Insights into the Heck Reaction with PCP Pincer Palladium(II) Complexes

Eberhard, Michael R.

doi:10.1021/ol049430a

Cited by 215 publications

(124 citation statements)

References 25 publications

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“…[21] Together these studies provide convincing evidence that these SCS palladacycles and other palladacycles are neither the catalytically active species nor a reversibly formed rest state of an active catalyst for Heck catalysis but rather are a precursor of another undefined but quite active catalyst.…”

Section: Introductionmentioning

confidence: 86%

Mechanistic Studies of SCS-Pd Complexes Used in Heck Catalysis

Bergbreiter

Osburn

Frels

2005

Advanced Synthesis & Catalysis

150

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Air-stable SCS palladacycles that can be used to promote C À C coupling chemistry were studied mechanistically. Using a small library of electronically varied SCS ligands, a collection of palladacycles was synthesized. Kinetic studies showed that these complexes all had induction periods, induction periods that were effected by concentration of substrates, products and trace impurities. Hammet correlations showed that electronically diverse palladacycles had identical 1 values, values that suggested that aryl halide electrophilic addition to a Pd species was not the rate-determining step. Phosphine addition experiments led to increased reactivity of the starting palladacycles, possibly by trapping an in situ generated Pd(0) species. Studies that examined reactivity in biphasic thermomorphic reactions showed residual activity in phases that do not contain polymer-bound palladacycle and provided convincing evidence that palladacycles are not the actual catalyst. Poisoning experiments using mercury metal to test for the presence of a Pd colloid were very effective with low molecular weight palladacycles, completely suppressing Heck chemistry. Similar studies with polymer-bound palladacycles showed mercury poisoning too. However, since so little decomposition of the palladacycle occurred, the polymer-bound palladacycle could still be recycled multiple times. However, mercury poisoned subsequent cycles of the experiment too. The conclusion is that SCS palladacycles are actually reservoirs of a catalytically active but ill-defined form of palladium(0).

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Section: Introductionmentioning

confidence: 86%

Mechanistic Studies of SCS-Pd Complexes Used in Heck Catalysis

Bergbreiter

Osburn

Frels

2005

Advanced Synthesis & Catalysis

150

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“…[59] Similar results were obtained on phosphite PCP pincers at the same time we reported on decomposition in SCS pincers. [41] Again, it was not possible to rule out small amounts of catalysis by intact metalligand complexes in that work, although it was clear that the vast majority of catalysis was promoted by Pd(0) species (nanoparticles, it was suggested). In contrast, by immobilizing SCS and PCP pincer complexes onto insoluble porous silica supports and soluble poly(norbornenes), it was possible to conclusively show for the first time that there was effectively no catalysis by intact pincer species.…”

Section: Poly(4-vinylpyridine) As a Poisonmentioning

confidence: 93%

“…Thus, in many contributions, authors have identified catalysis by soluble species but could not rule out some catalysis by supported sites. The Hg(0) test is capable of extinguishing catalysis by free Pd(0), [8,[22][23][24]26,40,41] although it is not believed that it can discriminate between heterogeneous, macroscopic palladium particles, soluble palladium nanoparticles, or homeopathic [35,36,42,43] palladium (vide supra). [8] What is needed is a poison that is selective for leached, homogeneous species.…”

Section: Introductionmentioning

confidence: 99%

Poly(4‐vinylpyridine) and Quadrapure TU as Selective Poisons for Soluble Catalytic Species in Palladium‐Catalyzed Coupling Reactions – Application to Leaching from Polymer‐Entrapped Palladium

Richardson

Jones

2006

Adv Synth Catal

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Poly(4-vinylpyridine) and Quadrapure TU were successfully used as poisons for palladium-catalyzed Heck couplings of iodoarenes and n-butyl acrylate using a polyA C H T U N G T R E N N U N G (urea) entrapped palladium precatalyst, Pd-EnCat 40, in a variety of reaction solvents and conditions. These results suggest that all of the Heck coupling takes place outside the polyA C H T U N G T R E N N U N G (urea) matrix via a soluble palladium species leached from the solid. This observation is consistent with previous conclusions that leached species are also solely active when using other supported catalysts such as Pd/C or with supported palladacycles. Control experiments including poisoning by Hg(0), hot filtration, 3-phase tests, and monitoring of reaction kinetics after recycle support this conclusion. The quantity of PVPy or Quadrapure TU needed for quenching depends on the amount of palladium that is leached and thus applying these poisons to other palladium catalysts in which the amount of soluble palladium is unknown requires an initial trial and error approach.

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“…The Heck reaction has been demonstrated to find wide utility in both, total syntheses of natural products in academia and synthesis in pharmaceutical and agrochemical industry [1][2][3][4][5][6][7][8][9][10] . Even though recent developments have considerably increased the activity of Heck catalysts [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] , a typical reaction protocol with aryl bromides as substrates still requires high reaction temperatures (140 °C), catalyst loadings in the range of 1 mol% and reaction times of up to 24 hr. Moreover, modified reaction conditions, including the reaction temperature, catalyst loadings, bases, solvents, and additives, e.g.…”

Section: Introductionmentioning

confidence: 99%

Mizoroki-Heck Cross-coupling Reactions Catalyzed by Dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium Under Mild Reaction Conditions

Oberholzer¹,

Frech²

2014

JoVE

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Dichloro-bis(aminophosphine) complexes of palladium with the general formula of [(P{(NC 5 H 10 ) 3-n (C 6 H 11 ) n }) 2 Pd(Cl) 2 ] (where n = 0-2), belong to a new family of easy accessible, very cheap, and air stable, but highly active and universally applicable C-C cross-coupling catalysts with an excellent functional group tolerance. Dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium [(P(NC 5 H 10 ) 3 ) 2 Pd(Cl) 2 ] (1), the least stable complex within this series towards protons; e.g. in the form of water, allows an eased nanoparticle formation and hence, proved to be the most active Heck catalyst within this series at 100 °C and is a very rare example of an effective and versatile catalyst system that efficiently operates under mild reaction conditions. Rapid and complete catalyst degradation under work-up conditions into phosphonates, piperidinium salts and other, palladium-containing decomposition products assure an easy separation of the coupling products from catalyst and ligands. The facile, cheap, and rapid synthesis of 1,1',1"-(phosphinetriyl)tripiperidine and 1 respectively, the simple and convenient use as well as its excellent catalytic performance in the Heck reaction at 100 °C make 1 to one of the most attractive and greenest Heck catalysts available.We provide here the visualized protocols for the ligand and catalyst syntheses as well as the reaction protocol for Heck reactions performed at 10 mmol scale at 100 °C and show that this catalyst is suitable for its use in organic syntheses.

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Insights into the Heck Reaction with PCP Pincer Palladium(II) Complexes

Cited by 215 publications

References 25 publications

Mechanistic Studies of SCS-Pd Complexes Used in Heck Catalysis

Mechanistic Studies of SCS-Pd Complexes Used in Heck Catalysis

Poly(4‐vinylpyridine) and Quadrapure TU as Selective Poisons for Soluble Catalytic Species in Palladium‐Catalyzed Coupling Reactions – Application to Leaching from Polymer‐Entrapped Palladium

Mizoroki-Heck Cross-coupling Reactions Catalyzed by Dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium Under Mild Reaction Conditions

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