Dibenzylideneacetone complexes

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]. 

Palladium olefin complexes are considerably less stable than the platinum analogues. The dibenzylideneacetone complex Pd2(dba)3 (18-H-IV), however, is air-stable it is a convenient source of Pd° for catalytic applications. 

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. ... 

Interaction of the dibenzylideneacetone complex of palladium with a high surface area carbon gave a supported complex which on heating produced a Pd/C catalyst having a uniform distribution of palladium metal particle sizes. The rhodium carbonyls, Rh4(CO) 2 and Rh6(CO) 6, were adsorbed on silica to give... 

Heck reactions are often carried out using palladium(O) complexes. Tetrakis(triphenylphosphine)palladium(0) is frequently used, but does not allow the chemist to vary either the identity of the ligand, or the lig-andipalladium ratio. A more convenient mixture is to use the air-stable palladium(O) dibenzylideneacetone complex with the added ligands of choice. It is, however, not necessary to use a palladium(O) pre-catalyst. Palladium(II) salts, especially the more soluble palladium(II) acetate, are often used, with added phosphines. The palladium(II) salts are reduced to palladium(O) in situ. 

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. 

The complex is obtained in high yield by reaction of dimethyl acetylenedicarbox-ylate with bis(dibenzylideneacetone)palladium. The complex is only slightly soluble in the usual solvents. 

Another instance of C—H- O hydrogen bond yielding a supramolecular assembly (at least in the solid state) is from complexes 82 and 83 between 1,3,5-trinitrobenzene and dibenzylideneacetone or 2,5-dibenzylidenecyclopentanone, respectively, which were investigated by X-ray diffraction199. C—H- O hydrogen bonds (such as those of 82 and 83) are three to five times weaker than N—H- O (and O—H- O) bonds200. 

The choice of catalyst is important, for instance the use of tetrakis(triphenylphos-phane)palladium(O) complex results in the quantitative cyclotrimerization of 3,3-dimethylcy-elopropene.17 In similar fashion 3,3-dimethoxycyclopropene cyciodimerizes to 3 (R = OMe, 74%) using bis(dibenzylideneacetone)paUadium(0) [Pd(dba)2] complex.18 The trisubstituted cyclopropene 4 is transformed to the head-to-head dimer 5 in the presence of copper(I) iodide.19... 

Other reactions of dienes with metal atoms are only of a limited synthetic use. Dibenzylideneacetone (PhCH=CH—CO—CH=CHPh DBA) reacts with palladium vapor to afford Pd2(DBA)3, a complex in which the coordination is through the two C=C units and does not involve the C=0 (5, 92). Cobalt vapor undergoes an extremely complicated reaction with 1,4-pentadiene, producing pentenes, C5H6, and various polymers as well as the organometallic product, HCo( 1,3-pen tadiene)2, which involves isomerization from a nonconjugated to a conjugated diene (104, 110). 

Highly selective transformation of terminal acetylenes to either linear or branched carboxylic acids or esters may be achieved by appropriately selected catalyst systems. Branched esters are formed with high selectivity when the acetylenes are reacted with 1-butanol by the catalyst system Pd(dba)2/PPh3/TsOH (dba = dibenzylideneacetone) or palladium complexes containing PPh3. Pd(acac)2 in combination with various N- and O-containing phosphines and methanesulfonic acid is also an efficient catalyst for the alkoxycarbonylation of 1-alkynes to yield the branched product with almost complete selectivity.307,308... 

The phase-transfer method has also been employed for the carbonylation of benzylic halides to carboxylic acids.477 The palladium(O) complexes [Pd(PPh3)4] (103), [Pd(diphos)2] (104) and [Pd(DBA)2] (105 DBA = dibenzylideneacetone) were used as catalysts. With (103) and (104) the carboxylic acid was the major product. Complex (105) gave little or none of the acid, the toluene and bibenzyl derivatives corresponding to the benzyl halide used being formed. Benzyl esters of the carboxylic acid were sometimes present as minor products. The reaction has been adapted to provide a new synthesis of anthranilic acid derivatives (equation 106).478 Tri-n-butylamine was used to neutralize the HBr formed. 

Methods (i) and (ii) require palladium(II) salts as reactants. Either palladium acetate, palladium chloride or lithium tetrachloropalladate(II) usually are used. These salts may also be used as catalysts in method (iii) but need to be reduced in situ to become active. The reduction usually occurs spontaneously in reactions carried out at 100 °C but may be slow or inefficient at lower temperatures. In these cases, zero valent complexes such as bis(dibenzylideneacetone)palladium(0) or tetrakis(triphenylphos-phine)palladium(O) may be used, or a reducing agent such as sodium borohydride, formic acid or hydrazine may be added to reaction mixtures containing palladium(II) salts to initiate the reactions. Triarylphosphines are usually added to the palladium catalysts in method (iii), but not in methods (i) or (ii). Normally, 2 equiv. of triphenylphosphine, or better, tri-o-tolylphosphine, are added per mol of the palladium compound. Larger amounts may be necessary in reactions where palladium metal tends to precipitate prematurely from the reaction mixtures. Large concentrations of phosphines are to be avoided, however, since they usually inhibit the reactions. 

It is worth noting at this point that, for stable DBA (dibenzylideneacetone) Pd(0) complexes, three types, namely Pd(dba)2, Pd2(dba)3, and Pd2(dba)3-CHCh are... 

Palladium is generally introduced in catalytic amounts as the stable complexes Pd(PPh3)4 and Pd(dba)2 (dba = dibenzylideneacetone), frequently accompanied by a stabilizing phosphine [PPh3, Ph2P — (CH2) — PPh2, or others], or as a form of Pd(II) such as acetate, chloride, or acetylacetonate plus a phosphine. In the last case, the Pd(II) is reduced in situ to the catalytically active Pd(0) species. 

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. 

Pd-catalyzed intramolecular ene reactions.s Both this complex and bis(dibenzylideneacetone)palladium, Pd(dba)2, have been used to effect this intramolecular cyclization of dienes capable of forming allylpalladium complexes. 

Dibenzylideneacetone Pt° complexes catalyze the arylation of Si—H bonds via C—H bond activation of substituted arenes.139... 

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