Unprecedented Carbene Ligands

Cyclic amino compounds known as N-hetero­cyclic carbenes (NHCs) are among the most versatile and popular ligands for stabilizing transition-metal catalysts. NHCs come in many flavors: The rings vary in size from three to eight atoms; include up to three nitrogen atoms and two carbenes per ring; and sometimes contain boron, phosphorus, or other heteroatoms. The latest version, created by Holger Braunschweig and coworkers of the University of Würzburg, in Germany, is a four-membered boron-containing cyclic (amino)(imino)carbene bound to chromium (Chem. Commun., DOI: 10.1039/c3cc38379b). The C₂BN ring is new for an NHC, Braunschweig says. And the reaction to make the new molecule is also unprecedented. The process proceeds through cycloaddition of a boron carbene analog (a borylene) to an isocyanide to form a three-membered BCN ring, followed by insertion of a second isocyanide to form the four-membered ring (shown). The ability to shift the lone pair of electrons—the carbenoid center—from boron to carbon opens up new possibilities for making carbenes, Braunschweig adds. The researchers are now studying the reactivity of the new chromium NHC complex and determining how to isolate the free carbene from the metal.