Cyclopropanes built by a [1+1+1] trimerization pathway

Among synthetic methods available to chemists, cyclotrimerization reactions are an efficient approach to assembling complex cyclic molecules in a single step from three simple building blocks. One limitation of the process is that known examples only allow synthesis of aromatic or heterocyclic compounds, such as the [2+2+2] cyclotrimerization of alkynes or acetophenones to make substituted benzenes or of aldehydes to make trioxanes. Srimanta Manna and Andrey P. Antonchick of the Max Planck Institute of Molecular Physiology have now expanded the cyclotrimerization strategy to make cyclopropanes (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201600807). The team stitched together a variety of substituted acetophenones (one example shown) using a copper iodide/2,2´-bipyridine catalyst and a peroxide oxidant. The [1+1+1] cascade reaction proceeds through a previously unknown radical pathway in which a copper enolate intermediate functionalizes unactivated C–H methyl bonds of two acetophenone molecules to form a diketone. The diketone subsequently couples with a third acetophenone molecule leading to the cyclopropane ring. Overall, the new method is counter to the way chemists typically think about making cyclopropanes from olefins.