Relative Aromaticity of Pyrrole, Furan, Thiophene and Selenophene, and Their Diels–Alder Stereoselectivity

Thiophene has been suggested to have better aromatic character than pyrrole and furan for its poorest reactivity under Diels–Alder cycloadditions conditions. Many theories have been developed around this notion over a period of time, prominent among them being aromaticity index based on interaction coordinates (AIBIC), nucleus-independent chemical shift (NICS), topological resonance energy (TRE), magnetic resonance energy (MRE), ring current (RC), and ring current diamagnetic susceptibility. However, they are not consistent because different aromaticity values derived from different indices lead to different aromaticity orders. The suitability of the NICS and some other approaches for the prediction of aromaticity has also been questioned. Aromaticity follows Huckel’s 4n + 2 (n = 0, 1, 2, 3, etc.) rule. The electron count, however, is not enough. Along with the number of requisite electrons, all the bond lengths in the ring must be the same or very similar, as in benzene. In five-membered heterocylces such as pyrrole, furan, and thiophene, the size of the heteroatom lone pair orbital must be similar to that of the p orbital on ring π bonds. Both the requirements are not fulfilled in thiophene because σC–S bond is significantly longer than σC–C bond and, in comparison to the p orbital on carbon, the size of lone pair orbital on sulfur is too large to allow effective overlap. It is demonstrated with enough experimental data available in the literature and new data from computations that the separation between the termini of 1,3-diene is a significant control factor as the p orbitals on these carbons must coaxially interact with the p orbitals on dienophile to result in σ bonds. This control factor has been named R-factor.