β-Lactam Ylidenes: An Overlooked Class of N-Heterocyclic Carbenes.

Abstract

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.