Neutral-cumulenic diazo (X[double bond, length as m-dash]C[double bond, length as m-dash]N2) compounds.

Reactive intermediates play a crucial role in organic chemistry, and their isolation and structural characterization have long been of interest to experimental chemists. Among all textbook summaries of reactive intermediates, neutral-cumulenic diazo compounds of the structure X[double bond, length as m-dash]C[double bond, length as m-dash]N2 (X = R2C, R3P, R2S) are of immense interest due to their unique bonding features and wide synthetic applications in main-group and transition-metal chemistry, even though their direct spectroscopic detection proved highly challenging. In particular, diazoalkenes (R2C[double bond, length as m-dash]C[double bond, length as m-dash]N2) attracted significant interest as synthetically important intermediates and in the late 20th century several research groups probed their existence, mainly indirectly through trapping experiments or directly through matrix-isolation studies. In 2021, the Severin group and our group independently reported the synthesis and characterization of the first room-temperature-stable diazoalkenes, followed by our recent discovery of the diazo heterocumulenes Ph3P[double bond, length as m-dash]C[double bond, length as m-dash]N2 and Ph2S[double bond, length as m-dash]C[double bond, length as m-dash]N2, which initiated a rapidly expanding research field. These bottleable neutral-cumulenic diazo compounds have proven to be pivotal synthons in organic transformations, single carbon atom doping (SCAD) reactions, transition metal chemistry, and low-coordinated main group complexes, with tremendous future promise.