Cycloisomerization of enynes catalyzed by iron(0)-ate complexes

The 18-electron half-sandwich iron(0) complex [CpFe(C2H4)2] [Li(tmeda)] (1a), which is readily available in multigram quantities from inexpensive starting materials (ferrocene, ethylene, Li sand), is shown to be an efficient catalyst for the Alder-ene reaction of various 1,6(7)-enynes. Thereby, the presence of the labile alkene ligands in the ferrate catalyst is essential since the analogous complex [CpFe(CO)2]Na is catalytically incompetent. The cycloisomerizations catalyzed by 1a are compatible with various functional groups and turned out to be highly diastereoselective with regard to the configuration of the newly formed alkenes as well as relative stereochemistry at the ring junction. The alkyne moiety in the substrates may be terminal, silylated, or substituted with various groups, including cyclopropane rings. Likewise, the alkene substructure can be varied to a large extent, with cycloalkenes of ring sizes >/=7 being particularly suitable.