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

The indenyltriptycenes, and , 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, and , by the [4 + 2] cycloaddition of benzyne to the anthracene fragment, and the rotational barriers about the indenyl-triptycenyl single bonds in (12 kcal mol(-1)) and (<9 kcal mol(-1)) have been measured. The precursor anthracenes, and , were prepared by using palladium-catalysed coupling reactions. Unexpectedly, the Heck-type reaction of 9-bromoanthracene, , with indene leads to the formation of 3-indenylanthracene ; moreover, this process is accompanied by a novel palladium-catalysed carbocyclisation reaction leading to the indenophenanthrylene . The addition of benzyne to 9-(3-indenyl)anthracene, , yields the corresponding indenyltriptycene, , and, surprisingly, the anthracenyl methano-bridged phenanthrene . It has been demonstrated that 2-arylindenes can act as 1,3-dienes in the [4 + 2] cycloadditions of benzyne. The products , , and have been characterised by X-ray crystallography.