Pergamon Tetrahedron Letters, Vol. 38, No. 44, pp. 7805-7808, 1997 © 1997 ElsevierSciencel.,td All rights reserved.Printed in Great Britain PII: S0040-4039(97) 10042-9 0040-4039197 $17.00 + 0.00 A Metallocene Molecular Gear Andrew M. Stevens and Christopher J. Richards* Department of Chemistry, University of Wales, Cardiff, PO Box 912, Cardiff, CFI 3TB, UK. Abstract: In a five step synthesis (ethynylcyclopentadienyl)(tetraphenylcyclobutadiene)cobalt is produced in 19% overall yield from sodium carbomethoxycyclopentadienylide. Subsequent cross- coupling with 9-iodoanthracene (76%) and addition of benzyne (52%) gave (9- triptyeylethynyleyclopentadienyl)(tetraphenylcyclobutadiene)cobalt, the first example of a metallocene containing molecular gear. © 1997 Elsevier Science Ltd. The quest for novel molecular components that function in an analogous fashion to macroscopic mechanical systems has recently led to the synthesis and description of such molecules as a shuttle, t a brake 2 and a turnstile. 3 These are providing a basis for the future design of nanoscale systems and materials to perform tasks as varied as catalysis and nanoscale manipulation, information storage, and replication. One of the earliest examples of a molecular component, reported simultaneously by Mislow and Iwamura, utilises oxygen or methylene linked triptycenes to give molecules 1 analogous to a three-toothed bevel gear. 4 Subsequent studies on ring substituted phase isomers of 1 established that the two triptycenes undergo correlated rotation with an energy barrier to this gearing mechanism of only 1-2 kcal mol 1. In contrast, gear slippage requires an activation energy of 30 - 40 kcal mol l such that disrotatory cog wheeling in these systems is as strictly specified as are rotations in symmetry-controlled reactions. 5 Simple carbon derived rotors such as triptycene are limited by valency to three cog-teeth. In order to increase the number of cog-teeth available in simple rotors, and to use these for the generation of molecular gears with non-equal gearing ratios, we became interested in the possibility of utilising metallocenes. The low energy barrier to rotation of the ~-fragrnent about the metal may be regarded as analogous to rotation about a low friction ball-bearing. To explore the possibility of using metallocenes in this way we embarked upon a synthesis of 2, in which an acetylene substituted triptycene is linked to a four-toothed tetraphenyl cyclobutadiene containing metallocene. 2 7805