Iodosulfuron-Methyl-Based Herbicidal Ionic Liquids Comprising Alkyl Betainate Cation as Novel Active Ingredients with Reduced Environmental Impact and Excellent Efficacy

Abstract

A new family of bio-based herbicidal ionic liquids (HILs) has been synthesized starting from the renewable resource glycine betaine (a derivative of natural amino acids). After esterification, the obtained alkyl betainate bromides containing straight alkyl chains varying from ethyl to octadecyl were combined with a herbicidal anion from the sulfonylurea group (iodosulfuron-methyl). The melting points of the iodosulfuron-methyl-based salts were in a range from 51 to 99 °C, which allows their classification as ionic liquids (ILs). In addition, the new HILs exhibited good affinity for polar and semipolar organic solvents, such as DMSO, methanol, acetonitrile, acetone, and chloroform, while the presence of bulky organic cations reduced their solubility in water. The synthesized products turned out to be stable during storage at 25 °C for over 6 months; however, at 75 °C they underwent fast, progressive degradation and released volatile byproducts. The values of the logarithm of the octanol–water partition coefficient of ILs with alkyls longer than hexyl occurred in the “safe zone” (between 0 and 3); hence, the risk of their migration into groundwater after application or the possibility of their bioaccumulation in the environment is lower in comparison with the currently available commercial form (iodosulfuron-methyl sodium salt). Greenhouse studies confirmed a very high herbicidal efficacy for the obtained salts toward tested plants of oilseed rape, indicating that they may become an attractive replacement for the currently available sulfonylurea-based formulations.

Transformation of Iodosulfuron-Methyl into Ionic Liquids Enables Elimination of Additional Surfactants in Commercial Formulations of Sulfonylureas

“…Hexane, being a less polar solvent, failed to dissolve most of the tested compounds and only 8 demonstrated intermediate solubility. The differences in solubility in the least polar solvents (toluene, hexane) are consistent with available literature data and support the thesis that ionic liquids, due to the presence of a polar ionic bond, are often immiscible with solvents of low polarity index [ 7 , 45 ]. Among the ILs obtained, only one product containing cocotrimethylammonium cation ( 1 ) showed good solubility in water, while the other salts demonstrated very poor solubility.…”

“…Sulfonylureas are known as selective herbicides, and are extremely effective against abroad spectrum of weeds, e.g., ryegrass, oats, wild radish, fireweed, toad rush, legumes, and a number of other troublesome weeds [ 5 , 6 , 7 ]. Sulfonylureas are absorbed by the above-ground parts of plants (LIT), and their mechanism of action involves inhibition of the enzyme acetolactate synthase (ALS), which is involved in the biosynthesis of the branched-chain amino acids (BCAAs)—e.g., leucine, valine, and isoleucine [ 8 ].…”

“…This concept is based on pairing appropriate organic cation—with herbicidal anion. Herbicidal ionic liquids (HILs) have numerous advantages such as adjustable physicochemical properties e.g., phase transitions (i.e., melting point), viscosity, polarity, water solubility or mobility in soils and waters [ 7 , 20 , 26 ]. The presence of an ionic bond in their structure reduces the volatility which positively affects the safety associated with their application [ 27 ].…”

“…Earlier publications reported the synthesis and characterization of HILs containing the following sulfonylureas anions: iodosulfuron-methyl [ 7 ], metsulfuron-methyl [ 33 ] and nicosulfuron [ 14 ]. However, from an economic point of view, new herbicidal formulations should be cost-effective and easy to prepare.…”

“…However, from an economic point of view, new herbicidal formulations should be cost-effective and easy to prepare. Thus, it is preferred to use already commercially available and inexpensive cations instead of preparing new ones in multistep synthesis [ 7 , 14 , 33 ]. Alkyltrimethylammonium halides and dialkyldimethylammonium halides are popular cationic surfactants that are commonly used in cosmetic, detergents, as fabric softeners, preservatives or in production of organic clays [ 34 , 35 , 36 ].…”