1 to the characteristic CN band at 1645 cm –1 in the presence of excess EDA in acetonitrile confirms the condensation between benzaldehyde and EDA (Figure S10). Actually, the imine group (>CN−) in the Schiff base can be readily hydrolyzed back to aldehyde and amine in water. , For instance, Soliman et al reported a high yield of 85% Schiff base in ethanol, but a low yield of 30% in water, indicating instability of the imine group (>CN−) in water. After the reaction between benzaldehyde and EDA in water, CCl 4 was used to extract the produced Schiff base in the reaction system, and Figure S11 shows the characteristic bands of CN at 1648 cm –1 and the CO of benzaldehyde at 1705 cm –1 in organic solvents, again demonstrating the presence of Schiff base and benzaldehyde in an aqueous system.…”

“…We, therefore, deduce that EDA reacts with a fraction of benzaldehyde to form the Schiff base in an aqueous solution, thus protecting it from overoxidation to benzoic acid. To obtain more benzaldehyde after reaction, the pH was also adjusted to acidic conditions after reaction in that the cleavage of imine bonds in Schiff bases is faster under acidic conditions than that under neutral conditions. ,, In this study, we adjusted the reacted solution to weak acidic for further hydrolysis of the Schiff base, and then the concentration of benzaldehyde was measured by HPLC. It is interesting to notice that a negligible difference can be observed in the quenching experiments when EDA was added in the reaction system (Figure S12), indicating that the oxidizing species in the oxidation process are not affected by EDA.…”

“…55 Actually, the imine group (>C�N−) in the Schiff base can be readily hydrolyzed back to aldehyde and amine in water. 56,57 For instance, Soliman et al 58 reported a high yield of 85% Schiff base in ethanol, but a low yield of 30% in water, indicating instability of the imine group (>C�N−) in water. After the reaction between benzaldehyde and EDA in water, CCl 4 was used to extract the produced Schiff base in the reaction system, and Figure S11 shows the characteristic bands of C�N at 1648 cm −1 and the C�O of benzaldehyde at 1705 cm −1 in organic solvents, 55 again demonstrating the presence of Schiff base and benzaldehyde in an aqueous system.…”

Changes in Protonation Sites of 3-Styryl Derivatives of 7-(dialkylamino)-aza-coumarin Dyes Induced by Cucurbit[7]uril

“…This macrocycle has been used in different studies due to its structural dimensions and its high affinity for electron-poor sites ( Assaf and Nau, 2015 ; Macartney, 2018 ). In this context, dialkylamino substituted coumarins or other dialkylamino substituted chromophores have played a leading role in several studies in the presence of CB7, forming stable inclusion complexes on the side of the dialkylamino group in its protonated or deprotonated form ( Barooah et al, 2012 , 2014 ; Manna and Chakravorti, 2015 ; Aliaga et al, 2017 ; Basílio et al, 2017 ; Jana et al, 2019 ; Paudics et al, 2020 ; Alcázar et al, 2021 ). On the other hand, the pyridinium group has also shown a remarkable affinity for CB7 ( Manna and Chakravorti, 2015 ; Jana et al, 2019 ; Paudics et al, 2020 ).…”

Effect of Na+ and K+ on the cucurbituril-mediated hydrolysis of a phenyl acetate

“…17 The catalytic activity of neat cucurbiturils can be rationalized by the excess electronic density on the two portals that accumulate hydroxonium ions around the reaction centre. 18 These macrocycles promote the hydrolytic cleavage of amides, 19 benzoyl chlorides, 20 esters, 21 ethers, 18 triazenes, 22 oximes, 19 and Schiff bases, 23 as well as the formation of hydrazones. 24 As a model reaction we choose the acid hydrolysis of esters (Scheme 1).…”