The cascade unzipping of ladderane reveals dynamic effects in mechanochemistry

Abstract

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 [4]-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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Fig. 1: The mechanochemistry of ladderane/ene consists of tandem cycloreversions.
Fig. 2: Mechanochemistry of chain-centred [4]-ladderene.
Fig. 3: Mechanochemistry of chain-centred [4]-ladderane.
Fig. 4: Mechanochemistry of a multi-mechanophore polymer containing multiple [4]-ladderane units.
Fig. 5: Computational analysis of the unzipping of [4]-ladderane using AISMD under different forces.
Fig. 6: Analysis of the origin of stereochemical product distribution in the mechanochemistry of [4]-ladderane.

Data availability

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.

Code availability

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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Acknowledgements

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.

Author information

Z.C., X.Z., T.J.M. and Y.X. conceived this project. Z.C. and Y.X. designed the experiments and X.Z. and T.J.M. designed the computations. Z.C. and J.Y. prepared the polymers and performed the mechanoactivation, characterizations and data analysis, under the guidance of Y.X. X.Z. performed calculations and data analysis under the guidance of T.J.M. J.A.M.M. synthesized starting materials 1 and 8 under the guidance of N.Z.B. Z.C., X.Z. and Y.X. wrote the manuscript. All authors discussed the results and commented on the manuscript.

Correspondence to Todd J. Martinez or Yan Xia.

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Supplementary information

Supplementary information

Supplementary information, including materials and methods, calculations and synthetic procedures.

Computational dataset

Calculated optimized molecular geometries, in XYZ format and in angstroms, of the minima and transition states of key structures during [4]-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.

Supplementary Video 1

One example of AISMD simulation showing tandem unzipping to 21 (EE, EE product) from 17.

Supplementary Video 2

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

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