Crystal Structure and Rotational Barrier of Octakis(bromomethyl)naphthalene Samah Simaan, † Vered Marks, ‡ Hugo E. Gottlieb,* ,‡ Amnon Stanger,* ,§ and Silvio E. Biali* ,† Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel, and Department of Chemistry, The Institute of Catalysis Science and Technology, and The Lise-Meitner-Minerva Center for Computational Quantum Chemistry, Technions Israel Institute of Technology, Haifa 32000, Israel gottlieb@mail.biu.ac.il; silvio@vms.huji.ac.il; stanger@tx.technion.ac.il Received August 22, 2002 Abstract: Octakis(bromomethyl)naphthalene (4) adopts in the crystal a chiral conformation with a helical central naphthalene core and the bromomethyl groups disposed in an alternate up-down “in” arrangement. According to MM3 calculations, this conformation is less stable than the corresponding all alternated “out” form, while B3LYP/ LANL2DZ calculations suggest the opposite stability order. The topomerization barrier (16.0 kcal mol -1 ) is ascribed to an enantiomerization process requiring 180° rotation of all the bromomethyl groups and reversal of the helical sense of the naphthalene core. Two of the simplest substituents of stereochemical interest are the bromomethyl and ethyl groups. 1-3 When attached to an aromatic ring, ethyl and bromomethyl groups prefer a perpendicular (i.e., with Br-CH 2 -C Ar - C Ar torsional angles equal to ca. (90°) arrangement over a coplanar one. Both hexaethylbenzene (1) 2 and hexakis- (bromomethyl)benzene (2) 3 adopt conformations with the side chains oriented perpendicularly to the central ring and arranged in an alternate up-down fashion. The sterically overcrowded octamethylnaphthalene (3) and its organometallic derivatives have been the subject of numerous studies. 4 X-ray crystallography indicates that the naphthalene core of 3 is nonplanar and adopts a helical conformation of D 2 symmetry. This helical conformation is adopted to alleviate the steric interac- tions between the methyl groups at the peri positions. Octakis(bromomethyl)naphthalene 4 is a multiarmed organic compound possessing a polysubstituted central naphthalene core. 5,6 Naphthalene 4 was prepared by Hart and co-workers in 1977 by electrophilic bromination of octamethylnaphthalene (3). 5 The published 1 H NMR data of 4 (two broad methylene signals in CDCl 3 at 41 °C, two sharp singlets in tetrachloroethylene at 100 °C) suggested restricted rotation of the bromomethyl groups on the NMR time scale at room temperature. 5 In this paper, we report the conformation, crystal structure, and rotational barrier of the crowded multiarmed naphthalene 4. “In” and “Out” Forms. By analogy to 3, it could be expected that the central naphthalene core of 4 should adopt a chiral helical conformation. As recently described for the octaethylfluorene 5 7 (which possess a helical central core), two diastereomeric fully alternated up- down forms are possible for 4. This can be rationalized by viewing those conformations as resulting from the superposition of two independent stereogenic elements: the two enantiomeric fully alternated up-down patterns of the bromomethyl groups and the two enantiomeric helical conformations (helicities) of the central naphtha- lene core. This superposition yields two diastereomeric forms (Figure 1), denoted “in” and “out”. MM3 calcula- tions have indicated that in 5 the “out” arrangement is of lower energy. X-ray Crystallography. A single crystal of 4 was grown from ethanol and submitted to X-ray crystal- lography. The molecule possesses crystallographic D 2 symmetry with the bromomethyl groups at the peri positions adopting the “in” conformation (Figure 2). The bromine atoms connected to C5 were disordered between two adjacent positions (Br2 and Br2′) with 50% occupancy each. The naphthalene core is helical, and its degree of twist can be characterized by the C1-C3-C3*-C1* torsional angle (20.4°), which is similar to the value obtained in the crystal structure of octamethylnaphtha- lene (21.3°). 4c Calculations. The relative steric energies of the fully alternated “in” and “out” conformers of 4 were initially estimated using MM3 calculations. 9,10 The MM3 calcula- † The Hebrew University of Jerusalem. ‡ Bar-Ilan University. § TechnionsIsrael Institute of Technology. (1) For a review on the conformation of alkyl groups see: Berg, U.; Sandstro ¨m, J. Adv. Phys. Org. Chem. 1989, 25, 1. (2) (a) Iverson, D. J.; Hunter, G.; Blount, J. F.; Damewood, J. R., Jr.; Mislow, K. J. Am. Chem. Soc. 1981, 103, 6073. (b) For a recent study on systems structurally related to hexaethylbenzene see Kilway, K. V.; Siegel, J. S. Tetrahedron 2001, 57, 3615. (3) Marsau, P. Acta Crystallogr. 1965, 18, 851. See also: Zuaretz, N.; Golan, O.; Biali, S. E. J. Org. Chem. 1991, 56, 2444. (4) (a) Oku, A.; Kakihana, T.; Hart, H. J. Am. Chem. Soc. 1967, 89, 4554. (b) Hart, H.; Teuerstein, A. Synthesis 1979, 693. (c) Sim, G. A. Acta Crystallogr. 1982, 38, 623. (d) Hull, J. W.; Gladfelter, W. L. Organometallics 1982, 1, 264. (e) Hull, J. W.; Gladfelter, W. L. Organometallics 1984, 3, 605. (5) Hart, H.; Reilly, J. L.; Jiang, J. B.-C. J. Org. Chem. 1977, 42, 2684. See also: Ashton, P. R.; Brown, G. R.; Foubister, A. J.; Smith, D. R.; Spencer, N.; Stoddart, J. F.; Williams, D. J. Tetrahedron Lett. 1993, 34, 8333. (6) For recent studies on crowded naphthalenes see: Qiao, X.; Padula, M. A.; Ho, D. M.; Vogelaar, N. J.; Schutt, C. E.; Pascal, R. A., Jr. J. Am. Chem. Soc. 1996, 118, 741. Zhang, J.; Ho, D. M.; Pascal, R. A., Jr. J. Am. Chem. Soc. 2001, 123, 10919. Christofi, A. M.; Garratt, P. J.; Hogarth, G.; Ibbett, A. J.; Ng, Y.-F.; Steed, J. W. Tetrahedron 2002, 58, 4543. (7) Marks, V.; Gottlieb, H. E.; Melman, A.; Byk, G.; Cohen, S.; Biali, S. E. J. Org. Chem. 2001, 66, 6711. (8) For other studies on polyethylated aromatic compounds see: (a) Marks, V.; Gottlieb, H. E.; Biali, S. E. J. Am. Chem. Soc. 1997, 119, 9672. (b) Gottlieb, H. E.; Ben-Ari, C.; Hassner, A.; Marks, V. Tetra- hedron 1999, 55, 4014. (c) Taha, M.; Marks, V.; Gottlieb, H.; Biali, S. E. J. Org. Chem. 2000, 65, 8621. 10.1021/jo026343s CCC: $25.00 © 2003 American Chemical Society J. Org. Chem. 2003, 68, 637-640 637 Published on Web 12/24/2002