Inducing and quantifying forbidden reactivity with single-molecule polymer mechanochemistry.

Area of Science:

• Chemical dynamics
• Physical organic chemistry
• Mechanochemistry

Background:

• Forbidden reactions, violating Woodward-Hoffmann rules, are challenging to study experimentally due to competing allowed pathways.
• Covalent mechanochemistry offers a novel approach to activate and investigate these inaccessible reactions.

Purpose of the Study:

• To experimentally quantify the 'forbiddenness' of reactions using single-molecule force spectroscopy.
• To investigate mechanically induced reactions along both symmetry-allowed and symmetry-forbidden pathways.

Main Methods:

• Single-molecule force spectroscopy was employed to study three distinct ring-opening reactions.
• Mechanical forces were applied to induce reactions on a ~0.1 s timescale, comparing allowed and forbidden pathways.

Main Results:

• The study quantified the force required to overcome activation barriers for symmetry-forbidden reactions.
• Benzocyclobutene, gem-difluorocyclopropane, and gem-dichlorocyclopropane reactions showed varying force requirements for their forbidden pathways compared to allowed ones.
• Specifically, forbidden reactions required ~130 pN less (benzocyclobutene) or ~560-1000 pN more (cyclopropanes) force than their allowed counterparts.

Conclusions:

• This research provides the first experimental benchmarks for mechanically induced forbidden reactions.
• The findings offer insights into the energetic landscape of forbidden reactions and may necessitate revisions to computational predictions.