A Stable Acyclic Ligand Equivalent of an Unstable l,3-Dithiol-5-ylidene

摘要

During the last two decades, N-heterocyclic carbenes (NHCs), such as A (Scheme 1), have played a prominent role as ligands for transition-metal catalysts. Their popularity is mainly due to their strong α-donor properties and the robustness of the corresponding complexes. These two features result from the presence of the electropositive carbon center and the strength of the carbon-metal bond. Therefore, other types of carbon-based L ligands are highly desirable. The simplest method for the preparation of metal complexes featuring a given L ligand is by ligand substitution at the metal center; however, the availability of stable compounds with a lone pair of electrons at a carbon center is very limited. It has recently been shown that mesoionic carbenes (MICs) B-D can be isolated as free species. In contrast to "normal carbenes", no obvious dimerization pathway can be foreseen for MIGs. Consequently, a variety of these unusual carbenes should in principle be available without the need for kinetic protection. No derivatives of the 1,3-dithiol-2-ylidene F are known owing to their dimerization into derivatives of tetrathiafulvalene G. Herein, we report our attempts to prepare a free MIC isomer by carbenes of type F, namely, a 1,3-dithiol-5-ylidene E. We show that this compound is unstable owing to spontaneous ring opening to form the corresponding ethynylcarbamodithioate. Importantly, the latter reacts with a variety of metals to give 1,3-dithiol-5-ylidene-metal complexes and therefore is a ligand equivalent of E.