Diverse chemical applications of N-heterocyclic carbenes.

N-Heterocyclic carbenes, or NHCs as they are commonly abbreviated, have attracted significant interest over the past decade due to their widespread utility in chemical transformations. As ligands on transition metals, NHCs have been found to have properties similar to those of the widely employed phosphine ligand, due to their ability to act as both a Lewis base and a Lewis acid at the same time. In addition, the ability to prepare a number of derivatives allows control of both sterics and electronics, also reminiscent of the capabilities of phosphines. JACS Select is a collection of previously published work, intended to capture recent interest in a topic of increased importance to the chemical community. In 2008-09, over 100 publications have appeared in JACS featuring chemistry involving NHCs, either attached to a transition metal complex or as integral components in the chemistry being reported. This sixth issue of JACS Select contains 20 reports chosen to demonstrate the variety of chemistry that can be accomplished with these versatile molecules. First, it is worth examining the structures of typical NHCs that are currently being examined. Structure A shows the presence of a carbene moiety stabilized by two adjacent π-donating atoms. The unsaturation in the backbone makes this an aromatic system, so that the carbine p-orbital is available to act as a π-acid. In comparison, use of the corresponding NHC with a saturated backbone (B) leaves the carbene with two π-donor atoms attached, decreasing its Lewis acidity. In fact, the back-filling of the carbene p-orbital has led to these species being drawn as shown in C, which represents the NHC as a strong σ-donor ligand with little π-acceptor character.