in tetrahydrofuran byproducts: methylenecyclopropane, 1,4-dimethylenecyclohexane; KOC(CH3)3 and n-BuLi were mixed in pentane under Ar; addn. of isobutylene (condensed at -78°C); removal of solvent after 1 h; dissolving in THF; addn. to Mo(CO)4Br2 in THF at 0°C; mechanism discussed;; removal of solvent after 1 h; extn. with benzene; removal of benzene under Ar;;