Mesoionic thiazol-5-yIidenes as ligands for transition metal complexes

摘要

In 2001, Crabtree et al. reported the preparation of complex B,1 featuring an imidazol-5-ylidene C as ligand. It was quickly demonstrated that these types of ligands2 are even stronger electron-donors than their imidazol-2-ylidene isomers, the so-called NHCs A3-4 (Fig. 1). Recently, it has been shown that derivatives of type C,5 as well as pyrazolin-4-ylidenes D6 and 1,2,3-triazol-5-ylidenes E,7 can be isolated. Since no reasonable canonical resonance forms showing a carbene can be drawn without additional charges, and since they are mesoionic compounds, they can be named mesoionic carbenes (MICs).8 Importantly, in contrast to regular carbenes, no obvious dimerization pathway can be foreseen, and therefore it was reasonable to believe that unhindered MICs with various heteroatom-containing skeletons could be stable. To test this hypothesis, we attempted the preparation of l,3-dithiol-5-ylidenes F, but observed a spontaneous ring opening yielding the corresponding ethynyl dithiocarbamate G.9 The formation of G is reminiscent of the ring-opening reaction observed in the deprotonation of isothiazolium salts,10 and therefore is probably due to the weakness of the sulfur-carbon bond. These results prompted us to investigate the preparation of thiazol-5-ylidenes H, since a similar ring-opening process would imply the cleavage of a carbon-nitrogen bond.