Hydroxyl Radical Promotes the Direct Iodination of Aromatic Compounds with Iodine in Water: A Combined Experimental and Theoretical Study

Abstract

It is still a challenge to develop green, simple and effective approaches to prepare aromatic iodides. Herein, a novel and green strategy for the direct mono-iodination of aromatic compounds starting with molecular iodine has been developed. The strategy uses ceria nanocrystals to decompose hydro-gen peroxide, giving hydroxyl radicals which are demonstrated experimentally and computationally to be crucial to promote the iodinations.

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“…Further EPR analysis revealed that a pH decrease lead to an increase of the EPR response which correlates with the amount of OH radicals. The pH required for oxidative iodination is lower than for bromination (pH ≤ 3.0 vs pH ≤ 7.0). For pH values ≥6.0 only hydroxylated rather than iodinated products (e.g., for benzene) were formed.…”

“…The pH required for oxidative iodination is lower than for bromination (pH ≤ 3.0 vs pH ≤ 7.0). For pH values ≥6.0 only hydroxylated rather than iodinated products (e.g., for benzene) were formed. Computational studies suggested a stepwise oxidation of one iodine molecule (I 2 ) to two HOI molecules with four different iodine compounds and iodites (I + ) involved in the actual oxidative iodination step.…”

“…The activity was attributed to the large surface area, the number of acidic sites and their acidity. Similarly, benzylic compounds such as dimethoxybenzene (DMB) could be iodinated with I 2 , H 2 O 2 in the presence of ceria nanorods in high yields (up to 97%, 0.5 h, mono‐iodination), but temperatures of 100 °C and an excess of H 2 O 2 were needed. A Fenton‐like decomposition of H 2 O 2 at Ce 3+ centers was proposed, as the presence of hydroxyl radicals could be demonstrated by electron paramagnetic resonance (EPR) spectroscopy with the spin trap agent 5,5‐dimethyl‐pyrroline N ‐oxide (DMPO).…”

“…For pH values ≥6.0 only hydroxylated rather than iodinated products (e.g., for benzene) were formed. Computational studies suggested a stepwise oxidation of one iodine molecule (I 2 ) to two HOI molecules with four different iodine compounds and iodites (I + ) involved in the actual oxidative iodination step. The electrophilicity of HOI is not sufficient to promote iodination, but protonation to H 2 OI + under acidic conditions increases the reactivity and drives the reaction …”

“…The long‐term stability of the CeO 2− x nanorods is very high. The stability and catalytic activity and was ensured under laboratory conditions in leaching tests (solubility near the detection limit of ICP‐MS because ceria is virtually insoluble) together with cycle tests (constant activity over many catalytic cycles), and it showed promising robustness . These properties remain unchanged for CeO 2− x nanorods embedded in polymers and boat paints.…”