Kinetics and mechanistic study of the Bamberger rearrangement of N-phenylhydroxylamine to 4-aminophenol in acetonitrile-trifluoroacetic acid: A substrate acid complex as para selectivity driver

• Bamberger rearrangement of phenylhydroxylamine (PHA) to 4-aminophenol in acetonitrile.
• Trifluoroacetic acid (TFA) an easily reusable acid catalyst.
• Kinetics of reactions is studied in relation to the equilibria of TFA and PHA and water.
• The complex PHA-TFA drives the reaction explaining both kinetics and para regioselectivity.
• Fittings of the kinetic model gives kinetic constants, equilibrium constants and activation parameters.

The paper deals with the kinetics of Bamberger rearrangement of N-phenylhydroxylamine to 4-aminophenol in acetonitrile as a solvent catalyzed by TFA. The influence of the water is also investigated being the rearranging moiety in the mechanism. It is evident that the presence of water depresses the rate of the rearrangement suggesting the rate-determining step does not involve water as the key reagent. In acetonitrile, at lower temperature than that of reaction, we evidence the formation of a PHA-TFA complex, and its equilibrium has been measured between 288 K and 298 K. We observe also the addition of water destroys this complex but the ternary equilibrium PHA, water and TFA cannot easily measured because of the complex solvent effect on the Uv–vis and NMR signals. Starting from this evidence, we have proposed a kinetic model, which takes into account all the equilibria, and the results of the fitting gives reliable thermodynamic and kinetic parameters of the entire process. Finally, we have suggested that the PHA-TFA complex is the key intermediate, which determines the regioselectivity of the rearrangement. In fact, preliminary quantum chemistry calculation have shown that the TFA interaction with the NHOH group causes the hindering of the ortho position, thus favoring the attack of water to the para one.

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