When does carbonylation of carbenes yield ketenes? A theoretical study with implications for synthesis.

Quantum-chemical calculations using DFT and ab initio methods have been carried out for 32 carbenes RR'C which comprise different classes of compounds and the associated ketenes RR'C═C═O. The calculated singlet-triplet gaps ΔE(S-T) of the carbenes exhibit a very high correlation with the bond dissociation energies (BDEs) of the ketenes. An energy decomposition analysis of the RR'C-CO bond using the triplet states of the carbene and CO as interacting fragments supports the assignment of ΔE(S-T) as the dominant factor for the BDE but also shows that the specific interactions of the carbene may sometimes compensate for the S/T gap. The trend of the interaction energy ΔE(int) values is mainly determined by the Pauli repulsion between the carbene and CO. The stability of amino-substituted ketenes strongly depends on the destabilizing conjugation between the nitrogen lone-pair orbital and the ketene double bonds. There is a ketene structure of the unsaturated N-heterocyclic carbene parent compound NHC1 with CO as a local energy minimum on the potential-energy surface. However, the compound NHC1-CO is thermodynamically unstable toward dissociation. The saturated homologue NHC2-CO has only a very small bond dissociation energy of D(e) = 3.2 kcal/mol. The [3]ferrocenophane-type compound FeNHC-CO has a BDE of D(e) = 16.0 kcal/mol.