C−C Reductive Elimination in Palladium Complexes, and the Role of Coupling Additives. A DFT Study Supported by Experiment

Abstract

A DFT study of R-R reductive elimination (R = Me, Ph, vinyl) in plausible intermediates of Pd-catalyzed processes is reported. These include the square-planar tetracoordinated systems cis-[PdR(2)(PMe(3))(2)] themselves, possible intermediates cis-[PdR(2)(PMe(3))L] formed in solution or upon addition of coupling promoters (L = acetonitrile, ethylene, maleic anhydride (ma)), and tricoordinated intermediates cis-[PdR(2)(PMe(3))] (represented as L = empty). The activation energy ranges from 0.6 to 28.6 kcal/mol in the gas phase, increasing in the order vinyl-vinyl < Ph-Ph < Me-Me, depending on R, and ma < "empty" < ethylene < PMe(3) approximately MeCN, depending on L. The effect of added olefins was studied for a series of olefins, providing the following order of activation energy: p-benzoquinone < ma < trans-1,2-dicyanoethylene < 3,5-dimethylcyclopent-1-ene < 2,5-dihydrofuran < ethylene < trans-2-butene. Comparison of the calculated energies with experimental data for the coupling of cis-[PdMe(2)(PPh(3))(2)] in the presence of additives (PPh(3), p-benzoquinone, ma, trans-1,2-dicyanoethylene, 2,5-dihydrofuran, and 1-hexene) reveals that: (1) There is no universal coupling mechanism. (2) The coupling mechanism calculated for cis-[PdMe(2)(PMe(3))(2)] is direct, but PPh(3) retards the coupling for cis-[PdMe(2)(PPh(3))(2)], and DFT calculations support a switch of the coupling mechanism to dissociative for PPh(3). (3) Additives that would provide intermediates with coupling activation energies higher than a dissociative mechanism (e.g., common olefins) produce no effect on coupling. (4) Olefins with electron-withdrawing substituents facilitate the coupling through cis-[PdMe(2)(PR(3))(olefin)] intermediates with much lower activation energies than the starting complex or a tricoordinated intermediate. Practical consequences are discussed.

The Negishi Catalysis: Full Study of the Complications in the Transmetalation Step and Consequences for the Coupling Products

“…In Negishi syntheses, where halides are always present (introduced in the initial oxidative addition step of ArX to Pd 0 ), the study of the undesired reactions shown in Scheme 1 (equations a and b) is obscured by the interference of re- 13 but in instances that disfavor reductive elimination (e.g. electron withdrawing groups in the carbon fragments, or large steric hindrance), the undesired transmetalations shown in Scheme 1a-c can become a serious competition to the expected Ar-Ar' crosscoupling.…”

Bimetallic Catalysis using Transition and Group 11 Metals: An Emerging Tool for CC Coupling and Other Reactions

“…[29] This ligand comproportionation reaction is possible only because the initial cis-A C H T U N G T R E N N U N G [PtR 2 L 2 ] complexes do not undergo easy R À R coupling and the thermodynamically favorable isomerization is much faster than the coupling. For this exchange reaction, the authors proposed two possible mechanisms www.chemeurj.org (Scheme 11) not requiring prior L dissociation (since dissociation should promote fast R À R coupling): [30] a) through a cyclic intermediate; or b) through what can formally be considered an oxidative additon/reductive elimination sequence.…”

Combined DFT and experimental studies of C–C and C–X elimination reactions promoted by a chelating phosphine–alkene ligand: the key role of penta-coordinate PdII

“…complexes. 12,14 In a similar fashion, the alternative direct pathway for C-C coupling from the tetracoordinate square planar dimethyl complex 1 was probed computationally (Scheme 5, Path A). These studies reveal that, as expected, the mechanism is concerted, 4,31 with an activation barrier of 28.1 kcal mol -1 ( Fig.…”

“…1), we have shown above that the Gibbs energy barrier to reaching TS1→2 is 28.1 kcal mol -1 , a value that lies within the range of activation energies reported previously for reductive elimination involving C(sp 3 )-C(sp 3 ) coupling reactions. 14,15 Furthermore, our calculations predict that reductive elimination from 1, leading first to the unstable low-coordinate species 2', is weakly exergonic (ΔG 1→2' = -7.1 kcal mol -1 Together these data indicate that the slow reaction observed on treating [PdMe 2 (tmeda)] with one equivalent of L1 is not only a result of kinetic control (activation barrier of 28.1 kcal mol -1 ), but also a consequence of thermodynamic constraints imposed by the weakly exergonic reaction leading to 2', which then evolves to the bis(L1) complex 2 after release of elemental palladium.…”

“…13 An analogous computational study by Maseras and co-workers explored R-R (R = Me, Ph, vinyl) coupling reactions from cis-[PdR 2 (L)(PMe 3 )] complexes via reductive elimination, where L is either acetonitrile, ethylene or maleic anhydride. 14 Here, the steric demands and the electronic character of the organyl group R, the phosphine and the mode of coordination of the co-ligand L all strongly influence the coupling mechanism. In particular, the influence of the coupling partners R was highlighted, with reductive elimination involving C(sp 2 )-C(sp 2 ) bond formation being computed to be more facile than that involving C(sp 3 )-C(sp 3 ) coupling.…”