Reaction of <i>N</i>‑Isopropyl‑<i>N</i>‑phenyl-2,2′-bipyridin-6-amine with K<sub>2</sub>PtCl<sub>4</sub>: Selective C–H Bond Activation, C–N
Bond Cleavage,
and Selective Acylation
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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 sp<sup>2</sup> C–H
bond activation in acetonitrile and low yield of sp<sup>3</sup> C–H
activation in acetic acid. The use of a base was found to effectively
suppress the competing sp<sup>3</sup> C–H bond activation in
acetonitrile, improving the selectivity of sp<sup>2</sup> 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 K<sub>2</sub>PtCl<sub>4</sub> 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 K<sub>2</sub>PtCl<sub>4</sub> 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