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 C₂symmetry. 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.