Metal cyclopropyl carbenes in the reactions of alkynes with alkenes and furans

Electrophilic transition-metal complexes catalyze the reaction of enynes in the presence of water or alcohols to give hydroxy- or alkoxycyclization derivatives. The reaction proceeds by the anti addition of the alkene and the metal to the alkyne. The key intermediates in this reaction are cyclopropyl metal carbenes, which are also probably involved in the metathesis-type rearrangement of enynes. A general scheme is proposed for the cyclization of enynes initiated by the coordination of the metal to the enyne by transition metals, which included 5-exo-dig and 6-endo-dig pathways. The intramolecular reaction of furans with alkynes also proceeds via cyclopropyl metal carbenes.