Ligands dibenzylideneacetone

Palladium(0) complexes with the weak ligand dibenzylideneacetone (Figure 5-4) not only are commercially available but also serve as valuable precatalysts for a number of reactions (e.g.. Scheme 5-38). Depending on the synthesis procedures, Pd(dba)2, Pd2(dba)3 CHCl3 and/or other stoichiometries can be obtained. Recrystallization provided the pure complexes. In some cross-coupling reactions, the commercially available modified dibenzylideneacetone ligand 3,5,3, 5 -... 

The synthesis of metal nanoparticles via the controlled decomposition of pre-prepared organometallic complexes or metal carbonyls where the metals are already in the zero valent or low-valent state has been known since 1970. The first examples were Pd- and Pt-dibenzylideneacetone complexes where the coordinated ligands detached using either hydrogen of carbon monoxide under mild conditions to give the respective metal nanoparticles [310]. 

Some other examples of metal-catalyzed substitutions are given in Scheme 11.10. Entries 1 to 3 are copper-catalyzed reactions. Entry 1 is an example of arylation of imidazole. Both dibenzylideneacetone and 1,10-phenanthroline were included as ligands and Cs2C03 was used as the base. Entry 2 is an example of amination by a primary amine. The ligand used in this case was (V,(V-diethyl sal icyl amide. These conditions proved effective for a variety of primary amines and aryl bromides with both ERG and EWG substituents. Entry 3 is an example of more classical conditions. The target structure is a phosphodiesterase inhibitor of a type used in treatment of asthma. Copper powder was used as the catalyst. 

The catalyst reported by Drent [48] was generated in situ by mixing a palladium source with the ligand. A palladium source is broadly defined as a complex or any form of palladium metal whereby upon mixing with the ligand an active catalyst is formed. Many palladium sources are possible, but the sources exemplified by Drent aretris(dibenzylideneacetone)dipalladium(0)(Pd2(dba)3),bis(dibenzylideneacetone) palladium(O) (Pd(dba)2), or palladium(II) acetate. 

Buchwald has shown that, in combination with palladium(II) acetate or Pd2(dba)3 [tris(dibenzylideneacetone)dipalladium], the Merrifield resin-bound electron-rich dialkylphosphinobiphenyl ligand (45) (Scheme 4.29) forms the active polymer-supported catalysts for amination and Suzuki reactions [121]. Inactivated aryl iodides, bromides, or even chlorides can be employed as substrates in these reactions. The catalyst derived from ligand (45) and a palladium source can be recycled for both amination and Suzuki reactions without addition of palladium. 

Recently, arylation of methyl acrylate was intensively studied [15-21], Phenylation with Ph3BiX2 was largely affected by X, as shown in Scheme 6. PdCl2 as well as Pd2(dba)3 (dba = dibenzylideneacetone) are efficient catalysts, whereas the addition of phosphine ligands retards the phenylation e.g., PdCl PPh3 afforded no... 

Cyclocarbonylation of o-iodophenols 503 with isocyanates or carbodiimides and carbon monoxide in the presence of a catalytic amount of a palladium catalyst (tris(dibenzylideneacetone)dipalladium(O) Pd2(DBA)3) and l,4-bis(di-phenylphosphino)butane (dppb) resulted in formation of l,3-benzoxazine-2,4-diones 504 or 2-imino-l,3-benzoxazin-4-ones 505 (Scheme 94). The product yields were dependent on the nature of the substrate, the catalyst, the solvent, the base, and the phosphine ligand. The reactions of o-iodophenols with unsymmetrical carbodiimides bearing an alkyl and an aryl substituent afforded 2-alkylimino-3-aryl-l,3-benzoxazin-4-ones 505 in a completely regioselective manner <1999JOC9194>. On the palladium-catalyzed cyclocarbonylation of o-iodoanilines with acyl chlorides and carbon monoxide, 2-substituted-4f/-3,l-benzoxazin-4-ones were obtained <19990L1619>. 

An N-heterocyclic carbene ligand, formed from l,3-bis-(2,4,6-trimethylphenyl)-3//-imidazol-l-ium chloride and cesium carbonate, with dipalladium tris(dibenzylideneacetone) gave excellent yields (93-96% yields) in the Suzuki coupling of 2-chloropurines and arylboronic acids in anhydrous dioxane <2001TL8751>. The combination of an imidazolium-carbene and nickel(O) bis(cyclooctadiene) formed a catalyst capable of insertion into the C-F bond of 6-fluoropurine nucleosides (Scheme 35) <20050L1149>. 

Abbreviations arene, i/6-benzene or substituted benzene derivative bipy, 2,2 -bipyridyl Bu, Bu", Bu, iso-, n-, or rerf-butyl COD, 1,5-cyclo-octadiene Cp, /5-C5H5 DAD, dimethyl-acetylene dicarboxylate dam, 1,2-bis(diphenylarsino)methane DBA, dibenzylideneacetone DMF, A. A -dimethylformamide dpe, l,2-bis(diphenylphosphino)ethane dpen, os-l,2-bis(di-phenylphosphino)ethylene dpm, 1,2-bis(diphenylphosphino)methane ESR, electron spin resonance F6-acac, hexafluoroacetylacetone FN, fumaronitrile MA, maleic anhydride Me, methyl MLCT, metal ligand charge transfer phen, 1,10-phenanthroline Pr , Pr", iso- or n-propyl py, pyridine RT, room temperature TCNE, tetracyanoethylene tetraphos, (Ph2PCH2CH2)jP THF, tctrahydrofuran Xylyl, 2,6-Me2C6H3. 

A dry two-neck flask was charged with amine (1.00 mmol), bromide (2.40 mmol), base (2.80 mmol), ligand (20.0 mol%), tris (dibenzylideneacetone)dipalladium (0) (Pd2dba3) (10 mol%), and toluene (2.00 mL/mmol of amine). The reaction was flushed with N2 and heated at 90 °C. The mixture was diluted with ether, filtered through a pad of Celite, and the volatile components were removed reduced pressure to give a crude product, which was purified by flash column chromatography on silica gel. 

A very useful extension of the de Mayo reaction has been recently introduced by Blechert et al. (Scheme 6.26) [78]. The retro-aldol fragmentation was combined with an intramolecular enantioselective allylation (asymmetric ring-expanding allylation) catalyzed by a chiral Pd complex. Bicycloheptane 68, for example, was accessible by intermolecular [2 + 2]-photocycloaddition of cyclopentenone 67 with allene. Further transformation in the presence of Pd2(dba)3 (dba = dibenzylideneacetone) and the chiral oxazoline ligand 69 (tBu-phox) resulted in the enantioselective formation of cycloheptadione 70. 

Acylation of aryl and alkenyl halides. The zinc salts of enol ethers and allenic ethers couple with aryl and alkenyl halides in the presence of several Pd(0) catalysts Pd[P(QH,),l4 or bis(dibenzylideneacetone)palladium with added phosphine ligands. 

For the palladium dibenzylideneacetone complex (1), NMR data to support the proposition that the bis-phosphine acts as a bidentate ligand has been reported. A triflate salt of the TT-allyl palladium complex has been isolated and is stable in the solid state. However, no crystals suitable for X-ray analysis were obtained. An X-ray crystal structure of the ligand and a bis-palladium complex has been reported. The palladium complexes are generated just before use under an inert atmosphere exposure to air affords a catalytically inactive tetra-coordinated palladium(II) species. ... 

Diaryl-substituted isoxazolidines were synthesized in a one-pot reaction starting from aryl halides and 0-homoallyl hydroxylamines through a diastereoselective cascade reaction catalyzed by Pd(0). The choice of the phosphine ligand was shown to affect the product distribution between the isoxazolidine and the Heck-type product. The best results were obtained when 1 mol% of Pd2(DBA)3/P(o-Tol)3 was used (DBA = dibenzylideneacetone). For example, under these conditions, isoxazolidine 520 was obtained in 79% yield together with a minor amount of the Heck coupling adduct 521 (Equation 83) <2006TL927>. 

The crystal structure of this complex is not known, but the structure of the tris(dibenzylideneacetone)dipalladium(o) precursor has been reported [167, 168]. The two palladium atoms have a trigonal planar geometry arising from bonding to the olefinic groups of the three ligands. 

In dichloromethane the dipalladium complex exhibits a ferrocenyl-based reversible oxidation (E° = -t- 0.62 V) and a palladium-centered irreversible oxidation ( p = -t-1.33 V) [166]. Under the same experimental conditions, the 4-ferrocenyl-dibenzylideneacetone ligand displays reversible one-electron oxidation at E° = + 0.64 V, thereby indicating that the complexation by palladium atoms does not appreciably perturb its electronic structure. 

Similar studies in an organic solvent yielded almost the same product [66]. Nanostructured particles of amorphous carbon-activated palladium metallic clusters have been prepared (in situ) at room temperature by ultrasound irradiation of an organometallic precursor, tris-//-[dibenzylideneacetone]dipalladium [(p-CH= CH-CO-CH=CH-5 )3Pd2] in mesitylene. Characterization studies show that the product powder consists of nanosize particles, agglomerated in clusters of approximately 800 A. Each particle is found to have a metallic core, covered by a carbonic shell that plays an important role in the stability of the nanoparticles. The catalytic activity in a Heck reaction, in the absence of phosphine ligands, has been demonstrated. 

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