Metal-mediated reaction modeled on nature: the activation of isothiocyanates initiated by zinc thiolate complexes

On the basis of detailed theoretical studies of the mode of action of carbonic anhydrase (CA) and models resembling only its reactive core, a complete computational pathway analysis of the reaction between several isothiocyanates and methyl mercaptan activated by a thiolate-bearing model complex Zn(NH(3))(3)SMe](+) was performed at a high level of density functional theory (DFT). Furthermore, model reactions have been studied in the experiment using relatively stable zinc complexes and have been investigated by gas chromatography/mass spectrometry and Raman spectroscopy. The model complexes used in the experiment are based upon the well-known azamacrocyclic ligand family (12]aneN(4), 14]aneN(4), i-14]aneN(4), and 15]aneN(4)) and are commonly formulated as (Zn(X]aneN(4))(SBn)]ClO(4). As predicted by our DFT calculations, all of these complexes are capable of insertion into the heterocumulene system. Raman spectroscopic investigations indicate that aryl-substituted isothiocyanates predominantly add to the C═N bond and that the size of the ring-shaped ligands of the zinc complex also has a very significant influence on the selectivity and on the reactivity as well. Unfortunately, the activated isothiocyanate is not able to add to the thiolate-corresponding mercaptan to invoke a CA analogous catalytic cycle. However, more reactive compounds such as methyl iodide can be incorporated. This work gives new insight into the mode of action and reaction path variants derived from the CA principles. Further, aspects of the reliability of DFT calculations concerning the prediction of the selectivity and reactivity are discussed. In addition, the presented synthetic pathways can offer a completely new access to a variety of dithiocarbamates.