Cyclic (alkyl)(amino)carbenes (CAACs): stable carbenes on the rise

CONSPECTUS: Carbenes are compounds that feature a divalent carbon atom with only six electrons in its valence shell. In the singlet state, they possess a lone pair of electrons and a vacant orbital and therefore exhibit Lewis acidic and Lewis basic properties, which explains their very high reactivity. Following the preparation by our group in 1988 of the first representative, a variety of stable carbenes are now available, the most popular being the cyclic diaminocarbenes. In this Account, we discuss another class of stable cyclic carbenes, namely, cyclic (alkyl)(amino)carbenes (CAACs), in which one of the electronegative and π-donor amino substituents of diaminocarbenes is replaced by a σ-donating but not π-donating alkyl group. As a consequence, CAACs are more nucleophilic (σ-donating) but also more electrophilic (π-accepting) than diaminocarbenes. Additionally, the presence of a quaternary carbon in the position α to the carbene center provides steric environments that differentiate CAACs dramatically from all other ligands. We show that the peculiar electronic and steric properties of CAACs allow for the stabilization of unusual diamagnetic and paramagnetic main group element species. As examples, we describe the preparation of room temperature stable phosphorus derivatives in which the heteroatom is in the zero oxidation state, nucleophilic boron compounds, and phosphorus-, antimony-, boron-, silicon-, and even carbon-centered neutral and cationic radicals. CAACs are also excellent ligands for transition metal complexes. The most recent application is their use for the stabilization of paramagnetic complexes, in which the metal is often in a formal zero oxidation state. Indeed, bis(CAAC)M complexes in which the metal is gold, copper, cobalt, iron, nickel, manganese, and zinc have been isolated. Depending on the metal, the majority of spin density can reside either on the metal or on the carbene carbons and the nitrogen atoms of the CAAC ligand. In contrast to diaminocarbenes, the higher basicity of CAACs makes them poor leaving groups, and thus they cannot be used for classical organocatalysis. However, because of their superior electrophilicity and smaller singlet-triplet gap, CAACs can activate small molecules at room temperature, such as CO, H2, and P4, as well as enthalpically strong bonds, such as B-H, Si-H, N-H, and P-H. Lastly, excellent results have been obtained in palladium, ruthenium, and gold catalysis. CAAC-metal complexes are extremely thermally robust, which allows for their utilization in harsh conditions. This property has been used to perform a variety of gold-catalyzed reactions in the presence of basic amines, including ammonia and hydrazine, which usually deactivate catalysts.