Synthesis of (Allyloxy)(alkynyl)carbene Complexes and Their Reaction with Octacarbonyldicobalt: An Unprecedented Retro-Fischer Reaction

Abstract

Pentacarbonyl[(allyloxy)((4-methylphenyl~eth~yl)carbenelchromium 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 nucleophilidelectrophilic 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 (((ally1oxy)butyl)oxy)carbene complex 3 as a byproduct. Both the alkene-alkyne complexes 1 and 2 and their COZ(CO)S adducts fail to undergo intramolecular Pauson-Khand reactions; instead, at room temperature, a l,2-elimination within the carbene ligand occurs, regenerating the metal hexacarbonyl with predominant formation of the 1,4-enynes 4 and 8-10. This process corresponds to an unprecedented retroversion of the customary Fischer carbene complex synthesis.