N/CO Exchange at a Vinylidene Carbon Center: Stable Alkylidene Ketenes and Alkylidene Thioketenes from 1,2,3-Triazole Derived Diazoalkenes.

We present a new class of room-temperature stable diazoalkenes featuring a 1,2,3-triazole backbone. Dinitrogen of the diazoalkene moiety can be thermally displaced by an isocyanide and carbon monoxide. The latter alkylidene ketenes are typically considered as highly reactive compounds, traditionally only accessible by flash vacuum pyrolysis. We present a new and mild synthetic approach to the first structurally characterized alkylidene ketenes by a substitution reaction. Density functional theory calculations suggest the substitution with isocyanides to take place via a stepwise addition/elimination mechanism. In the case of carbon monoxide, the reaction proceeds through an unusual concerted exchange at a vinylidene carbon center. The vinylidene ketenes react with carbon disulfide via a four-membered thiete intermediate to give vinylidene thioketenes under release of COS. We present spectroscopic as well as structural data for the complete isoelectronic series (RC═C═X; X = N, CO, CNR, CS) including (C-C) data. As N, CO, and isocyanides belong to the archetypical ligands in transition-metal chemistry, this process can be interpreted in analogy to coordination chemistry as a ligand exchange reaction at a vinylidene carbon center.