Homeopathic Ligand-Free Palladium as a Catalyst in the Heck Reaction. A Comparison with a Palladacycle

Vries, André H.M. de; Mulders, Jan M.C.A.; Mommers, John; Henderickx, Huub; Vries, Johannes G. de

doi:10.1021/ol035184b

Cited by 557 publications

(315 citation statements)

References 52 publications

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“…Thus, the higher the S C -1 ratio the higher the turnover frequency will be. 147 Both Cu(0) as well as Cu(I) and Cu(II) oxide can form nanoparticles. Moreover, as in most reactions halide anions, either from the substrate or from the used copper salt precursor, are abundant, even under apparently homogeneous reaction conditions, the presence of nano-particles of copper halide salts (copper in various oxidation states with bridging halides, cf.…”

Section: Discussionmentioning

confidence: 99%

The mechanism of the modified Ullmann reaction

et al. 2010

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The copper-mediated aromatic nucleophilic substitution reactions developed by Fritz Ullmann and Irma Goldberg required stoichiometric amounts of copper and very high reaction temperatures. Recently, it was found that addition of relatively cheap ligands (diamines, aminoalcohols, diketones, diols) made these reactions truly catalytic, with catalyst amounts as low as 1 mol% or even lower. Since these catalysts are homogeneous, it has opened up the possibility to investigate the mechanism of these modified Ullmann reactions. Most authors agree that Cu(I) is the true catalyst even though Cu(0) and Cu(II) catalysts have also shown to be active. It should be noted however that Cu(I) is capable of reversible disproportionation into Cu(0) and Cu(II). In the first step, the nucleophile displaces the halide in the LnCu(I)X complex forming LnCu(I)ZR (Z = O, NR¢, S). Quite a number of mechanisms have been proposed for the actual reaction of this complex with the aryl halide: 1. Oxidative addition of ArX forming a Cu(III) intermediate followed by reductive elimination; 2. Sigma bond metathesis; in this mechanism copper remains in the Cu(II) oxidation state; 3. Single electron transfer (SET) in which a radical anion of the aryl halide is formed (Cu(I)/Cu(II)); 4. Iodine atom transfer (IAT) to give the aryl radical (Cu(I)/Cu(II)); 5. p-complexation of the aryl halide with the Cu(I) complex, which is thought to enable the nucleophilic substitution reaction. Initially, the radical type mechanisms 3 and 4 where discounted based on the fact that radical clock-type experiments with ortho-allyl aryl halides failed to give the cyclised products. However, a recent DFT study by Houk, Buchwald and co-workers shows that the modified Ullmann reaction between aryl iodide and amines or primary alcohols proceeds either via an SET or an IAT mechanism. Van Koten has shown that stalled aminations can be rejuvenated by the addition of Cu(0), which serves to reduce the formed Cu(II) to Cu(I); this also corroborates a Cu(I)/Cu(II) mechanism. Thus the use of radical clock type experiments in these metal catalysed reactions is not reliable. DFT calculations from Hartwig seem to confirm a Cu(I)/Cu(III) type mechanism for the amidation (Goldberg) reaction, although not all possible mechanisms were calculated.

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Section: Discussionmentioning

confidence: 99%

The mechanism of the modified Ullmann reaction

et al. 2010

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“…using extremely low palladium acetate catalyst loadings, which they dubbed "homeopathic" ligand free palladium catalysts [29][30][31]. They reported that the Suzuki reaction of aryl bromides were catalysed in high yields using 0.02-0.05 mol% palladium and even the reactions of some aryl chlorides were catalysed using 0.005- However, further TEM analysis of the recycled MWCNT/Pd-DMAP NP composite shows a further aspect of interest within this reaction mixture.…”

Section: Interestingly De Vries Et Al Have Identified the Catalysis mentioning

confidence: 99%

Suzuki coupling activity of an aqueous phase Pd nanoparticle dispersion and a carbon nanotube/Pd nanoparticle composite

Sullivan¹,

Flanagan²,

Hain

2009

Catalysis Today

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An aqueous phase dispersion of Pd nanoparticles stabilised by 4-Dimethylaminopyridine (DMAP) promotes model Suzuki coupling reactions. The dispersion contains Pd nanoparticles of 3.4 0.5 nm and a Pd(II) species (Pd(DMAP) 4 (OH) 2 ) which forms following aerobic oxidation of the nanoparticles.The activity of the nanoparticle dispersion in promoting the Suzuki reactions is directly proportional to the size of the halogen on the substrate (as is usual for these coupling reactions) and also to the age of the nanoparticle dispersion.The Pd(DMAP) 4 (OH) 2 complex can be isolated from the dispersion and is found to be very active in promoting the reactions. Its formation following aerobic oxidation of the nanoparticles is proposed as the reason for the improved activity of the dispersion with age.The nanoparticles present in the dispersion can, through displacement of the stabilising ligand, be immobilised onto functionalised Multi Wall Carbon Nanotubes (MWCNT) and the composite formed is an active and recyclable catalyst. This MWCNT/Pd-DMAP NP composite acts as a reservoir of dissolved Pd species, which function as homogeneous catalysts under reaction conditions.

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“…Therefore, further work continued using the alternative route where the Heck reaction was involved in the initial step. Firstly, Heck reactions between 3-bromobenzaldehyde (12) and commercially available styrenes (23-27) were performed in Nmethylpyrrolidone containing palladium (II) acetate (0.06% mole) as catalyst and anhydrous sodium acetate as base [22]. Except for the reactant 4-aminostyrene 27, all styrenes (23-26) gave corresponding stilbenes (35-38) in good yields (Table 4).…”

Section: Resultsmentioning

confidence: 99%

Synthesis of stilbene-fused 2′-hydroxychalcones and flavanones

Akçok

Çağır

2010

Bioorganic Chemistry

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a b s t r a c tSynthesis of stilbene-fused chalcones and flavanones were successfully completed. Molecules were designed in a way to mimic the structural features of both ''stilbene and chalcones" or ''stilbene and flavanones" at the same time, and synthesized by three steps. Heck reactions of 3-bromobenzaldehyde with styrene derivatives gave corresponding (E)-stilbenes, which were reacted with acetophenones to furnish stilbene-fused 2 0 -hydroxychalcones under basic conditions. Finally, intramolecular cyclization reactions were performed to produce stilbene-fused flavanones.

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Homeopathic Ligand-Free Palladium as a Catalyst in the Heck Reaction. A Comparison with a Palladacycle

Cited by 557 publications

References 52 publications

The mechanism of the modified Ullmann reaction

The mechanism of the modified Ullmann reaction

Suzuki coupling activity of an aqueous phase Pd nanoparticle dispersion and a carbon nanotube/Pd nanoparticle composite

Synthesis of stilbene-fused 2′-hydroxychalcones and flavanones

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