The Role of Ligand Transformations on the Performance of Phosphite- and Phosphinite-Based Palladium Catalysts in the Suzuki Reaction

Abstract

The orthometalated complex [{Pd(μ-Cl){κ2-P,C-P(OC6H2-2,4-tBu2)(OC6H3-2,4-tBu2)2}}2] reacts with phenylboronic acid hydrate and K2CO3 in dimethylacetamide to give [Pd{κ2-P,C-μ2-O-P(O)(C6H2-2,4-tBu2)(C6H3-2,4-tBu2)(DMAc)}]. When the reaction is repeated in dimethylformamide 3,3‘,5,5‘-tetra-tert-butyl-2,2‘-biphenol is isolated. Both compounds have been characterized crystallographically. The reaction of palladium dichloride with PiPr2(OC6H4-4-Et) in 2-methoxyethanol followed by recrystallization in the presence of ethanol leads to the formation of trans-[PdCl2{PiPr2(OEt)}2], which was also characterized by crystallography. To determine whether related solvolytic processes have a bearing on catalytic activity, the performance of a range of catalysts with “hydrolyzed” and “nonhydrolyzed” ligands was assessed in the Suzuki coupling of aryl bromides. In some cases it was evident that hydrolysis plays a significant role on the catalytic activity; however, this depends not only on the ligand, but also on the combination of ligand and palladium precursor.

Crystal structure of bis(μ 2-chlorido)-bis(di-p-tolylhydroxyphosphine-κP)-bis(di-p-tolylphosphite-κP)dipalladium(II), C56H58Cl2O4P4Pd2

“…Palladium catalysts, more speci cally those containing phosphine ligands have been intensively studied in homogeneous catalytic reactions such as the Heck and Suzuki cross coupling reaction [1][2][3][4][5][6]. The title compound forms part of an ongoing research study on the synthesis of highly selective and air stable phosphine ligands coordinated to platinum and palladium to ensure minimal by-product formation and to strengthen catalyst stability.…”

New aryl phosphinite ligands avoiding ortho-metallation: Synthesis and molecular structures of trans-[PdCl2(PPh2OR)2] and trans-[Rh(CO)Cl(PPh2OR)2] (R=2,4,6-Me3C6H2; 2,6-Ph2C6H3)

“…The new aryl phosphinites PPh 2 OR (R = 2,4,6-Me 3 C 6 H 2 , 1; R = 2,6-Ph 2 C 6 H 3 , 2) (Scheme 2) are accessible according to a general method reported by Bedford [4], using chlorodiphenylphosphine and the corresponding phenol (Eq. 1).…”

“…The white precipitate was isolated by filtration and recrystallized from hexane (65% yield). 4 ] in methylene chloride (20 mL) was stirred at room temperature for 24 h. After filtration through filter-paper, the solution was reduced in vacuo to half of its volume. Upon dropwise addition of n-hexane, a yellow solid precipitated.…”

“…Of particular interest in this respect was the use of palladacyclic complexes for carbon-carbon coupling reactions, an area initiated by Beller et al [3] and the introduction of palladacyclic pincer compounds by Bedford et al [4][5][6]. However, these complexes are so reactive that it is generally not possible to detect the catalytic active species or even to recover the palladium catalyst at the end of the reaction.…”

“…In these ligands, the ortho-positions of the aromatic phosphite function are blocked by methyl and phenyl substituents, which allows coordination to metal centres without ortho-metallation. Thus, reaction with [PdCl 2 (cod)] leads to the complexes trans-[PdCl 2 (PPh 2 OR) 2 ] (R = 2,4,6-Me 3 C 6 H 2 , 3; R = 2,6-Ph 2 C 6 H 3 , 4), while the reaction with [Rh 2 (CO) 4 Cl 2 ] gives trans-[Rh(CO)Cl(PPh 2 OR) 2 ] (R = 2,4,6-Me 3 C 6 H 2 , 5; R = 2,6-Ph 2 C 6 H 3 , 6). The single-crystal X-ray structure analyses of 3 and 5 confirm the trans-coordination of the new ligands in these square-planar complexes.…”