Synthesis of N-heterocyclic carbene gold complexes using 2,4,6-trimethylphenyl sydnone as model substrate.

Sydnones are five membered heterocycles that belong to the class of dipolar compounds named "mesoionics" and represented as hybrids of several mesomeric/ionic forms. Their pka values are approximately 18, so they can be deprotonated with the appropriate base to give N-heterocyclic carbenes (NHC). These NHC can react with different gold precursors to obtain gold complexes. In this work the 2,4,6-trimethylphenyl sydnone was used as model substrate to synthesize gold complexes by a two-step reaction. This reaction is extremely sensitive to the presence of water thereby inert atmosphere is needed. The first reaction step was deprotonation of the sydnone with butillithium to obtain the corresponding NHC. In a second reaction step the metal precursor was added to afford the corresponding gold complex. At this point, the complex obtained depends on the nature of the metal precursor. A monocarbene complex (sydnone-Au-Cl) was obtained using AuthtCl as metal precursor. On the other hand, a biscarbene complex (sydnone-Au-imidazolium) was obtained using a gold imidazolium complex as metal precursor. The sydnone complexes were purified by precipitation or column chromatography and characterized by ¹H-, ¹³C- and two-dimensional NMR, Fourier transform infrared (FTIR), UV Spectroscopy and elemental analysis. The ¹³C-NMR resonance frequencies of the carbene carbon atom are extremely shifted upfield from 97 to 139 ppm.