~ ) Pergamon 0040-4020(95)00078-X Tetrahedron Vol. 51. No. 12, pp. 3627-3640, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0040-4020/95 $9.50+0.00 Synthesis and Dynamic NMR Studies of Heptachloro-7- (dihalomethyl) naphthalenes IL Garcia," J. Riera," J. Carilla, L. Julifi, Departamento de Materiales Orghnicos Halogenados, Centro de Investigaci6n y Desarrollo (CSIC), Jordi Girona 18-26, 08034 Barcelona, Spain. F.J. Sfinchez-Baeza," Departamento de Quimica Orglnica Biol6gica, Centro de Investigaci6n y Desarrollo (CSIC), Jordi Gimna 18-26, 08034 Barcelona, Spain. E. Molins Instituto de Ciencia de Materiales de Barcelona (CSIC), Campus Universitario de Bellaterra, 08193 Cerdanyola, Spain. Abstract: Photochlorinations of nonachloro-3-(chlommethyi)-l,4-dihydronaphthalene (1) and nonachloro-7-(chloro- methyl)- 1,4-dihydronaphthalene (2) give nonachloro-3-(dichlommethyl)-1,4-dihydmnaphthalene(3) and nonachloro-7- (dichloromethyl)-l,4-dihydronaphthalene (4), respectively. Dechlorination of both 3 and 4 by means of SnCI2.2H20 affords heptachloro-7-(dichloromethyl)naphthalene (5). Photobmmination of heptachlom-7-methylnaphthalene (6) with Br2 results in 2-(bromomethyl)heptachloronaphthalene (8) or heptaehloro-7-(dibromomethyl)naphthalene(7), depending on the reaction time. Intermediate 8 can be prepared in high yield by brominstion 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 tH NMR spectra orS, 7, and 2-(bromochlo- romethyl)heptachloronaphthalene (9) ate 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 XH NMR study shows that the barriers to rotation of CHCI2, CHBrCI, and CHBr 2 groups in 5, 9, and 7 are as high as 16.8, 18.2, and 19.3 kcal tool"t, respectively. The X-ray structure of 7 is commented, and spectral data of the synthesized compounds are reported. The essence of conformational analysis is the study of how molecules accommodate strain. A lot of research has gone into the conformational analysis of those substituted naphthalenes with substituents at the 1 and 8 positions (peri positions), t Relief of steric strain in these naphthalenes tends to favour out-of-plane distortions because of a buttressing effect exerted by the 2,7 hydrogens. In the context of our investigations on highly chlorinated compounds, we have reported the synthesis of many polychloronaphthalenes and their properties. 2 In this paper, principally we report the synthesis of (dichloromethyl)naphthalene $ and (dibromomethyl)naphthalene 7 which offer a good opportunity to study in solution the conformational analysis by dynamic nuclear magnetic resonance (DNMR) of perchloroaromatic 3627