SYNTHESIS OF (ALLYLOXY)(ALKYNYL)CARBENE COMPLEXES AND THEIR REACTION WITH OCTACARBONYLDICOBALT - AN UNPRECEDENTED RETRO-FISCHER REACTION

Pentacarbonyl[(allyloxy)((4-methylphenyl)ethynyl)carbene]chromium and tungsten complexes 1 and 2 have been prepared in moderate yields from the corresponding metal hexacarbonyl in a two-step, one-pot, Fischer type nucleophilic/electrophilic addition methodology using allyl triflate as alkylating reagent. As a consequence of its pronounced electrophilicity it also attacks the solvent THF, which results in the formation of the (((allyloxy)butyl)oxy)carbene complex 3 as a byproduct. Both the alkene-alkyne complexes 1 and 2 and their Co-2(CO)(6) adducts fail to undergo intramolecular Pauson-Khand reactions; instead, at room temperature, a 1,2-elimination within the carbene ligand occurs, regenerating the metal hexacarbonyl with predominant formation of the 1,4-enynes and 8-10. This process corresponds to an unprecedented retroversion,of the customary Fischer carbene complex synthesis.