Use of Phenyl Formate as a CO Surrogate for the Reductive Cyclization of Organic Nitro Compounds to yield Different N-Heterocycles: Avoiding the Use of Autoclaves and Pressurized Gases

The reductive cyclization of different organic nitro compounds by carbon monoxide, catalyzed by transition metal complexes, is a very efficient and clean strategy for the synthesis of many N-heterocycles. Indeed, only CO₂ is formed as a stoichiometric byproduct. Most often, suitably substituted nitroarenes are employed as substrates, but the use of b-nitrostyrenes and nitrodienes has also been described and a few cases of inter-molecular reactions of a nitroarene with a separate alkene or diene have been reported. Despite the advantages these reactions have with respect to alternative procedures, their use has not spread out of the few groups which developed them, undoubtedly mainly because they require the use of autoclaves and pressurized CO lines, which are not available in most synthetic laboratories. In this talk, the author will present the results obtained in his laboratories on the use of phenyl formate as a convenient CO surrogate, able to liberate carbon monoxide under the reaction conditions and allowing the use of a standard and cheap glass pressure tube as a reaction vessel. This way, both the need for autoclaves and pressurized CO lines are released. Four examples will be discussed: the synthesis of indoles from o-nitrostyrenes, the synthesis of indoles from b-nitrostyrenes, the synthesis of oxazines from nitroarenes and conjugated dienes, and the synthesis of carbazoles from o-nitrobiphenyls. In most cases, yields were better than those previously reported by the use of pressurized CO, proving that the use of CO surrogates can be a viable alternative to the gaseous reagent and not just a second-choice strategy.