Stability trends and tautomerization of chalcocyclopentadienes. The role of aromaticity

Abstract

The stability trends and tautomerization processes within the family of chalcocyclopentadienes (CpXH, X = O, S, Se, Te) have been investigated through the use of high-level G3B3 and G2 ab initio, as well as B3LYP density functional theory calculations. All methods predict the 5-substituted derivative to be the least stable tautomer, with the only exception of the Te derivative. For X = O, Se the dominant forms are the 2- and the 1-substituted derivatives, respectively, whereas for S, an equal distribution of the two forms is predicted. The enhanced stability of the 5-substituted compound on going from O to Te is due to a parallel increase of the aromaticity of the system, through a charge donation from the C–X bond toward the five membered ring. Independently of the nature of the chalcogen heteroatom, the global minimum of the PES is the asymmetric keto structure 2-cyclopenten-1-one. Its enhanced stability with respect to the symmetric keto form 3-cyclopenten-1-one is due to a more favorable conjugation between the CX and the CC π-systems. This interaction steadily increases on going from O to Te. The barriers connecting the keto and the enolic forms are rather high, as those connecting the enolic forms among each other. Hence, an important conclusion is that, in the gas-phase, if the 5-substituted derivative is formed, it should be observable no matter the nature of the chalcogen heteroatom.