Cyclic alkyl amino carbenes

A CAAC - The carbene functionality is shown as two dots which represent the two unpaired electrons capable of bonding to a vacant coordination site. The R groups show the potential for derivitisation of the CAAC at these points.

In chemistry, cyclic alkyl amino carbenes (CAACs) are a type of cyclic carbene which contain a single nitrogen atom within the cyclic structure.^[1] CAACs were first developed by Guy Bertrand at UCSD as an alternative to traditional N-heterocyclic carbenes which contain two nitrogen atoms with in the structure typically derived from imidazole. Replacing a single nitrogen in the NHC to a quaternary carbon in the CAAC makes a difference to the electron donating abilities of the carbene hence making it a stronger π-acceptor as well as σ-donor ligand.^[2]

Current research is investigating the use of these ligands in organometallic compounds as catalysts^[3] and into developing new derivatives of CAACs such as bicyclic CAACs.^[4]

CAAC synthesis

Cyclic alkyl amino carbenes can be prepared from 2,6-disopropylaniline and 2-methylpropanal. The initial step involves the deprotonation of 2,6-disopropylaniline with lithium diisopropylamide to give a aza-allyl anion. The aza-allyl anion then induces the ring opening of 1,2-epoxy-2-methylpro-pane leading to a lithium alkoxide. The lithium alkoxide is then treated with triflic anhydride at -78 celsius, this is then warmed to room temperature to generate the aldiminium salt. The production of the aldiminium salt gives a yield of 58% with respect to 2,6-disopropylaniline. The final step in the synthesis uses lithium diisopropylamide to generate the carbene which is a white solid.^[1]

CAAC synthesis

References