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New Insights Into the Solid-State Rearrangement of P-Bromobenzeneazotribenzoylmethane

The thermal solid-state rearrangement of p bromobenzeneazotribenzoylmethane (1), which has been thoroughly investigated in the past [e.g. Dimroth, O. ; Hattmann, M. Ber. 1907, 40, 2404. ; Puckett, R. T. ; Pfluger, C. E. ; Curtin, D. Y. J. Am. Chem. Soc. 1966, 88, 4637–4642. ; Curtin, D. Y. ; Byrn, S. R. ; Pendergrass, Jr., D. B. J. Org. Chem. 1969, 34, 3345–3349.] yields two products. In the first product, the benzoyl group of 1 migrates to the oxygen atom of another benzoyl group forming an enol benzoate (2), whereas in the second product, the benzoyl group migrates to the azo-nitrogen atom forming a benzoylphenylhydrazone (3). The mechanism of the rearrangement reaction, however, remained unknown owing to the difficulty to characterize the intermediates and the product within the reacted solid. In this work we have applied in situ time- and temperature-resolved X-ray powder diffraction (XRPD) to structurally characterise crystalline intermediate phases as well as the crystal structure of the final rearrangement product. The studies presented herein reveal conclusively that the intermediate crystalline phase of 2 also undergoes a thermal rearrangement reaction. Therefore, in a rough description, the mechanism of this reaction includes direct rearrangements 1 → 2 and 1 → 3 and parallel to them, the rearrangement 2 → 3. XRPD techniques have proved essential to determine the crystal structure of the final product as a different polymorph is obtained by solvent crystallisation. This work underpins the utility of modern time- and temperature-resolved XRPD in the elucidation of reaction mechanisms in the solid state.

solid-state reaction ; time-resolved powder diffraction ; variable temperature

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