Saturated N, X-Heterocyclic Carbenes (X = N, O, S, P, Si, C, and B): Stability, Nucleophilicity, and Basicity

Various saturated five-membered N, X-heterocyclic carbenes (X = N, O, S, P, Si, C, and B) have been studied by ab initio and density functional theory (DFT) methods. The substitutions alter the properties of the reference carbene from the viewpoint of electronic structure, stability, nucleophilicity, and basicity. Our study shows that the oxygen containing carbene (X = O) induces the highest HOMO-LUMO energy gap (Delta EHOMO-LUMO), while carbene with X = N has the widest singlet-triplet energy difference (Delta Es-t). The nucleophilicity of the carbene derivatives increased upon replacement of C, Si, and B, with the effect of the boron substituent being more pronounced. In addition, the basicity of the structure increased for the carbene derivatives with X = C and B with the latter substitution imposing a remarkably higher effect. Moreover, the substitution of boron at the alpha-position of the carbene increased the nucleophilicity and basicity, while inducing a reduction in the values of Delta Es-t and Delta EHOMO-LUMO.