π Complexes with Annellated Cyclobutene Rings: Bicyclo[3.2.0]heptadienyl Anion and Benzocyclobutene as Ligands for Transition Metals

As was first found on the basis of DSC measurements, bicyclo[3.2.0]heptadienyl cobalt complex 17 undergoes a ring opening - cycloaddition sequence at high temperature with formation of ring expansion products. Benzocyclobutene complexes react in a similar way only in one case. However, deprotonation of benzocyclobutenol complex endo-31 results in a dramatic decrease of the ring opening temperature and allows corresponding [4+2] cycloaddition reactions at low temperature. Benzocyclobutenedione complex 74 shows interesting structural features, its fragmentation in the mass spectrometer leads to a η⁶ coordinated benzyne chromium cation. Addition of vinylmetal reagents induces a dianionic oxy-Cope rearrangement at -78 °C, the first case of such a reaction at such mild reaction conditions. Products are benzannellated cyclooctenediones and their intramolecular aldol adducts, which are obtained with very high diastereoselectivity.

bicyclo[3.2.0]heptadienyl complexes - benzocyclobutene complexes - cobalt - chromium - dianionic oxy-Cope rearrangement