Clustering Modulates Reaction Mechanisms: A Case Study on Ester C-O Bond Cleavage

Area of Science:

• Chemical reactions
• Organic chemistry
• Polymer science

Background:

• Ester bond cleavage is crucial for polyester recycling and synthesis.
• Traditional electronic effects fail to explain selective C-O bond cleavage by ionic liquids.
• Ionic liquids, specifically halide-based ones, exhibit unexpected selectivity in polyester degradation.

Purpose of the Study:

• To investigate the mechanism behind selective C-O bond cleavage in ester bonds by ionic liquids.
• To propose and validate the role of solution clusters in determining reaction pathways.
• To elucidate the governing principles of C-O bond cleavage selectivity in ionic liquid-ester systems.

Main Methods:

• Excess infrared spectroscopy to identify aggregate species in solution.
• Molecular dynamics simulations to analyze reactant solvation and proximity.
• Density functional theory calculations to model reaction pathways and transition states.

Main Results:

• Experimental evidence of microheterogeneous solutions with distinct ionic liquid-ester clusters.
• Identification of a catalytically relevant cluster, [BMIm]Br(MB)3, with preferential C-alkoxy solvation around the bromide ion.
• Computational analysis revealing proximity effects and S_N2-like transition states governing C-O bond cleavage.

Conclusions:

• Solution clustering, not just electronic effects, dictates ester bond cleavage pathways.
• The proximity effect, mediated by clusters, is the primary driver of C-O bond cleavage selectivity.
• This finding offers new insights into designing catalysts for selective polyester recycling and organic synthesis.