C-C-bond formation by the palladium-catalyzed cycloisomerization/dimerization of terminal allenyl ketones: Selectivity and mechanistic aspects

The scope of the palladium-catalyzed cyclization/dimerization of terminal allenyl ketones 1 to the 2,4-disubstituted furans 3 has been investigated. Simplified and improved conditions almost exclusively provided the dimer 3, accompanied by only traces of the easily separable monomer 2. The formation of an isomer of 3, the unconjugated ketone 4, was completeley suppressed. Under these mild conditions, besides the normal functional group tolerance known for palladium-catalyzed reactions, an interesting selectivity was observed with functional groups that are known to react either in palladium-catalyzed reactions or reactions catalyzed by other transition-metals. Thus aryl halides, terminal alkynes, 1,6-enynes, and alpha-allenic alcohols were tolerated. In the latter example the selective reaction of only one out of two different allenes was achieved. Mechanistic investigation indicated a Pd(II)/Pd(IV)-cycle involving palladium(II)-gamma-alkoxyvinylcarbene and furylpalladium(IV) hydride intermediates, although a second pathway for the formation of the dimer 3 which also involves Pd(IV)-intermediates like the 3,4-dimethylenepalladacyclopentane 23 and the 3-methylenepalladacyclobutane-like structure 15 (respectively 25) could not completely be excluded.