H. Ubayama et al., GENERATION OF CYCLOBUTADIENE DERIVATIVES FROM 1-METALLO-4-HALOBUTA-1,3-DIENE DERIVATIVES, Chemical communications, (18), 1998, pp. 1931-1932
1-Zircona-4-halobuta-1,3-diene derivatives react in the presence of Cu
Cl to produce cyclobutadiene derivatives which afford their dimers, or
Diels-Alder adducts with dimethyl maleate or fumarate.