Dihydrophenanthrene Open-Shell Singlet Diradicals and Their Roles in the Mallory Photocyclization Reaction.

Area of Science:

• Computational chemistry
• Organic reaction mechanisms
• Photochemistry

Background:

• The Mallory photocyclization is a key organic reaction.
• Dihydrophenanthrene (DHP) intermediates are central to this reaction.
• Understanding the electronic states of DHPs is crucial for reaction control.

Purpose of the Study:

• To computationally investigate the electronic states of DHP intermediates in the Mallory photocyclization.
• To classify DHPs based on their closed-shell (CS) or open-shell-diradical (OS) character.
• To explore the factors influencing DHP stability, including aromaticity and triplet energy.

Main Methods:

• Utilized computational chemistry, specifically the ωB97X-D/6-31G(d) method.
• Analyzed properties of DHPs to classify them as OS, borderline-OS, borderline-CS, or CS.
• Employed the Harmonic Oscillator Model of Aromaticity (HOMA) to assess aromatic stabilization.
• Studied thermal decomposition pathways (cycloreversions) of DHPs.

Main Results:

• DHPs can exist as either CS or OS singlet ground states.
• A classification scheme for DHP intermediates was developed.
• The triplet electronic state and higher energy CS* isomer of OS diradicals were located.
• Established relationships between the diradical index (y_o), triplet energy, and OS-CS* energy gaps.
• Confirmed the significant role of aromaticity in stabilizing OS singlet diradicals.
• Characterized the cycloreversion mechanism as homolytic cleavage with anchimeric assistance.

Conclusions:

• Dihydrophenanthrene intermediates in the Mallory reaction exhibit diverse electronic ground states.
• Aromaticity is a key stabilizing factor for open-shell diradical intermediates.
• Computational methods provide valuable insights into reaction mechanisms and intermediate properties.