Tautomeric and E‐Z equilibria of the herbicide clethodim in water and organic solvents: A nuclear magnetic resonance and theoretical study

Abstract

The herbicide clethodim can exist as different tautomeric forms and also as E‐Z isomers. Previous works have made some approach to the matter of tautomerization and have always stated that the E form was the only one stable. In this work, the investigation on the tautomeric equilibrium of clethodim is carried out in solution by means of proton nuclear magnetic resonance (1H NMR), carbon‐13 nuclear magnetic resonance (13C NMR), and correlation spectroscopy (COSY) experiments in four solvents (CDCl3, acetone‐d6, dimethylsulfoxide (DMSO)‐d6, and D2O). Major tautomeric species have been identified, and equilibrium constants have been calculated from the integration of hydrogens of the chloroallyl radical of clethodim. E‐ketoenolimine is the only detectable tautomer in CDCl3. Mixtures of E‐ketoenolimine and E‐diketoenamine tautomers are observed in acetone‐d6 and DMSO‐d6 (in ratios of 8:92 and 53:47, respectively). In water, the most abundant neutral tautomers are E‐ketoenolimine, E‐diketoenamine, and Z‐ketoenolimine (with a tautomeric ratio of 66:20:14). Identification of the E and Z tautomers is reported. Thermodynamic parameters (obtained by analysis of the effect of temperature) and theoretical calculations (energies and chemical shifts) support the assignations and tautomeric ratios when considering specific clethodim‐solvent hydrogen bonds. In the case of water, calculations suggest double bonded complexes that would be the cause of stability of the tautomers. These new observations on the structure of clethodim in solution open a new series of questions on its absorption mechanism, and they are also a tool for further chemical modification of this herbicide.

Comment on “Study of enantioselective metolachlor adsorption by activated carbons” by A. Gomis-Berenguer, I. Laidin, S. Rononcial and B. Cagnon, RSC Advances, 2020, 10, 40321

“…An enantiomer interaction in solution could be conceivable, and we have considered possible configurations (by means of theoretical calculations) in order to find some strong interaction between R and S metolachlor, even including water molecules (as we have done in similar systems 8 ), but we could not find any specific low-energy array that justify such a hypothesis.…”

Assessing the Effect of Organic Amendments on the Degradation of Profoxydim in Paddy Soils: Kinetic Modeling and Identification of Degradation Products

“…Consequently, the E isomer is marketed in all herbicides. However, the two forms interconvert under different environments (Sandín-España et al, 2013;Caputo et al, 2020). Isomerization of the E-isomer into the Z-isomer has been reported to occur quite readily in dark, aqueous environments, as well as under light and in neutral or acidic environments (Roberts et al, 1998;Sevilla-Morán et al, 2008).…”