Radialenes are a unique class of star-shaped cyclic hydrocarbons that have double bonds radiating out from each carbon in the ring. Chemists have made the three-, four-, and six-membered ring versions of these polyenes in the past, but never radialene. It has always been deemed too unstable.
An Australian research team including Emily G. Mackay and Michael S. Sherburn of Australian National University and Michael N. Paddon-Row of the University of New South Wales has done a little computational detective work to find out why. Using the clues they turned up, the researchers developed a synthetic strategy to make radialene for the first time.
“The structure and properties of radialene are of great interest for chemists, not only from a synthetic perspective, but also for the physical organic chemistry of the molecule,” comments Norihiro Tokitoh of Kyoto University, in Japan, whose group has synthesized radialenes. “With this missing molecule in hand, the systematic comparison of the structures and properties with other radialenes will provide us with a new look at how these unique cross-conjugated systems can be used as π building blocks.”
Chemists typically synthesize radialenes from acyclic polyenes by using elimination and rearrangement reactions at temperatures reaching 1,000 °C. The Australian team studied previous attempts to make radialene and found that it rapidly decomposes in the presence of oxygen and that its size and shape make it highly susceptible to undergo Diels-Alder dimerization leading to polymerization. Their findings suggested a milder touch would be necessary.
Building on their earlier work to make acyclic branched polyenes called dendralenes, the researchers used an iron tricarbonyl complex to stabilize a dendralene so they could carry out a ring-closing procedure. At the end they removed the iron complex at cold temperature to obtain the elusive, and still short-lived, radialene (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b07445).
“This is a beautiful and important contribution, not just to hydrocarbon chemistry, but to organic chemistry in general,” says Henning Hopf of the Technical University of Braunschweig, in Germany. Radialene provides a “missing link” in polyolefin chemistry, Hopf notes, a gap that many chemists—including himself—have tried to close in vain. The success of the new synthetic strategy, he says, demonstrates the practicality of “soft” metal-mediated low-temperature reactions over “hard” flash vacuum pyrolysis reactions at high temperature.