Effects of α-cyclopropyl on heterocyclic carbenes stability at DFT

α-Cyclopropyl stability impacts on singlet and triplet heterocyclic carbenes with acyclic, cyclic, and cyclic-unsaturated structures are compared and contrasted to di-t-butyl as well as t-butylcyclopropylcarbenes through appropriate isodesmic reactions at B3LYP/AUG-cc-pVTZ level. Substitution of one of the t-butyl groups of di-t-butylcarbene with a cyclopropyl alters the ground state multiplicity from triplet to singlet with a singlet-triplet energy separation (ΔE_s-t) of 7.2 kcal/mol. Additional heteroatom substitution increases ΔE_s-t values for the resulting α-heteroatom cyclopropylcarbenes in the following order: amino > oxy > thio > phophino. α-Cyclopropyl group stabilizes singlet states of all our carbenes two to three times more than their corresponding triplet states. The ΔE_s-t values of all the carbenes are increased through cyclization, while the introduction of unsaturation in the rings causes small and rather random changes. To probe the kinetic stability of the species, we calculated the transition states for the opening of cyclopropyl through 1,2-C shift. Interestingly, the 4.1 kcal/mol energy barrier in cyclopropylcarbene is significantly increased in the presence of heteroatoms to 31.2 kcal/mol for aminocyclopropylcarbene. The reactivity of the species is discussed in terms of nucleophilicity and electrophilicity issues showing our carbenes, especially acyclic ones, more nucleophilic than the common N-heterocyclic carbenes.