Dithiophosphorus-Palladium Complexes as a Catalyst in the Heck Reaction via Pd(II)/Pd(IV) Catalytic Cycle: A Combined Experimental and Computational Study

作者：Gholivand Khodayar^*; Salami Rasoul; Rastegar Somayeh F; Roe S Mark

来源：ChemistrySelect, 2018, 3(27): 7822-7829.

DOI：10.1002/slct.201801185

• Dithiophosphorus Ligands
• DFT
• Heck Reaction
• Pd(II)/Pd(IV) Mechanism
• Palladium-Catalyzed

The palladium-catalyzed Heck reaction is the most effective method for vinylation of unsaturated halides. Here, new thermally stable palladium(II) complexes {(4OMe-Ph) RPS2}(2)Pd [R= C5H11NH (1); R= CH3CH2NHCH3 (2); R=OCH3 (3)] were readily synthesized in a high yield by the reaction of PdCl2 with dithiophosphorus ligands. The molecular structures of 1 and 2 were determined by means of X-ray diffraction analysis. The catalytic activity of all complexes was evaluated in Heck reaction. The results revealed that the complexes are highly active catalysts in coupling of aryl bromides with styrene. The Hammett plot of the competition experiments of various aryl bromides with styrene, catalyzed by the complexes, displayed very good linear correlation with a positive slope (rho) within a range of 3.99-4.39 for 1-3. The absence of any decomposition signs together with negative Hg-poisoning tests indicated that the liberation of Pd(0) species in the catalytic cycle is unlikely. So, it was supposed that the palladium(II)-catalyzed Heck reaction might not proceed via classical Pd(0)/Pd(II) catalytic cycle. To find the most probable mechanism, density functional theory calculations were performed and the predicted data compared with the experimental results. Upon the obtained results, the Pd(II)/Pd(IV) catalytic cycle was proposed as the most plausible mechanism. The overall free energy profile showed that the first step of the mechanism (oxidative addition) with a free energy barrier of 33.8kcal/mol involves the highest energy barrier and can be considered as the rate determining step of the mechanism.

• 出版日期2018-7-23