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)tricarbonylchromi um(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 c ase the unexpected formation of benzocycloheptenedione complexes is observe d. Treatment of 4 with an excess of 1-ethoxy-1-lithioethene gives the produ ct of a dianionic oxy-Cope rearrangement followed by an intramolecular aldo l addition. This is also the case with lithiated methoxyallene, and as the result the first head-to-head coupling product 12 of two methoxyallene mole cules is isolated in good yield. 12 is used as a diene in Diels-Alder cyclo additions, and its molecular structure is compared to that of the similar m olecule 13, lacking the two exo methylene substitutents.