Greening Up The Aromatic Finkelstein

Researchers at McGill University, in Montreal, have reworked the aromatic version of the classic Finkelstein reaction to make it a more straightforward and greener method for preparing iodide reagents (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b03220). Aryl and heteroaryl iodides are sought-after building blocks in organic synthesis for the ease in which they undergo metalation, nucleophilic substitution, and cross-coupling reactions. In addition, radiolabeled versions of the compounds are important for treating hypothyroidism and as contrast agents in medical imaging. But the reactivity that makes the compounds useful makes them difficult to prepare. A leading method is the Finkelstein halogen-exchange reaction, which is carried out by treating an aromatic chloride or bromide with sodium iodide and a copper or nickel diamine catalyst at high temperature. The McGill researchers led by Chao-Jun Li and Zetian Mi realized they could achieve better results without a metal catalyst and at room temperature by using ultraviolet light. By carefully excluding oxygen and adding a dash of elemental iodine, they minimized radical side reactions to prepare high yields of aromatic iodides and diiodides, polycyclic iodides, and vinyl iodides on a gram scale.