During the course of chemical reactions, reactant molecules need to surmount an energy barrier to allow their transformation into products. The energy needed for this process is usually provided by heat, light, pressure or electrical potential, which act either by changing the distribution of the reactants on their ground-state potential energy surface or by moving them onto an excited-state potential energy surface and thereby facilitate movement over the energy barrier. A fundamentally different way of initiating or accelerating a reaction is the use of force to deform reacting molecules along a specific direction of the reaction coordinate. Mechanical force has indeed been shown to activate covalent bonds in polymers, but the usual result is chain scission1. Here we show that mechanically sensitive chemical groups make it possible to harness the mechanical forces generated when exposing polymer solutions to ultrasound2, and that this allows us to accelerate rearrangement reactions and bias reaction pathways to yield products not obtainable from purely thermal or light-induced reactions. We find that when placed within long polymer strands, the trans and cis isomers of a 1,2-disubstituted benzocyclobutene undergo an ultrasound-induced electrocyclic ring opening in a formally conrotatory and formally disrotatory process, respectively, that yield identical products. This contrasts with reaction initiation by light or heat alone3, in which case the isomers follow mutually exclusive pathways to different products. Mechanical forces associated with ultrasound can thus clearly alter the shape of potential energy surfaces4 so that otherwise forbidden or slow processes proceed under mild conditions, with the directionally specific nature of mechanical forces providing a reaction control that is fundamentally different from that achieved by adjusting chemical or physical parameters. Because rearrangement in our system occurs before chain scission, the effect we describe might allow the development of materials that are activated by mechanical stress fields.
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We thank T. Martínez, K. Suslick and P. Beak for discussions. This work was supported by grants from the Air Force Office of Scientific Research and The Petroleum Research Fund.
Author Contributions J.S.M. conceived the BCB experiment. C.R.H. performed all experiments and initial computational analyses. J.B. performed the final computational analysis. S.R. Wilson performed the crystallography. N.R.S., S.R. White and J.S.M. directed the research and all authors wrote the manuscript.
The atomic coordinates for cis-1,2-bis[(3-carboxypropanoyl)oxy]-1,2-dihydrobenzocyclobutene 2 (accession number 628670), N-(1-pyrene)-2,3-naphthimide (accession number 628377), and 4,9-diacetoxy-2-(1-pyrenyl)-(3aR,4c,9c,9ac)-3a,4,9,9a-tetrahydro-benzo[f]isoindole-1,3-dione 6 (accession number 628671) have been deposited in the Cambridge Crystal Structure Database.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
This file contains Supplementary Figures 1-10 with Legends, Supplementary discussion and additional references. The file contains all of the supporting information including results of computational studies, results of all control experiments, procedures for the synthesis and characterization of all new compounds reported in the manuscript, representative GPC traces of ultrasound experiments, and 13C NMR spectra of all new reported compounds. (PDF 1266 kb)
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Hickenboth, C., Moore, J., White, S. et al. Biasing reaction pathways with mechanical force. Nature 446, 423–427 (2007). https://doi.org/10.1038/nature05681
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