Ring carbo-mers of “aromatic” heterocycles

Abstract

The structural and magnetic aromaticity of the ring carbo-mers of fully conjugated heterocycles has been investigated for their calculated geometries at the B3PW91/6-31G** level. carbo-pyridine and carbo-phosphinine, C₁₇H₅X with X = N and P, respectively, are found to be aromatic according to orbital (18 conjugated π_z electrons), structural (planarity, bond length equalization) and magnetic (NICS) criteria. As expected from the Hückel rule, the ring carbo-mers C₁₄H₄X of furan, pyrrole, thiophene and phosphole with X = O, NH, S and PH, respectively, exhibit no marked structural or magnetic aromatic character. For X = NH, PH, formal hydride abstraction gives the corresponding cationic structures C₁₄H₄Y⁺, with Y = P, N and 14 conjugated π_z electrons, which are calculated to be aromatic according to structural and magnetic criteria. carbo-phosphole oxide, formally C₁₄H₄P⁺–O⁻ with 14 (homo)conjugated out-of-plane π_z electrons, is found to be homoaromatic on the basis of NICS calculations, orbital and electron localization function (ELF) analyses. Likewise, the three-membered carbo-meric structures C₈H₂X, with X = O, NH or PH and 10 conjugated π_z electrons, are found to be structurally and magnetically aromatic. The latter neutral structures are expected to be viable synthetic targets.