Direct Arylation of 2-Methylthiophene with Isolated [PdAr(μ-O₂CR)(PPh₃)]_n Complexes: Kinetics and Mechanism

Abstract

The palladium-catalyzed direct arylation of aromatic compounds with aryl halides has been proposed to involve an arylpalladium carboxylate intermediate. However, isolated arylpalladium complexes, which undergo C−H bond cleavage of aromatic substrates without the aid of additional activators or promoters, have been scarcely documented. This paper reports that [PdAr(μ-O 2 CR)(PPh 3 )] n complexes (1: Ar = Ph, 2-MeC 6 H 4 , 2,6-Me 2 C 6 H 3 ; R = Me, t Bu) successfully react with 2-methylthiophene (2) in the absence of additives to afford 5-aryl-2-methylthiophenes (3) in high yields. The reactivity increases with increasing bulkiness of the Ar group, whereas the bulky pivalate ligand (R = t Bu) reduces the reactivity as compared with the acetate ligand (R = Me). Complex 1 is in equilibrium with the monomeric species [PdAr(O 2 CR-κ 2 O)(PPh 3 )] (5) in solution, as confirmed by IR spectroscopy. Kinetic examinations have suggested that the direct arylation proceeds via 5, which undergoes C−H bond cleavage of 2. Complex 1 serves as a good catalyst for direct arylation of 2 with aryl bromides.

Optimization of direct arylation polymerization (DArP) through the identification and control of defects in polymer structure

“…Such a difference could be attributed to the fact that while the Fagnou conditions used in this study did not employ any phosphine ligands, the Ozawa catalytic system contained bulky and strongly coordinating ligands. In a recent paper focused on the mechanistic aspects of small molecule coupling through direct arylation, Ozawa et al described an interesting phenomenon of decreasing the degree of homocoupling with increasing the bulk of substituents at palladium center . Although largely unexplored at this point, such control of homocoupling regiodefects through tuning the bulk of substituents at palladium catalytic center could be useful in the case of DArP polymers.…”

Catalyst Efficacy of Homogeneous and Heterogeneous Palladium Catalysts in the Direct Arylation of Common Heterocycles

“…By comparison, a pseudo-first-order rate constant of 3.5(2)·10 -4 s -1 M -1 for the arylation of 2-methylthiophene has been reported, measured directly with [PdAr( -OAc)(PPh 3 )] 2 in 1,4-dioxane at 90 °C. 21 In conclusion, the effects of four nominally distinct palladium catalysts in direct arylation reactions, mediated by homogeneous and heterogeneous palladium, have been examined. 8 Initial-rate kinetic analysis showed similarities in catalytic behaviour between apparently distinct palladium catalysts, implying that dissimilar catalyst structures can function as precatalysts for the formation of a comparable active catalyst phase; speciation to form PdNPs or clusters is proposed as one possible mechanism for this process, under the reaction conditions tested.…”

In vitro and In silico Studies on the Base Effect in Palladium‐Catalyzed Direct Arylation

“…2‐Bromo‐3‐alkylthiophene is used as a demonstration example (Scheme ). It is accepted that the DAr proceeds through concerted metalation‐deprotonation (CMD) mechanisms, in which deprotonation of thiophene unit and metalation to Pd would occur simultaneously (Scheme ) . Two distinct transition states (TSs) might take place in the CMD.…”

“…It is accepted that the DAr proceeds through concerted metalation-deprotonation (CMD) mechanisms, in which deprotonation of thiophene unit and metalation to Pd would occur simultaneously (Scheme 2). [38,[41][42][43][44][45][46][47] Two distinct transition states (TSs) might take place in the CMD. Given that a base coordinates to Pd in the TS, it is termed as basecoordinated CMD (C-CMD) (Scheme 2).…”