Journal
SCIENCE
Volume 329, Issue 5995, Pages 1057-1060Publisher
AMER ASSOC ADVANCEMENT SCIENCE
DOI: 10.1126/science.1193412
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- U.S. Army Research Laboratory
- Army Research Office [W911NF-07-1-0409]
- American Chemical Society Division of Organic Chemistry Schering-Plough
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Transition state structures are central to the rates and outcomes of chemical reactions, but their fleeting existence often leaves their properties to be inferred rather than observed. By treating polybutadiene with a difluorocarbene source, we embedded gem-difluorocyclopropanes (gDFCs) along the polymer backbone. We report that mechanochemical activation of the polymer under tension opens the gDFCs and traps a 1,3-diradical that is formally a transition state in their stress-free electrocyclic isomerization. The trapped diradical lives long enough that we can observe its noncanonical participation in bimolecular addition reactions. Furthermore, the application of a transient tensile force induces a net isomerization of the trans-gDFC into its less-stable cis isomer, leading to the counterintuitive result that the gDFC contracts in response to a transient force of extension.
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