How does aromaticity rule the thermodynamic stability of hydroporphyrins?

Area of Science:

• Computational Chemistry
• Organic Chemistry
• Quantum Chemistry

Background:

• Hydroporphyrins are partially hydrogenated porphyrins with varying stability.
• Understanding their stability and reactivity is crucial for synthetic and biological applications.

Purpose of the Study:

• To investigate the stability sequence of hydroporphyrins from porphin to octahydroporphin.
• To rationalize preferred hydrogenation pathways using aromaticity concepts.
• To explain the synthetic and natural inaccessibility of octahydroporphin.

Main Methods:

• Topological resonance energies (TREs) and circuit effects.
• Bond resonance energies and multicenter delocalization indices.
• Ring current maps, magnetic susceptibilities, and nuclear-independent chemical shifts (NICS).
• Density Functional Theory (DFT) for hydrogenation energies.

Main Results:

• Multiple aromaticity measures (energetic, magnetic, electron density) provide consistent information.
• All methods successfully explain the stability order of hydroporphyrins.
• Calculated hydrogenation energies align with observed stability and inaccessibility.

Conclusions:

• Aromatic stabilization is the key factor governing hydroporphyrin stability and reactivity.
• The inaccessibility of octahydroporphin is directly linked to its aromatic character.
• Consistent results across diverse aromaticity metrics validate their utility in porphyrinoid chemistry.