Aromaticity enhancement of cyclopentadiene in piperidine derivatives: a DFT study on combination of the anomeric effect and Schleyer hyperconjugative aromaticity

Abstract

Schleyer introduced hyperconjugative aromaticity and antiaromaticity in 5-substituted cyclopentadienes. In this study, the cyclopentadiene hyperconjugative aromaticity on the anomeric carbon atom of piperidine derivatives was computationally investigated through structural, electronic, energetic and magnetic indices of aromaticity. The analysis is based on 11 optimized structures at the B3LYP/6-311++G(d,p) level of theory. The results show that the anomeric effect induces substantial aromaticity in cyclopentadiene. According to geometry-based indices (HOMA and ΔBL), the hyperconjugative aromaticity follows the trend: 9ax > 7ax > 2ax > 8ax > 3ax > 1ax > 5ax > 4ax > 6ax. The second order perturbation energy NBO analysis revealed that the highest stabilization energies belong to the structures bearing cyclopentadiene with electron withdrawing groups. These conclusions are supported by calculated aromatic and isomerization stabilization energies and magnetic descriptors.