Effect of Lewis Acids on Photoenolization/Diels-Alder Reactions

Area of Science:

• Organic Chemistry
• Photochemistry
• Catalysis

Background:

• Ortho-methyl benzaldehydes and acetophenones undergo photoenolization, forming intermediates for Diels-Alder reactions.
• The role of Lewis acids in the photoenolization step preceding Diels-Alder reactions is not well understood.

Purpose of the Study:

• To investigate how Lewis acid coordination influences photoenolization/Diels-Alder (PEDA) reactivity.
• To elucidate the mechanisms by which Lewis acids modulate PEDA reactions.

Main Methods:

• Combined theoretical calculations and experimental studies.
• Investigated Lewis acid coordination effects on excitation energies, hydrogen-atom-transfer (HAT) barriers, and conformer equilibrium.
• Studied model systems and analyzed literature cases.

Main Results:

• Lewis acid coordination can red-shift excitation energies and alter HAT barriers.
• Coordination shifts the equilibrium between reactive syn and unreactive anti conformers.
• Observed variable effects of Lewis acids (e.g., Li+, Ca2+) on photoenolization, depending on substrate chelation and Lewis acid properties.

Conclusions:

• Lewis acid coordination profoundly impacts PEDA reactivity through electronic and conformational changes.
• The influence of Lewis acids is highly dependent on factors like substrate basicity, Lewis acid stoichiometry, and coordination environment.
• Lewis acids can either promote or inhibit photoenolization, highlighting the complexity of their catalytic role.