A new technique makes it possible to add carbon-nitrogen bonds at different spots in some organic compounds by making simple changes to a catalyst. The technique controls how and where amine formation reactions occur in a compound by altering the catalyst-ligand ratio in a silver-based catalytic system (J. Am. Chem. Soc. 2013, DOI: 10.1021/ja406654y).
Nitrogen-containing groups such as amines are crucial in many pharmaceuticals and other biologically active molecules. Researchers have long used transition-metal catalysts to introduce amines at reactive C–H sites or C=C sites. But the ability to favor one reaction over the other was previously possible only by changing reagent combinations.
Studies suggest that silver catalyst complexes undergo unique alterations in geometry when their composition changes in certain ways. Jennifer M. Schomaker and coworkers at the University of Wisconsin, Madison, wondered whether that feature of silver-based complexes could alter their reactive properties and, thus, be used to control where C–N bonds formed.
They now report that a silver triflate catalyst with a phenanthroline ligand in a 4:5 catalyst-ligand ratio catalyzed almost exclusively the addition of aziridine (a three-membered ring with a nitrogen) to the site of a C=C group in carbamate compounds. But changing the catalyst-ligand ratio to 1:3 transformed the reaction predominantly to amine formation at a C–H bond in the same carbamates.
Schomaker suspects that cutting the catalyst-ligand ratio congests the catalyst’s local steric environment, making the C–H reaction more likely and vice versa. Her group is working on extending this silver-based catalysis technique to chemoselective amination of other organic compounds.
Jennifer L. Roizen, an organic chemist at Duke University, comments that “other examples where ligand stoichiometry is linked to or causes indirect or direct changes in reactivity or chemoselectivity are few and far between.”