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DFT Study of Cysteine-Mediated ortho-Specific Deuteration of Azobenzene Using Cyclopalladated Azobenzenes as Precursors

Acid-mediated hydrogen–deuterium exchange reactions (H/D exchange) are among the oldest methods known for labeling of aromatic compounds. Nevertheless, site-selective high-degree deuteration of small molecules is still a complex procedure even though labelled substrates are in high demand. Activation of the aromatic C–H bond by metal centers leads to cyclometalated complexes that in acidic environments can be converted back to the starting ligand. If a deuterated acid is used, deuterium can be introduced to the ligand at the site of the metalation. Recently our group prepared and studied a series of mono- and dicyclopalladated azobenzene complexes (Scheme 1). Experiments involving amino acid cysteine as a side ligand showed that this reaction leads to isolation of the azobenzene ligand. If a deuterated cysteine (Cys4D) is used, site-specific deuteration at ortho position to the azo group of the ligand occurs. Here we present the mechanistic DFT study of cleavage of the cyclopalladated complex by cysteine and formation of the ortho-deuterated azobenzene. Reaction involves formation of the cysteine cyclopalladated complex followed by a transfer of the deuteron from uncoordinated cysteine to the ortho-carbon atom.

deuteration ; azobenzene ; cysteine ; cyclopalladated complexes ; DFT

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