Synthetic strategies towards c₆₀. Molecular mechanics and MNDO study on sumanene and related structures

Abstract

Strain energies of the hydrocarbons derived from the fragments of C₆₀ along the C₅, C₃ and C₂ symmetry pathways indicate that the strain energy per carbon (E_s) increases gradually along the C₃ and C₂ paths, but has a maximum at C₄₀H₁₀ along the C₅ path. C₂₁-Sumanene, 10, has been recognised as a leading fragment along C₃ route with an inversion barrier of 24.2 kcal mol^-1 and a bowl depth of 1.15 Å at MNDO level. This contrasts with C₂₀-corannulene, 6, which is a more shallow bowl with a lower inversion barrier. The bond alternation in the central six membered ring of sumanene resembles that of C₆₀. The higher fragments en route to C₆₀ are deep bowls with higher barriers for inversion. An approach to sumanene via a triphenylene derivative, 26, has been probed and an increase in strain through sequential placement of methylene bridges has been estimated, using MNDO and MM2 calculations. Synthetic routes to a doubly bridged triphenylene, 28, and schemes for further elaboration of 26a and sumanene derivative, 29, towards C₆₀ are considered.