Cocondensation reactions of heterocyclic aromatic compounds with lithium, calcium and magnesium atoms at 77 K

Calcium and magnesium atoms were cocondensed with aromatic heterocycles containing five- and six-membered rings in the presence of THF at 77 K. In the case of calcium the cocondensation with five-membered heterocyclic compounds resulted in C–H bond activations and led to the corresponding aryl calcium compounds, while magnesium did not show comparable reactions. When six-membered heterocyclic compounds, e.g., pyridine and 4-methylpyridine (4-picoline) were cocondensed with calcium, magnesium and lithium atoms, all reactions led to the formation of non-metallated aromatic products and the formation of metal hydride. DFT calculations at the B3LYP/6-31G** level of theory were carried out in order to interpret the pathways of the cocondensation reactions using calcium atoms and identify the possible intermediates involved. In all reactions π- and σ-complexes between calcium atoms and the heterocyclic aromatic reactant were found as stable intermediates on the energy hypersurface.

Graphical abstractLithium, calcium and magnesium atoms were cocondensed with aromatic five-membered heterocycles in the presence of THF. In the case of calcium the reaction led to heteroaryl calcium hydride and the reaction mechanism of this unusual CH activation reaction was investigated using DFT calculations.Figure optionsDownload full-size imageDownload as PowerPoint slide