Structure–property relationships for the force-triggered disrotatory ring-opening of cyclobutene

Abstract

Symmetry forbidden reactions are notoriously difficult to study experimentally, for the simple reason that their competing symmetry allowed pathways typically dominate. Covalent polymer mechanochemistry offers an opportunity to broaden access to symmetry forbidden reactions, through the judicious placement of polymer handles on mechanophore reactants. Here, single molecule force spectroscopy and computation are used to evaluate substituent effects on the disrotatory ring opening reaction of cyclobutene to butadiene. Theory and experiment reveal that the formally forbidden reaction is more sensitive to substituents on the scissile carbon–carbon bond than on the alkene, with each of two Me substituents providing approximately 1.5–2 kcal mol⁻¹ of stabilization and a trimethylsilyl alkyne substituent approximately 4.5–6.5 kcal mol⁻¹.