Reaction of <i>N</i>‑Isopropyl‑<i>N</i>‑phenyl-2,2′-bipyridin-6-amine with K₂PtCl₄: Selective C–H Bond Activation, C–N Bond Cleavage, and Selective Acylation

Abstract

The selective C–H bond activation of <i>N</i>-isopropyl-<i>N</i>-phenyl-2,2′-bipyridin-6-amine promoted by Pt­(II) was complicated by the low selectivity of sp² C–H bond activation in acetonitrile and low yield of sp³ C–H activation in acetic acid. The use of a base was found to effectively suppress the competing sp³ C–H bond activation in acetonitrile, improving the selectivity of sp² C–H bond activation from 70% to 99%. In the reaction in acetic acid, the low yield was due to the competing C–N bond cleavage. The use of a base reduced the C–N bond cleavage, but not completely. The reaction of <i>N</i>-<i>tert</i>-butyl-<i>N</i>-phenyl-2,2′-bipyridin-6-amine with K₂PtCl₄ in acetic acid produced the cyclometalated complex with complete C–N bond cleavage and its acylated derivative. These results indicated that the C–N bond cleavage might proceed via heterolytic C–N bond dissociation. The acylation following the C–N cleavage in the reaction in acetic acid is regioselective. Further experiments showed that the reaction of <i>N</i>-phenyl-2,2′-bipyridin-6-amine with K₂PtCl₄ in acetic acid produced the cyclometalated complex, while the reaction in a mixture of acetic anhydride and acetic acid produced the acylated cyclometalated complex. An X-ray crystal structure study revealed strong intramolecular H bonding in the acylated complexes. The regioselectivity was explained in terms of H bonding and the electron distribution predicted by the DFT calculations