Solution photochemistry of the potent pyrethroid insecticide alpha-cyano- 3-phenoxybenzyl cis-2,2-dimethyl-3- (2,2-dibromovinyl) cyclopropane-carboxylate
Ruzo, LO; Holmstead, RL; Casida, JE
PESTAB. Photolysis of (S)-alpha-cyano-3- phenoxybenzyl cis (1R,3R)-2,2-dimethyl-3- (2,2-dibromovinyl) cyclopropanecarboxylate (I) in methanol, hexane, or 40% water-acetonitrile was carried out. The products were isolated, identified, and quantitated by thin-layer and gas chromatography, infrared and mass spectrometry, and nuclear magnetic resonance. In hexane, I underwent isomerization, decarboxylation, and debromination. Major products from the acid moiety in methanol and hexane were dibromovinyl and monobromovinyl acids and, in methanol, their methyl esters. Photolysis in aqueous acetonitrile yielded only the dibromovinyl acid from the acid moiety. Vinylic free radicals were probable intermediates in the photoreductions. On photolysis in methanol or hexane, the alcohol moiety yielded the nitrile, dimers, the cyanohydrin, and the aldehyde. The major product of I in hexane, absent in methanol, was the benzoyl cyanide, while the major product in methanol was the methyl benzoate. The only major products from the alcohol moiety in aqueous acetonitrile were the aldehyde and 3-phenoxybenzoic acid. Products not retaining the original ester linkage can be explained by homolytic cleavage of the carboxyl-carbon or acyl-oxygen bonds. The photochemical pathways of I must include at least partial intersystem crossing to the triplet and reaction from that excited state.