Oxidative Cyclization Reaction of 2‐Aryl‐Substituted Cinnamates To Form Phenanthrene Carboxylates by Using MoCl5

The oxidative cyclization reaction of 2‐aryl cinnamates and derivatives thereof can be easily performed with MoCl₅ as the oxidant. This powerful reagent allows oxidative coupling reactions for which other reagents fail. The best results are obtained when the 2‐phenyl substituent of the cinnamate is equipped with two methoxy groups. Even iodo moieties in the bay region of phenanthrene are tolerated under the reaction conditions. If naphthalene moieties are involved, a rearrangement of the skeleton occurs, providing an elegant route to highly functionalized angular arenes. The cyclization is demonstrated for 15 example substrates with isolated yields of up to 99 % for the phenanthrene derivative. The broad scope of the reaction underlines the usefulness of MoCl₅ and MoCl₅/TiCl₄ in the oxidative coupling reaction.