Four-Square Nitrogen

Australian chemists have spotted what they say is the first definitive example of a tetrazetidine, a molecule containing an unprecedented four-membered nitrogen ring (J. Am. Chem. Soc., DOI: 10.1021/ja303019y). The tetrazetidine system is one of the few simple chemical structures that has not yet been synthesized with any certainty. Over the past 20 years, ­David Camp and Ian D. Jenkins of Griffith University and Graeme R. Hanson of the University of Queensland have used electron paramagnetic resonance spectroscopy to study the formation of radicals when triphenylphosphine and diisopropyl azo­dicarboxylate are combined in the Mitsunobu reaction. In addition to a phosphine-azodicarboxylate radical, they sometimes noticed a second persistent radical with what they describe as a “rather beautiful, almost symmetrical nine-line spectrum.” Now, in conjunction with Griffith’s Marc Campitelli, and using more advanced and affordable computational methods, the researchers believe the tetrazetidinetetracarboxylate radical cation shown is being formed by a Michael-type addition of a phosphine-azodicarboxylate radical to a diisopropyl azodicarboxylate molecule. The tetrazetidine radical is surprisingly long-lived, they note, lasting several hours at room temperature.