Synthesis of 2-pyranylidene or (2-furyl)carbene–chromium complexes from conjugated enyne carbonyl compounds with Cr(CO)5(THF)

The reaction of β-ethynyl α,β-unsaturated esters or amides with Cr(CO)5(L) (L=Et2O, THF), derived from chromium hexacarbonyl, gives 2-pyranylidene–chromium complexes in good yields. The conjugated ene-carbonyl-vinylidene complex is a key intermediate to these Fischer-type oxacarbene complexes. In contrast, β-ethynyl α,β-unsaturated ketones react with Cr(CO)5(THF) to give (2-furyl)carbene complexes via nucleophilic attack of the carbonyl oxygen atom to the intermediate π-alkyne–chromium complexes. Some chemical transformation, such as [4+2] type cyclization of 2-pyranylidene complexes with alkynes and aldehyde formation from (2-furyl)carbene complexes, has also been described.