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How Stereochemistry Considerations can Improve Pesticide Safety
Citation:
Garrison, Wayne. How Stereochemistry Considerations can Improve Pesticide Safety. Presented at 4th International Symposium on Pesticides and Environmental Safety, September 16 - 19, 2012.
Impact/Purpose:
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Description:
About 30% of pesticides are chiral molecules and therefore exist as two or more stereoisomers, which can differ significantly in their toxicity, biodegradation, and persistence. Such differences can impact their relative safety to humans and environmental species. Enantiomers, mirror image optical isomers, are the best known examples of stereoisomers that can show these differences. However, the great majority of chiral pesticides are manufactured and applied as their racemates; i.e., equal mixtures of each enantiomer. Use of the appropriate single or enriched enantiomer (or other type of stereoisomer) can be practical and effective pesticide safety measure and contribute to the sustainability of pesticide use. In fact, the agrochemical industry and government regulators have begun to take enantioselectivity into account. For example, the (R)-enantiomers of the phenoxypropionic acid herbicides such as dichlorprop and mecoprop are the active enantiomers, killing the weeds, while the (S)-enantiomers are inactive. To reduce the amount of herbicide used, as well as avoid possible adverse impacts from the unnecessary (S)-enantiomer, several European countries decreed years ago that only the (R)-enantiomers will be used. Fipronil is an important phenylpyrazole insecticide used on rice, turfgrass and residentially for tick, flea and termite control. It has a chiral center at its sulfoxide moiety. Its two enantiomers have been shown to be selective in both acute and chronic toxicity, and it is selectively oxidized to the sulfone in fish and reduced to the sulfide in sediments. o,p’-DDT shows selective estrogen activity, with the (R)-enantiomer being a stronger agonist. All pyrethroid insecticides are chiral molecules with 2 or 3 chiral centers, and exhibit selectively in their toxicity and biotransformation. Twelve or so pyrethroids are marketed as single- or enriched-anantiomer (or steroisomer) pesticide products. Several other classes of pesticides include chiral