Development on the Synthesis of Ring Expanded N-Heterocyclic Carbene Precursors

As a kind of novel developed ligands, N-heterocyclic carbenes (NHCs) play very important roles in the development of modern organic chemistry. Early studies on NHCs have been mainly focused on imidazole derived five membered heterocycles, due to their easy accessibility and comparably stable properties. Recently, ring expanded NHCs, such as six- or seven-membered NHCs became another research hot spot due to their steric characteristics and chemical properties. Because deprotonation of the heterocyclic onium salt precursors is by far the most commonly used method to access NHCs, the synthesis of the precursors remains a key factor affecting the development of ring expanded NHCs. This review will summarize the progress on the synthesis of ring expanded NHC precursors.