Synthesis and dynamic NMR studies of heptachloro-7-(dihalomethyl)naphthalenes

Abstract

Photochlorinations of nonachloro-3-(chloromethyl)-1,4-dihydronaphthalene ( 1) and nonachloro-7-(chloromethyl)-1,4-dihydronaphthalene ( 2) give nonachloro-3-(dichloromethyl)-1,4-dihydronaphthalene ( 3) and nonachloro-7-(dichloromethyl)-1,4-dihydronaphthalene ( 4), respectively. Dechlorination of both 3 and 4 by means of SnCl 2.2H 2O affords heptachloro-7-(dichloromethyl)naphthalene ( 5). Photobromination of heptachloro-7-methylnaphthalene ( 6) with Br 2 results in 2-(bromomethyl)heptachloronaphthalene ( 8) or heptachloro-7-(dibromomethyl)naphthalene ( 7), depending on the reaction time. Intermediate 8 can be prepared in high yield by bromination of 6 with NBS in the presence of AIBN. Theoretical calculations in 7 indicate the existence of two energetically favoured conformers and that one of them seems to be slightly more stable than the other one. Accordingly, the 1H NMR spectra of 5, 7, and 2-(bromochloromethyl)heptachloronaphthalene ( 9) are temperature dependent, displaying two singlets that collapse in a sharp singlet about 100°C. Although the two conformers of 5, 7, and 9 can be detected in solution, only the most stable of each pair, at least in the case of 7, is present in solid form (X-ray analysis). A dynamic 1H NMR study shows that the barriers to rotation of CHCl 2, CHBrCl, and CHBr 2 groups in 5, 9, and 7 are as high as 16.8, 18.2, and 19.3 kcal mol −1, respectively. The X-ray structure of 7 is commented, and spectral data of the synthesized compounds are reported.