Issue in Honor of Professor Kalevi Pihlaja ARKIVOC 2001 (iii) 95-113
Introduction
Toxaphene is an insecticidal mixture, which is produced by the controlled chlorination of camphene (2,2-dimethyl-3-methylene-bicyclo[2.2.1]heptane) under UV light.1 The initial ionic step of the chlorination leads via Wagner-Meerwein rearrangement pathway to bornane (1,7,7trimethyl- bicyclo[2.2.1]heptane) skeleton. The main intermediate products, 2-exo,10dichlorobornane (2-exo-chloro-1-chloromethyl-7,7-dimethylbicyclo[2.2.1]heptane) and 2exo, 10,10-trichlorobornane (2-exo-chloro-1-dichloromethyl-7,7-dimethyl-bicyclo[2.2.1]heptane), are accompanied by other low chlorination hydrocarbons.2, 3 Further radical chlorination leads to a complex mixture of highly chlorinated C10-terpenes.
For many years it was believed that toxaphene mixture primarily consists of chlorinated bornanes next to small amounts of chlorinated bornenes and even smaller amounts of chlorinated bornadienes.4 The existence of polychlorinated bornenes and bornadienes had been postulated due to the GC/NCI-MS detection of substances with fragment masses being 2 or 4 amu below the [M-Cl]-ions of bornanes. Actually, these mass spectra must probably be attributed to camphenes, because new insights in synthetic pathways of technical toxaphene indicate the formation of camphenes and dihydrocamphenes.5
Previously, Landrum et al.6, 7 reported the identification of a heptachlorodihydrocamphene in technical toxaphene and Hainzl et al.8 reported the isolation of five polychlorocamphenes from a toxaphene standard. In addition, Tribulovich et al.9 showed that cis- and trans-5-exo,10dichlorocamphenes (5-exochloro-3-chloromethylene-2,2-dimethyl-bicyclo[2.2.1]heptane) are possible precursors of polychlodihydrocamphenes in technical toxaphene.
It is known that the various toxaphene components are differently transformed in the environment; while some congeners are highly persistent, others are easily metabolized and not detectable in environmental samples.10 This leads to a simpler toxaphene residue pattern compared to the technical mixture. The composition in biota also varies depending on the trophic level and the ability of a species to metabolize selected toxaphene components.11 This implies that the orientation of the chlorine atoms may influence the chemical stability of toxaphene congeners.12 In addition, individual toxaphene congeners may exhibit different toxicity.13 Accordingly, the position and the number of the chlorine atoms on the molecule determines the magnitude and nature of biological effect.14 Consequently, an exact assignment of the stereochemistry of the chlorine atoms on the primary carbons is necessary for correct prediction of chemical reactivity and for correct assessment of degradation or accumulation in environment.
In this study, we report the syntheses, isolation and characterization of nine novel polychlorinated dihydrocamphenes 1-9. An exact description of position of the chlorine atoms on the primary carbons has been achieved by extensive one- and two-dimensional 1H and 13C NMR spectroscopy. In the case of 2, the structure has been determined by X-ray methods. The experimental results are compared with those obtained from ab initio optimizations performed at the HF/6-31G* level. In addition, rotation barriers of compounds 7 and 9 have been characterized with the semiempirical AM1 method.15
ISSN 1424-6376 Page 96 ©ARKAT USA, Inc
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