REARRANGEMENT OF 1-(1-ALKYNYL)CYCLOPROPANOLS TO 2-CYCLOPENTENONES VIATHEIR HEXACARBONYLDICOBALT COMPLEXES - A NEW USE OF ALKYNE-CO-2(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 octacarbon yldicobalt (Co-2(CO8)). 1-(1-Alkynyl)cyclopropanols with a wide range of substituents on their alkyne terminus rearranged to the correspondi ng 3-substituted 2-cyclopenten-1-ones in good yield. In case of the re actions of 1-alkynylcyclopropanols with an alkyl substituent on the cy clopropane ring, either 4-substituted or 5-substituted 2-cyclopenten-1 -ones could be selectively obtained by appropriate choice of stereoche mistry and protective group of the substrates. This rearrangement was successfully applied to cyclopentenone annulation reactions onto cyclo alkenes. An efficient synthesis of alkynyl-substituted bicyclo[n.1.0]a lkanol derivatives from the corresponding cycloalkenes according to Da nheiser's protocol was developed, and bicyclic cyclopentenones were ob tained in moderate to good yield by applying to these the cobalt-media ted rearrangement. Furthermore, the rearrangement was found to proceed catalytically on addition of triaryl phosphite as a ligand. In partic ular, when tri(o-isopropylphenyl) phosphite was used as ligand, 5-10 m ol % 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 th ermal or oxidative dissociation of a carbonyl ligand from the alkyne-C o-2(CO)(6) complex and that the coordinately unsaturated cobalt moiety so-formed inserted into the neighboring carbon-carbon bond of the cyc lopropane to give a metallacyclic intermediate. This further rearrange d to a metallacyclohexanone intermediate, which gave the cyclopentenon e with liberation of a coordinatively unsaturated cobalt species.