The Shapiro reaction of barrelene derivatives: the influence of annelation on acene formation

The possible formation of pentacene from a tosylhydrazone of 6,13-dihydro-6,13-ethenopentacene under the conditions of the Shapiro reaction is explored, as previous work demonstrated that the tosylhydrazone of barrelene (bicyclo[2.2.2]octatriene) yields benzene under these conditions [C. Weitemeyer, T. Preuss, and A. de Meijere, Chem. Ber., 1985, 118, 3993]. The computational analyses based on homodesmotic equations involving the anions, and monomeric (including the dimethyl ether solvate) and dimeric organolithium compounds reveals that benzene formation is exothermic, but pentacene formation is endothermic due to the increased stability of the lithium derivative and the decreased stability of pentacene. The computational predictions are confirmed by experimental investigations.