Dinuclear Tetrapyrazolyl Palladium Complexes Exhibiting Facile Tandem Transfer Hydrogenation/Suzuki Coupling Reaction of Fluoroarylketone

Herein,
we report an example of dinuclear pyrazolyl-based Pd complexes
exhibiting facile tandem catalysis for fluoroarylketone: tetrapyrazolyl
dipalladium complexes with varying Pd–Pd distances efficiently
catalyze the tandem reaction involving transfer hydrogenation of fluoroarylketone
to the corresponding alcohol and Suzuki–Miyaura cross-coupling
reaction of the resulting fluoroarylalcohol under moderate reaction
conditions, to biaryl alcohol. The complex with the shortest Pd–Pd
distance exhibits the highest tandem activity among its dimetallic
analogues, and it exceeds, in terms of activity and selectivity, the
analogous mononuclear compound. The kinetics of the reaction indicates
clearly that reductive transformation of haloarylketone into haloaryalcohol
is the rate-determining step in the tandem reaction. Interestingly,
while fluoroarylketone undergoes the multistep tandem catalysis, the
chloro- and bromo-arylketones undergo only a single step C–C
coupling reaction, resulting in biarylketone as the final product.
Unlike the pyrazole-based Pd compounds, the precursor PdCl2 and the phosphine-based relevant complexes (PPh3)2PdCl2 and (PPh3)4Pd are found
to be unable to exhibit the tandem catalysis.