Understanding Anionic “Ligandless” Palladium Species in the Mizoroki–Heck Reaction

Abstract

The anionic complex [NBu][Pd(DMSO)Cl], as a "ligandless" system, was shown to be an active catalyst in the Mizoroki-Heck coupling of aryl chlorides in the absence of strongly σ-donating ligands. To investigate the experimentally observed influence of halides and the amount of water on the catalytic activity, we employed a combination of experiments and theoretical calculations. The presence of water was shown to be critical for the formation of the active palladium(0) species by oxidation of in situ generated tributylamine. Oxidative addition to an anionic palladium(0) species was found to be the rate-determining step of the catalytic cycle. For the ensuing steps, both neutral and anionic pathways were considered. It was shown that, in the absence of strongly σ-donating neutral ligands, chloride ions stabilize the catalytic intermediates. Therefore, an anionic pathway is preferred, which explains the need for tetrabutylammonium chloride as an additive. The study of the influence of bromide ions on the catalytic activity revealed that the strongly exergonic displacement of the neutral substrates by bromide ions lowers the catalytic activity.

Tailored Palladate Tunable Aryl Alkyl Ionic Liquids (TAAILs)

“…This way, the butyl groups form a cavity, which contains the Pd 2 Br 6 ‐dianion. The anion adopts a V‐shape (Figure S2, Supporting Information), as evident from the Br1‐Br3‐Br5 angle of 160.09(1)°, which diverges significantly from the ideal planar arrangement expected for the isolated Pd 2 Br 6 ‐dianion …”

“…The aniona dopts aV -shape ( Figure S2, SupportingI nformation), as evident from the Br1-Br3-Br5 angle of 160.09(1)8,w hich divergess ignificantly from the ideal planar arrangement expected for the isolatedP d 2 Br 6dianion. [31] The solid-state structure of 3b differs from that of 3a (Figure 3). Given that the aryl group does not contain an ortho-substituent, the torsion versus the imidazolium ring is reduced to 20.1 (8)8 (C8-N1-C5-C6).…”

“…Considering that compound 3d crystallized with two molecules of THF (Figure 4), the solid-state structure differs substantially from the other Pd 2 Br 6 compounds 3a,b.A sad onor molecule, the THF oxygen atom interacts with the backbone hydrogen atoms, leading to an O2···H(C10) distance of 2.31 ,r ough- [8], and torsions [8]: Pd1ÀBr1 2.455(1), Pd1ÀBr2 2.412 (1), Br3···H(C8) 3.25, O2···H(C10)2. 31,Br2ÀPd1ÀBr3' 176.61 (1), C8ÀN1ÀC5ÀC6 30.03(3). ly similar to the O1···H(C10)d istance in 2a of 2.30 .T his way, the interaction between the imidazolium hydrogen atom and the anion is only weak [3.25 for Br3···H(C8)].I nterestingly,t he dianion is nearly planar in this compound.A dditionally, pstacking of the phenyl rings becomes possible, asi ndicatedb y the distance of 3.417(3) between C5 of one ring and C3 of a neighboring ring ( Figure S4, Supporting Information).…”

“…Repeated attempts to crystallize 5a led to the formation of [NBu 4 ] 2 [Pd 2 Br 6 ] ( Figure 7). This is in stark contrast to compounds 2a and 5b.T he main reasons might be the increased tendency of bromide ions to act as bridging ligands, [31][32] and the absence of as tabilizing hydrogen bond donorf or the DMSO oxygen atom. This effect is reflected by the DFT calculations (see below).…”

Is the Mechanistic Scheme of the Heck‐Type Coupling with Unsaturated Substrates needed to be revised?

“…As to the possible ways to change the compositions of the species containing Ar and X moieties participating in the steps B , B ′, C , C ′, and D , D ′ (Scheme 1), the addition of halide salt to the reaction system and the varying aryl moiety can be considered. The addition of halide salt influences the nature of counterion in ArPdX because of entrance of halide anions in the coordination sphere of Pd intermediates including anionic ones [22–26] …”