Synthetic strategies towards C70: molecular mechanics and MNDO calculations on pinakene, C28H14 and related molecules

Abstract

Molecular mechanics (MM2) calculations have been performed to estimate the strain energies of hydrocarbons derived from the fragments of C₇₀ which retain C₅ or C₂ symmetry. There are two ways of dissecting C₇₀ retaining C₂ symmetry, C₂(a) and C₂(b). The key fragment along the C₂(a) route is pinakene. C₂₈H₁₄(12). It exists in a bowl shape. Its strain energy is close to that of the corresponding C₂₈ fragment of C₇₀. The bowl depth was calculated to be 2.03 Å at the MNDO level, and the ‘bowl-to-bowl’ inversion barrier calculated to be about 48.6 kcal mol^–1, implying that this is a very rigid bowl. The strain energy build-up from the sequential placement of methylene bridges in going from dibenzo[fg,op] naphthacene (26) to pinakene was estimated. Methyl derivatives of dibenzo[fg,op]naphthacene were taken as the key precursors in this study. Strain energies of various positional isomers were also evaluated. Synthetic strategies towards pinakene are delineated.