Scientists in China have prepared functionalised (arylcyclobutenyl)carbinols and hydrogenated furans in good yields and selectivities under mild conditions. 

Min Shi and colleagues from Shanghai Institute of Organic Chemistry and East China University of Science and Technology have developed an efficient method for ring expansion of 2-(arylmethylene)cyclopropylcarbinols catalyzed by palladium(II) chloride in the presence of copper(II) bromide. They produced (arylcyclobutenyl)carbinols or hydrogenated furans in good yields depending on the substituents on the benzene ring. 

Methylenecyclopropanes (MCPs) are very reactive owing to the relief of ring strain and are therefore utilised in organic synthesis. 2-(Arylmethylene)cyclopropylcarbinols are a kind of MCP that bear an additional hydroxymethyl group and can undergo a variety of transformations triggered by the nucleophilic hydroxyl group under milder conditions. Shi and colleagues thought that the tethered hydroxyl group might accelerate transition metal-assisted reactions or trap the reaction intermediates of MCPs. 

They found that substrates with electron-donating groups on the benzene ring undergo rearrangement smoothly to provide (arylcyclobutenyl)carbinols. While, substrates with electron-withdrawing groups on the benzene ring produce dihydrofurans and tetrahydrofurans. All the reactions proceed in good yields and selectivities under mild conditions. 

Shi and colleagues have also proposed mechanisms accounting for the distinct products on the basis of control and deuterium labeling experiments. It is thought that carbene species might be involved in the formation of (arylcyclobutenyl)carbinols and a ?-allylpalladium intermediate would account for the production of the dihydrofuran and tetrahydrofuran products. 

Future challenges will be 'using metal catalysts to synthesize some interesting molecules with these small rings,' says Shi. 

Kathryn Sear