Direct Synthesis of Organometallics, IV. The Mechanism of the Activation of Aromatic Hydrocarbons with Lithium Atoms: Spectroscopic Results and Their Ab initio Interpretation

Abstract

In the presence of THF, lithium atoms are known to be able to activate benzene derivatives under cryogenic reaction conditions. This synthesis affords solid lithium hydride and ring-metalated products selectively. For the determination of the reaction mechanism, the cocondensation reaction was repeated on a reduced scale in a 77-K cryostate, which allowed the measurement of UV/Vis and IR spectra. The spectroscopic results exclude any generation of free electrons; their interpretation suggests that small solvated lithium clusters activate the hydrocarbon bonds. A model reaction was developed by using the spectroscopic results and ab initio calculations. Thus, benzene was added to a Li2 cluster, which reduces the carbon-hydrogen bond to the phenyl anion and hydride on the surface without the formation of free radicals. In the last step the intermediate mixed organolithium species dismutate to solid lithium hydride and phenyllithium. The reduction process on the cluster surface is facilitated by coordinating THF to the lithium, which results in a higher redox potential. The complete mechanism is discussed in the terms of geometries of the intermediates and reaction enthalpies in connection with the experimental values. Thus, a distinct new reaction pathway for a substitution on the benzene ring is established.