Aromaticity of dihetero analogues of pentalene dianion. X-Ray and ab initio studies of eight methyl furo[3,2-b]pyrrole-5-carboxylate derivatives and five methyl furo[2,3-b]pyrrole-5-carboxylate derivatives

The crystal and molecular structures of eight methyl furo[3,2-b]pyrrole-5-carboxylate derivatives (4H; 2,3-dimethyl-4H; 2-formyl-4H; 2-(4-chloro-2-methyl)phenoxy-4-methyl; 4-benzyl-2-methyl; 4-benzyl-2,3-dimethyl; 4-benzyl-2-formyl; 4-benzyl-2-(4-chloro-2-methyl)phenoxy and five methyl furo[2,3-b]pyrrole-5-carboxylate derivatives (2-formyl-6H, 2-formyl-6-methyl, 2-cyano-6-methyl, 6-benzyl-2-formyl-, 6-benzyl-2-cyano) have been solved by X-ray diffraction and supplemented by ab initio RHF/6-311+G∗∗ and B3LYP/6-311+G∗∗ calculations. The molecular geometries of the furo[3,2-b]pyrrole and furo[2,3-b]pyrrole fragment were used to study the aromatic character of these systems leading to the conclusion that the aromaticity of the rings practically does not depend on the nature of the atom in a neighbouring ring if a similar topological pattern is concerned. In line with the observed stability of the systems, the aromaticity of the furo[2,3-b]pyrrole derivatives is definitely smaller in comparison with the structural isomer the furo[3,2-b]pyrrole, implying that the most important factor which determines the aromatic character of a moiety is of topological nature. The stability and aromaticity of the compounds reveal a strong dependence on the substituent effect, much stronger than in the case of benzene derivatives.