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

The reaction of beta-ethynyl alpha,beta-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, beta-ethynyl alpha,beta-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 n-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. (C) 2002 Elsevier Science B.V. All rights reserved.