Abstract
M.Sc. (Chemistry)
A series of bulky pyrazole-based compounds (1-7) were synthesized by reaction of methyl 4-(bromomethyl) benzoate or benzyl bromide with phenyl, tert-butyl, ferrocenyl and methyl substituted pyrazole compounds. These pyrazolyl compounds were utilised as ligands in the synthesis of new bis(pyrazolyl) palladium(II) complexes (8-12) of the type [PdCl2(L)2] using [PdCl2(MeCN)2] as precursor complex. The palladium complexes and ligands were characterized using multinuclear NMR and infrared spectroscopy, mass spectrometry, melting point, elemental analysis and single crystal X-ray diffraction for complexes 9 and 10.
The palladium complexes (8-12) were evaluated as catalyst precursors in Suzuki-Miyaura cross-coupling reactions, and were found to be very active and selective. Complexes bearing phenyl substituents (8, 10 and 12) were slightly more active for the Suzuki-Miyaura cross-coupling reaction as compared to complexes with tert-butyl substituents (9 and 11). These experimental observations were also corroborated with computational studies. The presence of ester groups on (pre)catalysts 8 and 9 resulted in slightly reduced activity for the Suzuki-Miyaura cross-coupling reaction as compared to (pre)catalysts for which the ester groups were absent (10 and 11). In addition, (pre)catalysts 8, 9, 10 and 11 showed tolerance to functional groups present on substrates, including 4-CN, 4-CHO, 4-Cl and 2-CH3, in the Suzuki-Miyaura cross-coupling reactions.
Complexes 8-11 were also evaluated as (pre)catalysts in Mizoroki-Heck cross-coupling reaction, and displayed high activity and selectivity. Additionally, these high conversions were achieved using low (pre)catalyst loading and short reaction times for 8-11. The mercury drop test confirmed that (pre)catalysts 8-11 act as homogenous molecular catalysts in these Mizoroki-Heck cross-coupling reactions and do not convert to heterogeneous colloidal particles during the reaction.
The catalyst systems generated with (pre)catalysts 8-11) exhibited similar conversions and selectivity for the Mizoroki-Heck compared to Suzuki-Miyaura cross-coupling reactions performed with these catalysts. In addition, (pre)catalysts 8-11 were tolerant to the functional groups 4-CF3, 4-CH3, 4-CO2Me and 4-CO2Et that were present on substrates used for the Mizoroki-Heck cross-coupling reactions.