Trapping a Diradical Transition State by Mechanochemical Polymer Extension

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 po...

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Bibliographic Details
Published in:Science (American Association for the Advancement of Science) 2010-08, Vol.329 (5995), p.1057-1060
Main Authors: Lenhardt, Jeremy M, Ong, Mitchell T, Choe, Robert, Evenhuis, Christian R, Martinez, Todd J, Craig, Stephen L
Format: Article
Language:English
Subjects:
Activation
Applied sciences
Backbone
Chemical bonding
Chemical modifications
Chemical reactions
Chemical reactions and properties
Chemicals
Contracts
Exact sciences and technology
Isomerization
Isomers
Lead
Leaves
Materials
Materials science
Organic polymers
Phase transitions
Physicochemistry of polymers
Polybutadienes
Polymer chemistry
Polymers
Reactivity
Rock cleavage
Trajectories
Trapping
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Summary: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.
ISSN:0036-8075
1095-9203
DOI:10.1126/science.1193412