Using sunlight to create carbon-carbon bonds the green way
Creating carbon-carbon bonds is a basic challenge for synthetic chemists, but it can be difficult if the carbon atoms are isolated from the functional groups that activate these reactions. Radicals needed to make the reactions work better are traditionally formed using metal catalysts - reagents that green chemists try to avoid.
Seeking an alternative green chemistry route to carbon-carbon bond formation, Roisin Doohan and Niall Geraghty at the National University of Ireland, Galway, have found a way to generate carbon radicals photochemically. A photomediator (typically benzophenone) is essential to the reaction - it contains a carbonyl group that, when subjected to light, will produce the radicals needed to make the bond-forming reactions work.
As well as avoiding toxic reagents, this reaction has other environmental attractions: sunlight can be used to trigger the reaction and the photomediator can be attached to a solid support, which means it might be recycled.
In their work, Doohan and Geraghty carried out the same reactions either in a photoreactor or using solar radiation. Cyclopentane and cyclohexane in alkyne solutions were reacted using either a soluble or a supported photomediator.
The reactions produced a mixture of isomers and there was no evidence that any secondary photochemical isomerisation had taken place. Although the solar reactions took longer to work than those carried out in the photochemical reactor, they gave comparable yields in most cases. However, the supported photomediators required longer reaction times and performed poorly compared to the soluble benzophenone photomediator.
If efficient and robust supported photomediators can be developed, this green chemistry methodology could have a bright future.