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 (R2CCX; X = N2, CO, CNR, CS) including 1 J(13C–13C) data. As N2, 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.

Recently, stable diazoalkenes have received significant attention as a new substance class in organic chemistry. While their previous synthetic access was exclusively limited to the activation of nitrous oxide, we here establish a much more general synthetic approach utilizing a Regitz‐type diazo transfer with azides. Importantly, this approach is also applicable to weakly polarized olefins such as 2‐pyridine olefins. The new pyridine diazoalkenes are not accessible by the activation of nitrous oxide, allowing for a considerable extension of the scope of this only recently accessed functional group. The new diazoalkene class has properties distinct from the previously reported classes, such as photochemically triggered loss of dinitrogen affording cumulenes and not C−H insertion products. Pyridine‐derived diazoalkenes represent the so far least polarized stable diazoalkene class reported.

Singlet vinylidenes (R 2 C=C:) are proposed as intermediates in a series of organic reactions, and very few have been studied by matrix isolation or gas-phase spectroscopy. Triplet vinylidenes, however, featuring two unpaired electrons at a monosubstituted carbon atom are thus far only predicted as electronically excited-state species and represent an unexplored class of carbon-centered diradicals. We report the photochemical generation and low-temperature EPR/ENDOR characterization of the first ground-state high-spin (triplet) vinylidene. The zero-field splitting parameters ( D = 0.377 cm –1 and | E| / D = 0.028) were determined, and the 13 C hyperfine coupling tensor was obtained by 13 C-ENDOR measurements. Most strikingly, the isotropic 13 C hyperfine coupling constant (50 MHz) is far smaller than the characteristic values of triplet carbenes, demonstrating a unique electronic structure which is supported by quantum chemical calculations.

Kürzlich haben stabile Diazoalkene als neue Substanzklasse in der organischen Chemie große Aufmerksamkeit erhalten. Während ihr bisheriger synthetischer Zugang ausschließlich auf die Aktivierung von Distickstoffoxid (N 2 O) beschränkt war, etablieren wir hier einen allgemeineren synthetischen Ansatz, der einen Regitz-Diazotransfer-artigen Ansatz mit Aziden ausnutzt. Wichtig ist, dass diese Strategie auch auf schwach polarisierte Olefine wie 2-Pyridinolefine anwendbar ist. Die neuen Pyridindiazoalkene sind nicht durch die Aktivierung von Distickstoffoxid (N 2 O) zugänglich, was eine beträchtliche Erweiterung des Anwendungsbereichs dieser erst seit kurzem zugänglichen funktionellen Gruppe bedeutet. Diese neue Diazoalkenklasse hat Eigenschaften, die sich von den bisher bekannten Substanzklassen unterscheiden, wie z. B. der photochemisch ausgelöste Verlust von Distickstoff, der zu Cumulenen und nicht zu C-H-Insertionsprodukten führt. Von Pyridin abgeleitete Diazoalkene stellen die bisher am wenigsten polarisierte, stabile Diazoalkenklasse dar.