Accelerating a Mechanically Driven anti-Woodward-Hoffmann Ring Opening with a Polymer Lever Arm Effect

Area of Science:

• Polymer Chemistry
• Mechanochemistry
• Organic Synthesis

Background:

• Orbital symmetry rules, such as the Woodward-Hoffmann rules, dictate the feasibility of pericyclic reactions.
• Mechanical forces have emerged as a tool to bypass these electronic constraints, enabling symmetry-forbidden transformations.
• Benzocyclobutene (BCB) derivatives are known to undergo ring-opening reactions, often influenced by external stimuli.

Purpose of the Study:

• To investigate the role of a polymer "lever arm effect" in facilitating symmetry-forbidden reactions.
• To quantify the reduction in force required for the disrotatory ring opening of BCB using mechanochemical methods.
• To demonstrate the application of single-molecule force spectroscopy in studying polymer-assisted mechanochemistry.

Main Methods:

• Utilized single-molecule force spectroscopy (SMFS) to apply and measure picoNewton forces.
• Designed and synthesized benzocyclobutene (BCB) functionalized with an α-E-alkene as a mechanophore.
• Employed polymer chains to act as a "lever arm" to amplify applied forces on the BCB unit.

Main Results:

• The polymer lever arm effect significantly reduced the force required for the disrotatory ring opening of BCB.
• The force needed decreased from 1370 pN to 920 pN under experimental conditions (∼0.1 s timescale).
• This demonstrates a substantial mechanical advantage conferred by the polymer architecture.

Conclusions:

• A polymer lever arm effect can mechanochemically drive symmetry-forbidden reactions, specifically the ring opening of BCB.
• This approach offers a method to lower the energy barrier for reactions that are electronically disfavored.
• Mechanochemistry, aided by polymer design, provides a powerful platform for novel synthetic transformations.