Monoolefin and Diene Cycloaddition Induced by Transition-Metal Carbonyls. Cyclodimerization of 5,6-Dimethylidene-7-oxabicyclo[2.21]hept-2-ene Catalyzed by Dodecacarbonyltriosmium

The products generated by heating 5,6-dimethylidene-7-oxabicyclo2.2.1]hept-2-ene (1) with Fe₂(CO)₉, Ru₃(CO)₁₂, Os₃(CO)₁₂, Cr(CO)₃(MeCN)₃, (or W(CO)₅(MeCN) or by treatment with Fe-atoms have been characterized by spectroscopic methods. Apart from the expected η²- and η⁴-complexes of the triene 1 , condensation products are formed which arise from the formal 4 + 2]-cyclodimerization of 1 involving the endocyclic double bond of one molecule and the diene moiety of a second. The 4 + 2]-cyclodimerization is catalyzed by Os₃(CO)₁₂ in MeOH and gives 1,4-epoxy-7-methoxy-2,3-dimethylidene-1,2,3,4,4a,9,9a,10-octahydroanthracene (15) ). Fe-Atoms induce a stereoselective 2 + 2]-cyclodimerization pf 1 which involves its endocyclic double bond and produces the dimer 8 .