Joining the rings: the preparation of 2- and 3-indenyl-triptycenes, and curious related processes

Abstract

The indenyltriptycenes, 1 and 2, where the 3- or 2-indenyl, respectively, is attached at the 9-position of the triptycene, are attractive prototypes of molecular gearing systems that can also incorporate a brake. These molecules have been prepared from their respective indenylanthracenes, 3 and 4, by the [4 + 2] cycloaddition of benzyne to the anthracene fragment, and the rotational barriers about the indenyl–triptycenyl single bonds in 1 (12 kcal mol⁻¹) and 2 (<9 kcal mol⁻¹) have been measured. The precursor anthracenes, 3 and 4, were prepared by using palladium-catalysed coupling reactions. Unexpectedly, the Heck-type reaction of 9-bromoanthracene, 5, with indene leads to the formation of 3-indenylanthracene 3; moreover, this process is accompanied by a novel palladium-catalysed carbocyclisation reaction leading to the indenophenanthrylene 9. The addition of benzyne to 9-(3-indenyl)anthracene, 3, yields the corresponding indenyltriptycene, 1, and, surprisingly, the anthracenyl methano-bridged phenanthrene 16. It has been demonstrated that 2-arylindenes can act as 1,3-dienes in the [4 + 2] cycloadditions of benzyne. The products 2, 7a, 9 and 16 have been characterised by X-ray crystallography.