Applications of Carbenes in Organometallic Chemistry
When the first stable carbene was isolated in 1988 it was initially seen as simply a laboratory curiosity, but since this seminal report, carbenes have proven to be powerful tools in all fields of molecular science. This can be attributed to the breadth of stereoelectronic properties carbenes can possess which allows for a wide array of applications. These neutral carbon-based species have had the most impact in the field of homogeneous transition metal catalysis, where their strong σ-donating ability has allowed for the development of increasingly robust catalyst systems. Indeed, by using strongly-donating mesoionic carbenes in gold-catalyzed hydroamination, parent hydrazine could be exploited to develop a new method for the incorporation of nitrogen-nitrogen bonds into molecules.
The stabilizing ability of carbenes is further exemplified by cyclic (alkyl)(amino)carbenes, which are both strong σ-donors and π-acceptors. Using these carbenes, key bis(copper) active species in the ubiquitous Click reaction were isolated, which allows for a better understanding of the catalytic cycle to guide development of current methodologies.
Although carbenes are most widely known for their ability to modulate reactivity at metal centers, they are also capable of displaying transition metal-like reactivity on their own. Ambiphilic carbenes — whose frontier orbitals resemble to some extent the electronic situation at a metal center — can activate small molecules and perform the oxidative addition of strong chemical bonds. Furthermore, it was found that this transition metal mimicry can even be extended to the steric properties of a carbene. Just as for transition metals, the steric environment is a key parameter in promoting the reductive elimination from a carbon center. With this knowledge in hand, reactions that were traditionally under the purview of transition metals can now be extended to carbon-based molecules.