Why does perfluorination render bicyclo[2.2.0]hex-1(4)-ene stable toward dimerization? Calculations provide the...

Area of Science:

• Computational Chemistry
• Organic Chemistry
• Physical Chemistry

Background:

• Bicyclo[2.2.0]hex-1(4)-ene (1a) undergoes facile dimerization.
• Perfluorobicyclo[2.2.0]hex-1(4)-ene (1b) dimerization has not been observed.
• Understanding the factors influencing these reactions is crucial for predicting reactivity.

Purpose of the Study:

• To elucidate the reasons behind the differing dimerization behaviors of 1a and 1b.
• To quantify the energetic contributions to the observed reactivity differences.

Main Methods:

• B3LYP density functional theory calculations.
• Utilized two distinct basis sets for computational rigor.
• Analyzed reaction enthalpies, intermediate formation, and cyclization energies.

Main Results:

• Dimerization of 1a is exothermic (37.2 kcal/mol), while 1b dimerization is endothermic (7.4 kcal/mol).
• The pi bond in 1b is significantly stronger than in 1a (16 ± 1 kcal/mol difference).
• Fluorine's influence includes hyperconjugation and unfavorable syn-periplanar interactions with radical centers.

Conclusions:

• The enhanced pi bond strength in 1b, due to hyperconjugation and steric/electronic repulsion from fluorine atoms, prevents its dimerization.
• The calculated energy difference between dimerization reactions of 1a and 1b is 44.6 kcal/mol.