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Neonicotinoids are a new class of insecticides whose potential side-effects on the human health and the environment have not yet been well investigated. Recently, the EU approved the Decision 2015/495 establishing a watch list of substances for Union-wide monitoring in the field of water policy, which includes five neonicotinoids: thiamethoxam (TMX), imidacloprid (ICP), clothianidin (CTD), thiacloprid (TCP) and acetamiprid (AMP). The photodegradation of neonicotinoids by monochromatic UV irradiation (254 nm) has been investigated. The photolysis rate followed pseudo-first order kinetics, with rate constant values depending on the nature of the specific compound, pH, and the presence or absence of radical scavengers. A method based on atrazine actinometry provided the following quantum yields at pH = 7: (77 ± 3) × 10−3, (43 ± 2) × 10−3, (48 ± 1) × 10−3, (17 ± 2) × 10−3 and (5 ± 2) × 10−3 mol E−1 for TMX, CTD, ICP, TCP and AMP, respectively. The effects of the presence of bicarbonate, nitrate, nitrite and chloride ions on the simultaneous photodegradation of nicotinoids were not very significant, and only the removal of AMP, the least photosensitive neonicotinoid, was negatively affected. However, the photodegradation of neonicotinoids was clearly inhibited by the addition of humic acids. The results obtained in experiments performed in the presence of hydrogen peroxide or persulfate indicated that while direct UV photolysis was the main degradation pathway for TMX and even for CTD, ICP and TCP, the contribution of indirect photolysis (radical pathway) was important in the case of pollutants with low values of quantum yield such as AMP. Although the presence of natural organic and inorganic matter in real water matrices decreased the degradation rate of the selected neonicotinoids, UV photolysis and the AOPs UV/H2O2 and UV/PS processes constitute efficient technologies for the removal of these pollutants from contaminated waters.

Imine and enamine bonds decorate the skeleton of numerous reagents, catalysts, and organic materials. However, it is difficult to isolate at will a single tautomer, as dynamic equilibria occur easily, even in the solid state, and are sensitive to electronic and steric effect, including π-conjugation and H-bonding. Here, using as model Schiff bases generated from salicylaldehydes and TRIS in a set of linear free energy relationships (LFER), we disclose how the formation of either imines or enamines can be controlled and provide a comprehensive framework that captures the structural underpinning of this prediction. This work highlights the potentiality of tailor-made designs en route to compounds with desirable functionality.

A one-pot procedure for preparing a series of chiral imines by direct condensation of D-glucamine with 2-hydroxyacetophenones is described. Under conventional acetylation an unexpected mixture of two different peracetylated molecules is obtained, one with an open enamine structure, and the other incorporating an N-acetyl-1,3-oxazolidine into the acyclic skeleton. Surprisingly, both molecules coexist within the crystal's unit cell, as inferred from single-crystal X-ray analysis of a 5-bromo-substituted aryl derivative. Moreover, the 1,3-oxazolidine ring exists as rotational conformers (E,Z) owing to the restricted rotation around the N-acetyl bond. The equilibrium involving imine and enamine structures has been assessed in detail, providing in addition linear free-energy relationships between the tautomerization constants (KT) and the electronic effect of the substituents.

Tautomerism plays a pivotal role in structural stabilization and reactivity. Herein we investigate in detail, aided by DFT simulations, the case of gossypol, a naturally occurring atropisomeric dialdehyde showing promising properties as a male contraceptive and an antineoplasic agent. Its toxicity linked to reactive aldehydo groups can be reduced through amino conjugation. The occurrence of either imino or enamino structures is puzzling indeed and a clear-cut rationale is missing yet. N-enamine-N-enamine structures are prevalent or exclusive tautomers for Schiff bases from gossypol, while their corresponding hydrazones only possess N-imine-N-imine structures both in solution and the solid state. The modification of interactions between the lone pairs on the nitrogen atoms by altering the steric hindrance of the non-iminic nitrogen can favor enamine tautomers. This assumption has now been confirmed and, in the solid state, hydrazones from N-aminopiperidine and their cis-2,6-dimethylderivative present bis-imine and bis-enamine structures, respectively. In solution, these compounds exist in equilibrium between both structures. The tautomerization mechanism, analysis of axial chirality and aromaticity in such H-bonded pseudorings are discussed as well.