Twin Triptycyl Spinning Tops: A Simple Case of Molecular Gearing with Dynamic C2 Symmetry

Abstract Lithiation and subsequent oxidation of the recently described prototype molecular paddlewheel 9‐(2‐indenyl)triptycene ( 1 ) leads to the racemic dimer 2,2′‐bis(9‐triptycyl)‐1,1′‐biindenyl ( 4 ). In the solid state this very congested molecule adopts a conformation in which the two triptycenyl paddlewheel fragments are attached to the C 2 ‐symmetric biindenyl core. However, the orientation of the triptycene blades, dictated by their size and shape, is gear‐like and breaks the nascent C 2 ‐symmetry of the whole molecule. Nevertheless, effective C 2 ‐symmetry is restored in solution when the triptycene paddlewheels, although apparently tightly interlocked, undergo rapid contra‐rotation on the NMR time‐scale. The analogous reaction of (2‐phenylindenyl)lithium ( 6 ), likewise yields racemic ( RR / SS ) 2,2′‐diphenyl‐1,1′‐biindenyl( 7 ). Interestingly, although the treatment of 6 with a Cu II salt leads to both rac ‐ 7 and its diastereomer meso ‐ 7 , the reaction of 6 and 2‐phenylindenyl bromide furnishes selectively rac ‐ 7 . Overall, these data suggest that the oxidative dimerisation to yield racemic products occurs through a nucleophilic substitution mechanism. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)