How fluorination collapses inversion barriers and increases electrophilicity in bicyclo[1.1.0]butanes

Area of Science:

• Organic Chemistry
• Computational Chemistry
• Materials Science

Background:

• Bicyclo[1.1.0]butane (BCB) possesses a highly strained structure due to its central C-C bond with significant p-character.
• The inherent strain of BCB dictates its unique reactivity patterns.

Purpose of the Study:

• To investigate the impact of progressive fluorination on the structural and electronic properties of BCB.
• To elucidate the structure-reactivity relationships governing fluorinated BCB analogues.
• To establish a predictive framework for designing novel fluorinated strained molecules.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to model the electronic structure and energy barriers.
• Natural Bond Orbital (NBO) analysis was used to understand bonding characteristics and stabilization effects.
• Computational simulations of nucleophilic attack quantified reactivity changes.

Main Results:

• Progressive fluorination markedly reduces BCB inversion barriers, from ~64 kcal mol⁻¹ in BCB to ~3.6 kcal mol⁻¹ in perfluorinated analogues.
• Bridgehead (C1/C3) substitution enhances hyperconjugation but incurs steric penalties; C2/C4 substitution offers a better balance.
• Fluorination lowers LUMO energies, increasing electrophilicity and facilitating nucleophilic attack, with activation barriers dropping from 44.2 kcal mol⁻¹ to 6.5 kcal mol⁻¹.

Conclusions:

• Highly fluorinated BCBs exhibit enhanced reactivity under mild conditions due to lowered inversion barriers and increased electrophilicity.
• The study establishes key structure-reactivity relationships for fluorinated BCBs.
• Findings provide a foundation for designing fluorinated strained building blocks in synthetic and medicinal chemistry.