Addition and Ring Expansion Reactions of Tricarbonyl-(1,2-dioxobenzocyclobutene)chromium(0) with Carbon Nucleophiles: Unexpected Formation of Benzocycloheptene Derivatives and the First Head-to-Head Coupling of Two Methoxyallene Molecules

Addition of carbon nucleophiles to (benzocyclobutenedione)tricarbonylchromiuni(0) (4) results in the formation of exo mono- and diadducts as well as 1,2-diketones as the consequence of proximal ring-opening reactions. In one case the unexpected formation of benzocycloheptenedione complexes is observed. Treatment of 4 with an excess of 1-ethoxy-1-lithioethene gives the product of a dianionic oxy-Cope rearrangement followed by an intramolecular aldol addition. This is also the case with lithiated methoxyallene, and as the result the first head-to-head coupling product 12 of two methoxyallene molecules is isolated in good yield. 12 is used as a diene in Diels-Alder cycloadditions, and its molecular structure is compared to that of the similar molecule 13, lacking the two exo methylene substitutents.