Force can induce remarkable non-destructive transformations along a polymer, but we have a limited understanding of the energy transduction and product distribution in tandem mechanochemical reactions. Ladderanes consist of multiple fused cyclobutane rings and have recently been used as monomeric motifs to develop polymers that drastically change their properties in response to force. Here we show that -ladderane always exhibits ‘all-or-none’ cascade mechanoactivations and the same stereochemical distribution of the generated dienes under various conditions and within different polymer backbones. Transition state theory fails to capture the reaction kinetics and explain the observed stereochemical distributions. Ab initio steered molecular dynamics reveals unique non-equilibrium dynamic effects: energy transduction from the first cycloreversion substantially accelerates the second cycloreversion, and bifurcation on the force-modified potential energy surface leads to the product distributions. Our findings illustrate the rich chemistry in closely coupled multi-mechanophores and an exciting potential for effective energy transduction in tandem mechanochemical reactions.
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Optimized geometries for the structures discussed in the text are available in the supplementary files in the online version of the paper. The experimental and simulation data that support the findings of this study are available from the authors upon request.
The program TeraChem (v1.92), used for steered ab initio molecular dynamics calculations and reaction path optimization, is available from PetaChem, LLC (http://www.petachem.com/products.html). The scripts used to analyse the trajectories are available upon request from T.J.M.
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This work was supported by the US Army Research Office under grant no. W911NF-15-1-0525. J.A.M.M. thanks the National Science Foundation for a graduate fellowship. T.J.M. acknowledges support from Office of Naval Research grant no. N00014-12-1-0828. This work used the XStream computational resource supported by the National Science Foundation Major Research Instrumentation programme (ACI-1429830). We thank B. M. Trost for the use of the ozone generator and S. R. Lynch for advice on NMR spectroscopy.
The authors declare no competing interests.
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Supplementary information, including materials and methods, calculations and synthetic procedures.
Calculated optimized molecular geometries, in XYZ format and in angstroms, of the minima and transition states of key structures during -ladderane unzipping under different external forces are provided in separate files. The geometries are calculated at UB3LYP/6-31g* level using TeraChem, available from PetaChem, LLC.
One example of AISMD simulation showing tandem unzipping to 21 (EE, EE product) from 17≠.
One example of AISMD simulation showing tandem unzipping to 22 (EE, EZ product) from 17≠.
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Chen, Z., Zhu, X., Yang, J. et al. The cascade unzipping of ladderane reveals dynamic effects in mechanochemistry. Nat. Chem. (2020) doi:10.1038/s41557-019-0396-5