Relative migratory aptitudes of hydrogen, benzoyl, 4-methoxyphenyl and 4-nitrophenyl groups in some unsaturated carbenes (vinylidenes)

([5-13C] Tetrazol-5-yl)methyl ketones were prepared and subjected to oxidative fragmentation induced by lead tetraacetate. The resulting intermediate [1-13C]-3-oxoprop-1-en-1-ylidenes rearrange, depending on the relative migratory aptitudes of the benzoyl group and the ligands R, either to [3-13C]prop-2-yn-1-ones or to [2-13C]prop-2-yn-1-ones or to mixtures of the two isomers. The 1H and/or 13C NMR spectra of the products allow the three cases to be distinguished. The relative migratory aptitudes were found to be H>PhCO, 4-MeOC6H4>4-O2NC6H4.