Abstract
Pyrazole-containing compounds have been used in recent times as ligands to stabilize metal
complexes
used as pre-catalysts in cross-coupling reactions. With various substituents at various positions
in the pyrazole ring, the overall electrophilic and steric properties of the metal complexes can be
fine-tuned. Herein, we report the synthesis of bulky pyrazole-based ligands by condensation of
methyl 4- (bromomethyl)benzoate or benzyl bromide with various substituted pyrazole compounds.
These ligands were utilised in the synthesis of bis(pyrazolyl)palladium(II) complexes. The
complexes' catalytic activity in Suzuki–Miyaura cross-coupling reactions was evaluated. Phenyl
bearing pre-catalyst 7, at a catalyst loading of 0.33 mol%, successfully converted 98% of
bromobenzene and phenylboronic acid to biphenyl in 4 h at 140 o C, while the tertiary butyl
bearing pre-catalyst 8 converted up to 81% of the same substrates to biphenyl. An increase in
conversion was seen for all pre-catalysts when an electron- withdrawing substituent was present on
the aryl halide substrate, and the opposite was observed when
the electron-withdrawing group was present on the phenyl boronic acid.