ESI-MS~(n)-, IRMPD-Spectroscopy and Computational Modeling of Solvated Organozinc Cations: Structure Elucidation of Gas-Phase Negishi Cross Coupling Reagents

摘要

The extensive study of functionalized alkylzinc complex ions with tandem MS, IRMPD-spectroscopy and computational modelling demonstrates the value of this analytical approach. The series of compounds investigated have allowed detection of quite dramatic changes in structure through relatively minor changes to the molecules in question, which were followed in detail by IRMPD-spectroscopy and high level computations. Thus, we were able to produce in-depth structural information on this class of organic reagents, helping to understand the influence of specific protecting groups and chelating ligands on the subtle balance between intra- and intermolecular coordination of ligands to zinc cations. Besides that new alkylzinc compounds derived from Tyr and Trp have been introduced and examined, showing the compatibility of the carbon-zinc bond with both a phenol and indole functionality in the context of these experiments. From our results we conclude that complex alkylzinc cations either carrying DMF- or TMEDA-ligands show a preference to form a tetrahedral coordination sphere around zinc in the gas phase. However, the experimental findings also show that not only the zinc cation determines the conformation of the complex ions but structural constraints and additional non-covalent interactions are found to be important for stabilization, too. Finally, this detailed investigation of structural features of TMEDA complex ions delivers additional insights eventually valuable for the understanding of the reactivity of TMEDA stabilized alkylzinc iodides in solution, i.e. their tolerance towards the presence of water. Our results point towards an extensive ionization of the zinc-iodine bond by formation of stable cationic TMEDA organozinc complex ions, which in turn could be the reason for the low basicity, which effectively prevents protonation.