Medium-sized cyclophanes. Part 56. 8-Substituted 5-tert-Butyl[2.2]metaparacyclophane-1,9-dienes. Preparation, X-ray diffraction study and their treatment with Lewis and protic acids

Abstract

The preparation of various 8-substituted 5-tert-butyl[2.2]metaparacyclophane-1,9-dienes 1, using the thiacyclophane method, and an X-ray diffraction study of 5-tert-butyl-8-cyano[2.2]metaparacyclophane-1,9-diene 1e are described. Lewis and protic acid-catalyzed reactions of 8-substituted [2.2]metaparacyclophane-1,9-dienes 1b–g in dichloromethane proceeded by isomerization and transannular cyclization to afford the strainless pyrenes 10 in good yields. In contrast, similar treatment of 5-tert-butyl-8-methoxy[2.2]metaparacyclophane 11 with Lewis acids in dichloromethane under the same conditions only led to recovery of the starting compound. Treatment of 8-methoxy[2.2]metaparacyclophane-1,9-diene 1d with TiCl₄ in dichloromethane also led to isomerization and transannular reactions to afford 2-tert-butylpyrene 10b within 1 min in almost quantitative yield. However, the TiCl₄ catalyzed reaction of an electron-poor [2.2]metaparacyclophane-1,9-diene such as 1e did not afford any product and the starting material was recovered in almost quantitative yield. These results suggest that the present novel isomerization reaction might be attributed to the bridged double bonds, which increase the strain in the molecule in comparison with the corresponding saturated [2.2]metaparacyclophane 11. The characterization and the reaction pathway of these products are also discussed.