The Rational Development of Organometallic Reagents through Fluorescence Microscopy Experiments
Inorganic Molecular Chemistry - Synthesis and Characterisation
Final Report Abstract
The overall question intended to be answered with this research project was how additives enable or facilitate the formation of organometallic reagents from metal powder. For this investigation fluorescence microscopy was applied using a fluorescent beacon attached to a chemically reactive group. The conditions to attach this beacon to metal particles were investigated by observing the particles via fluorescence microscopy. It was found that indium powder that contains 2% of magnesium oxide as anticaking agent binds the reactive organic compounds in the magnesium oxide layer surrounding the indium particles. Indium powder that did not contain magnesium oxide, or indium powder in which it had been washed away, did not persistently bind the organic compounds. This feature might be an explanation for the observation of indium containing the anticaking agent being more effective in the formation of organoindium reagents. A similar observation was made in the case of aluminium, only that here no anticaking agent was added, but the oxide layer of the aluminium was the reason of the organic compounds being trapped one the metal particles. In this case the oxide layer could not be washed away. Furthermore, a theoretical model could be established that explains how different additives impact the rate and the final product structure of organozinc reagents. In this model an effective additive binds to a surface organozinc intermediate and facilitates its solubilization from the metal surface. Depending on the additive different solution structures with different properties for further conversion are formed. That model is extended in that it was found that highly polar solvents increase reaction on the zinc surface. Specifically, reaction in the highly polar solvent dimethylsulfoxide gave a higher content of surface organozinc intermediates than a reaction in the less polar tetrahydrofuran. In this research topic, the observation of the reactivity in these elementary steps could be matched with the reactivity in a real macroscale synthesis. These results are important for a deep understanding of the synthesis, structure, and reactivity of organozinc reagents which are the key coupling partners in the palladium-catalyzed Negishi coupling. Finally, the reaction of copper powder with chlorotetramethylsilane gave small crystals of copper(I) chloride that, as opposed to copper alone, captured one organic compound selectively over two others. Neither the reason behind this selective binding nor the binding situation are known, yet.
Publications
- "Microscopy Reveals: Impact of Lithium Salts on Elementary Steps Predicts Organozinc Reagent Synthesis and Structure." J. Am. Chem. Soc. 2019, 141, 9879–9884
K. Jess, K. Kitagawa, T. K. S. Tagawa, S. A. Blum
(See online at https://doi.org/10.1021/jacs.9b02639)
