In this study, the underappreciated class of β-lactam carbenes (BLCs) is revisited and systematically explored. The required precursors are available in a highly modular synthetic approach from simple starting materials. Two routes may be followed for the generation of the free BLCs: thermal fragmentation of spiro-fused oxadiazoles (110 °C, toluene), or deprotonation of carbene HCl-adducts at -80 °C with a strong base. TEP values (2065-2072 cm-1), 77Se NMR shifts of selenium-adducts (812-995 ppm) as well as DFT calculations reveal moderate σ-donor and strong π-acceptor character of BLCs, leading to a pronounced ambiphilic reactivity. The formation of a wide variety of products under thermal conditions in the cause of oxadiazole fragmentation (110 °C, toluene) is rationalized by a combination of computational reaction discovery and experimental validation. Products include olefinic dimers as well as unprecedented N2-bridged dimers arising from bimolecular reactions, while sterically more demanding precursors are converted in a monomolecular fragmentation reaction via CO release to form ketenimines as intermediates, which finally form amides. With substituents providing appropriate steric protection, a persistent BLC is available via the low temperature route, characterized by its 13C NMR shift of 287 ppm at -20 °C.
Read more in:
β-Lactam Ylidenes: an Overlooked Class of N-heterocyclic Carbenes
Leonard Karl, Daniel Deißenbeck, Jan Meisner, Christian Ganter
Chem. Eur. J., 2025