Cycloelimination Reactions of SIMes and Related N-Heterocyclic Carbenes to Generate N-Heterocyclic Carbene Carbodiimide Adducts

Technology

Since the first demonstration of their potential stability, isolation, and storability, NHCs have become pervasive in both experimental and commercial applications as electron-rich, neutral ligands. One of the most widely used classes of NHCs—1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene, known as “SIMes”—are commercially available and widely used as ligands and catalysts. Upon heating, NHCs of the SIMes family can be converted via a [3+2]-cycloelimination process to a novel class of NHCs known as NHC-CDI amidinates, notably useful as novel ligand scaffolds for Lewis acidic metals. An even broader array of NHC-CDIs could be prepared by the direct addition of the NHC to the CDI.  This method allows the steric bulk and electronics of both components to be independently tuned for desired NHC-CDI performance. Isolable NHC-CDI adducts have been reported only once previously, and were structurally distinctive from those generated as described above, rendering them less practical for commercial use. NHC-CDI adducts generated as described above are uniquely stable at both ambient conditions and in the presence of moisture. Additionally, NHC-CDIs absorb visible light and emit violet-indigo fluorescence, conferring optical sensor functionality. A much broader array of NHC-CDIs could be prepared by the direct addition of the NHC to the CDI.  This method allows the steric bulk and electronics of both components to be independently tuned to achieve the desired NHC-CDI chemical and photophysical properties, including air-stability, metal ligation, and UV-vis absorbance profile.   

Problem Addressed

The development of ligands with distinctive properties and reactivity is critical to the discovery of new metal catalysts. This method of production of a novel metal ligand class (NHC-CDIs) is easily performed and results in a uniquely bench-stable construct that exhibits robust binding and ligand properties.  

Advantages

• Easily synthesized via commercially available and air-stable NHCs
• Stable under ambient conditions for months and in the presence of moisture for at least 3 days
• Novel ligand for Lewis acidic metals
• Optical sensor capabilities