Hydrogen-Bond-Assisted Controlled C–H Functionalization via Adaptive Recognition of a Purine Directing Group

We have developed the Rh-catalyzed selective C–H functionalization of 6-aryl­purines, in which the purine moiety directs the C–H bond activation of the aryl pendant. While the first C–H amination proceeds via the N1-chelation assistance, the subsequent second C–H bond activation takes advantage of an intramolecular hydrogen-bonding interaction between the initially formed amino group and one nitrogen atom, either N1 or N7, of the purinyl part. Isolation of a rhodacycle intermediate and the substrate variation studies suggest that N1 is the main active site for the C–H functionalization of both the first and second amination in 6-arylpurines, while N7 plays an essential role in controlling the degree of functionalization serving as an intramolecular hydrogen-bonding site in the second amination process. This pseudo-Curtin–Hammett situation was supported by density functional calculations, which suggest that the intramolecular hydrogen-bonding capability helps second amination by reducing the steric repulsion between the first installed ArNH and the directing group.