Monoheteroatom substituted six-membered carbenes: A computational survey of stability and reactivity

1-(X)-3,3-dimethyl-2-cyclohexanylidenes, where X = amino, oxy, phosphino, and thio, are compared and contrasted to probe the effect of monoheteroatom substitution on the stability, multiplicity and reactivity of six-membered saturated and unsaturated cyclic carbenes. The stabilizing effects of substituents are discussed through geometrical parameters, and AIM analysis. The strong π-donor/σ-acceptor amino group exerts singlet-triplet energy separation (ΔE_S-T) of 38.4 kcal/mol while more electronegative oxy group induces a ΔE_S-T of 23.0 kcal/mol. Aminoalkylcarbenes rebuff dimerization while oxyalkylcarbenes show a high dimerizing affinity. The reactivity of species is discussed in terms of nucleophilicity and electrophilicity indices as well as proton affinities. It seems that our six-membered carbenes are more nucleophilic than common five-membered N-heterocyclic carbenes.