N-heterocyclic carbenes (NHCs) have been drawing great attention for its excellent roles in catalysis. For past two decades, a variety of NHCs have been developed by not only adjusting electronic and steric properties but also by replacing one nitrogen atom of imidazole ring with carbon atom or sulfur atom. Especially, the steric factor is important because it enhances regio? or stereoselectivity of various reactions and promotes reactivity of certain reactions by stabilizing low-valent intermediates.1
However, synthesis of bulky NHC usually requires some harsh reaction conditions and the multiple reaction steps to afford low chemical yields.2 Therefore, a facile and efficient synthetic route to bulky NHC is highly desired.
Here we report the the synthesis of bulky NHC ligands 1a and 1b with imidazo[1,5-a]pyridin-3-ylidene moiety, and their coinage metal complexes. The bulky NHCs were synthesized in four steps with approximately 40% of overall yield depending on the substituents. More interestingly, the method does not require harsh reaction conditions or any chromatographic purification process, which is sustainable for large scale synthesis.3
Copper(I), silver(I), and gold(I) complexes were synthesized and fully characterized by NMR and single crystal X-ray analysis. The percent buried volume4 for copper(I) complex is 55.5%, which indicates the high steric effect (compared to commonly used IPr and IMes with %VBur = 44.5 and 36.5 respectively).
 Christmann U.; Vilar R. Angew. Chem. Int. Ed. 2005, 44, 366-374.
 Dierick, S.; Dewez, D. F.; Mark?, I. E. Organometallics 2014, 33, 677?683.
 Espina, M.; Rivilla, I.; Conde, A.; D?az-Requejo, M. M.; P?rez, P. J.; ?lvarez, E.; Fern?ndez, R.; Lassaletta, J. M. Organometallics 2015, 34, 1328?1338.
 Calculated via SambVca with following parameters: Radius of sphere = 3.5 ?; Distance from sphere 2.0 ?; Mesh step 0.5 ?; H atoms omitted.