Novel Pyranylidene Complexes from Group 6 Transition Metals and β-Ethynyl α,β-Unsaturated Carbonyl Compounds

The reaction of conjugated enyne−carbonyl compounds such as 1-(alkoxycarbonyl)-2-ethynylcycloalkenes and 1-carbamoyl-2-ethynylcycloalkenes with Mo(CO)5·NEt3 and W(CO)5·THF gave the corresponding 6-alkoxy- and 6-amino-2H-pyranylidene−molybdenum and −tungsten complexes, respectively. The cycloisomerization reaction of the conjugated vinylidene−ene−carbonyl complexes generated from 1-(alkoxycarbonyl)- or 1-carbamoyl-2-ethynylcycloalkenes and metals is a key route to these Fischer-type oxacarbene complexes. The crystal structure of the 6-methoxy-2H-pyranylidene−tungsten complex 2b has been determined by X-ray diffraction. New pyranylidene complexes undergo [4 + 2] cycloaddition reactions with acetylenes to give aromatic rings together with the liberation of metal hexacarbonyls.