Catalytic Deuteration of C(sp2)-H Bonds of Substituted (Hetero)arenes in a Pt(II) CNN-Pincer Complex/2,2,2-Trifluoroethanol-d1 System: Effect of Substituents on the Reaction Rate and Selectivity

Thirty four (hetero)arene derivatives have been tested in catalytic H/D exchange reactions involving their C(sp(2))-H bonds and 2,2,2-trifluoroethanol-d(1) (TFE-d(1)) in the presence of the homogeneous Pt(II) complex 1 supported by a sulfonated CNN-pincer ligand at 80 degrees C. The 18 substrates, including one pharmaceutical (naproxen), that are stable in the presence of 1 and are active in the H/D exchange reaction have been characterized by their position-specific extent of deuteration and, in a number of cases, the reaction kinetic selectivity. For the most reactive substrates the extent of deuteration approaches the expected statistical distribution of the exchangeable H and D atoms: e.g., 67-69% for phenol after 23 h and 88% for indole beta-CH bonds after 45 min. For a few substrates (N,N-dimethylaniline, indole, nitrobenzene) the H/D exchange is highly position selective. No satisfactory correlation was found between the position-specific (meta, para) H/D exchange rate constants for X-monosubstituted benzenes and Hammett sigma(x) constants. This observation was proposed to be related to the concerted nature of the CH bond activation, the rate-determining CH bond oxidative addition at a Pt(II) center. A novel scale of Hammett sigma(M)(X) constants was introduced to characterize the reactivity of C(sp(2))-H bonds in transition-metal-mediated reactions. The experimentally determined position-specific Gibbs energies of activation of the H/D exchange in substituted benzenes meta and para positions) as well as in thiophene (alpha and beta positions) were matched satisfactorily using DFT calculations.