Rearrangement of 1-(1-alkynyl)cyclopropanols to 2-cyclopentenones via their hexacarbonyldicobalt complexes.: A new use of alkyne-Co2(CO)6 complexes in organic synthesis

A novel rearrangement of 1-(1-alkynyl)cyclopropanols to 2-cyclopenten-1-ones proceeded on complexation of their alkynyl part with octacarbonyldicobalt (Co-2(CO8)). 1-(1-Alkynyl)cyclopropanols with a wide range of substituents on their alkyne terminus rearranged to the corresponding 3-substituted 2-cyclopenten-1-ones in good yield. In case of the reactions of 1-alkynylcyclopropanols with an alkyl substituent on the cyclopropane ring, either 4-substituted or 5-substituted 2-cyclopenten-1-ones could be selectively obtained by appropriate choice of stereochemistry and protective group of the substrates. This rearrangement was successfully applied to cyclopentenone annulation reactions onto cycloalkenes. An efficient synthesis of alkynyl-substituted bicyclo[n.1.0]alkanol derivatives from the corresponding cycloalkenes according to Danheiser's protocol was developed, and bicyclic cyclopentenones were obtained in moderate to good yield by applying to these the cobalt-mediated rearrangement. Furthermore, the rearrangement was found to proceed catalytically on addition of triaryl phosphite as a ligand. In particular, when tri(o-isopropylphenyl) phosphite was used as ligand, 5-10 mol % of Co-2(CO)(8) and 10-20 mol % of the phosphite sufficed for the efficient conversion of 1-alkynylcyclopropanols to 2-cyclopentenones, Mechanistic studies revealed that the rate-determining step was the thermal or oxidative dissociation of a carbonyl ligand from the alkyne-Co-2(CO)(6) complex and that the coordinately unsaturated cobalt moiety so-formed inserted into the neighboring carbon-carbon bond of the cyclopropane to give a metallacyclic intermediate. This further rearranged to a metallacyclohexanone intermediate, which gave the cyclopentenone with liberation of a coordinatively unsaturated cobalt species.