UC San Diego

Initially considered as laboratory curiosities, singlet carbenes have rapidly become one of the most powerful tools in organic, inorganic, and organometallic chemistry. The widespread use of carbenes can be attributed to their readily tunable electronic and steric properties. Indeed, modification of steric hindrance is simply achieved by changing the substituents around the carbene center. Moreover, the atoms adjacent to the carbene center, the nature of the backbone, and the size of the heterocycle have an impact on the relative energies of the HOMO-LUMO gaps and thus on the electronic properties of the carbene. A better understanding of the influence of these parameters on the behavior of carbenes allows to obtain the desired reactivities and selectivity. For example, it has been shown that cyclic bent allenes are extremely basic and can compete with the strongest organic superbases. This opens new avenues for carbon-centered superbases. Ring size also has an impact on the sigma-donation of carbenes. Ring-expanded N-heterocyclic exhibit higher reactivity than their 5-membered counterparts and can lead to metal-like reactivities. Finally, the neighboring atoms of the carbene allow the introduction of a chiral center which can result in asymmetric reactions. They can also be good leaving groups, for example in the case of halocarbenes, making carbenes the synthetic equivalent of R2NC+.