Preference of intra- and intermolecular cation–p interaction: cis–trans geometrical effects of amide bond on the interaction mode Shinji Yamada, * Yuka Morimoto and Tomoko Misono Department of Chemistry, Ochanomizu University, Bunkyo-ku, Tokyo 112-8610, Japan Received 18 May 2005; revised 15 June 2005; accepted 17 June 2005 Available online 5 July 2005 Abstract—An intermolecular cation–p interaction is observed in trans-amide 3, whereas an intramolecular interaction is observed in cis-amide 4, suggesting that cis–trans conformational difference plays a critical role in the preference of the interaction modes. Ó 2005 Elsevier Ltd. All rights reserved. Cation–p interaction 1 between a pyridinium cation and an aromatic ring plays an important role in the forma- tion of intramolecular complexes, 2,3 inclusion com- plexes 4 and supramolecules, 5 because of its larger interaction energy than that of related p–p and CH–p interactions. 6 Since the structure of molecules is respon- sible for the formation of complexes, elucidation of the relationship between the molecular structure and the interaction mode will provide insight into the design of various new complexes and supramolecules. In our previous studies, we have clarified a considerable substi- tuent effect on the formation of the desired intramolec- ular pyridinium–p complex in N-methylnicotinium amides. 2 Tanaka and his co-workers reported that the steric bulkiness of the substituent in bis(pyridinioprop- yl)benzene derivatives has a significant effect on the pref- erence of the intra- and intermolecular interaction mode in crystal. 7 To clarify further the factors governing the interaction mode, we focused on very simple nicotinic amides 1 and 2 and the corresponding N-methyl salts 3 and 4, the structures of which were investigated by 1 H NMR measurements and X-ray structural analyses. In this let- ter, we describe that cis and trans geometrical differences in the amide linkage significantly affect the interaction modes. 0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2005.06.088 Keywords: Cation–p interaction; Pyridinium–p interaction; cis–trans effect; Amide bond. * Corresponding author. Tel.: +81 3 5978 5349; fax: +81 3 5978 5715; e-mail: yamada@cc.ocha.ac.jp Table 1. d Values for 1–8 and Dd values for 1–4 a d1 d5 Dd1 d2 d6 Dd2 d3a d7 Dd3a d4a d8 Dd4a H2 8.85 8.96 À0.11 8.55 8.67 À0.12 10.37 9.70 0.67 8.02 9.07 À1.05 H4 8.05 8.12 À0.07 7.55 7.77 À0.22 9.04 9.07 À0.03 7.56 8.41 À0.85 H5 7.37 7.39 À0.02 7.30 7.33 À0.03 8.00 8.20 À0.20 7.87 8.12 À0.25 H6 8.69 8.73 À0.04 8.62 8.63 À0.01 8.84 9.13 À0.29 9.45 9.36 0.09 Me — — — — — — 4.62 4.62 0.00 4.57 4.74 À0.17 a Measured at 400 MHz in 10 mM solution of CDCl 3 . Tetrahedron Letters 46 (2005) 5673–5676 Tetrahedron Letters