*2.2. Rotational Barriers in di-Indenyl Anthracenes*

Having established convenient palladium-catalysed routes to 9-(3-indenyl)anthracene, 6, and 9-(2-indenyl)anthracene, 7, this work was then extended to include their di-indenyl counterparts, starting from 9,10-dibromoanthracene (Scheme 8). Thus, 9,10-di(2-indenyl)anthracene, 18, was prepared in 83% yield via Suzuki cross-coupling with 2-indenylboronic acid mediated by (dppf)PdCl2 in ethanol-toluene. The attempted Heck-type route to 9,10-di(3-indenyl)anthracene led predominantly to formation of dihydroaceanthrylenes, but the Stille reaction of 1-(trimethylstannyl)indene, 9,10-dibromoanthracene and dichloro-bis(tri-*O*-tolylphosphine)-palladium(II) in 1,4-dioxane gave the desired product, 19, in 63% yield. The X-ray crystal structures of 18 and 19 appear in Figure 9, and exhibit interplanar indenyl-anthracenyl dihedral angles of 81.7◦ and 81.5◦, respectively [19].

**Scheme 8.** Pd-catalysed formation of di-indenylanthracenes, **18** and **19**. Reagents and conditions: (**i**) 2-indenylboronic acid, (dppf)PdCl2, Na2CO3, ethanol-toluene, 80 h, 80 ◦C; (**ii**) 1- (trimethylstannyl)indene, (O-tol3P)2PdCl2, 1,4-dioxane, 40 h, 120 ◦C.

**Figure 9.** Structures of 9,10-di(2-indenyl)anthracene, **18**, and *anti*-9,10-di(3-indenylanthracene, **19**.

It was apparent that 9,10-di(3-indenyl)anthracene, 19, exists as an almost 50/50 mixture of *syn* and *anti* atropisomers that, fortunately, are separable by column chromatography. Their barrier to interconversion was evaluated in a kinetic experiment monitored by NMR as ca. 105 kJ mol−1, approximately twice that found in 9-(3-indenyl)triptycene. Likewise, 9,10-di(2-indenyl)anthracene, 18, exists as an *syn*/*anti* mixture; these atropisomers are not separable but their interconversion can be monitored by V-T 13C NMR that yielded a rotational barrier of ca. 55 kJ mol−1. At first sight, it may be surprising that the 3-indenyl rotation barrier in the anthracene, 19, is so much greater than in the corresponding triptycene, 8. However, simulated virtual rotation in 8 reveals that, as the benzo ring of the 3-indenyl unit passes a triptycyl blade, the five-membered ring can bend into the space between the other two blades, thus minimising the energy cost of the rotation. This "duck and dodge" mechanism is not available in the 3-indenylanthracenes in which two unfavourable coplanar H•••H contacts are attained simultaneously [19].
