Iron complex overcomes radialene’s enthusiasm for reacting with itself, filling decades-old gap in series
Thirty-eight years after the last member of the [n]radialene class of star-shaped hydrocarbons was first reported, chemists have made its missing member. To isolate cyclopentane-based radialene the team led by Mick Sherburn at the Australian National University in Canberra had to overcome its tendency to polymerise extremely rapidly. Their success provides both a potentially powerful synthetic reagent and an important example of producing and characterising highly unstable compounds, Sherburn tells Chemistry World.
[n]Radialenes, cycloalkane rings with each corner decorated with just one outward-pointing carbon–carbon double bond, had previously been made for three, four and six membered central rings. Radialene was the last of these, originally described in 1977, with attempts to prepare radialene using similar methods proving unsuccessful.
‘The approaches were too harsh for this sensitive hydrocarbon to survive,’ Sherburn says. He and collaborator Michael Paddon-Row from the University of New South Wales in Sydney gravitated to this ‘star’ molecule after previous work synthesising acyclic conjugated polyalkenes. ‘Radialene looked like a significant synthetic challenge. Mike’s theoretical insights and predictions based upon computations gave us the confidence to give it a try.’
Paddon-Row calculated that radialene would react with itself with a rate constant four orders of magnitude greater than 1,1-divinylallene, whose half-life is just 40 hours in 20mM concentration at 25°C. The team therefore called upon Fe(CO)3 complexes, which have previously stabilised similar molecules. Using this strategy, they devised two different synthetic routes starting from opposite diastereomers of a modified dendralene, the straight-chain analogue of radialene.
The double Fe(CO)3 radialene complex proved stable enough for Sherburn’s students Emily Mackay and Christopher Newton to get good x-ray crystallography data. ‘It was quite a day when Chris and Emily brought the crystal structure to me,’ Sherburn recalls. However, when they de-complexed it, it only had a half-life of around 16 minutes at -20°C and 30µM concentration.
‘It is great to see that, 50 years after the quest began, the [n]radialene series has finally been completed,’ comments Rik Tykwinski from the University of Erlangen-Nuremberg. ‘It is too bad that radialene is not sufficiently stable to study in detail, but calculations fill in experimental gaps nicely.’
Radialene’s reactivity could be useful in a synthetic context, Sherburn underlines, as [n]radialenes behave as multi-1,3-butadienes. ‘Such dienes participate in ring-forming cycloadditions, the most powerful synthetic transformations known,’ he says. ‘These factors should whet the appetite of anyone interested in step-economic synthesis of complex multi-cyclic structures.’
E G Mackay et al, J. Am. Chem. Soc., 2015, DOI: 10.1021/jacs.5b07445