Effect of catalyst on zwitterionic intermediacy in additions of dimethyl diazomalonate to vinyl ethers

Abstract The reaction of dimethyl diazomalonate with 2-alkoxy-6-methyl-3,4-dihydro-2H-pyran in hot fluorobenzene has been studied in the presence of some transition metal catalysts. The characteristic addition-elimination reaction that afforded the 3-malonyl-pyran derivative 3 was found to proceed with the intervention of highly polar species, as indicated by the appearance of skeletal rearrangement products 4a,b that stem from the fragmentation of a carbenium ion at C-2 on the pyran ring. Molecular reorganization was strongly dependent on lingands and metal atom of the catalyst. This was explained as being the consequence of either a polar open-end transition state containing metal catalyst and dimethoxycarbonyl methylene sigma bonded to C-3 of the pyran ring, or as a sigma olefin-metal carbene complex where stabilization of the positive charge is provided by a strong polar interaction with high electron density centers of appropriate ligands.